Flexible circuit materials Coverlay

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. 

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. 

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. 

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. 

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. 

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... 

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