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94 PCB007 MAGAZINE I SEPTEMBER 2019 Introduction Flexible circuits were first introduced as a re- placement for wire harnesses. The earliest ver- sions date back to World War II. Today, flex and rigid-flex circuits are filling an important role across multiple industries, including ap- plications in the medical, automotive, and tele- communications fields. Even though they are a smaller part of the circuit board industry, flex and rigid-flex cir- cuits have been growing in popularity over the last decade, and for good reasons. These cir- cuits are made to be thin, flexible, and durable. However, in addition to the opportunities that come with flex and rigid-flex circuits, there are also challenges. Generally, these occur with the processing part of the technology. Characteristics of Flex and Rigid-flex Circuits Flex and rigid-flex circuits have become a go-to solution for a variety of applications be- cause they offer capabilities that simply aren't available from alternatives. They can be manu- factured to very thin specifications, and they will survive bending and folding without error. These circuits can be run over long distances to make a connection. For example, some us- ers have designed 14–18-foot flexible cables. Some of the most sought-after features of flex and rigid-flex circuits include the following: • Thin-core capability • Improved dielectric constant • Low dielectric constant (Dk) and dissipation factor (Df) critical concerns • Ultra-fine line capable (L/S decreasing to less than 15 microns) • Shorter interconnect distances Flexible circuits have the same capabilities of their rigid counterparts, including repeat- ability, reliability, and high density. In addi- tion, they have characteristics that make them more versatile than rigid circuits. For example, Working With Flexible Circuits Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY