12 Flexible Printed Circuit Boards
12.2 Construction of Flexible Printed Circuit Boards
Figure 12.2 shows the constructional parts of flexible printed circuit boards. They are made of a dielectric substrate (film) which is coated with an adhesive over which the copper foil forms the conducting path. The copper foil is protected from corrosive media by a cover layer or special coating.
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Copper Coverlayer Adhesive Polyimide Fig. 12.2 Schematic view of flexible printed circuit board — constructional parts
12.2.1 Films — Types and Their Characteristics
Flexible printed circuits make use of flexible laminate. The properties of the laminate are crucial not only to its manufacturing process but also to the performance of the finished circuit. The flexible laminate consists of a conducting foil and dielectric substrates. The dielectric substances are of two types which are used for flexible printed circuits:
a Thermosetting Plastics: These are polyimide, polyacrylate, etc.
a Thermoplastics: These include materials which, after curing, will soften by heat input, such as some types of polyester, fluorinated hydrocarbon, polymers, etc.
Copper is the most commonly used foil, while virtually all flexible circuitry is built on polyimide or polyester film. For special purposes, aramid and fluorocarbon films are used.
The selection of a particular film depends upon a number of factors. These are enumerated below.
a High performance flexible circuits, particularly those for military applications, are manufactured with polyimide films because they offer the best overall performance.
a Commercial, cost-sensitive circuits are built on polyester films that provide polyimide performance at a lower cost, but with reduced thermal resistance.
a Aramid non-woven fibre is inexpensive and has excellent mechanical and electrical properties, but exhibits excessive moisture absorption.
a Fluorocarbons, though expensive and difficult to handle, offer superior dielectric properties.
They are most suitable for controlled impedance applications.
12.2.1.1 Polyimides
The most common choice of film in flexible circuits is polyimide film. This is because of its favourable electrical, thermal and chemical characteristics. This film can withstand the temperatures encountered in soldering operations. The film is also used in wire insulation and as insulation in transformers and motors.
The polyimide film used in flexible circuit is Kapton, which is a trademark of Du Pont Co., USA.
Kapton/modified acrylic has a temperature rating of –65 to 150 °C, though circuits will discolour after a long-term exposure at 150 °C. Kapton type H film is an all purpose film that can be used in applications requiring working temperature ranging from –269 °C to 400 °C. There are some specialized versions of Kapton film which are required for use in applications requiring special
properties. One of them is Kapton “XT”, which is thermally conductive with twice the heat dissipating capacity of Kapton type “H” film, enabling higher speeds in thermal-transfer printers. Polyimide films are available in standard thicknesses of 0.0005, 0.001, 0.002, 0.003 and 0.005 inches (0.0125, 0.025, 0.050, 0.075 and 0.125 mm)
The main reason for the large usage of polyimide film is its ability to withstand the heat of manual and automatic soldering. Polyimides have excellent thermal resistance and have continuous use ratings approaching 300 °C. At these temperatures, copper foils and solder joints are quickly destroyed through oxidation and inter-metallic growth. The behaviour of the laminate is determined by the combined properties of adhesive and supporting film. It is therefore important to understand the influence of both adhesive and film properties while selecting a laminate.
Polyimides are inherently non-burning and when combined with specially compounded fire- retardant adhesives, produce laminates which can withstand high temperatures. However, many flexible circuit adhesives have much less resistance. Although they can withstand soldering, these adhesives constitute the weak link in a polyimide film laminate. Table 12.1 shows the characteristics of polyimide films.
