Failure and reliability
Failure in lighting applications is closely tied to the concept of reliability, defined as a product's ability to perform its intended function over a specified time under expected conditions If any expected lighting characteristic—such as light output, color appearance, or light distribution—is not met, the system is deemed unreliable and may be considered to have reached the end of its useful or rated life.
A failure mode is defined as the direct outcome of a failure mechanism, where the latter serves as the cause of the former (Collins et al., 2013) An example of this is the browning or discoloration of an LED package lens material, which leads to lumen depreciation as the failure mode When this failure mode occurs gradually over time, it is categorized as a parametric failure Conversely, failure modes that lead to sudden and irreversible disruptions in device operation are termed catastrophic failures Both types of failure modes are relevant to LED systems and their components.
Generally, LED systems are composed of a number of subsystems and elements, including:
• LED(s) and printed circuit boards
• LED driver and power regulation source
• Electrical contacts, interconnections, and standardized bases
• Optical elements, including primary, secondary and tertiary lenses, optical guides, reflectors, and diffusers
Assuming a system series configuration, the failure of any of these components or subsystems could result in the malfunction or failure of the entire system [Goel and Graves, 2006].
Lamp Rated Life
Lamp life for traditional light sources is defined as the duration until 50 percent of a large batch of lamps fails under controlled laboratory conditions This rated lamp life is a statistically determined estimate of median operational life, expressed in hours and specified by the manufacturer, reflecting certain operating conditions and failure criteria For instance, a median life of 10,000 hours indicates that half of the tested lamps have lasted that long without failure, and this definition applies to large installations where at least 50 percent are expected to remain operational by the end of their rated life.
The operational definition of electric lamps and luminaires encompasses both catastrophic failure modes, such as lamp burnout in incandescent lamps, and parametric failure modes, like luminous flux depreciation and chromaticity shifts The International Electrotechnical Commission (IEC) outlines these considerations in definition 845-07-63, which describes "life to X % failures" as the duration during which a specified percentage of lamps, tested under defined conditions, reach the end of their operational life based on established criteria Traditional light sources have distinct failure mechanisms, with lifetime tests reflecting these differences; for instance, incandescent lamps undergo continuous operation testing, while fluorescent lamps are tested with specific on-off cycles, such as 2 hours and 45 minutes on followed by 15 minutes off, or a 3-hour on and 20-minute off cycle in the U.S.
9 were conducted under controlled ambient temperature, typically 25°C In addition to reporting light source lifetime defined this way, manufacturers also provided the expected lumen depreciation at
40 percent of the rated lamp life.
LED Rated Life
LEDs generate light through a process called electroluminescence, which allows them to efficiently convert electricity into light, resulting in high energy efficiency Unlike traditional electric light sources that rely on high temperatures or excited mercury atoms, LEDs gradually degrade in light output over time instead of burning out The lighting industry, following guidelines from the Alliance for Solid-State Illumination Systems and Technologies (ASSIST), typically rates LED life based on maintaining 70% of their initial light output, known as "L70." Additionally, other lumen maintenance criteria, such as 80% and 90%, as outlined in IEC 62612, can also be used to determine when an LED lighting system is no longer performing adequately and has reached the end of its useful life.
Similarly, light source color shift has been used as a criterion to define the end of LED life [ASSIST,
2005; IEC 62612, 2018] Light source color appearance is measured by the chromaticity (i.e., in the
The 1976 CIE u’,v’ chromaticity space is crucial for assessing light source chromaticity consistency, which is vital for effective architectural lighting Consistency in chromaticity, both among similar products and over time, has been highlighted since the early days of commercial phosphor-converted white LEDs, with varying criteria based on specific applications Recently, chromaticity maintenance has become an essential component in standards and specifications for market transformation programs, underscoring its significance in lighting quality, as noted in documents like IEC 62612 (2018) and guidelines from the United States Environmental Protection Agency.
Agency, 2020] Chromaticity shift tolerance zones are typically specified as a function of the radius of a circle in the u’v’ chromaticity space, for example a 2-step u’v’ circle [CIE, 2014].
LED System Rated Life
Present practice
Briefly, the L70 value is determined by operating individual LEDs under a standard set of conditions
(continuous operation; at least two different LED junction temperatures) for at least 6,000 hours [IES
The LM-80 test provides crucial data for evaluating lighting systems, allowing for the estimation of LED operating temperatures This information is incorporated into an exponential decay model to predict when the system will reach 70% of its initial light output.
The IES TM-21 standard, established in 2019, primarily focuses on the evaluation of LED lifespan; however, luminaire and replacement lamp manufacturers frequently utilize this procedure to assess and communicate the rated life of their products.
Limitations
LED lighting systems are often marketed for their extended lifetimes and slow lumen depreciation; however, studies indicate that catastrophic failures are prevalent and frequently the primary failure mode in replacement lamps and luminaires Common causes of these failures include issues with solder joints, electrical interconnects, and driver components.
Defining the lifetime of LED products solely through lumen maintenance assumes that their primary failure mode is a gradual decline in luminous flux over time It also implies that cycling the power on and off does not adversely affect the performance of LEDs or their systems.
Research indicates that power cycling significantly affects the lifespan of LEDs, LED arrays, and LED systems, highlighting the importance of managing power cycles to enhance the longevity of LED lighting solutions.
The performance of a lighting system relies on various critical factors, including operating temperature, humidity, on-off cycling, thermal management, luminaire housing, secondary optics, driver components, and the LEDs themselves A failure in any single component can lead to the failure of the entire system.
Using lumen depreciation as a predictor for LED product lifespan is problematic, as it focuses solely on the LEDs and overlooks other critical components in an LED lamp or luminaire In many cases, the overall lifetime of the LED system is more significant than the expected lifespan of the LEDs alone End users prioritize having a reliable lighting system available when needed, and other system components are likely to fail before the LEDs do.
To accurately predict the lifetime of LED systems, it is essential to consider both parametric and catastrophic failures; however, there is a lack of consensus among existing standards In North America, standards primarily focus on lumen depreciation of LED packages (IES LM-80) or complete LED products (IES LM-84, 2014) In contrast, international standards incorporate endurance tests to assess initial catastrophic failures (IEC 62612, 2018; IEC 62717, 2019; European Union, 2019).
Additional considerations
Temperature and humidity significantly impact the lifespan of LED systems, making it essential to consider the specific operating conditions of various applications when defining and testing LED system lifetime.
LEDs are utilized in a wide range of applications, each presenting distinct operating conditions for lighting systems, as illustrated in Table 1.1.
• Thermal environment The operating temperature of the LED and other system components is determined by these factors:
• Method of installation: e.g., fully ventilated (table lamps, street lights), semi-ventilated (recessed downlights, no ceiling insulation), enclosed (recessed downlights with ceiling insulation, in-ground outdoor fixtures)
• Installation site: solar exposed vs sheltered
• Geographical location: e.g., tropical vs temperate locations
• Seasonal variations: e.g., winter vs summer
• Total power dissipated by the system
• Relative humidity of the environment The relative humidity of the environment where the
LED and other system components is determined by these factors:
• Method of sealing the luminaire (hermetic) to prevent water reaching the LED die and package components
• Geographical location: e.g., tropical vs temperate locations; seaside with high salt, and relative humidity
• Pattern of use The pattern of use includes the number and the duration of cycles and is determined primarily by the application itself, for example:
• Residential: short but several on/off cycles per day
• Commercial, industrial: long but few on/off cycles per day
• Outdoor: long and typically single on/off cycle per day; failed daylight sensors
(photocells) can result in the lights operating during the day at a higher than normal temperature because of the solar thermal gain
Lighting usage patterns reflect the necessity and timing of illumination in a space, influenced by automatic controls such as occupancy sensors and scheduling Previous research has explored the usage times of various light sources in typical applications, including dimming techniques Key findings from these studies, detailed in Tables 2.1-2.5, provide a foundational understanding for establishing testing cycles for LED systems in life testing.
Table 2.1 Sample average daily operating hours by residence type and room (abridged data from United
Table 2.2 Sample average daily operating hours per lamp type by commercial building type (abridged data from United States Department of Energy, 2012)
INC HAL CFL LFL HID
Table 2.3 Sample average daily operating hours per lamp type by industrial building type (abridged data from United States Department of Energy, 2012)
INC HAL CFL LFL HID
Table 2.4 Default annual operating hours and expected average installation life for sample indoor applications in European Standard EN 15193-1 [EN, 2017] (after Lighting Europe, 2018)
Indoor application Default annual operating hours Average installation life
Table 2.5 Default annual operating hours and expected average installation life for sample outdoor applications in European Standard EN 13201-5 [EN, 2015] (after Lighting Europe, 2018)
Indoor application Default annual operating hours Average installation life
To accurately assess the operating temperature of LED systems, thermocouples can be utilized to monitor thermal profiles during multiple on/off cycles in their intended applications This data is essential for replicating conditions in laboratory life testing For outdoor applications, leveraging official weather reports, such as those from the European Environment Agency, can help establish a baseline for ambient temperature and relative humidity based on geographical regions and seasonal variations.
LED components and systems can experience various failure mechanisms, primarily due to environmental factors such as temperature and humidity, as well as power supply cycling It is crucial to define the useful lifetime of these systems and their testing methods by considering realistic worst-case scenarios that align with the intended application.
3 LED Package Related Failure Mechanisms, Test Methods, and
Introduction
Commercial white phosphor-converted (pc-) LEDs made their market debut in the mid-1990s, generating significant excitement in the semiconductor industry Initial claims touted their impressive lifespans of up to 100,000 hours, relying on metrics like Mean Time to Failure (MTTF) and Mean Time Between Failures (MTBF) that are standard for assessing the longevity of electronic components.
