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PCBD-June2014

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28 The PCB Design Magazine • June 2014 flexing applications, the vast majority of flex cir- cuits do not bend, fold, or flex once they are in- stalled. This obviously begs a few questions like, "what are all those other applications?"; "why do they use flexcircuits?"; and "when should a flex circuit be used rather than a rigid PCB?" The answers vary as diversely as the appli- cations themselves. Answering these questions requires an examination of why past products have incorporated flex circuits. What utility is offered by this novel interconnection packag- ing option? Answers include space savings, weight reduction, high-performance cable de- sign, multi-planar interconnection, and cost. Cost vs. Hardboard One of the questions I field most frequently is, "How does flex circuitry compare in cost to a rigid printed circuit?" And my honest answer is generally is that it is more expensive. The spe- cific premium paid for flex circuitry is a multi- dimensional factor and a function of material costs, handling requirements, applications en- gineering support, volume, and the construc- tion. I generally advise that, if a design doesn't take advantage of the bending characteristics or the thinness of a flex circuit, it is likely that a rigid PCB will be a lower-cost solution. But taking advantage of a flexible circuit's charac- teristics involve considerations for multiplanar interconnections while eliminating connectors or hand soldered wires, simplified assembly, improved reliability, or providing a pre-tested component/subassembly. Applications adopt- ing flex circuits have cleared the hurdle of look- ing at total cost of ownership and decided the technology is cost effective. In fact, the market for flex circuits is estimated to be about 10% of the overall printed circuit market. It is a perva- sive technology. Power Circuits A lesser known segment of the flex circuit market is found in high-power applications. Most applications utilize .5 or 1 ounce copper (.0014" = one ounce) but some products require consid- erably more current-car- rying capacity than can be reasonably designed in with these copper thick- nesses. One option is to consider thicker copper. Copper thicknesses of .010"–.015" thick are not uncommon but flexibil- ity is dramatically reduced with these materials. Using polyimide thick- nesses of .001" (dielectric strength at 6000volts/mil per ASTM D149) does al- low a considerable thick- ness reduction in applica- feature FLExIBLE CIRCUITRY...A 3D PACkAgINg TooL continues Figure 2: Folded circuit with components. Figure 3: Circuit with integrated pins.

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