Table 12.1 Characteristics of Polyimide Films (after Stearns, 1992)
Property, Units Upilex-S Kapton-H Apical
Density, g/cm3 1.47 1.42 1.42
Tensile strength, psi 56,800 25,000 35,000
Elongation, % 30 75 95
Tensile modulus, psi 1,280,000 430,000 460,000
Flammability 94 VTM-0 94 VTM-0 94 VTM-0
Moisture absorption, % 1.2 3.0 3.0
Oxygen permeability, ml /m2/mil 0.8 380 ~380
Moisture permeability, g/m2 1.7 84 ~84
Dielectric strength, V/mil 6800 7000 7800
Dielectric constant 3.5 3.5 3.4
Dissipation factor 0.0013 0.0025 0.0014
Volume resistivity, MW -cm 1 ¥ 1011 1¥ 1012 3¥ 1011
Note: *Typical values for 1 mil thick (25 mm), at 25 °C
Some polyimide films absorb a great deal of moisture. Prior to exposure to elevated temperatures, such as soldering temperatures, the laminate must be baked dry by keeping it at least one hour at 100 °C or higher for single layer circuits and longer for multi-layer constructions. As the moisture re-uptake is very rapid, the laminate should be stored under dry conditions if the process cannot be completed within an hour’s time.
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Dimensional Stability: A crucial property of flex circuits is their dimensional stability. Flexible laminates inherently expand and shrink more during exposure to various process conditions than the glass-reinforced rigid system. The stability of a flexible laminate depends upon the film properties, degraded by the properties of the adhesives and process conditions used to form the laminate (Stearns, 1992). Careful laminate manufacture, using low web tensions, vacuum evacuated lamination and thermally stabilized films minimize the chances of shrinkage. After-etch shrinkage of 0.1 per cent is achievable with high performance films having high tensile strength, but shrinkage for laminates made with conventional polyimide film is generally of the order of 0.15 per cent. These shrinkage values may seem trivial, tolerable and predictable if not accompanied by other errors. But many times, they are undesirable and costly to neutralize.
Tear Resistance: Flexible circuits commonly have complex geometries with multiple stress concentration points. That makes tear resistance an important property. For example, a torn circuit cannot be repaired. The adhesive can enhance laminate performance in terms of tear resistance since most flexible adhesives have better tear resistance than polyimide films.
Unfortunately, film characteristics that are essential for better dimensional stability result in lowering tear values because dissipation of tear energy requires a softer film with greater elongation and yield before failures.
In flexible laminates, the primary insulation is provided by the adhesive which has its own insulation resistance and dielectric strength. Thus, the flexible circuit designer must look carefully at the properties of the laminates and not the film, when designing the conductor pattern in a PCB layout.
Polyimide films and adhesives have relatively poor electrical properties for use in controlled- impedance applications because of their high dielectric constant, (3.7 or greater), and dissipation factor (greater than 0.03). This limitation suggests that some other type of laminate should be used in such applications.
12.2.1.2 Polyesters
Polyester dielectric substrate films are mechanically similar to polyimide and electrically superior, and absorb far less moisture. However, they fail to match polyimides in the crucial area of thermal resistance as the maximum temperature upto which they can be used is less than 125 °C for most polyester. Their melting point is below the soldering temperature. Even so, by using special techniques like crimp or pressure, polyester can cut flex circuit cost without lowering circuit performance and quality. Polyester film is most commonly used in automotive and communication circuitry.
As compared with polyimides, polyesters have a lower dielectric constant, higher insulation resistance, greater tear strength and lower cost. The moisture absorption of polyester is well under 1 per cent with excellent dimensional stability. Polyesters have limitations only in the area of thermal resistance, but offer a great cost advantage. Polyester films are highly resistant to solvents and other chemicals. Polyester has a high tensile strength (25,000 psi) and a good dielectric strength (7.5 KV
¥ 10–3 in. for 0.001 inch film).
Polyester film is a polymer. One of the most commonly used polyester films is “Mylar”, which is a trade name for the product produced by M/s Du Pont, USA. The temperature range of the polyester film for use is 75 to 150 °C, making it unsuitable for soldering temperatures over 230 °C. This problem can be circumvented by using large solder pads, wide traces and foil thickness of 0.00275 inch (0.07 mm) and using an appropriate mask or jig to keep the heat away from all parts of the circuit except the portion being soldered.
