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22 The PCB Design Magazine • June 2014 smoothly than the first order; if it doesn't, start looking for another vendor because good ven- dors learn from mistakes and all others should be avoided. Much of the high cost of first-run FPC comes from the engineering supervision that is needed to assure a decent yield on a new design; this extra oversight goes away when an acceptable yield is achieved and unique prob- lems have been seen and neutralized. This brings us to the high-volume situation. Because FPC is so good at reducing produc- tion cost in dense electronic packages, it fre- quently appears in high-volume production. What happens there? Given a good, fully pro- ductionized design (i.e., one with rational tol- erances and adequate room for uncontrollable variation in films and adhesives; well-set process conditions with functional limits; mature qual- ity control procedures with their limits) FPC can be produced reliably at competitive cost. Summary Keep in mind that vendor choice is more important in FPC applications than anywhere else. Building FPC involves chemical, thermal and mechanical processes, which means the FPC vendor has to be several times more skilled than other vendors with a grasp of much more technology. The raw materi- als of FPC production are manufactured, not natural. This brings in another layer of poten- tial variation and out- of-spec products, along with latent defects like varying shrinkage in polymer films, out- of-date adhesives and other reactive chemi- cals, and so forth. Your vendor not only has his hands full controlling his own factory, but is exposed to variations anywhere upstream in his supply chain. FPC has been his- torically overspecified because it started out as a military component. Overspecification, (i.e., specs that force the production to be "on the edge of state-of-the-art" will rebound on you when yield drops. Your vendor is a partner and his problems are your problems. Set reasonable tolerances and inspection limits; it will help you. Ever had a house built? Remember dealing with your contractor? FPC is also a custom- built product and this means you don't "buy" it, but contract for its manufacture sometime in the future. Price and delivery may be stated on the PO and embellished with boilerplate, but reality rules on the production line. Therefore, choose your vendor carefully. PCBDESIgN Thomas Stearns is a 57-year veteran of the FPC business and the holder of 23 patents. Stea- rns is the president of Brander int'l Consultants, providing design, process and quality sup- port in flex and rigid-flex circuitry. he may be reached at thomasstearns@comcast.net. Figure 6: relay circuit: FPC enables dense packaging. relay on the left shows the FPC as folded/potted. The circuit is shown as manufactured on the right. WHY PRoCURINg FLExIBLE PRINTED CIRCUITRY IS DIFFERENT continues feature