Failure Mechanisms
Research on the reliability and lifespan of gallium nitride (GaN) based LEDs and phosphor-converted (pc) white LEDs began in the late 1990s Following the introduction of white pc-LEDs in 1999, the Lighting Research Center (LRC) started testing 5-mm white LED devices, revealing that lifetime estimates based on Mean Time To Failure (MTTF) or Mean Time Between Failures (MTBF) were not applicable to the lighting industry Initial laboratory tests indicated that at a nominal current of 20 mA, these pc-LEDs diminished to 50% light output in about 6,000 hours This observation highlighted that LED light output decreased before a catastrophic failure occurred, prompting a new definition of the lifespan of white pc-LEDs based on the duration it takes to reach this critical light output level.
In 2001, Narendran et al reported that early 5-mm type pc-white LEDs exhibited rapid lumen depreciation, primarily due to the browning of the encapsulant surrounding the GaN blue chip, with a lumen maintenance of only 70 percent However, in 2002, Steigerwald and his team at Lumileds demonstrated improved lumen maintenance in their Luxeon high-power white pc-LEDs compared to the initial models Following this, researchers from Padova University, led by Levada, began investigating the failure and reliability of GaN-based pc-white LEDs in 2005 Their research included accelerated life tests of GaN LEDs under direct current, where they examined the failure mechanisms associated with high current stress.
Lumen depreciation and changes in light chromaticity are key parametric failure modes caused by various mechanisms, such as defect growth in the semiconductor chip and degradation of encapsulants and phosphors Notably, these issues are primarily driven by operation at elevated junction temperatures.
2000, 2001; Narendran, 2005; Narendran and Gu, 2005; Narendran et al., 2007; Cai et al., 2017] The literature reports increased degradation when the LEDs operate under high temperature and high humidity conditions
Operating under high humidity conditions can cause defects in the active region of LED chips, resulting in significant photometric and colorimetric alterations Research indicates that increased humidity may also damage the edges of LED chips, leading to diminished light output Furthermore, elevated temperature and humidity, along with short-wavelength irradiance, have been linked to the degradation of phosphor energy conversion and the darkening of encapsulation materials at the package level.
Volatile organic compounds (VOCs) trapped in silicone encapsulants of LED packages can lead to lumen depreciation and alterations in light color Additionally, studies indicate that these VOCs can cause changes in thermal resistance characteristics, impairing the heat transfer efficiency from the junction.
The main reasons for catastrophic failures in LED chips are inherent defects within the chips and the electromigration of metals into the chip, which is triggered by the metal contacts that are attached to them.
Semiconductor failures in LEDs often result from electrical and thermal stresses, leading to short-circuit issues (Meneghini et al., 2005; Lu et al., 2009) A prevalent cause of failure at the LED package level is solder joint failures, which disrupt the electrical path and create open circuits (Chang et al., 2010) Additionally, elevated temperature and humidity can cause interconnect corrosion at the LED chip and package level, where electromigration may remove silver from beneath the LED contact pads on the circuit board, increasing series resistance (Tan and Singh, 2014) Furthermore, humidity significantly reduces interfacial adhesion strength by 40–60% within the LED package, potentially leading to delamination at critical interfaces, including the LED chip's die attach and the chip-encapsulant interface (Hu et al., 2007; Zhou et al., 2009; Luo et al., 2010; Fan and Yuan, 2013).
Test Methods
In 2001, Narendran et al established the 70 percent lumen maintenance criterion (L70) based on assumptions regarding average lumen values and the economic lifespan of conventional light sources This definition was reinforced by human factors research indicating that observers typically require a 30 percent change in ambient illumination before perceiving noticeable differences.
Cree, Lumileds, and Nichia were among the first companies to adopt the L70 concept as a measure of LED life, leading to the recommendation by the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) to use L70 and a maximum chromaticity shift defined by a 4-step MacAdam ellipse for evaluating LED life in general illumination applications ASSIST's publication proposed a method for testing LED lumen maintenance performance at three different junction temperatures (Tj) and calculating L70 values using an exponential decay function This information is primarily intended for LED system manufacturers to aid in the proper design of LED lighting systems The testing methods outlined in ASSIST's 2005 publication laid the groundwork for the industry standard established by the Illuminating Engineering Society (IES) in IES LM-80-08, which details the approved method for measuring lumen maintenance of LED light sources.
The ASSIST and IES LM-80 testing methods identify key LED package failure modes, including lumen depreciation and chromaticity shift However, other potential failure modes, such as increased forward voltage (Vf), elevated thermal resistance (Rtheta), and catastrophic failure, should also be considered These failure modes arise from various stress factors, including thermal, electrical, mechanical, environmental, chemical, and radiation influences, which are critical in lighting applications.
The IES LM-80 standard has long been utilized to assess and report L70 values for new LED packages Recent advancements in encapsulant materials and packaging techniques have notably reduced lumen depreciation in pc-LEDs, necessitating data projection methods to estimate the duration until luminous flux diminishes to 70 percent In response to these developments, the IES has introduced TM-21.
The Technical Memorandum outlines the process of projecting long-term lumen, photon, and radiant flux maintenance of LED light sources, specifically utilizing LM-80 data to determine the L70 lifespan for LED packages The most recent updates in IES LM-80 (2020) and TM-21 provide essential guidelines for accurately extrapolating lumen maintenance data, ensuring reliable performance assessments for LED technologies.
21 (2019) do not mention L70 as a criterion to define “rated lumen maintenance life” [IES, 2008a,
2011, 2015, 2019a] Rather, IES TM-21-19 states that the extrapolation methods shall not be used to project lumen maintenance beyond L70 [IES, 2019a]
White LED lighting products began to gain traction in the mid-2000s, prompting the need for lifetime information from lighting specifiers for effective application Due to the lack of a standardized testing method for LED system lifetimes, the lighting industry relied on IES LM-80 and its modified versions to assess LED package lumen maintenance and determine system longevity.
IEC standards globally outline the test methods for LED modules, lamps, and luminaires, focusing on lumen depreciation at the test's conclusion and assigning a corresponding lumen maintenance category (e.g., IEC 62612, 2018; IEC 62717, 2019; IEC 62722, 2014) It is important to note that these IEC test methods do not aim to predict the lifespan of the products.
Various programs, such as the US EPA's Energy Star® and initiatives from the European Union, utilize rapid cycle test data—like 5 minutes on and 5 minutes off—to validate the reliability of LED products, alongside lumen maintenance test data.
Standards
For over a decade, the industry has relied on lumen maintenance values derived from the IES LM-80 test procedure to estimate LED system life These values serve as the foundation for projecting the time to reach L70, as outlined in IES TM-21 It is crucial to apply TM-21 methods cautiously, as projection accuracy is influenced by the number of samples and the duration of testing Notably, lumen maintenance projections should not exceed six times the LM-80 test duration Overall, LM-80 and TM-21 remain essential for estimating LED system lifetime, a topic further explored in section 6.
To address color shift, ANSI/IES TM-35, Technical Memorandum: Projecting Long-Term Chromaticity
The recently published standard, "Coordinate Shift of LED Packages, Arrays, and Modules" [ANSI/IES, 2019], introduces a method for predicting chromaticity shifts in LED packages based on extensive research into various color shift mechanisms It utilizes differential chromaticity analysis, applying curve-fitting techniques to chromaticity data over time to extrapolate changes in u’ v’ chromaticity space However, due to its recent release, there is limited public information available regarding its practical application.
4 LED Driver Failure Mechanisms, Test Methods, and Standards
Introduction
As LED package technologies have advanced, other system components are increasingly identified as the weak links, according to literature reports A frequently referenced study highlights that more than
A significant 70 percent of failure modes in a specific outdoor LED luminaire model, analyzed over multiple product generations and 212 million operational hours, are attributed to the driver The primary causes of driver failure include issues with solder joints, electrical connections, and critical circuit components such as MOSFETs, electrolytic capacitors, rectifying diodes, and opto-isolators.
[United States Department of Energy, 2011, 2014]
Research indicates that the proper functioning of key components significantly enhances the lifespan of LED drivers, as evidenced by existing literature on the subject.
The failure of components can occur in two main forms: parametric failure, which has been extensively studied (Han 2009; Han and Narendran, 2009; Sun et al., 2016; Zhang, 2017; Niu et al., 2018a, 2018b; Keil and Hofmann, 2019), and catastrophic failure, as highlighted in various reports (RTI International, 2013, 2019; Lall et al., 2015).
The literature discussed in this section attributes the following list of stressors as affecting the lifetime of LED driver components:
• Electrical overstressing (due to poor product design)
Electrolytic Capacitors
Research on the parametric failure of electrolytic capacitors in the output stage of LED drivers has been conducted by various scholars, including Han (2009), Han and Narendran (2009), Sun et al (2016), Zhang (2017), and Niu et al (2018a,b).
The failure mechanism of electrolytic capacitors involves the evaporation and deterioration of the dielectric material, leading to reduced capacitance and increased equivalent series resistance (ESR) Elevated temperatures are a primary stressor that accelerates dielectric dry out As ESR rises and capacitance falls, the degradation of the electrolytic capacitor becomes evident Manufacturers typically define the parametric end-of-life for these capacitors as a 10% to 20% decrease in capacitance and a 200% increase in ESR, measured at 120 Hz Research indicates that higher ESR and lower capacitance result in increased LED driver output current ripple, which can serve as a predictor of LED driver lifetime concerning parametric failure.
In their research, Han (2009) and Han and Narendran (2011) investigated the effects of constant elevated ambient temperatures, ranging from 150°C to 205°C, on various groups of capacitor samples They monitored the capacitor positive lead temperature as an indicator of internal temperature, while also assessing capacitance, equivalent series resistance (ESR), and output current ripple, which is defined as the percentage ratio of peak-to-peak current amplitude to mean current amplitude.