12.2.1.3 Aramids
Commonly used as a motor and generator insulation, non-woven aramid fibre materials are inexpensive and have outstanding dielectric strength and thermal properties. They are rated for continuous use at 220 °C and when quoted with the suitable laminating adhesive, form a very good flexible laminate.
The product has good tensile and tear strength as well as dimensional stability. However, it is very hygroscopic. Like polyimide-based laminates, aramid-based laminate must also be thoroughly dried before solder assembly, and kept dried through the assembly process.
Aramids have an undesirable property of stainability. It develops when the laminate is exposed to the liquid process wherein the process chemistry may wick into the fibre structure, leaving a permanent stain and potential insulation resistance problems. Aramids have many desirable properties and are inexpensive. But their shortcomings make them difficult to use in volume flex circuit applications.
One of the common aramid materials is “Nomex”, a trade mark of M/s Du Pont. Nomex is a high temperature paper which can withstand soldering temperatures very well. It has a very low initiation and propagation tear strength.
12.2.1.4 Fluorocarbons
The first flexible circuits were supported by high performance fluorocarbons long before Kapton came on to the scene. Unmatched chemical inertness, extremely high thermal resistance, outstanding dielectric and tough mechanical properties suggested that fluorocarbons would be ideal for flexible circuitry.
Fluorocarbon dielectrics, which, are formed with the fusion process, suffer from dimensional instability. Lamination at the required temperatures (near 300 °C) creates stresses on a semi-molten dielectric that can destroy fine and delicate conductor patterns.
Fluorocarbons have superior characteristics for flexible circuits, specially since their tear values are very good. Because of this property, fluorocarbon patches are sometimes used to reinforce weak corners of polyimide circuit. Fluorocarbon laminates are essentially inert to all common chemistries and inherently incombustible, and do not pose any problem in the production process or in use.
Fluorocarbons are not easily adaptable to the plated through-hole process because they have excellent chemical resistance. Baths used to promote adhesion of electroless copper onto hole walls have little effect, requiring the use of additional process steps.
Today, for ease of circuit and laminate manufacture, fluorocarbons can be assembled with adhesives instead of the use of the fusion process, giving an improved dimensional stability, though not of the
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level of polyimides. If the adhesive is kept as thin as possible, the circuit will display some of the excellent electrical characteristics of a fusion-made circuit, at a lower cost.
12.2.1.5 Choice of Dielectric
The dielectric substrate of a flexible laminate has a significant effect on the manufacturing cost and performance of the finished circuit. Polyimide films offer the best combination of cost and properties for this use.
Polyester films come a close second, falling short only in thermal resistance. Aramid non-woven fibre has unique properties that suggest use in applications where cost is important and slight imperfections can be overlooked. Fluorocarbons have superior dielectric properties and are suitable for use in demanding controlled impedance applications. Table 12.2 gives the characteristics of polyester, fluorocarbon and aramid films.
Table 12.2 Characteristics of Polyester, Fluorocarbon and Aramid Films (after Stearns, 1992)
Property Polyester Fluorocarbon Aramid
FEP Du Pont 410
Tensile strength, K psi 20 – 40 2.5–3 6–10*
Elongation, % 60 –165 300 9
Tear strength, g./mil 50 –130 125 550
Propagation 50 –300 125 45–80**
Moisture absorption, % 0.25 < 0.01 3–7
Moisture permeability:
1–1.3 0.4 NA
g-mil/100 sq. in./24 hrs.
Dielectric strength, V/mi, 1 mil 7500 6500 530
Dielectric constant, 1 kHz-1 GHz 3.2–2.8 2.0–2.05 2.3
Dissipation factor, 1Hz-1 GHz 0.003– 0.016 0.0003 –0.0015 0.007
Chemical resistance Good Excellent Good
Notes: *Per mil based on 2 mil thickness **Per mil based on Elmdorf test of 2 mil
12.2.2 Foils
The use of copper foil as a base material in flexible circuits is well known. Knowledge of how it is manufactured, however, is not as common. The production of copper foil requires a number of processing steps to provide the flexible circuits industry with quality foil products.
