Embedded Passives Final report - FY042

Embedded Passives Technology Final Testing Report FY’04 Work Co-Author: Danielle Ator University of Idaho ator8374@uidaho.edu Co-Author/ David Gerke Task Manager: Jet Propulsion Laboratory david.gerke@jpl.nasa.gov 818-393-6372 http://nepp.nasa.gov Date: November 2004 Table of Contents Executive Summary 2Background 3Printed Wiring Board (PWB) .4 3.1Embedded Resistors 3.2Embedded Capacitors .5 4Low Temperature Co-Fired Ceramic (LTCC) 4.1Embedded Resistors 4.2Embedded Capacitors .7 5Environmental Testing .7 5.1 Thermal Coefficient of Resistance/Capacitance 5.2Thermal Aging 5.3Humidity Exposure 5.4Thermal Shock 10 6Characterization .10 6.1CSAM 11 6.2 Dissipation Factor vs Frequency 11 6.3Cross sectioning 12 6.4Capacitance vs Frequency .13 7Results 13 7.1Thermal Coefficient of Resistance (TCR) 14 7.2Thermal Coefficient of Capacitance (TCC) 14 7.3Thermal Aging 15 7.3.1Effects on Resistance 15 7.3.2Effects on Capacitance 16 7.4Humidity Exposure 17 4.7.1Effects on Resistance 17 4.7.2Effects on Capacitance 18 7.5Thermal Shock 18 7.5.1Effects on Resistance 18 7.5.2Effects on Capacitance 19 8Discussion: .19 8.1General Summary of Results Section .19 8.2Effects of Component Size .21 8.3Effects of Embedded Component Orientation 24 8.4Effects of Resistors on Surface .24 9Recommendations: 25 10Future Work 26 11Acknowledgements: .26 12References: 26 Executive Summary Embedded resistors and capacitors were purchased from two technologies; organic PWB and inorganic low temperature co-fired ceramic (LTCC) Small groups of each substrate were exposed to four environmental tests and several characterization tests to evaluate their performance and reliability Even though all passive components maintained electrical performance throughout environmental testing, differences between the two technologies were observed Environmental testing was taken beyond manufacturers’ reported testing, but generally not taken to failure When possible, data was quantitatively compared to manufacturer’s data Both technologies performed favorably with some nuances noted for each material set The resistors were not embedded deep into the substrate structures but were placed on the surface and coated This served two purposes: the first was that resistors could later be trimmed if they reside on the surface and the second was that it represented worst case for protection of the resistive elements for the reliability testing, mainly moisture exposure Typically, the PC board solder resist is sufficient to protect the resistors in the PWB resistors Should there be a pin-hole or damaged area, the environmental protection could be compromised During the moisture environmental testing, a resistor in the PWB technology failed due to corrosion The level of concern for this failure mechanism is elevated only for laser trimmed resistors where the coating would be opened and an additional coating is applied following the adjustment The failed resistor in this study failed at the 1000 hour readpoint of 85%RH/85oC and the failure was not an open but an increase in resistance The capacitors exhibited a size relationship to reliability where small capacitors varied in capacitance more than large capacitor sizes selected in this study The best physical size for the capacitors was found to be between and cm on a side which agrees with literature Background Passive components generally refer to electrical components without gain or current-switching capability such as resistors, capacitors and inductors A majority of the passive devices used in electrical circuitry today are directly mounted on the surface of a printed circuit board (PCB) and are referred to as surface mount passives Such passives can account for 80%-95% of the total number of circuit components and can consume up to 40% of the surface area of the PCB [1, 2] Embedded passives are passives that have been integrated within the printed circuit board, or substrate material This embedding can take place on a single layer of material, a combination of material layers or even can be achieved by placing a component within a cavity in a substrate.[3] Common embedded substrate materials include, but are not limited to ceramic, silicon, polyimide and FR-4 boards Research into embedded passives technology originated from the demand for new devices with smaller size, less cost and more features Although capacitors, resistors and inductors are all candidates for embedding, the greatest interest is currently focused on capacitors and resistors [1] By embedding such passive components within the substrate material it becomes possible to create smaller circuit boards Embedded passives also make it possible to shorten the distance between the passive components and the active components in a circuit assembly By shortening this distance the circuit receives better signal transmission producing less noise which leads to better electrical performance [1, 2] This work utilized thirty (30) printed wiring boards (PWB) and fourteen (14) low temperature cofired ceramic (LTCC) substrates embedded with resistors and capacitors that were designed/layed out at JPL and purchased for the purpose of assessing reliability of these two technologies in the embedded passives subject area [1] The goal of this task was to investigate the integrity of embedded components (specifically capacitors and resistors), as well as to evaluate the reliability of the PWB substrates and the LTCC substrates that the components are embedded within This was accomplished by dividing the substrates into sub-groups and then subjecting each sub-group to a specific environmental stress test If technology of this sort is found to be reliable, it will allow NASA Programs and Projects to reduce the weight and size required for electronics assemblies within systems by building functioning circuitry into PCB’s and/or substrates using embedded passives [1] A description of the two types of substrates used in this evaluation below: Printed Wiring Board (PWB) A printed