Issue link: https://iconnect007.uberflip.com/i/324117
June 2014 • The PCB Magazine 45 cuits with this feature require a multi-step etch- ing process to produce a flexible circuit having thinner and more flexible finished conductors in regions that are bent, but with thicker copper pins for the soldered connections. High-Speed Cables High-performance cables with controlled impedance requirements can also be created with flexible printed circuits. These cables are designed for use in high-speed digital applica- tions. A common microstrip circuit, easily con- structed with PCB materials and requiring con- trolled impedance, uses a flat conductor over a ground plane. The conductor and ground plane are separated by a dielectric—usually polyimide and adhesive. The surface microstrip transmis- sion line also has free space (air) as the dielectric above the conductor. Stripline designs with two reference planes are also easily designed with circuit board materials and processes. There are several options on the Internet for finding de- FLExIBLE CIRCUITRy...A 3D PACKAgINg TooL continues sign assistance when calculating controlled im- pedance. One option is located here. Component Assembly Component assembly on flex circuitry is common and similar to methods used for rigid PCBs. More than 50% of flex circuits are sold with value-added component attachment. This can be done with hand soldering techniques or hand placement of SMT components for low volume. Both traditional tin/lead solders and lead-free solders (used to meet RoHS require- ments) are common as polyimide constructions are very robust at high temperatures. With high- er-volume parts there is custom fixturing used which can get complicated as it is adopted to accommodate the handling nuance character- istic of thin, flexible materials. Both high-vol- ume wave soldering and surface mount solder- ing are common assembly techniques. Selective stiffeners are attached with either thermoset or high temperature pressure sensitive adhesives table 1.