Two types of copper foils are used for flexible laminates today: (i) rolled annealed (also known as wrought foil), and (ii) electrodeposited foil. The manner in which foils are manufactured, either rolled annealed or electrodeposited, determines their mechanical characteristics. Each type of foil is further categorized into grades, on the basis of its mechanical properties and applications. The copper foil classification is shown in Table 12.3 providing four separate classifications for both electrodeposited and wrought types (Savage, 1992). Generally, electrodeposited foils identified as grades 1-4, are used for rigid printed circuit boards. Flexible circuits use both electrodeposited and wrought copper foils (grades 5-8). Typically, grades 2, 5, 7 and 8 are used in flexible laminates.
Table 12.3 Copper Foil Classification (after Savage, 1992)
Type IPC Grade Description Application
Electrodeposited 1 Standard Rigid laminates
Electrodeposited 2 High ductility Automotive flex
Electrodeposited 3 High temp. elongation Multi-layer board inner layer
Electrodeposited 4 Super high ductility
Wrought 5 As rolled Commercial flex
Wrought 6 Special temper
Wrought 7 Rolled annealed Military flex
Wrought 8 Low temp. annealable Commercial flex
12.2.2.1 Rolled Annealed Foils These are made by first heating copper ingots, then sending them through a se- ries of rollers that reduce them into foils of specified thicknesses. This is shown in Figure 12.3. Rolling creates a grain structure in the foil that looks like over- lapping horizontal planes. Both pressure and temperature are used to create stresses between different sizes of cop- per grains. These produce copper foil properties such as ductility and hardness while also providing a smooth surface.
This manufacturing technique yields foils with greater resistance to repeated flex-
ing than that of electrodeposited foils. However, its disadvantage is its higher cost and lack of avail- ability of various thicknesses and widths.
Rolling operation
Raw foil Pressure
Copperingot
Temperature
Copper base foil Ingot
Fig. 12.3 Foil production by rolling method
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12.2.2.2 Electrodeposited Foils
They are made by plating copper ions into a cylindrical cathode, from which the foil is continuously stripped. Electrodeposition creates a columnar grain structure. When the foil is flexed, the grains separate. This results in less flexibility and a lower resistance to cracking when folded than in the case of rolled annealed foils. Figure 12.4 shows a schematic diagram of the process for making electrodeposited foil.
Copper feed stock
Digestion (HSO ,°T, Air,
copper)
2 4
Electroforming cell(EFC)
Electrodeposition Copper sulfate
Copper
Cu° Cu +° H2SO4 CuSO4 Cu2+ Cu (foil)° Raw foil
Fig. 12.4 Process for making electrodeposited foil (Savage, 1992)
The process begins with the dissolution of copper metal in a sulphuric acid solution. Both temperature and agitation are used to control the rate of dissolution. The profile and mechanical properties of the foil can be controlled by using various types of additives.
The copper solution is continuously pumped into an electroforming cell, wherein the application of current between the anode and cathode causes copper ions from the chemical bath to plate on the cathode surface. The cathode is a cylindrical drum that rotates while being partially submerged in the solution. As it enters the solution, copper begins to deposit on the drum surface and continues to plate until it exits. The copper foil is stripped from the cathode as it continues to rotate. The thickness of the foil is determined by the rotation speed of the cathode drum. The electrodeposition process is capable of producing copper foil in many thicknesses and widths.
After raw foil production, both wrought and electrodeposited foils are treated in three treatment stages as shown in Figure 12.5.
Bonding (Anchoring) Treatment: This treatment usually consists of a copper metal /copper oxide treatment, which increases the surface area of the copper surface for better wetting of the adhesive or resin.
Thermal Barrier Treatment: This allows the adhesion of the clad laminate to be maintained in spite of the thermal processing conditions involved in PCB manufacture.
