Flexible coverlay

These plasticizers tend to depress the overall glass transition temperature of the Pl-based adhesive, improve flame retardancy, and helps to produce both a flat and flexible coverlay coating. Other plasticizers, such as phthalate esters, aryl sulfonamides, and adipates may result in less flame resistant properties. The plasticizer is incorporated into the adhesive by dissolving it into the coating solution prior to casting and curing. 

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... 

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. 

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. 

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 ... 

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. 

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. 

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. 

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. 

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. 

FIGURE 62.5 Cross section of a single-sided flexible circuit showing (a) adhesives-based laminate and film overlay (b) adhesiveless laminate with liquid coverlay. 

Basically, the same construction systems of coverlay, stiffeners, and surface treatments as single-side flexible circuits could be available for double-side flexible circuits. 

A dynamic flexing area of a double-sided flexible circuit should have only one conductor layer. Coverlay on the other side of conductor should also be eliminated to enable symmetrical layer constructions. Copper... 

Most of the major materials for flexible circuits, such as copper laminates and coverlay films, are supplied in rolled form. The first process of manufacturing is to sUt and cut the rolled materials into workable sizes. It is possible to cut with a manual sharing tool however, an automatic machine is recommended to get an exact size without handling damage. 

The coverlay construction of flexible circuits demands another special process. It is very different from the solder mask used for rigid circuit boards. Furthermore, recent high-density flexible circuits need new coverlay technologies. Table 63.7 summarizes the comparison of traditional coverlay technologies and new technologies. Traditional film coverlay has an excellent balance of physical properties and can provide high reliability, especially for long dynamic flexible endurance. 

It takes more than one and a half hours for the whole heat press cycle to complete the chemical curing reaction of the adhesive resins, therefore the batch size and the lamination equipment have been becoming larger to make the productivity higher. But it makes the process flexibility lower. A quick press system has been developed to have a better flexibiUty. One set of coverlay films and an etched flexible circuit panel is hold in a small vacuum press for less than two minutes as the preliminary lamination. After that, several numbers of panels are baked in a thermal oven to complete the chemical curing of the adhesive resin. This process needs a special epoxy resin for the adhesives, but it provides a high flexibility for the small volume and prototype productions. 

Basically, the same process and the same screen printer are available for applying liquid coverlay materiais as a solder mask on rigid circuit boards. The solder mask materials have to have appropriate flexibility to avoid the cracks during the bending.The only difference in the processes is the... 

The coverlay process could be the typical example. RTR lamination of film coverlay is almost impossible.The screen-printing for RTR processing has developed for relatively rough circuit patterns. The photo-imageable coverlay system could be the right solution for the RTR process of the coverlay of the flexible circuits. However, there have been several limitations to apply. 

The conductive patterns are generated by the traditional etching process withont through holes. All traces are covered with plain coverlay films, which have no openings. The multiple covered flexible circuits are bonded with bonding sheets, which are aheady routed to make openings for nonbonding flexible areas. 

Figure 66.1 indicates two basic structures of dual-access conductors, in both the single sided and double-sided types of flexible circuits. Figure 66.2 shows actual examples of the fine flying leads built on the single side circuits and double side circuits. Basically, a coverlayer is applied to the side of the conductive layer opposite the substrate.The coverlayer can be processed easily to create the top-side exposure. The flying lead is created when the base substrate is also removed in the same area. 

The basic concept of quality assurance for flexible circuits is the same as that for rigid boards, in the sense that any critical defect must be eliminated before shipping. This includes failures due to opens and shorts dimensional degradation of pad arrays, serious defects on conductors, substrates, coverlay, and so on. These defects need to be inspected for each circuit, whether it is flexible or rigid. Technically, however, there are several differences in quality assmance for flexible circuits because of their additional functions, such as dynamic flexing capability. 

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