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56 DESIGN007 MAGAZINE I DECEMBER 2023 ere are two general areas of application that may take the designer down different paths. e first is a circuit that will be bent or formed into a rigid package during assembly. e second is where the circuit will be flexed continuously or intermittently during opera- tion. Insulating Materials e choice of material must take into con- sideration conditions of operation and manu- facture, electrical characteristics, mechanical requirements, and cost. Some materials have qualities that may make them more suitable for certain applications, but they may not be uni- versally useful. Kapton® is perhaps the most widely used material, dimensionally stable, and compatible with all PCB processes. It does not support combustion. It is flexible and adherable, has a high tensile strength, and about the same coef- ficient of expansion as copper. It does, how- ever, absorb significant amounts of moisture. Mylar is inexpensive, flexible, adherable, and dimensionally stable, but is sensitive to some chemicals used in PCB processing and to sol- dering heat. Teflon® FEP is an excellent dielectric, does not absorb water, and is flexible and adher- able, but it's not dimensionally stable and it's expensive. Nomex® is dimensionally stable, flexible, and adherable, has high tensile strength, and resists soldering temperatures. However, Nomex reacts to processing chemicals and absorbs water. Design Procedures If the actual unit to use the circuit is unavail- able, construct a full-size model from card- board, sheet metal, or wood. Open the model to lay flat and place or sketch components in their required positions. Make a "paper doll" out of the circuit shape that will provide all connections and conductor runs in a size that fits the packaging constraints (Figure 1). Prepare a single-entry wire list showing each conductor in a continuous run to every termination point. Establish current-carry- ing capacity and resistivity requirements and determine copper conductor width and thick- ness required for current capacity and voltage drops. Next, condense the wiring list and deter- mine the best layout of conductor runs to minimize the required circuit layers and the circuit's mechanical area. Separate analog and digital signals to minimize crosstalk by plac- ing them on opposite sides of the circuit or on different layers. Shielding may be neces- sary. With these preliminary steps completed, begin generation of specific artwork for cir- cuit production. Spacing between conductors is determined by the voltage between conductors (DC or AC peak volts) and ranges from a minimum of 0.005" for 15v or less to 0.020" for up to 500v. A general rule of thumb is 0.00012"/volt. Larger than minimum spacing is preferred. Routing of conductors should be as direct as possible with no nonessential overlaps that create the need for additional layers. Prepare a detailed drawing with all dimensions speci- fied and all components positioned. Specify terminal areas without assigning pin addresses when possible. Keep edge distance as large as possible; 0.050" is recommended. Figure 1: The physical model of the circuit and package.