Narendran (2011) estimated capacitor lifetime by analyzing the second derivative of current ripple changes during testing The authors employed exponential extrapolation to predict application lifetime, specifically at a positive pin temperature of 100°C, by plotting capacitor lifetime against positive pin temperature using data from accelerated life tests.
Cracking and melting of metallized film capacitors, as identified by Keil and Hofmann (2019), are failure mechanisms linked to elevated humidity, which poses a significant stressor alongside high temperatures, echoing findings from multiple studies including Han (2009) and Zhang (2017) While not directly associated with LED drivers, research by Wang and Blaabjerg (2014) also highlights the impact of humidity on metallized film capacitors within power electronic converters.
Liang et al (2020) identified that LED drivers experience parametric failures caused by electrical stress, alongside temperature and humidity factors They established a failure criterion for output current ripple from the constant current driver, setting it at ≤10% The study conducted accelerated life testing on LED drivers under specific test conditions to evaluate their performance and reliability.
• Constant temperature (120°C) and humidity (75% and 95% RH)
• Constant temperature (120°C), humidity (85% RH), electrical stress (38 V)
• Constant humidity (85% RH) and temperature cycle (25°C to 85°C at a cooling or heating rate of 2°C/min, soak time 30 minutes, and cycle time 2 hours)
The authors observed that the output current of the LED driver decreases with increasing temperature, yet there was no significant failure or degradation noted They found that high humidity temperature cycles did not accelerate aging, although degradation increased notably within a relative humidity range of 55% to 85% Liang et al attributed the output current changes to a decrease in the Equivalent Series Resistance (ESR) of the output filter capacitor as temperature rises, highlighting interactions among devices that could lead to failure In 2018, Niu et al conducted tests at temperatures of 85°C and 100°C, measuring normalized capacitance and ESR over 3,000 hours They established a failure criterion based on a 20% capacitance reduction, noting that ESR changes significantly affect electrical and thermal performance, which is vital for estimating the LED driver's lifespan through Monte Carlo simulations.
A 2016 study by Sun et al investigated the aging of electrolytic capacitors in an RC linear driver at a high ambient temperature of 125°C The research measured the relative output power of these linear drivers connected to a stable LED load over time Utilizing a physics of failure (PoF) methodology, the authors employed Monte Carlo simulations to predict the lifetime and assess the probability of failure for the output stage electrolytic capacitor.
Zhang (2017, 2018) conducted accelerated life tests on quasi-flyback LED driver samples, subjecting them to temperature cycling between –40°C and 85°C The temperature changed at a rate of 5°C per minute, with a soak time of 2 hours at –40°C and 22 hours at 85°C To measure degradation, the authors quantified changes in capacitance, defining the lifetime as the time taken to reach a ±25% change from the initial capacitance value.
Zhang (2017, 2018) introduced an enhanced part stress analysis (PSA) method that effectively predicts the reliability and lifespan of LED drivers This improved PSA demonstrated a strong correlation with data from accelerated life testing, achieving accurate lifetime estimates at a 90% survival rate for LED drivers, in contrast to the predictions from the Military Handbook on reliability.
The improved PSA method, utilizing the MIL-HDBK-217F exponential failure distribution, significantly enhances the reliability of LED drivers by addressing the operating conditions of critical components like electrolytic capacitors A study conducted by Zhang in 2017 validated this improved model using data from Chelminski (2016), demonstrating that the failure rates of electronic equipment were approximately two times higher than previously estimated.
Lall et al (2015) conducted tests on LED drivers using high temperature storage life acceleration at 135°C, along with constant temperature and humidity conditions at 85°C and 85% relative humidity They found no signs of parametric degradation in capacitance and ESR measurements during the humidity test, although 4 out of 10 catastrophic failures occurred In contrast, the high temperature storage testing revealed measurable degradation in the ESR and capacitance of aluminum electrolytic capacitors.
In 2013, RTI International reported that 2 out of 17 tested 6-inch downlights experienced catastrophic failures during their Hammer testing of fifteen 6-inch downlights and two 2×2 troffers, with detailed testing conditions outlined in Section 6 of the report.
In 2019, RTI International conducted an accelerated life test on LED drivers, exposing them to high stress conditions of 75°C and 75% relative humidity, while implementing a power cycling regimen with a 50% duty cycle over two hours The study revealed that 2 out of 11 LED driver samples experienced catastrophic failures due to capacitor issues, occurring between 2,800 to 4,000 hours of operation Overall, there were 7 driver failures attributed to various components, with the EMI filter capacitors in Stage 1 identified as the most prone to failure.
MOSFETs (Metal Oxide Semiconductor Field Effect Transistors)
According to Lan et al (2012, 2014), parametric failure of MOSFETs can be caused by:
• Hot carrier injection (HCI) at the MOSFET
• Electromigration (EM) at the output of the MOSFET
Lan et al found that high output voltage and elevated operating temperatures significantly accelerate failure mechanisms in LED drivers In their research conducted in 2012 and 2014, they tested LED drivers at 120°C with a maximum output voltage of 17 Vdc and a maximum output current of 45 mA They recorded the I-V curves of the drivers every 24 to 48 hours, utilizing a thermoelectric cooling device to maintain a stable temperature of 18.5°C during measurements The authors noted alterations in the current to voltage relationship of the MOSFETs used in linear mode LED drivers, including a noticeable shift in the knee-point of the I-V curve.
V curve over time, which the authors claim to be useful for predicting MOSFET failure
Lan et al (2012) demonstrated that black box testing and degradation models effectively explain LED driver degradation phenomena without requiring detailed circuit knowledge, which is crucial for LED systems testing However, access to the critical parameter dI/dV, associated with the degradation index for LED degradation, is essential By analyzing the degradation rate of dI/dV over time, it is possible to identify the corresponding failure mechanisms, such as Hot Carrier Injection (HCI) or Electromigration (EM).
A study by Lall et al (2015) found that consistent exposure to a temperature of 85°C and 85% relative humidity resulted in short-circuit failures in electronic components, ultimately leading to the catastrophic failure of the driver Specifically, 3 out of 10 IGBT/MOSFETs experienced top blow-off, likely caused by electrical surges from moisture infiltration, which severely compromised the system's integrity.
A 2019 study by RTI International revealed that out of seven total failures in LED drivers, two catastrophic failures were linked to MOSFETs during a power cycling test conducted at 75°C and 75% relative humidity.
Driver Printed Circuit Board (PCB)
Keil and Hofmann (2019) highlighted that elevated ambient humidity can lead to galvanic corrosion at the solder interconnects on PCB assemblies in LED drivers They suggest that cleaning the PCB assembly post-soldering can significantly mitigate this type of corrosion.
Vandevelde et al (2018) discovered that high power LEDs mounted on insulated metal substrates have a limited lifespan due to fatigue fracturing of solder connections during temperature cycles They noted that the number of cycles until failure decreases with higher temperature excursions, as well as with increased maximum solder temperature and longer dwell times at peak temperatures The authors asserted that the failure cycles adhere to a Weibull distribution, indicating a consistent function shape across various thermal cycling tests, thereby confirming that solder joint failures occur irrespective of the temperature profile.
Soltani et al (2018) conducted a study on substrate and surface-mounted LED devices, examining their performance under two drive currents (75 mA and 100 mA), three ambient temperatures (25°C, 85°C, and 105°C), and three substrate materials, including FR-4 The research revealed that the coefficient of thermal expansion (CTE) mismatch between the materials in the LED package and the substrate led to mechanical failures, such as die attach delamination and lens cracking, resulting in both catastrophic and parametric failures Utilizing L70 as the failure criterion, the authors applied an Arrhenius-type equation to estimate the lifetime of the tested samples.
In 2013, RTI International conducted a hammer test (described in section 6) and found that 6 out of
A study of 17 LED luminaires revealed that 12 failures were due to catastrophic PCB failure, primarily attributed to extreme cyclical thermal stress during testing Notably, six of these failures originated from two specific 6-inch downlight product families designed for indoor use.
To the best of our knowledge, no further investigations have been undertaken to predict LED driver lifetime based on this failure mechanism.
Standards
In the absence of LED driver-specific standards, and given that most LED drivers presently are electronic in nature, manufacturers have used the Telcordia Reliability Prediction Procedure
When estimating the lifetime of LED drivers, it is essential to reference established guidelines such as Telcordia SR332 (2016) and the Military Handbook MIL-HDBK-217F (1991) from the United States Department of Defense Additionally, adherence to the IEC 62384 standard, which pertains to DC or AC supplied electronic control gear for LED modules, is crucial for ensuring reliability and performance.
Performance Requirements includes guidance on how to quote product life and failure rate in Annex
Manufacturers are urged to disclose the maximum temperature at the critical location of their products that allows for a lifespan of 50,000 hours Additionally, to facilitate product comparisons, they should report the number of failures that occur over time when the product is continuously operated at this maximum temperature.
5 LED System Reliability and Life Test Methods and Standards
Background
LED lighting products are often praised for their potential long service life when utilized correctly However, the lighting industry requires a standardized definition of LED system life, along with a reliable testing method to accurately assess their longevity across various applications.
LED systems consist of various components such as LED packages, printed circuit boards (PCBs), secondary optics, drivers, heat sinks, and mechanical housings The specific arrangement of these components varies by product, and the failure of any single component or subsystem can result in the overall system malfunctioning.
Figure 5.1 Anatomy of a sample LED product
Until around 2007, failure analysis studies primarily focused on individual components like LED packages, secondary optics, and drivers Since then, research has shifted towards understanding LED system reliability and lifespan, aiming to identify system-level failure modes, the frequency of component failures, and their underlying mechanisms A key aspect of these system-level studies is the examination of interactions among components, which are often overlooked in component-level analyses but are vital for assessing overall system reliability In 2010, the U.S Department of Energy released a significant report on LED technology.
