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36 The PCB Design Magazine • June 2015 Flexible circuits are designed to be bendable, but bending rigid PCBs is a little unusual. How- ever, many applications that do not use flex cir- cuit technology will also require bending and forming the circuit. Some of these applications use high-frequency circuit materials to create a circuit in a form that enables improved anten- na functionality. Another application involves wrapping a circuit around a structure, which sometimes functions as an antenna as well. Bending and forming a circuit with dynamic flexing action will require understanding a few basic principles, regardless of the circuit materi- al used. Of course, the circuit material used can make a huge difference in the success of form- ing circuits without causing conductor or mate- rial fracturing. As a general statement with a few exceptions, a circuit material used for bending, forming and flexing cannot have woven glass reinforcement. Because of this, typical FR-4 ma- terials with woven glass are not recommended. Several materials used in high-frequency rigid board applications do not have glass reinforce- ment and have been used successfully for bend- ing, forming and flexing. LCP circuit materials are quite suitable for applications where bending, forming and flex- ing is necessary, and they offer very good high- frequency electrical performance as well. These materials are made as relatively thin laminates, typically less than 5 mils. This thinness aids in the successful bending of the circuits. However, another set of high-frequency ma- terials has been on the market for many years and used in forming applications: PTFE-based laminates, without glass reinforcement. These materials typically use fillers with the PTFE sub- strate to help lower the high CTE of PTFE, and this does not detract from the material's bend- ing capabilities. The basic idea of bending circuits is based on mechanical beam composite theory. As an example, a simple double-sided circuit will be used to demonstrate the concepts. This circuit will be considered a microstrip transmission line with a signal conductor on the top con - ductor layer and ground plane on the bot- tom as shown in Figure 1. The beam composite concept con- siders the cross-sectional area of a by John Coonrod RogERs CoRPoRATIon LIGHTNING SPEED LAMINATES The Art of Bending and Forming PCBs column

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