wiring board is the platform upon which electrical components and devices are mounted A PWB is not only the physical structure for mounting, but is also the interconnection between components [4] The printed wiring boards used for this study were organic polyimide boards The embedded components used in the manufacture of this board are commonly used in the PC board industry, but not many high-reliability PCB shops combine both resistors and capacitors in the same structure This task was a continuation of last year’s NEPP task where a survey was conducted to obtain a record of board shops that could build embedded resistors and capacitors in the same substrate Boards were then designed and manufactured by selected shops See Figure below that illustrates the PWBs used in this study: Figure 1: Photo of PWB substrate with Embedded Resistors and Capacitors The visible components are the surface resistors Dimensions of the substrate are 1.5”x 1.8”x 062” 3.1 Embedded Resistors The embedded resistors were designed using the material Ohmega-Ply manufactured by Ohmega Technologies [1] This material has been in production for decades and has been widely used in the Aerospace Industry The PWB substrates contain 24 embedded resistors (12 per side) The resistors reside on the surface of the PWB and are covered by the solder resist, as recommended by the manufacturer for environmental protection The Ohmega-Ply material allows for the resistors to be placed on any layer of the board The main advantage to placing the resistors on the surface of the PWB is that they can then be laser trimmed to a tighter tolerance Resistors placed on the interior of the PWB not allow this luxury Since many of NASA’s future uses will require more precise resistor tolerances, these test PWBs were designed with the resistors on both surfaces, protected only by the solder mask This also represents a worst case situation as far as the coverage and environmental protection of these devices Ohmega Technologies informed the design team of the fact that there is a slight resistance difference due to preferential grain structure orientation caused by the manufacturing process in the raw material Therefore, the resistor layout and design were chosen based on resistor size and orientation (x and y) on the relatively small PWB It was hoped that any difference in orientation could be quantified and the reliability assessed by designing the PWB in such a manner The resistor dimensions can be seen in Table The ratio column describes the number of squares in the resistor (3 squares of 50Ω/ would yield a resistor of 150Ω, while a resistor made of a ratio of ½ square would yield a resistor of 25Ω) The year end report for the FY’03 work described the resistor values measured on these substrates in more detail Table 1: Informational table for Ohmega-Ply resistors Resistor sizes for Ohmega Ply Largest Length = 0.500" Smallest Length = 0.020" Orientatio # n Ratio Length (in) Width (in) Area (in^2) Area (mils^2) x 0.500 0.167 0.08333 83333.33 x 0.283 0.141 0.04000 40000.00 y 0.289 0.289 0.08333 83333.33 y 0.289 0.289 0.08333 83333.33 y 0.346 0.115 0.04000 40000.00 x 0.283 0.141 0.04000 40000.00 y 1/2 0.020 0.040 0.00080 800.00 y 0.346 0.115 0.04000 40000.00 800.00 x 0.028 0.028 0.00080 10 x 0.028 0.028 0.00080 800.00 11 x 0.500 0.167 0.08333 83333.33 12 y 1/2 0.020 0.040 0.00080 800.00 3.2 Embedded Capacitors There are 10 capacitors embedded within the subject PWB substrates The capacitors are made of a material called Interra HK04 manufactured by DuPont [1] The capacitor material comes as two parallel copper sheets laminated on both sides of a polyimide sheet (the dielectric of the capacitor plate) During the manufacture of the PWB this material is laminated to the PWB material (FR-4 or as in this case, polyimide) and then subtractively etched to form the parallel plate capacitor Connection to each plate can be made by vias or copper traces This technology is limited by the relatively low capacitor value created by the material set Therefore, it is typically used as a large capacitive plane inside of a PWB which is connected to form another level The capacitor dimensions used in this study are small in size and value but were designed to illustrate what limitations, if any, the material exhibited The year end report for the FY’03 work described the capacitor values measured on these substrates in more detail The capacitor layout/design can be seen in Table that follows: Table 2: Information Table for capacitors Capacitor Sizes Largest Length = 0.75" Smallest Length = 0.04 # x & y Length (in) Area (in^2) Area (mils^2) 0.75 0.5625 562500 0.51 0.2601 260100 0.51 0.2601 260100 0.28 0.0784 78400 0.28 0.0784 78400 0.16 0.0256 25600 0.16 0.0256 25600 0.04 0.0016 1600 0.04 0.0016 1600 10 0.04 0.0016 1600 Low Temperature Co-Fired Ceramic (LTCC) Low temperature co-fired ceramic substrates are an alternative to PWBs that offer increased reliability, cost efficiency for high volumes, and high packaging density LTCC has large benefits in microwave applications The LTCC manufacturing process starts with a slurry mixture of recrystallized glass and ceramic powder in binders and organic solvents It is then cast under “doctor blades” to obtain a desired tape thickness The dried tape is then coiled on to a carrier tape and is then ready for production The metallization pastes are screen printed layer by layer upon the un-fired or “green” ceramic tape Then the un-fired ceramic layers are stacked and laminated under pressure Next, the multilayer stack is fired during the final manufacturing step The firing temperature is around 900oC for the LTCC glass-ceramic substrate materials The melting point of the gold metallization is 960oC The LTCC substrates used for this investigation were manufactured using the Ferro A6M material with gold interconnect metallization [1] The dielectric in Ferro A6M LTCC tape is a calcium borosilicate, crystallizing glass The A6-M has a dielectric constant of and very low dielectric loss (