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June 2014 • The PCB Design Magazine 29 feature FLExIBLE CIRCUITRY...A 3D PACkAgINg TooL continues tions with space constraints. And the materials 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- 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 appli- cations. A common microstrip circuit, easily constructed with PCB materials and requir- ing controlled impedance, uses a flat conduc- tor over a ground plane. The conductor and ground plane are separated by a dielectric— usually polyimide and adhesive. The surface microstrip transmission 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 materi - als and processes. There are several options on the Internet for finding design assistance when calculating controlled impedance. One option is located here. Table1.

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