Flexible Printed Circuits: A Design Primer

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Flexible circuits consist of conductive strips in a sandwich of insulating or dielectric material. They resist moisture and contamination and are insulated from external shorts, with holes or contact surfaces for interconnection.

Understanding a package’s electrical requirements and not over-designing permits means taking full advantage of a flexible circuit’s potential compared to conventional wiring. Among the benefits are weight and bulk reduction, shorter assembly time and fewer errors, reproducible electrical characteristics, custom shielding, improved reliability, ability to interconnect moving parts, and an engineered appearance.

The mechanical and electrical characteristics of flexible circuits determine what type should be used. Total installed cost, including inspection, interconnection, fixturing and testing, must be weighed against the advantages of each:

• Single-sided flexible circuits have one layer of copper on dielectric.
• Double-sided flexible circuits have copper on both sides.
• Sculptured circuits are single- or double-sided circuits created from thick copper that allows connections such as fingers and pads to be relatively thick and very rigid extensions of the flexible conductors.
• Multilayer flexible circuits have several layers that are registered to each other and are separated by a cured layer of adhesive during lamination, with interlayer connections via plated through-holes.
• Multilayer rigid-flex is the combination of flex within and extending from rigid sections of the PCB.

There are two general areas of application that may take the designer down different paths. The first is a circuit that will be bent or formed into a rigid package during assembly. The second is where the circuit will be flexed continuously or intermittently during operation.

To read the rest of this article, which appeared in the December 2023 issue of Design007 Magazine, click here.