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12 SMT Magazine • March 2017 by John Talbot TRAMONTO CIRCUITS Introduction A flexible circuit assembly consists of a bare circuit with conductive patterns etched onto it that allow the attachment of components that, when completed, results in a working electronic product. Components are available in different shapes, sizes and methods of attachment. There are through-hole components that have wire leads that, as the description states, go through- holes in the circuit and are soldered. Surface mount components, intuitively, are placed on surface pads and soldered to the circuit. The number of different configurations of through- hole and surface mount components are vast. The typical method of attachment is by solder- ing the component to a conductive pattern. Some components are soldered one lead at a time by hand. Others are placed automatically by machine and run through an oven to solder the entire board at the same time. No matter the method, the assembly of electronic flexible circuits is tedious because of its inherent flexi- ble nature. Therefore, we must give extra atten- tion to the assembly process. Flexible Circuits Flexible circuits are made from a thin plas- tic substrate of polyimide or polyester. The most widely used material for flexible assemblies is polyimide (poly im id) because it can withstand the high temperatures required to solder com- ponents without negative effects. Polyester cir- cuits shrivel up when exposed to those tempera- tures and make them less appropriate for flexible assemblies. The polyimide material is laminated to conductive layers and insulating layers with epoxy or acrylic based adhesives. The final prod- uct is a very thin, typically .010", overall thick- ness. This is the characteristic that makes assem- bly on flexible circuits more difficult than tra- ditional printed circuit boards which are 1/16" thick and sturdy. The inherent nature of a flex- ible circuit demands high attention to handling during the assembly process. Whether hand sol- dering or automated soldering, the circuits must be supported for consistent results. The discus- sion below will focus on the processes that will make any flexible circuit assembly successful. Moisture and Flexible Circuits Before we can start any assembly on a flexi- ble circuit, we must first bake out the moisture. Assembly of Flexible Circuits FEATURE

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