Luminaire Lifetime: Recommendations for Testing and Reporting [United States Department of
Energy, 2010] This report put forward one very important clarification, namely that “reliability” and
The terms "lifetime" and "life" are often used interchangeably in publications about LED systems, but they are not synonymous According to a DOE report, a luminaire or lamp consists of various interdependent components and subsystems, each with unique life and reliability characteristics Therefore, it is essential to evaluate the entire system as a whole to understand its performance and longevity accurately.
“lifetime” or “end of life” was defined by when there is no light emitted
With the ongoing introduction of LED products into the market, the demand for reliable system-level testing and longevity assessment has been addressed using traditional methods previously employed by manufacturers for various electronic systems.
• Highly accelerated life testing (HALT)
• Step stress accelerated degradation testing (SSADT)
Accelerated life testing methods are designed to identify weak components and address infant mortality issues, typically functioning as pass-fail tests While these tests may not replicate the actual conditions faced by systems in their intended applications, the assumption is that if a product withstands 1000 cycles under overstress, it should perform reliably under normal use Recent research has contributed to the creation of short-period life testing methods that can accurately estimate the reliability of LED systems across various applications.
This section summarizes accelerated life testing methods, followed by an overview of testing techniques designed to estimate the lifespan of LED systems under conditions akin to typical lighting applications.
LED System Reliability and Life Test Methods
Accelerated life test methods
This section describes four types of accelerated test methods that have been used for assessing LED lighting systems’ reliability or lifetime
5.2.1.1 Highly accelerated life testing (HALT)
HALT, or Highly Accelerated Life Testing, is a crucial pass/fail test designed to identify the upper thermal destruction limits of various products, including mechanical and electronic devices The primary goal of the HALT process is to expose the device to stress environments that exceed typical application conditions, allowing for the assessment of its operational and destruction thresholds.
Developed in the 1980s, HALT (Highly Accelerated Life Testing) is an experimental testing method designed to uncover design vulnerabilities in electronic devices This technique is most effective during the product development phase, where devices are exposed to various stressors to identify potential weaknesses.
HALT (Highly Accelerated Life Testing) is not suitable for systems requiring detailed degradation mechanisms, such as LED products where lumen maintenance and chromaticity shift are crucial for lifetime estimation Instead, HALT is recommended as an initial step before conducting step-stress accelerated testing, which helps identify the upper and lower destruction limits of the device For instance, Cai et al suggest this method to determine destructive limits for LED systems and subsystems, where three samples, including a golden sample, were subjected to temperatures ranging from 55°C to 135°C in 11 steps, each with a 12-hour dwell time After stabilization at 25°C, the total light output was measured using an integrating sphere, while the junction temperature was assessed through a pulsed current method.
Cai et al (2012) utilized HALT test data to identify additional degradation mechanisms and modes under specific stressors for future testing Their analysis of light output and junction temperature revealed that LED products exhibited irregular behavior at temperatures exceeding 110°C, indicating potential stress conditions that could contribute to further degradation mechanisms compared to data collected below this threshold.
5.2.1.2 Highly accelerated decay testing (HADT)
HADT, akin to HALT, is utilized to determine the upper operating limits of products, as demonstrated in Figure 5.2 [Cai et al., 2016a] In this method, the sample device undergoes a gradual increase in stress levels while maintaining an approximate dwell time of 12 hours During this period, degradation parameters such as estimated junction temperature (Tj), luminous flux, and chromaticity are measured at specific intervals, including after the first hour and at the beginning of the last hour, paralleling the HALT testing approach outlined by Cai et al (2012).
HADT, like HALT, is utilized to ensure consistency in degradation mechanisms by estimating the goodness of step-fitness based on the predetermined degradation mechanism of the accelerated product (α), aiming to keep the value between 0 and 0.2 For instance, in a study on LED lifetime testing, Cai et al (2016a) achieved a Δα of less than 0.1.
Figure 5.2 Comparison between HALT and HADT tests [Cai et al., 2016a]
The HADT methods serve as an essential initial approach for determining a product's maximum operating limits and its output parameters under these conditions This critical data informs the stress limits for subsequent testing methods, like SSADT, which are used to evaluate the lifespan of LED systems or subsystems [Cai et al., 2012, 2016a].
5.2.1.3 Step-stress accelerated degradation testing (SSADT)
Accelerated degradation testing (ADT) is an alternative approach to accelerated life testing used to assess the lifetime of devices The ADT process assumes:
• Reliability is related to product quality characteristic degradation over time
• Collected degradation data at higher levels of stress can be used to predict a product’s lifetime at a use-stress level
Step stress accelerated degradation testing (SSADT) is an advanced form of accelerated degradation testing (ADT) known for its ability to reduce testing durations and accommodate small sample sizes, leading to significant resource and cost savings.
The SSADT method, introduced by Nelson in 1980, utilizes experimental data and cumulative exposure modeling to forecast product lifetimes In 2000, Tseng and Wen applied SSADT techniques to estimate the useful lifespan of indicator LED packages Over the last decade, several research efforts have led to the development of two variants of the SSADT test, as documented in studies by Cai et al (2012, 2015, 2016a, 2016b, 2017), Hao et al (2016, 2017), Gong et al (2012), Ren et al (2012), and Yang et al (2012).
• Step-up stress accelerated degradation testing (SUSADT)
• Step-down stress accelerated degradation testing (SDSADT)
There are key assumptions in the formulation of the SSADT, including [Tseng and Wen, 2000; Cai et al., 2012, 2015, 2016a, 2016b, 2017; Tian and Yang, 2014; Hao et al., 2017]:
• The performance degradation of test samples is irreversible
• The failure mechanism and failure mode of test samples remain unchanged in each of the accelerated stress levels
• Under different stress levels, ADT data have the same distribution pattern and pseudo failure life, and these data should be subject to the same type of distribution
• Test samples have “no memory characteristics,” i.e., they cannot be returned through any conditioning/processing to their initial performance characteristics
• Residual life is not affected by the damage accumulation method
• Residual life has nothing to do with the accumulation method; depending on the loaded stress level and accumulated partial failure
• The process of performance degradation can be described by a linear or linearized expression
Recent studies have explored the effects of multiple stressors, such as temperature and humidity, to enhance the understanding of failure mechanisms [Cai et al., 2012, 2015, 2016a, 2016b, 2017; Hao et al., 2016, 2017] For instance, Cai (2016a) investigated light output depreciation at a relative humidity of 85%, while varying ambient temperatures at 65°C, 85°C, and 95°C in the SUSADT, and decreasing from 95°C, 85°C, to 65°C in the SDSADT The researchers emphasized the necessity of ensuring that these multi-stress conditions effectively accelerate typical degradation mechanisms without introducing atypical degradation factors.
Before performing the SSADT test, it is essential to carry out HALT or HADT tests to identify the maximum stress levels for each degradation condition in the SSADT process, as highlighted by Tseng and Wen (2000) and various studies by Cai et al (2012-2017) and Hao et al (2016-2017).
The Hammer test is a HALT method designed for rapid screening and pass/fail testing to assess the reliability of devices under extreme conditions, particularly for solid-state lighting luminaires Intended to induce failures within a testing period of less than 2,000 hours, the test aims to provide qualitative insights into potential failure modes While the designers caution against using the Hammer test as a universal accelerated life test, data from tested luminaires can be analyzed using a Weibull model, which supports the assertion that the Hammer test functions as an accelerated test based on its shape parameter (β).
The Hammer test consists of 42-hour loops with four stages of different environmental stresses modeled after common stress tests used in the electronics industry The four stages include:
Stage 1 involves a 6-hour test under stable environmental conditions at 85°C and 85% relative humidity, incorporating a power cycling pattern of 1 hour ON followed by 1 hour OFF This testing protocol is based on the standards set by the Electronics Industry Association (EIA) and the Joint Electronic Devices Engineering Council (JEDEC) under EIA/JEDS22-A101-B.
Stage 2 of the testing process involves cycling temperature shock, where the device is subjected to a range of -50°C to +125°C for 15 hours This stage includes a 30-minute hold time at each temperature extreme while maintaining constant power ON operation Additionally, the temperature transition must occur in less than 5 minutes, with a controlled relative humidity of 40% This test adheres to the JEDEC standard JESD22-A104D.
• Stage 3: Repeat of stage 1 for a 6-hour duration with 1-hour ON and 1-hour OFF power cycling under steady-state at 85°C and 85% RH
Stage 4 involves a high-temperature soak test lasting 15 hours at 120°C, with an operational cycle of 1 hour ON and 1 hour OFF, while maintaining a relative humidity of 40% This testing phase is based on the JEDEC standard JESD22-A103C, test condition A Notably, the test duration of 15 hours is significantly shorter than the minimum recommended 1,000 hours outlined in the JEDEC standard.
Predictive life test methods
The lighting industry requires a standardized definition and testing method to accurately estimate the lifespan of LED systems across various applications Effective testing should replicate real-world conditions, including environmental factors and power switching patterns, while addressing key failure modes identified by research from the Lighting Research Center (LRC) Although accelerated life test methods have been somewhat helpful, initial system-level research primarily focused on lumen maintenance tests, such as the IES LM-80 method, to assess LED lifespan In 2006, the LRC initiated a multi-year study for the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) to establish comprehensive testing methods beyond lumen depreciation This research highlighted that testing LED products, like downlights, at room temperature may not accurately reflect their performance in practical applications Consequently, relying solely on lumen maintenance data from IES LM-80 for estimating LED system life can lead to inaccuracies, as it overlooks multiple factors influencing lumen depreciation, including optical and electrical degradation.
In 2009, a collaborative study by Sari and colleagues from Singapore, the Netherlands, and the UK focused on LED systems, particularly examining the testing and modeling of flux degradation over time The researchers highlighted the possibility of multiple degradation mechanisms affecting LED performance.
