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86 DESIGN007 MAGAZINE I MARCH 2021 Flexible printed circuit boards, by their very nature, are designed to be flexible. is pres- ents problems in securely and reliably attach- ing the ends of the flex circuits to a solid, stiff, main PCB or other electronic devices. A com- bination of hard, as well as semi-flexible stiff- eners, is used for this purpose. Hard stiffen- ers are FR-4, aluminum, and 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). ick flex cover-coat material is used as a semi-stiffener where the flex circuit needs to retain some flexibility close to the attachment point. e slightly less flexible stiffener allows for attachment of the flex to the FR-4 PCB, while providing an area of reduced flexibility, allowing for a more reliable connection with- out 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. e cover coat material allows for some stress tran- sition from highly bendable to rigid, reducing the chance of a crease crack in the copper, similar to a short, tapered cord protector on a power tool. Flex circuitry can rip or create openings in the copper traces if the bend radius Reaching the Limit on Stiffeners Consider This by John Talbot, TRAMONTO CIRCUITS Figure 1: Solid FR-4 stiffener with locating tabs.

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