FoilTreatment
Raw foil
Treated foil StageI: Bonding (anchoring) treatment
StageII:
StageIII:
Thermalbarrier (yellow) Stabilization (anti-oxidation)
Fig. 12.5 Stages for treatment and stabilization of foils (after Savage, 1992)
Foil Stabilization Treatment: Also called passivation or anti-oxidation, this treatment is applied on both sides of the copper to prevent oxidation and staining. All stabilization treatments are chrome- based. However, some manufacturers use nickel, zinc and other metals in combination with chrome.
After treatment, copper rolls are cut to desired widths and wound on a core after encapsulation in a plastic film to prevent oxidation. The ductility of copper foil is as follows:
a Electrodeposited copper foil : Elongation 4–40 per cent a Rolled annealed copper foil : Elongation 20– 45 per cent
The copper foil is usually covered with a film made of polyimide or liquid polymer solution. The coating of conductors with such type of treatment serves as both a long-term protection against corrosive environments and a solder resist.
12.2.3 Adhesives
The function of adhesives in flexible circuits is to bond copper foil to the dielectric substrate, and in multi-layer flex designs, to bond the inner layers together. A flexible laminate’s performance depends upon the combined properties of its adhesive and supporting dielectric film. Bond strength, dimensional stability and flexibility after soldering are the key factors determining an adhesive’s suitability for a particular application (Wallig, 1992).
Adhesives such as acrylics, polyimides, epoxies, modified polyesters and butyral phenolics have been used with varying degrees of success to bond the flex circuits. Since polyimide and polyester
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dielectric films are the two most commonly used substrate materials, the adhesives typically used with these materials are covered in the following sections.
12.2.3.1 Acrylic Adhesives
Acrylic adhesives offer high heat resistance and good electrical properties. They have been used successfully on polyimide film substrates and have made polyimide/acrylic a preferred choice for dynamic flex applications. However, many flex circuits manufacturers are finding acrylic adhesives thicknesses and high Z-axis expansions as limiting factors for more demanding electronic packaging applications. In addition, polyimide/acrylic adhesive laminates are vulnerable to attack by some of the solvents used in the photo-resist process, and to alkaline solutions used in plating and etching.
Absorbed solvents are especially difficult to remove prior to multi-layer lamination, resulting in de- lamination or blistering problems, if these volatiles are not removed.
In high density designs, dimensional stability and small hole drilling problems (drill smear) can decrease yields which, in turn, increase unit cost. If not controlled, plated through-hole failures could result.
Most rigid flex systems are made by using acrylic adhesive systems. For these, the most popular etch-back or hole cleaning process in use is the plasma system. The plasma system works with ionized gases, which are generated by a radio frequency source with the ionization of Freon (CF4) mixed with oxygen. This action de-smears the flexible circuit portion of the assembly but does not harm glass fibres that might be in the holes of the rigid portion. After the plasma treatment, the organic residue left in the holes is removed with an alkaline cleaner at 140 °C for 2-3 minutes in an ultrasonic cleaner.
12.2.3.2 Polyimides and Epoxies
Polyimide substances may also be successfully paired with polyimide adhesives. The chemical resistance and electrical properties of polyimide adhesive are as good or better than those of acrylic adhesives. Additionally, they offer better heat resistance than any of the other adhesives used with flex circuits.
Some polyimide-based flex circuit laminates incorporate epoxies as adhesives. Epoxies generally give good electrical, thermal and mechanical performance. However, they are limited to static flexing applications due to the resin cross-linking that occurs on curing.
The reduced dynamic flexing ability for polyimide and epoxy adhesives is not a serious limitation since the majority of flex circuits produced are used in static flexing applications. The trade-offs for this increased laminate stiffness are better dimensional stability, better processability and lower overall adhesive thickness in multi-layer flex and rigid flex board fabrication.
Epoxy remains in good condition during soldering operations. They exhibit long-term stability at elevated temperatures in environmental conditions upto 120 °C. Epoxy systems include modified epoxies known as phenolic butyrals and nitrile phenolics. They are widely used and are generally lower in cost than acrylics, but higher in cost than polyesters.