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APRIL 2019 I FLEX007 MAGAZINE 47 for your design. Rigid-flex designs with one or more flex arms that terminate in flex rather than hardboard take longer to build and gener- ally have lower yields. To build rigid-flex boards with flex arms, we have to build the flexible layers to completion somewhat akin to building a whole board. Then, we take that flex circuit and bury it within the rigid board, using a common tech- nique called "pouching." The pouch protects the inner flexible circuit during outer layer processing. If we were to leave the flexible cir- cuit exposed during outer layer processing, the etching and plating chemistries would attack the circuits and pads on the flexible circuit. Figure 4 shows how a typical rigid-flex with a flex arm is processed. The core material on the external layers forms a protective pouch for the circuitry on the internal flex layers. This pouch remains throughout the entire rigid-flex manufactur- ing process and isn't removed until the board is completed through all imaging, drilling, etching, plating, solder mask, etc. It is usu- ally removed just before electrical test so that the connections on the flexible circuits can be tested at the same time as the rest of the board. Although these designs are very common, their manufacturing sequencing is slower than conventional rigid-flex boards. First, we have to build the flex layers to completion—drill- ing, imaging, plating, coverlayer lamination, laser route, etc. Often, buyers think we build the whole board all at the same time and don't understand why these designs take longer. The only way to get them to yield effectively is to build them in sequence. Generally, pouched constructions don't cost much more than conventional rigid-flex, but they are much slower builds. The pouches are usually removed by hand, though under cer- tain parameters can be removed by laser or controlled depth routing. If you have a design with four flex arms that are pouched, that is eight pieces of material that are being removed on each part. If your order is for 1,000 pieces, Figure 3: Rigid-flex with flex arm (L) and conventional rigid-flex (R).

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