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14 The PCB Design Magazine • June 2014 At that time, almost all of the applications were in military or space equipment and nobody had a lot of experience with FPC. These quality- oriented customers were understandably sus- picious—how much heat could it withstand? Could you flex it for long periods of time? Did salt spray affect it? And so forth. So naturally, there was a need for complete characterization and quality control which led to overspecifica- tion. This was the era of "If you can build one feature WHY PRoCURINg FLExIBLE PRINTED CIRCUITRY IS DIFFERENT continues Figures 1 and 2: Bookbinder, shown flat (l) and bent (r). in rigid-flex circuits with bookbinder construction, which is very difficult to produce, FPC layers that will be on the outside of bent areas are designed slightly longer to provide ease in the formed position. Figure 3: Stiffeners: FPC mimics PWBs when reinforced by an added structural layer; in this case (left end), epoxy-glass. The right end shows a ground plane or shield, which is part of the circuitry.