Rigid/flexible circuits Constructions

There are many different concepts in the construction design of multilayer rigid/flexible circuits. 

There are many varieties of manufacturing processes for multilayer rigid/flexible circuits because of their complicated structures. Figures 65.5 shows a typical layer construction using... 

Although some substrates may be molded plastic (3-D or molded circuits) or flexible film plastic materials (flexible circuits), the largest group of substrates is of rigid laminate construction. The laminate construction consists of many layers of resin-treated fibers, fabrics, or papers, which are called reinforcements. 

The basic difference between flexible circuits and rigid circuit boards is the thin and flexible materials used in the substrates of flexible circuits. Furthermore, because the flexible circuits have complicated constructions, supplemental materials other than copper-clad materials have been required to build a whole circuit. 

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. 

Because of thin base layers, the flexible circuits can have many sjrecial constructions that are impossible to build by traditional manufacturing processes of the rigid circuit boards. These special constructions generate further functional values for the flexible circuits. This chapter introduces the major constructions in use, along with their manufacturing processes. 

There are several choices of places to bnild micro bnmps on flexible circuits, as described in Chap. 64 (see Sec. 64.3.8). The same processes nsed to build ball grid arrays on rigid circuit boards are appropriate for flexible circuits with flexible photo-imageable solder mask. It is possible to build micro bumps on bare conductors using plating masks. Many micro bumps are built on fine flying leads without solder masks. A valuable construction of a flexible circuit is micro bumps built on the other side of the circuit through the dielectric layer.This can provide a reliable micro bump array because of the stable construction. 

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