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92 DESIGN007 MAGAZINE I OCTOBER 2020 Heavy-Copper Flex Circuits See Large Growth Flex circuits can be designed and manufac- tured to handle heavy currents. Thicker cop- per—3 oz. to 20 oz.—prevents true high cycle flexing. However, the circuit can be used in "bend to fit" and limited bend applications in high-current situations. Heavy copper—as thick as 20 oz.—has been manufactured in single-sided and double-sided applications. Multi-level flex copper circuits are used when the board must have a heavy, bendable cop- per section for power applications and a thin flex section for attachment purposes. This is similar to rigid-flex but all bendable, and some parts are as normally flexible (1 oz. RA copper). The flex base material has a very high volt- age breakdown rating of 2000 volts per mil of thickness, which allows higher voltage use in a thinner, bendable package. The heavy cop- per can present difficulty during the etching operation, as the etch solution will etch down through the thick copper, creating a tapered effect. Typically, the amount of sidewall width lost is 1.5–2 times the copper thickness. This etching undercut means your line width to space is much larger than a typical 1-oz. cop- per circuit. A 6-oz. copper line and space minimum start- ing width is around 30 mils. When you start with a 30-mil-wide trace in 6-oz. copper, the final trace width will be in the area of a 20-mil- wide trace. This loss of sidewall width must be accounted for in your Gerber data. Addi- tionally, the spaces will widen by the same formula, creating wider spaces between tracks. Each manufacturer has its own set of design rules for etching heavy copper. Before you start to design your heavy-copper flex circuit, talk to your flex PCB shop to make sure you understand their dif- ferent production guidelines and heavy copper limits as to thickness, trace, and space etching losses. Consider This by John Talbot, TRAMONTO CIRCUITS

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