In recent studies, researchers have focused on enhancing the reliability and lifespan of LED systems Sari et al (2009) developed a degradation model that improves system lifetime estimates by considering parametric failures Building on this, Luo et al (2010) investigated the impact of moisture on LED module reliability, revealing that moisture infiltration into packaging materials not only reduces light output but also heightens the risk of electronic failure This underscores the significant role of moisture in lumen depreciation, indicating that multiple factors contribute to parametric failures in LED systems.
Power cycling has become a crucial factor in system-level research for life testing of LEDs, as highlighted by Wu (2010), who found that an increased temperature difference (ΔT) between maximum and minimum temperatures during power cycling leads to a reduced lifespan of LED arrays due to failures in solder interconnects Subsequent studies confirmed a strong correlation between ΔT and cycles to failure under power cycling conditions for high-power LEDs Notably, rapid power cycling was shown to inflict minimal damage on LED systems, as the quick cycles result in minor temperature changes at the LED-to-PCB interface, thereby limiting thermal stress Additionally, while the maximum junction temperature (Tj) had little impact on cycles to failure when maintained below the breakdown threshold, it was significantly linked to lumen depreciation.
In 2010, a significant advancement in the literature on lighting systems highlighted the critical role of power cycling, a common condition in lighting applications This research revealed that power cycling can lead to catastrophic failure mechanisms that are often overlooked in lumen maintenance tests such as IES LM-80 and LM-84.
Follow-up studies examined the variations in LED junction temperature when an LED A-lamp is turned on and off The findings revealed that the junction temperature rises to a peak and stabilizes after approximately 60 minutes of operation Conversely, when switched off, the temperature cools down to room temperature and also stabilizes after about 60 minutes.
The experiment demonstrated that rapid cycling of LED A-lamps (2 minutes on, 2 minutes off) causes minimal temperature changes, resulting in low thermal stress and no damage to the LED components However, the study highlighted the necessity of slower cycling tests to accurately replicate real-life applications and identify potential failure modes It was found that to predict LED system lifetime effectively, slow power cycling is essential to ensure maximum temperature difference (ΔT) between stabilized maximum operating temperature and average room temperature Research indicated that frequent rapid cycling could shorten LED lifespan by causing significant thermal stress at solder joints, leading to catastrophic failure The importance of slow thermal cycles has been supported by multiple LED life test studies over the past decade.
The temperature profile of the LED A lamp during standard on-off cycles is depicted on the left, while the right illustrates the temperature changes when the LED A lamp is switched on and off rapidly, with intervals of 2 minutes on and 2 minutes off (Lighting Research Center, 2010).
In 2011, the LRC initiated a study aimed at developing a shorter test method to predict the failure of LED luminaires under real-world conditions, focusing on catastrophic and parametric failures The research acknowledged that lighting systems often experience on/off cycles, such as 12 hours on and 12 hours off in offices, or 4 hours on and 8 hours off in home settings Contrary to the prevailing belief that LED products do not fail catastrophically, LRC findings revealed that power cycling can lead to significant failures in LED systems Consequently, it is crucial to incorporate slow power cycling in life testing LED systems to accurately estimate their lifespan in practical applications.
In 2011, Li et al from Philips Lighting in Shanghai published a study on LED systems testing, proposing a reliability prediction model that considers failures in mechanical, optical, and electronic subsystems They emphasized that each subsystem has various failure modes, each with distinct failure distributions, necessitating reliability tests that address these stresses The paper provides a thorough analysis of failure modes, including both parametric and catastrophic failures Notably, the authors referenced established standards like MIL-STD-217 and Telcordia SR332 for estimating electronic component lifetimes based on known variables such as temperature, current, voltage, and power They calculated individual component failure rates to estimate the overall failure rate of LED drivers However, it is important to note that recent unpublished results from LRC indicate discrepancies in lifetime estimates for LED drivers using these standards.
The United States Department of Defense (1991) and Telcordia SR332 (2016) methods lack accuracy in predicting the time to failure and identifying failing components To achieve more precise lifetime estimates for LED drivers, we recommend testing the entire driver in an environment that closely resembles the actual application conditions and usage patterns.
Between 2012 and 2015, researchers focused on LED lumen depreciation and the development of models to determine L70 A significant study conducted by Meneghini and colleagues at the University of Padova in 2012 revealed that high power LEDs experience considerable degradation in their electrical and optical characteristics when operated near their current and temperature limits This finding indicates that LED lumen depreciation is closely linked to both optical and electrical factors Therefore, when creating extrapolation models for lumen depreciation, it is crucial to account for various degradation functions and rates, as well as the fact that degradation may occur at different stages throughout the LED's operational lifespan.
In 2013, Koh et al proposed using the L95 criterion to reduce the standard testing time of 6,000 hours commonly employed in the industry However, this approach has a significant flaw: it assumes a constant depreciation rate post-testing, which contradicts evidence from their own research and can lead to inaccurate lifetime projections Additionally, while Koh et al focused solely on lumen depreciation, they neglected to consider the risk of catastrophic failures that may not manifest within the shortened testing period This oversight raises concerns that such failures could occur later, further compromising the reliability of lifetime estimates.
Shailesh et al (2018) published a paper titled “Understanding the Reliability of LED Luminaires,” aimed at educating manufacturers and end users about the overall reliability of LED luminaires and their subsystems The authors highlight that various subsystems introduce critical reliability issues that affect the system's lifespan They present a comprehensive theory for assessing the reliability of optical, electrical, and thermal subsystems, which is beneficial for designing experiments to evaluate the remaining life of LED luminaires Additionally, the paper summarizes research from notable institutions worldwide, including Philips Lighting, the Chinese Academy of Sciences, and the Lighting Research Center in the U.S., making it a valuable resource for those interested in different testing methodologies.
Laboratory validation studies
This section describes recent and relevant studies that aimed at validating a life test method for LED systems by comparing test results from large sample sets to expected life values
5.2.3.1 LRC large scale laboratory validation study of a lifetime test method
In 2013, with support from the Bonneville Power Administration (BPA), New York State Energy Research and Development Authority (NYSERDA), and ASSIST, the Lighting Research Center (LRC) initiated a long-term study aimed at developing an accelerated test method to accurately predict the lifespan of LED systems across various lighting applications This innovative method evaluates the entire LED system, incorporating on-off power cycling with adequate dwell time, and takes into account both catastrophic and parametric failures, specifically the L70 standard The study focuses on commercially available LED A-lamps and MR fixtures.
16 lamps, and integrated LED downlights (a total of 287 samples) were subjected to different test conditions of delta temperature and dwell time [Lighting Research Center, 2016; Narendran et al.,
2016, 2017; Narendran, 2017] Appendix A describes this study in greater detail
Products selected for long-term life testing: The following types of LED products were tested in the three stages of this study:
• Commercially available, Energy Star® rated LED A-lamp product, rated as a 75 W incandescent replacement (90 samples tested)
• Commercially available, Energy Star rated LED MR-16 lamp product, marketed as a 50 W incandescent replacement (90 samples tested)
• Two commercially available, Energy Star rated LED downlight luminaires: Downlight 1 was 14
W and marketed as a 75 W incandescent replacement (80 samples tested), and Downlight 2 was 11 W and marketed as 60 W replacement (27 samples tested)
Samples of various LED product types were installed in luminaires under typical operating conditions Thermal sensors were attached to the LEDs to measure thermal profiles, allowing for the determination of lower and upper junction temperatures (Tj) commonly encountered in applications using these products, along with the time needed for stabilization.
The study determined the time required for the system to achieve maximum temperature (full stabilization) after activation and the duration needed to cool down to room temperature (full stabilization) after deactivation Based on these results, three delta temperatures (ΔT) and three dwell times were chosen as independent variables for the long-term study.
A summary of the results for the A-lamp products tested is provided, with similar analyses conducted for two other systems, detailed in the full project publication by the Lighting Research Center (2016).
The summary of catastrophic failures for LED A-lamps under various test conditions is presented in Table 5.1 The median life, defined as the average time between the fifth and sixth lamp failures, indicates that higher ΔT conditions lead to shorter times to failure for both dwell time scenarios Notably, shorter dwell times correlate with reduced time to failure at 80°C and 90°C, although the median life at a ΔT of 100°C shows a shorter duration for the 4-hour dwell time compared to the 2-hour dwell time This anomaly arises from cumulative damage incurred during each transition, influenced by temperature changes Further analysis reveals that 84% of failures were attributed to broken solder joints between the LED and the PCB, while 16% resulted from driver failures.
Table 5.1 LED A- lamp catastrophic failure times for each test condition (ΔT and dwell time)
Delta time-averaged temperature (°C) Time to failure (median life)
(hours) ΔT/Dwell Condition 2 hours 4 hours 2 hours 4 hours
The life of an LED system is significantly influenced by its on-off switching, as demonstrated in Figures 5.4 (a) and (b) The left panel illustrates an inverse linear relationship between the number of cycles to failure (median life) and delta time-averaged temperature, with a strong goodness-of-fit (R² > 0.9) This relationship allows for the inference of cycles to failure for 1-hour and 3-hour dwell times, which were then converted to time to failure in the right panel It is evident that shorter dwell times lead to faster failure rates due to more frequent switching In continuous-on scenarios, where the lamps were not switched, the cycles remained at one, resulting in zero failures at 80°C, 7,000 hours at 90°C, and 1,100 hours at 100°C These findings underscore the importance of considering power cycling as a critical factor in LED system life tests, rendering the number of cycles to failure an irrelevant parameter.
