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44 FLEX007 MAGAZINE I APRIL 2019 In my last column, I discussed how manu- facturers of rigid-flex boards use techniques similar to those used by manufacturers of hard boards and flexible circuits, and how tech- niques vary. That column's discussion was for a standard, straightforward rigid-flex design. This column will talk more about non-standard designs, which can present process difficulties and require extra care for effective yields. Asymmetrical Rigid-flex Material Layups Asymmetrical rigid-flex designs are fairly common but not recommended (IPC- 2223A.8.1), as they can be very difficult, and depending on the design, impossible to build. Using materials with differing properties with- out balance and symmetry can cause manu- facturing issues with your board, and there are two common types. One is where the flexible layers are off cen- ter of the neutral axis in the material layup, putting more of the glass-reinforced layers on one side versus the other (Figure 1). Manufac- turing panels with this construction are prone to warp during manufacturing. Since most PWB processes are planar, a warped panel can be very difficult to drill, image, and plate correctly, resulting in reduced yields. Once removed from the production panel, those parts can present great difficulty at assem- Alternative Constructions in Rigid-f l ex Designs Flex Time by Bob Burns, PRINTED CIRCUITS LLC Figure 1: Six-layer rigid-flex with flex layers off-center of the neutral axis.

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