Coverlay Material

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 ... 

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 61.18 Properties of Different Photoimageable Coverlay Materials... 

Coverlays and surface treatments are very difficult for current AOI systems to inspect because of the small optical contrast between coverlay materials and conductors or substrate materials. It may be better to apply AOI after a surface treatment, which wiU increase the contrast. 

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 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... 

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. 

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. 

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. 

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. 

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. 

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. 

Unfortimately, it requires very complicated, labor-intensive processes, and it is difficult to introduce automatic manufactming systems. Another big issue is that the standard film coverlay process cannot make small openings with high dimensional accuracy because of the manud handling of thin film materials. 

A typical coverlay mannfactnring process is shown in Fig. 63.26. Generally the same film materials are chosen for the base film and the coverlay film. A B-stage adhesive such as epoxy... 

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... 

An adhesive film is laminated on a rigid board with a release sheet first. The boards are then routed by punching or by NC router. It is a very simple process if the adhesive material is pressure-sensitive adhesives (PSAs). Each piece of stiffener is placed on the flex circuits with appropriate pressure, mostly by hand.The process is more compUcated when a thermo-set type adhesive material is required. A temperature higher than 160°C with a pressure higher than 20 kg/cm is required for more than 30 min. A similar heat press used for multilayer circuit boards or film coverlay is necessary. Dummy boards should be prepared to make the pressure uniform. 

Coverlayer Selection. Coverlayer selection should be considered the second important material factor for controlling dimensional stability. Generally this layer is adhered to the circuitry using heat and pressure, which can induce additional dimensional distortion. 

Materials Design Manufacturing process Base film, copper, coverlay, bonding material Layer constructions, circuit density, drcuit balance Wet processes, heat processes, mechanically stressed processes... 

Coverlay processes There are several coverlay processes that have been developed with varying process conditions for different concept materials. Film coverlay undergoes a manufacturing process with a temperature greater than 160°C and a pressure over 20 kg/cm. ... 

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