Coverlays

DuPont Pyralux coverlay act as polymer foil. The composites are made of DuPont Kapton polyimide film, coated on one side with a proprietary B-staged modified acrylic adhesive. 

PI resins find use as coverlays. A coverlay is generally used to protect printed circuits during subsequent processing, primarily solder operations, or from environmental effects during use. The Pis can be used in combination with epoxies. In addition, additives, such as phosphorous-based flame retardants or adhesion promoters, may be used in the formulations. 

Flexible circuits. Flexible circuits (flex circuits) are analogous to rigid printed-circuit boards except they are fabricated from a thin flexible dielectric film to which is adhesively bonded a thin copper foil. The copper is then photoetched to form a circuit pattern using normal photolithography processes. A plastic film (coverlay) is then adhesive bonded to the etched... 

Solder resist surface coating, coverlayer, and coverfilm imperfections such as misregistration, blisters, bubbles, separations, adhesive squeeze out, delamination, adhesion, physical... 

Coverlayer Characteristics. Flexible and rigid/flexible PCBs are covered with either a coating similar to solder resist or a coverfilm adhered to by acrylic adhesive. Cover-coat acceptability requirements are the same as those for solder resist. 

Pitches smaller than 150 via holes smaller than 150 in diameter Base 25 jum or thinner, conductor thinner than 12 /rm Small opening on coverlay, tin or lead-free plating High dimensional accuracy (<0.3 mm diameter)... 

Coverlays and stiffeners are typical examples of the special components needed to build flexible circuits. Many types of materials have been introduced to accommodate the designs of the end applications. 

Base substrates Conductors Copper-clad laminates Coverlay Adhesive layer Stiffener Polyimide film (PI), polyester film (PET), polyester telephthalate Thin glass-epoxy, fluorinated ethylene propylene (FEP), resin-coated paper Electrodeposited (ED) copper foil, RA copper foil, stainless steel foil, aluminum foil, etc. Epoxy-based, acrylic-based, phenol-based (adhesive-based) PI film, PET film, flexible solder mask Acrylic resin, epoxy resin, phenol resin, pressure-sensitive adhesives (PSAs) PI film, PET film, glass-epoxy, metal boards, etc. 

Base dielectrics Conductor materials Copper-clad laminates Coverlay Adhesive sheets Polyimide films (Kapton K, E, EN, KJ Apical NP, FP Upilex S) Liquid polyimide resin, PEN film, LCP films Ultra-thin copper foils, sputtered copper, copper alloys, stainless steel foil Adhesiveless laminates (cast type, sputtered/plated type, laminated type) Photoimageable coverlay (PIC) (dry film type, liquid ink type) Hot-melt polyimide film... 

Properties of Polyimide Films. Table 61.8 shows major properties of typical poly-imide films, Kapton H and Apical AV. These have good mechanical properties and provide excellent performance as base substrate films and coverlay films in dynamic flexing appUca-tions.They have flame-retardant characteristics, and it is not difficult to achieve Underwriters Laboratories (UL) flame class 94-V-O or 94-VTM-O. The largest disadvantage of polyimide film is its higher cost compared to other common plastic films such as PET. 

Several liquid polyimide resins have been developed as the base materials of high-density flexible circuits. Some of these liquid polyimide resins can be photoimaged, and have been used in high volume as the dielectric layer and coverlay in the wireless suspension of hard disk drives. 

One of the major differences between flexible circuits and rigid circuit boards is coverlay. In addition to the solder mask used for assembly, coverlay is the mechanical protector for the fragile conductors on flexible circuits. Film-based coverlay and flexible solder mask have been standard materials for traditional flexible circuits. Several types of photoimageable coverlay materials have been developed to satisfy the fine-resolution requirements of FCDI flexible circuits ... 

Photoimageable coverlay (dry-film type and liquid type)... 

Several types of the materials are available for each coverlay type. 

Usually the same films are chosen for the coverlay films as are used for the substrate layers. The films are coated with semicured acrylic or epoxy resin, which is used as an adhesive layer. The surfaces of the adhesive layers are covered with release films when the materials are delivered to circuit manufacturers. Semicured adhesive resins are unstable at room temperature, so film coverlay must be kept in refrigerators. The adhesive resins of the coverlay usually have only weeks of pot life even when they are kept in low-temperature environments, and exact conditioning is required for processing. 

The process of manufacturing with film coverlay is very complicated, which makes process automation difficult and increases the cost. The manual registration process and the use of unstable film material are major issues when considering film coverlay for HDI flexible circuits due to small hole capability and lower dimensional accuracy. Basic properties of the typical materials are shown in Table 61.14. 

TABLE 61.14 Basic Properties of Rim Coverlay (Polyimide Rim Base)... 

Screen-Printable Coverlay Ink (Flexible Solder Mask)... 

A screen-printing process similar to that used for rigid circuit boards is available for flexible circuits as the low-cost coverlay process. However, specially conditioned liquid solder mask materials are required to produce appropriate flexibility for the circuits. Standard solder mask materials designed for the rigid boards have small cracks and de-laminations when bending. 

TABLE 61.15 Basic Properties of Screen-Printable Coverlay... 

The basic idea of the photoimageable coverlay is to satisfy both of fine resolutions of the opening and physical flexibility by one single material. Also, its simple manufacturing process should reduce costs. 

Dry-film-type and liqnid ink-type photoimageable coverlay materials have been developed with different resin matrixes to satisfy different technical requirements of HDI flexible circuits. A comparison of the materials is given in Table 61.16. Properties of the materials are shown in Tables 61.17 and 61.18. 

TABLE 61.17 Properties of Liquid/Epoxy-Type Photoimageable Coverlay... 

Acrylic or Epoxy/Dry-Film Type. Acrylic or epoxy/dry-fibn types were the first materials developed to act as the flexible photoimageable coverlay for HDI flexible circuits. They have the same product concept as the photoimageable solder mask of rigid circuit boards. The same vacuum laminators and imaging equipment are available for these materials. 

TABLE 61.18 Properties of Different Photoimageable Coverlay Materials... 

Several different types of adhesive materials can be used in flexible circuits depending on the application, except for the adhesive layer of copper laminates and fihn coverlays. The major applications are the bonding of stiffeners and multilayer constructions. 

There have been technical barriers to developing new materials with a good balance of performances for coverlay eliminating bromine compounds. Fihn coverlay have to have an adhesive layer with some flame-retardant reagent. Few new epoxy resins have good balance for aU of the physical performances without bromine compounds. 

New materials of photoimageable coverlay have been developed without bromine molecules. Not many choices offer a balanced performance, especially high flexibihties for dynamic flexing. Polyimide-based photoimageable coverlays are the solution to satisfy the all of the physical requirements, but these polyimide materials are remarkably more expensive. 

Conductor trace patterns and coverlay opening patterns are designed to minimize the... 

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