Flexible printed circuit

A flexible printed circuit is a circuit board that bends. Instead of a stiff fiberglass board, the base is a thin, amber plastic film with copper traces on it. It can fold around edges, flex in a hinge or connect parts that sit at different angles.

It starts with a film of polyimide, often 25 to 50 micrometers thick, chosen because it stays strong and flat through soldering heat. A thin layer of copper is bonded to it, either laminated as foil or plated straight onto the film. The copper is coated with photoresist, exposed to a pattern of light and etched, leaving only the traces. For display drivers the traces are extremely fine, a few tens of micrometers apart, and are tin-plated so they bond well. A protective cover goes over most of the circuit, leaving the contact pads bare.

In this build it's the carrier film under each display driver chip, linking the chip's hundreds of outputs to the panel's edge. Flex circuits are also inside phones, cameras and laptop hinges. They're related to, but different from, the simpler flat flexible cable, which is just straight conductors sealed in plastic.