Figure 5.4 (a) Cycles to failure as a function delta time- averaged temperature (ΔTavg); (b) Time to failure as a function of dwell time for the different ΔT values
LED A-lamp lumen depreciation studies revealed that most lamp samples experienced catastrophic failures before reaching the L70 light output threshold, indicating that these failures occurred more rapidly than parametric failures To assess parametric life, L70 values were extrapolated from lumen depreciation data collected prior to catastrophic failures The median L70 lamp life, expressed in hours, is detailed in Table 5.2 Figure 5.5 illustrates an inverse linear relationship between median life and maximum operating temperature, with a high goodness-of-fit (R² > 0.9) As maximum operating temperature increased, estimated L70 values decreased Notably, the projected L70 values across various test conditions were consistent, suggesting that temperature cycling during the short test duration had minimal impact on lumen depreciation, contrasting with the significant correlation observed with maximum operating temperatures at each ΔT.
Table 5.2 Maximum operating temperature (ΔTavg) values and time to failure values for the different ΔT and dwell time conditions
Maximum operating temperature (°C) Time to L70 (hours) ΔT/Dwell
Condition 2 hours 4 hours Continuous-on 2 hours 4 hours Continuous-on
Figure 5.5 Time to failure due to lumen depreciation, L70, as a function of maximum operating temperature
In conclusion, the studies revealed the presence of both catastrophic and parametric failure types in LED systems By utilizing the proposed accelerated test procedure, it is possible to predict the lifespan of LED systems within a 3000-hour testing period by monitoring the pin temperature and the on-off switching cycles.
The mass of the LED system and the surrounding ambient air significantly impact the final junction temperature and stabilization time of the fixture Power cycling can lead to catastrophic failures in susceptible products due to excessive thermal stress from differing coefficients of thermal expansion (CTE) between adjacent components This indicates that the life of LED systems is adversely affected by on-off switching, challenging previous assumptions Additionally, the maximum operating temperature plays a crucial role in the lumen depreciation rate By analyzing catastrophic failure lifetimes in relation to time-averaged temperature differences (∆T) and parametric failure lifetimes (L70) against maximum temperatures (Tmax), one can estimate the system's lifespan under specific pin temperature and switching cycle conditions.
The ASSIST document titled “Recommendations for Testing LED Lighting Systems and Projecting System Lifetime in Different Applications” outlines how testing results can estimate the expected lifetime of LED products under varying conditions, such as operational temperature and power cycling patterns A subsequent study confirmed the accuracy of this method, showcasing that a low-cost, short-duration testing setup is feasible for validating these lifetime predictions.
5.2.3.2 CPUC large-scale laboratory LED lamp test
In 2017, the results of a large-scale, multi-year laboratory test of LED lamps, conducted for the California Public Utilities Commission (CPUC), were published [Itron and Erik Page & Associates,
The study aimed to evaluate the impact of two prevalent temperature-related stress conditions on LED performance and longevity, while also examining the real-world factors affecting LED reliability It sought to gather data to refine the effective useful life (EUL) assumptions for LED lamps in California's investor-owned utilities' energy efficiency programs A research plan was established to investigate the primary stressors in residential settings, specifically high operating temperatures and thermal cycling caused by frequent on-off switching To develop the testing procedure for the CPUC product testing, researchers from the LRC were consulted during the study.
The study’s authors evaluated the impacts of the stress test conditions on efficacy, color quality, useful life, and differences in performance between LED lamps compliant with the “California
A study evaluated the performance of 92 lamp models, including A-lamps and reflector types, across three residential fixture types Initial photometric results indicated that California Quality Spec-compliant lamps performed 20% lower in efficacy than non-compliant lamps Maintenance tests revealed that 24% of the tested units failed or showed pre-failure signs within 4,500 hours, with A-lamps exhibiting the highest failure rates However, none of the retrofit trim kits failed Final testing indicated that only 1.5% of surviving lamps experienced significant light output decreases, and 2.2% showed objectionable color temperature changes The analysis identified contact failures due to poor solder connections as the primary issue, highlighting that elevated operating temperatures and frequent on-off switching are critical stressors leading to early LED lamp failures The findings suggest that current industry testing standards may not adequately account for these common field conditions.
The study highlights the potential for certain models of LED lamps to exhibit latent manufacturing defects, particularly under specific operating temperatures and switching patterns It suggests that incorporating short-run reliability tests focused on temperature-related early failure modes could enhance current standardized performance tests The authors recommend investigating the necessary total on-time and switching cycles to accurately project failure rates, as well as developing formal adjustments to the Effective Useful Lives (EULs) for LED lamps in various investor-owned utility programs.
The CPUC study plays a crucial role in developing and validating LED product testing methods; however, it is important to note that the cycles used did not achieve full thermal stabilization or include dwell time, leading to potentially overestimated results regarding catastrophic failure timelines The failure of the solder joint between the LED terminal and the PCB was attributed to fatigue damage from thermal expansion coefficient mismatches Incorporating a dwell time is essential for achieving complete stabilization, as it increases stress levels beyond those observed at 95% stabilization, resulting in longer solder failure times and extended catastrophic failure lifetimes.
Figure 5.6 Thermal switching cycle used in the CPUC large-scale laboratory study [Itron and Erik Page & Associates,
5.2.3.3 European Commission 3600-hour lifetime test validation studies
To validate a proposed life test method focused on lumen maintenance and endurance for the European Union, the Swedish Energy Agency, the Australian government’s Department of Environment and Energy, and CLASP Europe conducted testing on various LED products The findings from three laboratories indicated that the new method is more effective for life and endurance testing, revealing a higher failure rate compared to the existing IEC method, which employs a rapid switching cycle of 30 seconds on and off Notably, most failures were observed after 1000 hours, suggesting that shorter test durations of 500 or 1000 hours, as proposed by some groups, are inadequate.
The value of field evaluations to inform predictive models and test methods
In the early research phases of LED systems, there was a lack of comprehensive knowledge regarding failure modes and frequently failing components Real-world data on LED lighting system failures is crucial for identifying common issues and weak components A 2011 report by the United States Department of Energy (USDOE) revealed that over 70% of failures in a specific outdoor LED luminaire model were attributed to the driver, based on 212 million field operation hours However, failure results cannot be universally applied, as variations in component quality and application environments can lead to different failure modes in other LED systems.
Summary of research to develop predictive lifetime test methods
Studies on LED system failure modes reveal both parametric and catastrophic failures across all components, including LEDs, LED arrays, PCBs, optics, and drivers Each component has distinct failure mechanisms and breakdown thresholds Key failure mechanisms include degraded optics leading to lumen degradation and color shifts, weakened solder interconnects increasing series resistance and potentially causing current interruption, and failed components like capacitors and diodes in the driver or power supply, which can result in both lumen degradation and catastrophic failures These insights highlight the critical factors influencing LED system longevity.
LED systems can experience both catastrophic and parametric failure modes, making it essential to evaluate both types during life testing to accurately assess their lifespan Common failure points in LED products include drivers, LED packages, solder joints connecting the LED to the printed circuit board, driver components, and both primary and secondary optics.
Life testing of LED systems is essential and should incorporate power cycling and elevated temperature conditions The correlation between LED chip temperature and lumen depreciation is significant, with higher junction temperatures leading to increased lumen loss Factors contributing to lumen depreciation include degradation of the LED package, secondary optics, and changes in the electrical characteristics of the driver, which can occur independently or together Additionally, power cycling can adversely impact the lifetime of LED systems, potentially leading to catastrophic failures.
Power cycling causes solder joint failure because of the mismatch in coefficient of thermal expansion (CTE) of adjacent layers of materials
Power cycling conditions must include dwell time (full stabilization at the lower and upper end of the temperature) for an accurate failure estimate due to catastrophic failure
Power cycling rates have minimal effect on LED package lumen depreciation rate.
Standards
ANSI/IES LM-80 + TM-21
ANSI/IES LM-80-20, Approved Method: Measuring Luminous Flux and Color Maintenance of LED
The latest version of the industry standard for measuring and projecting the lifespan of LED packages is outlined in Packages, Arrays and Modules [IES, 2020a] This standard employs a lumen maintenance criterion to assess luminous flux data, which serves as the foundation for long-term lumen projections, as detailed in ANSI/IES TM-21-19, Technical Memorandum: Projecting Long-Term Lumen.
The IES documents Photon and Radiant Flux Maintenance of LED Light Sources (2019) have been widely used to project LED product life; however, it is crucial to note that the LM-80 method is designed for LED packages, arrays, and modules, rather than complete LED systems, and does not account for power cycling This leads to significant issues when using extrapolated data specific to LED packages to assess the lifetime of LED systems Several publications have highlighted the inappropriateness of this approach, which assumes that LEDs are the primary failure point and overlooks catastrophic failure modes To address these concerns, the IES has published two additional documents, LM-84 and TM-28, focusing on LED systems.
ANSI/IES LM-84 + TM-28
Originally published in 2014 [IES, 2014a], IES LM-84, Approved Method: Measuring Optical Radiation
The Maintenance of LED Lamps, Light Engines, and Luminaires [IES, 2020b] expands upon the LM-80 document, which focuses on assessing luminous flux maintenance in LED systems This measured luminous flux data is essential for predicting lumen maintenance values, following the methodology outlined in IES TM-28.
Projecting Long-Term Luminous Flux Maintenance of LED Lamps and Luminaries [IES, 2014b, 2020c]
The combination of LM-80 and TM-21 methodologies offers comparable testing methods and lifetime projections for LED packages, while LM-80 results can shorten testing durations However, despite advancements in the IES LM-84 standard, which accounts for system performance, it does not address catastrophic failures or power cycling Even after six years since its introduction, the industry still relies on LM-80 data to estimate LED product lifetimes.
