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APRIL 2021 I DESIGN007 MAGAZINE 113 the flex area may be omitted to assure maximum flexibility (Figure 2). If the core layers must be unbonded, this should be noted in the documentation. ose areas where bonding is to be avoided should be identified in the design artwork package. e unbonded areas must have a cov- erlayer applied to each exposed side (Figures 2 and 3). In lami- nated areas, it is not required and arguably a liability when plated through-hole reliability through the assembly process is considered. Obv iously, in areas where interconnection is required between multiple lay- ers of internal circuits, a dielectric is required as shown in Figure 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. at is, the construction should be bal- anced from center to surface on both sides of the circuit board, whether rigid or flex. Bal- anced construction is for the rigid section of rigid-flex. It is preferred that flex cores be balanced, too; however, it is not mandatory. is 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. e 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. e 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. e first thing to keep in mind is perhaps the most obvious: e 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. is is especially true for dynamic flex circuit appli- cations. FLEX007 To download your copy of e Printed Circuit Designer's Guide to…Flex and Rigid-Flex Fun- damentals, click here. Dave Lackey is vice president of business development at American Standard Circuits. Anaya Vardya is CEO of American Standard Circuits. Figure 3: Completed PCB flex belts separated. Dave Lackey Anaya Vardya