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82 DESIGN007 MAGAZINE I JANUARY 2020 Feature by Flexible Circuit Technologies Flexible circuit designs share many of the same challenges of rigid PCB designs, but there are also many differences and addition- al challenges. The very nature of a flex circuit being able to bend and flex make it as much a mechanical device as an electrical one. This creates a special set of requirements unique to flexible circuitry. Understanding how these re- quirements interact will allow the PCB design- er to create a flex circuit that balances the elec- trical and mechanical features into a reliable, cost-effective interconnect solution. Overview Scrutinize your design for stress concentra- tion features. Stress concentration features are the single predominant cause for mechanical failures in flexible circuits (i.e., cracked/bro- ken conductors, torn insulating material, etc.). To avoid stress concentration points, the con- struction of the circuit should not change in, or immediately adjacent to, the bend area. In a bend area, there should be no change of con- ductor width or thickness or direction, no ter- mination of plating or coatings, no openings in covers or outer insulating materials, and no holes of any kind in a bend zone. Bend Ratio Determine and evaluate the minimum bend ratio of your design. This will be your single best indicator of whether your flex circuit may experience problems in service. Bend ratio is bend radius: circuit thickness (Table 1). Conductor Routing When possible, conductors should be routed through bending or flexing areas with the con- ductors perpendicular to the bend (Figure 1). This will minimize stress on the conductors during flexing and maximize circuit life. Con- ductors should always change directions with soft curves rather than sharp corners. When Flexible Circuit Design Guidelines Table 1: Preferred bend ratios.

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