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66 DESIGN007 MAGAZINE I JANUARY 2024 Five Best Practices for Designing Flex and Rigid-flex Circuits Of course, adapting to these new materials requires learning about their capabilities and restrictions, as well as learning a new set of design rules and best practices. Here are five best practices to get you started on your first flex or rigid-flex PCB design. Before exploring these best practices, con- sider whether your PCB will be used in a static flex application—that is, it just needs to flex once during installation—or if it will be in a dynamic flex application. is important dis- tinction can impact your design decisions. 1. Use curved paths for your flex traces. Sharp angles can cause weak points in your traces across flex PCB segments. ey tend to concentrate mechanical stress along the angles, so if you use curved paths for flex points, you will avoid cracking and weakening connec- tions. is is particularly important in dynamic flex applications, where traces and conductors are subjected to repeated bending. 2. Use fillets to connect your traces to pads. A widened fillet can help reduce stress at connection points between traces and pads, greatly increasing the robustness of your design. Teardrop and rounded trace designs can reduce stress concentration points and prevent potential cracking or peeling around connections. 3. Make allowances for the coverlayer. Unlike rigid PCBs, flex PCBs require an additional coverlayer to protect circuits and maintain flexibility. It is important that the coverlayer leaves room for various features of the PCB and for the interaction between the coverlayer and the flex board. First, the coverlayer needs to be drilled to create access to the pad for soldering. However, it is important that the adhesive isn't squeezed out onto the pads. To accommodate for this, coverlayer drill holes need to be 0.005–0.010" larger than the land area. In addition, this same allowance needs to be made for clearance holes in the coverlayer around through-holes in the flex PCB. 4. Layout considerations to be made for conductors and traces in dynamic flex areas. Conductors need to be evenly staggered across areas of the board that will see regular flexing. If the circuit is repeatedly flexed per- pendicular to the conductors, stress will occur Figure 1: The example shows a cover-coated pad area in a Type 1 (single-sided) application. 1 Figure 3: A coverlayer pad area in a Type 1, 2, or 3 application. Figure 2: The example shows a cover-coated pad area in a Type 2 (double-sided) application.