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Page 54 of 109

AUGUST 2019 I SMT007 MAGAZINE 55 is one mil of adhesive (coverlayer) for every ounce of surface copper it is covering. The same holds true for bond ply. Figure 2 illus- trates a four-layer flex circuit and demonstrates the use of coverlayer and bond ply. Matching the Coverlayer to the Application Coverlayer and cover coat are terms normally reserved for flexible circuit constructions, and by default, they are a defining structural ele- ment of both flex and rigid-flex circuits (Fig- ure 3). Coverlayers serve as a flexible coating of sorts that protect the delicate surface traces from physical damage and potential wicking of solder along circuit traces. The coverlayer offers protection while leaving open access to design features where interconnections are to be made to components by soldering. It is important to determine the thickness of a cov- erlayer as a balance needs to be made between allowing for maximum flexibility while also ensuring a sufficient amount of adhesive on it to accommodate the copper weight (the more adhesive, the less flexible). This is where it is critical to understand the application and working environment the flex board will be subject to in the field. The cov- erlayer choice will depend on how the flex cir- cuits are utilized; will the circuit be bent just one-time during installation, intermittently, or dynamically millions or even billions of times over its useful life? In the latter case, the dimensions and make of the flexible circuit coverlayer are critical. In dynamic flex circuits, Figure 2: Four-layer flex circuit with coverlayer and bond ply. Figure 3: Flex circuit coverlayer.

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