In their simplest form, flexible circuits are PCBs made of materials that allow for a non-planar positioning within the end product. Typical materials are polyimide-based, and can go under trade names such as Kapton (DuPont).
Applications for Flexible PCBs
Often flexible circuits are used for applications that have repeated flex cycles, such as cellular phones, doors, etc. A growing application for flexible circuits are their use as a connector in LED-based lighting systems. Often, there are two components: a metal substrate LED PCB; an FR-4 based digital driver PCB. These two devices are often connected with a high profile and sometimes costly connector cable; however, a flexible PCB in its place provides a lower profile, slimmer design, and higher reliability.