IEC 62612
IEC 62612:2013+AMD1:2015+AMD2:2018 outlines the performance requirements and testing methods for self-ballasted LED lamps used in general lighting services with supply voltages greater than 50 V The standard recognizes that validating the long product lifetimes of LED lamps is impractical, thus it does not provide a specific test method for lifetime determination Instead, it introduces lumen maintenance codes (9 for ≥ 90%, 8 for ≥ 80%, 7 for ≥ 70%) based on a testing period of 25% of the rated life, capped at 6,000 hours It clarifies that the pass/fail criteria differ from common lifetime expectations, meaning these lumen maintenance categories do not predict actual product longevity Additionally, performance claims may vary when LED lamps are installed in different luminaires, and any claims regarding operation under varying temperature or humidity conditions necessitate testing under those specific conditions.
IEC 62717
IEC 62717:2014/AMD2:2019 outlines the performance requirements for LED modules used in general lighting, detailing the conditions, testing methods, and minimum test durations necessary for assessing lumen maintenance The standard defines the median life of LED products based on the achievement of specific lumen maintenance categories (L70, L80, L90) by 50% of the samples, although it does not provide a method for determining this value A key feature of the standard is its differentiation between two failure mechanisms—abrupt failure and lumen maintenance—along with the establishment of testing conditions for endurance assessments.
IEC 62722
IEC 62722-2-1:2014 outlines the testing conditions, methods, and minimum duration required for evaluating the lifespan of LED luminaires This initial edition introduces key technical updates to the publicly available specification, ensuring that life definitions and testing durations related to system life are now consistent with IEC 62717.
Commission Delegated Regulation (EU) 2019/2015 of 11 March 2019
Regulation (EU) 2017/1369 of the European Parliament
In December 2019, the EU Commission's Lighting Regulation introduced Annex V, which established a formal test method to evaluate the lifetime and endurance of lighting products This method incorporates longer switching cycles that better reflect real-life conditions in residential and office environments Additionally, it combines a thermal stress "endurance" test to assess performance under realistic usage scenarios.
A comprehensive 3,600-hour lumen maintenance test was conducted on the same sample, significantly streamlining the process by combining the previously required separate switching cycle and 6,000-hour lumen maintenance tests into one efficient procedure.
Assessment of Test Methods
Table 6.1 outlines the test features from various methods in the literature aimed at estimating the life and reliability of LED systems These methods were assessed based on specific criteria to identify the most effective approaches for meeting the IEA 4E SSL Annex's goals An ideal testing method for estimating LED product lifetime should evaluate products as a complete system, maintaining the integrity of the samples, and simulate the stress factors found in their intended applications.
• Power cycling profiles (on-off) b Consider two failure modes o Parametric, based on
Chromaticity shift (e.g., 2-step u’v’ circle) o Catastrophic, based on
Total failure (e.g., samples fail to produce any light output or produce a drastically low output)
End-of-life behavior of LED products, such as intermittent operation, cycling on-off, and flickering, can significantly impact their performance By analyzing the expected operating conditions, it is possible to predict the lifetime of the LED product in its specific application.
To ensure comprehensive testing, it is essential to include at least three conditions that represent various application environments for effective interpolation, or alternatively, conduct a single test focused on the worst-case scenario The life estimate should derive from the shorter predictions for both parametric and catastrophic failure modes Testing should be as brief as possible, ideally accelerated, without introducing new failure mechanisms Industry feedback suggests that tests exceeding 6,000 hours are impractical due to the fast-paced development cycles of new and existing products.
When defining system life, the two primary factors to consider are lumen maintenance and chromaticity shift While other parameters like flicker, dimming range, and intensity distribution are more challenging to measure and predict, they can still be relevant to system life if they significantly impact the application.
Testing for worst-case scenarios provides a lower limit for the expected lifespan of representative LED products, although it does not enable lifetime predictions based on a single test condition.
Table 6.1 Relevant test methods to estimate LED system lifetime or reliability that have been described or proposed in the literature
Test method or approach (see reference at foot of table)
Test feature/variable ALT/ADT 1 SSADT 2 HALT,
HADT HAST 3 Hammer 4 ANSI/IES
Regulation EU 6 LRC ASSIST recommends 7
Can predict system lifetime for parametric and catastrophic failure modes
1 [Cai 2012; Shepherd 2014; Hao 2016b, 2019; Padmasali et al 2019]
6 European Commission’s Ecodesign Lighting Regulation [European Union 2019]
The ALT, ADT, SSADT, and ANSI/IES LM-84+TM-28 methods fail to account for power cycling stress factors and do not adequately capture catastrophic failure modes Furthermore, as detailed in section 6.2, testing methods such as highly accelerated life testing (HALT), accelerated degradation testing (HADT), and highly accelerated stress testing (HAST) are primarily used to qualitatively identify potential weak components during the design phase or to compare alternative systems The literature advises against using these methods to estimate the useful life of products or systems, as referenced in sections 6.2.1.2 and 6.2.1.3 Consequently, these methods are not suitable for estimating LED system lifetime and are not pertinent for further investigation by the IEA 4E SSL Annex.
The Hammer test is a HALT method used for rapid screening of luminaires, functioning as a pass/fail testing procedure that offers qualitative insights into potential failure modes within a testing period of under 2,000 hours While parameters like luminous flux, chromaticity, CCT, and flicker are monitored, the test does not aim to estimate or predict the system's lifetime concerning parametric or catastrophic failures Notably, the test conditions do not accurately reflect the real-life applications of LED systems.
The European Union has proposed a test method that stands out as one of the two most promising approaches for estimating the lifetime of LED systems.
The 39 method lacks predictive capability as it only considers a single environmental factor—temperature—without accounting for relative humidity, and it evaluates only one usage pattern, power cycling To accurately estimate product lifespan under varied application conditions, tests should be conducted across multiple environmental scenarios, such as three different temperatures and dwell times Incorporating relative humidity as an independent variable is crucial, especially for assessing the longevity of specific products like outdoor luminaires.
To ensure accurate testing, it is essential to analyze and validate the 30-minute off time during each cycle Both dwell and off times should be timed to replicate the full thermal excursion that the system would encounter in real-life applications, especially for systems with larger thermal masses, such as outdoor luminaires Experimental results indicate that these luminaires may require several hours to achieve complete thermal excursion.
The Lighting Research Center's method for estimating LED system life, formalized by ASSIST in 2020, is a promising approach that predicts LED lifetime based on specified environmental conditions and usage patterns This method has been validated through multiple studies, including research for the California Public Utilities Commission and the IEA 4E SSL Annex by the Swedish Energy Agency and Australia's Department of Environment and Energy To enhance this method, it is crucial to incorporate relative humidity as an additional variable, which could lead to three more test conditions Additionally, further exploration of the proposed 1500-hour test duration is recommended.
Identified Areas for Further Research (including investigative product testing)
In addition to the previously mentioned recommendations for the development of the relevant test methods, our literature review and analysis of existing and promising methods, as outlined in Table 6.1, indicate the need for further research initiatives.
1) Collect data from large field installations of replacement lamps (A, linear, etc.) and luminaires (especially outdoors and those in harsh environments)
Collecting data from real-life field installations is crucial for understanding the actual lifetime and failure modes of lamps and fixtures This approach offers valuable samples for analyzing specific components and subsystems, revealing failure mechanisms that may not be evident in controlled laboratory studies Additionally, field installations encompass various uncontrolled variables, making it essential to investigate these real-world conditions to accurately identify potential failure modes.
2) Conduct a laboratory study at different off times of the endurance test cycles
The off time during endurance testing is crucial as it influences the delta temperature—the difference between the maximum temperature of the product and the ambient baseline This delta varies based on the thermal mass of each LED product, which can lead to inaccurate lifetime predictions if not properly accounted for Current European Commission regulations stipulate a 30-minute off time, but extending this duration to 45, 60, or even 90 minutes is advisable for a more thorough investigation of potential catastrophic failures.
3) Conduct a laboratory study at different humidity levels
Humidity negatively impacts light output depreciation in LED packages and electronic components, but its effects on LED systems remain unclear This test aims to clarify these impacts and improve predictions regarding LED performance in humid environments.
This article highlights 40 significant failures of lamps designed for damp and wet environments, specifically in residential bathrooms and outdoor fixtures like street and parking lot lights The recommendations provided align with guidelines from both the European Commission and the Lighting Research Center, emphasizing the importance of using suitable lighting solutions in these challenging conditions.
4) Verify the accuracy in the predictions of LED driver life based on the models in the Telcordia SR332 standard and the Military Handbook 217F
The two publications focus on predicting the performance of electronic components under various stress conditions, such as electrical and thermal factors While these methods have been utilized by the lighting industry to estimate the lifespan of LED subsystems, including drivers, their accuracy remains unverified.
Akashi, Y., and J Neches 2004 “Detectability and acceptability of illuminance reduction for load shedding.” Journal of the Illuminating Engineering Society, 33 (1), 3–13
ANSI/IES 2017 Nomenclature and Definitions for Illuminating Engineering ANSI/IES RP-16
ANSI/IES 2019 Technical Memorandum: Projecting Long-Term Chromaticity Coordinate Shift of LED
Packages, Arrays, and Modules ANSI/IES TM-35
Appaiah, P., N Narendran, I.U Perera, Y Zhu, and Y Liu 2015 “Effect of thermal stress and short- wavelength visible radiation on phosphor-embedded LED encapsulant degradation.” Optical
ASSIST has published several important recommendations regarding LED lighting In 2005, they released a document titled "LED Life for General Lighting," which serves as a foundational guide for understanding the lifespan of LED technology In 2020, ASSIST expanded on this topic by providing detailed recommendations for testing LED lighting systems and estimating their longevity across various applications Both publications are available through the Lighting Research Center's website, offering valuable insights for industry professionals.
