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44 The PCB Magazine • June 2014 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 less well 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 considerably more current-carrying capacity than can be reasonably designed in with these copper thicknesses. One option is to consider thicker copper. Copper thicknesses of .010"– 015" thick are not uncommon but flexibility is dramatically reduced with these materials. Us- ing polyimide thicknesses of .001" (dielectric strength at 6000volts/mil per ASTM D149) does allow a considerable thickness reduction in ap- plications with space constraints. And the ma- terials do remain bendable if necessary. Current carrying capacity is a function of a copper traces cross sectional area and the fol- lowing chart (Table 1) provides a quick refer- ence for design considerations. Circuit features with integrated pins are sometimes favored in applications with high current requirements. The thicker copper cre- ates robust pins that are not easily bent or dam- aged during handling. These integrated pins are often used in high reliability applications serv- ing as jumpers from one PCB to another, elimi- nating the need and cost of connectors. Cir- flexing applications, the vast majority of flex circuits do not bend, fold, or flex once they are installed. This obviously begs a few questions, such as: What are all those other applications? Why do they use flex circuits? 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- FLExIBLE CIRCUITRy...A 3D PACKAgINg TooL continues Figure 2: Folded circuit with components. Figure 3: circuit with integrated pins.