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March 2016 • The PCB Magazine 59 direCt metallization system for flexible printed CirCuits The degree of crosslinking determines the ther- mal properties of the polyimide, such as glass transition temperature (Tg). Table 2 shows Tg for commonly used polyimides. The specific attributes of polyimides, which make it highly desirable for use in flexible cir- cuit applications, also make it highly challeng- ing for the metallization of drilled holes. Espe- cially with resin/woven glass reinforced com- posite structures of a rigid flexible substrate. Fig- ure 3 shows a typical four-layer rigid-flex circuit with two rigid and two flexible layers [6] . Figure 4 shows an expanded view of the glass reinforced rigid layer. The conditioner needs to prepare the resin surface along with glass fibers to be recep- tive to deposition of MnO 2 , which is the key to the polymerization of the conductive polymer. Table 1: Characteristics of thin-film dielectrics [4] . Figure 2: simplified structure of polyimides. Table 2: Glass transition temperature for PI substrate [5] .