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18 FLEX007 MAGAZINE I JULY 2018 Feature by Dave Lackey and Anaya Vardya AMERICAN STANDARD CIRCUITS Designing Flex Circuits for First-Pass Success, Part 2 Circuit Layup Symmetry It is a long-standing practice to design mul- tilayer circuit structures with special attention given to the symmetry of the layers in the cir- cuit. That is, the construction should be bal- anced from center to surface on both sides of the circuit board, whether rigid or flex. This is most easily accomplished by choosing core materials that feature two metal layers. It's more difficult to control this balance when using core materials with an odd number of metal layers. The symmetry requirement applies to the layer count as well as the overall copper area of the different layers. In this regard, the reten- tion of maximum copper in the circuit pattern is beneficial to manufacturing, because cop- per is a dimensionally stable element of the construction. The base polymer is by its very nature flexible, and not intrinsically dimen- sionally stable. Designing for Bending: Understanding the Important Issues Bending and flexing are hallmark functions of a flex circuit, whether the circuit is bent once or flexed millions of times. Understand- ing specific design rules for flex circuits is cru- cial for success in the field. The first thing to keep in mind is perhaps the most obvious: The thicker the cross-section of the material stack in the bend area, the less flexible it will be. It is important to keep the area where bending or flexing is to occur as thin as possible. Ideally it should be a single metal layer, if possible. This is especially true for dynamic flex circuit applications. In other applications where simple bending to shape is required, thickness is less impor-

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