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64 PCB007 MAGAZINE I MARCH 2018 The most commonly spec'd materials tend to be ½ and 1 oz. copper with 1 mil or 2 mil poly- imide and, because they are the most common materials, pricing tends be lower and fabri- cators will often have this material in stock. Adhesiveless materials are most often recom- mended for higher layer count flex designs and rigid-flex construction. Rigid-Flex Rigid-flex construction consists of a flexible section and rigid section on the same board. What differentiates this construction from flex with a stiffener is that plated through-holes ex- tend through both the rigid and flexible sec- tions. This construction is most often used when the design requires dense surface mount pads on both sides of the circuit. If rigid-flex is a new technology for you, there are a few key things to keep in mind. The term "bikini cut" is important. It is recommended to keep the adhesive within 0.050" of the edge of the rigid portion of the design. Adhesive- less flex materials should be used and cover- lay should not extend into the plated through areas. There is a Z-axis mismatch between the rigid materials and the adhesive that can im- pact the reliability of the design. The simplest version of a rigid-flex construc- tion is to keep all plated through-holes in the rigid area of the designs. It is certainly possi- ble to create a rigid-flex with plated through- holes in the flex regions as well, but this type of design requires additional processing, add- ing cost to the design. The flex layers can also be bonded or un- bonded. If there are several flex layers or flex- ibility is a concern, one common solution is to eliminate the adhesive between selected flex layers, providing more flexibility to the overall design (Figure 1). Often, this is confused with bookbinder rigid-flex construction. Bookbinder Rigid-Flex Bookbinder construction has been around for decades but seems to be regaining popu- larity in the market. A bookbinder rigid-flex is similar to a hard-covered book. The flex layers are staggered, each flex layer gaining length as it is stacked on the bend so that when the flex area is bent, it does not buckle and cre- ate stress on the flex layers. Bookbinder con- struction is both labor and engineering inten- sive and there are only a handful of fabricators that specialize in this construction. If bookbinder rigid-flex is new to you, atten- tion should be given to the variables that need to be considered to allow the proper fit. It is ad- visable to add extra length if air circulation is required to keep the flex cool in a high current application rather than tightly nest the layers. It is also important to plan for the mechani- cal space this bulge will require in final assem- bly. Moving along the technology scale would Figure 1: Unbonded layers improve flexibility. (Source: The Printed Circuit Designer's Guide to... Flex and Rigid-Flex Fundamentals)

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