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100 DESIGN007 MAGAZINE I SEPTEMBER 2020 Consider This by John Talbot, TRAMONTO CIRCUITS Flexible PCBs, by their very nature, are designed to be flexible. This presents problems in securely and reliably attaching the ends of the flex circuits to a solid, stiff, main PCB or other electronic devices. A combination of hard, as well as semi-flexible, stiffeners is used for this purpose. Hard stiffeners used include FR-4, aluminum, as well as heat conductive aluminum-backed metals. A hard stiffener has a limit as to how close the flex circuit bends to the stiffener without stressing the joint or causing the flex circuit to crease (Figure 1). Thick, flexible cover coat material is used as a semi-stiffener where the flex circuit needs to retain some flexibility close to the attach- ment point. The slightly less flexible stiffener allows for attachment of the flex to the FR-4 PCB while providing an area of reduced flex- ibility, allowing for a more reliable connection without creasing the flex near the attachment point (Figure 2). Double layers of thicker cover coat stiffen- ers create a slightly bendable flex attachment point, such as a printer head to a fixed PCB. The cover coat material allows for some stress transition from highly bendable to rigid, reduc- ing the chance of a crease crack in the cop- per, similar to a short tapered cord protector on a power tool. Flex circuitry can rip or create Let's Talk About the Basics of Flex Figure 1: Solid FR-4 stiffener with locating tabs. Figure 2: A 5-mil, double-sided, thick cover coat used as a stiffener on a 1-mil very flexible circuit.

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