Cai, M., et al 2012 “Highly accelerated life testing of LED luminaries.” 2012 International
Conference on Electronic Packaging Technology & High Density Packaging, pp 1659–1664
Cai, M., et al 2015 “Step-stress accelerated testing of high-power LED lamps based on subsystem isolation method.” Microelectronics Reliability, 55, pp 1784–1789
Cai, M., et al 2016a “Determining the thermal stress limit of LED lamps using highly accelerated decay testing,” Applied Thermal Engineering 102, pp 1451–1461
Cai, M., et al 2016b “Thermal degradation kinetics of LED lamps in step-up-stress and step-down- stress accelerated degradation testing.” Applied Thermal Engineering 107, pp 918–926
Cai, M., et al 2017 “Color shift modeling of light-emitting diode lamps in step-loaded stress testing.” IEEE Photonics Journal 9 (1); doi: 10.1109/JPHOT.2016.2634702
Chang, C.-C., Y.W Lin, Y.W Wang, and C.R Kao 2010 “The effects of solder volume and Cu concentration on the consumption rate of Cu pad during reflow soldering.” Journal of Alloys and Compounds 492 (1–2), pp 99–104
Chelminski, M 2016 “Extending the life of aluminum capacitors: How to calculate the effect of operating conditions.” Future Technology Magazine, May 2016
CIE 2004 15:2004 Colorimetry Commission Internationale de l’Eclairage
CIE 2011 ILV: International Lighting Vocabulary CIE S 017/E: 2011
CIE 2014 Technical Note 001, Chromaticity Difference Specification for Light Sources CIE TN
Collins, D.H., et al 2013 “Accelerated test methods for reliability predictions.” Journal of Quality Technology 45 (3), pp 244–259
EN 2015 Road lighting, Part 5: Energy performance indicators EN 13201-5:2015 European
Committee for Standardization, CEN-CENELEC: Brussels
The 2017 standard EN 15193-1 outlines the energy performance requirements for building lighting, focusing on specifications within Module M9, established by the European Committee for Standardization in Brussels Additionally, the 2019 legislation from the European Union, detailed in Annex V of the Official Journal, addresses functionality post-endurance testing, ensuring compliance and reliability in energy performance For further details, visit the EUR-Lex website.
42 content/EN/TXT/?uri=OJ:L:2019:315:TOC and http://www.legislation.gov.uk/eur/2019/2020/contents/2020-01-31
In their 2013 conference paper, Fan and Yuan investigate the influence of temperature gradients on moisture diffusion in high-power devices, specifically focusing on LED packages The research, presented at the IEEE 63rd Electronic Components and Technology Conference in Las Vegas, provides insights into how temperature variations can affect moisture behavior, which is critical for the reliability and performance of electronic components Their findings highlight the importance of managing temperature gradients to enhance the longevity and efficiency of LED packaging.
General Motors Company 2011 GM Worldwide Engineering Standards (GMW8287): Highly
Goel, A., and R.J Graves 2006 “Electronic system reliability: Collating prediction models.” IEEE Transactions on Device and Materials Reliability 6 (2), pp 258–265
In their 2012 study presented at the 13th International Conference on Electronic Packaging Technology & High Density Packaging, Gong et al conducted a reliability assessment of LED luminaires utilizing the Step-Stress Accelerated Life Test methodology The research, detailed on pages 1546-1549, focuses on evaluating the durability and performance of LED lighting systems under accelerated stress conditions, contributing valuable insights to the field of electronic packaging and high-density applications.
Gullo, L.J 2012 Design for Reliability: Highly Accelerated Life Testing [M] John Wiley & Sons, Inc., pp 169–181
Han, L 2009 An Accelerated Test Method for Predicting the Useful Life of an LED Driver Master’s thesis, Lighting Research Center, Rensselaer Polytechnic Institute
Han, L., and N Narendran 2009 “Developing an accelerated life test method for LED drivers.” Ninth International Conference on Solid State Lighting, August 3-5, 2009, San Diego, Proceedings of SPIE 7422: 742209
Han, L., and N Narendran 2011 “An accelerated test method for predicting the useful life of an LED driver.” IEEE Transactions on Power Electronics 26 (8), pp 2249–2257
Hao, J., et al 2016 “The design of two-step-down aging test for LED lamps under temperature stress.” IEEE Transactions on Electron Devices, 63 (3), pp 1148–1153
Hao, J., et al 2017 “Step-down accelerated aging test for LED lamps based on Nelson models.” Optik
Hathaway, N 2020 “Long-term performance evaluation of LED lighting products.” LD+A Magazine, May 2020, pp 64–68
Hobbs, G K 2008 HALT and HASS: The Accepted Quality and Reliability Paradigm Technical Report Westminster, CO: Hobbs Engineering
Hu, J., L Yang, M W Shin 2007 “Mechanism and thermal effect of delamination in light-emitting diode packages.” Microelectronics Journal 38 (2), pp 157–163
IEC International Electrotechnical Vocabulary IEC 60050-845 Internet: http://www.electropedia.org/
IEC 2014 Luminaire Performance - Part 2-1: Particular Requirements for LED Luminaires IEC 62722- 2-1:2014
IEC 2018 Self-ballasted LED lamps for general lighting services with supply voltages > 50 V -
Performance requirements IEC 62612:2013+AMD1:2015+AMD2:2018 CSV
IEC 2019 LED modules for general lighting - Performance requirement IEC
IEC 2020 DC or AC Supplied Electronic Control Gear for LED Modules – Performance Requirements 62384:2020
IEEE 2010 IEEE 1413: IEEE Standard Framework for Reliability Prediction, second ed New York: IEEE
IES 2008a Approved method: Measuring lumen maintenance of LED light sources IES LM-80-08, Illuminating Engineering Society, New York
IES 2011 Technical Memorandum: Projecting Long Term Lumen Maintenance of LED Light Sources IES TM-21-11, Illuminating Engineering Society, New York
IES 2014a Approved method: Measuring luminous flux and color maintenance of LED lamps, light engines, and luminaires IES LM-84-14 Illuminating Engineering Society of North America, New York
IES 2014b Projecting long-term luminous flux maintenance of LED lamps and luminaires IES TM-28-
14 Illuminating Engineering Society of North America, New York
IES 2015 Approved method: Measuring luminous flux and color maintenance of LED packages, arrays and modules IES LM-80-15, Illuminating Engineering Society, New York
IES 2019a Technical Memorandum: Projecting Long-Term Lumen, Photon, and Radiant Flux
Maintenance of LED Light Sources IES TM-21-19, Illuminating Engineering Society, New York
IES 2019a Approved method: Optical and electrical measurements of solid-state lighting products IES LM-79-19, Illuminating Engineering Society, New York
IES 2020a Approved method: Measuring luminous flux and color maintenance of LED packages, arrays, and modules IES LM-80-20, Illuminating Engineering Society, New York
IES 2020b Approved method: Measuring optical radiation maintenance of LED lamps, light engines, and luminaires IES LM-84-20, Illuminating Engineering Society, New York
IES 2020c Projecting long-term luminous flux maintenance of LED lamps and luminaries IES TM-28-
20, Illuminating Engineering Society, New York
Itron and Erik Page & Associates 2017 Final report: 2013-2014 Work Order ED_I_Ltg_1: LED Lab
Test Study Final Project Report to the California Public Utilities Commission Prepared by Itron and
Erik Page & Associates Related test method: https://pda.energydataweb.com/api/view/1275/LED_Lab_Test_Study_Public_Final_Research_Plan_ WithAppendices.pdf
Jayawardena, A., D Marcus, X Prugue, and N Narendran 2013 “Long-term lumen depreciation behavior and failure modes of multi-die array LEDs.” Proceedings of SPIE 8835: 883510
In their 2019 study, Keil and Hofmann conducted a comparative analysis of the lifetimes of high-end versus low-cost offline LED drivers under accelerated testing conditions This research was presented at the 16th China International Forum on Solid State Lighting and the International Forum on Wide Bandgap Semiconductors in China The findings, documented under the DOI 10.1109/SSLChinaIFWS49075.2019.9019814, provide valuable insights into the durability and performance differences between various LED driver categories.
Koh et al (2013) conducted a study on accelerated lifetime testing for indoor LED luminaires, presented at the 14th International Conference on Thermal, Mechanical, and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems (EuroSimE) in Wroclaw, Poland, from April 14-17.
Lall, P P Sakalaukus, L Davis 2015 “Reliability and failure modes of solid-state lighting electrical drivers subjected to accelerated aging.” IEEE Access 3, pp 531–542; doi:
Lall, P., Sakalaukus, P., and Davis, L (2016) presented significant advancements in the IES TM-28-14 Lumen Maintenance Standard by proposing a generalized acceleration factor approach tailored for solid-state lighting at the 2016 IEEE 66th Electronic Components and Technology Conference in Las Vegas Additionally, Lall and Zhang (2013) explored the effects of temperature and humidity on LED lumen degradation and projected remaining life, as detailed in their publication in Volume 10 of Micro- and Nano-Systems Engineering and Packaging.
Lan, S., C.-M Tan, and K Wuc 2012 “Reliability study of LED driver – A case study of black box testing.” Microelectronics Reliability 52, pp 1940–1944; doi: 10.1016/j.microrel.2012.06.023
I don't know!
Lee, M., Hillman, C., & Kim, D 2005 “Industry News: How to Predict Failure Mechanisms in LED and Laser Diodes.” Military & Aerospace Electronics
Leslie, Russell P., and Kathryn M Conway 1996 Lighting Pattern Book for Homes New York:
Levada, S., Meneghini, M., Meneghesso, G., & Zanoni, E 2005 “Analysis of DC current accelerated life tests of GaN LEDs using a Weibull-based statistical model.” IEEE Transactions on Device and Materials Reliability 5(4), pp 688-693
In recent studies, Li et al (2011) presented a method for predicting the lifetime of LED lamp systems at the IEEE International Conference on Quality and Reliability, highlighting the importance of reliability in lighting technology Furthermore, Liang et al (2020) investigated the impact of various stress conditions on the accelerated life testing of LED drivers, contributing valuable insights to the field as published in the Journal of Physics: Conference Series.