There are numerous, different structures available. The more common ones are defined below:
Single sided flex (IPC-60103 type 1) Coverlay (polyimide + adhesive) bonded onto an adhesiveless single sided FPC core. With or without stiffeners.
Double sided flex (IPC-6013 type 2) Coverlay bonded onto both sided of an adhesiveless double-sided FPC core (two conductive layers) with plated through holes. With or without stiffeners.
Multilayer flex (IPC-6013 type 3) Coverlay bonded on both sides of an adhesiveless construction containing three or more conductive layers with plated through holes. With or without stiffener. Capability is 4L.
Traditional rigid flex construction (IPC-6013 type 4) Multilayer rigid and flexible circuit combination containing three or more layers with plated through holes. Capability is 22L with 10L flex layers.
Asymmetrical rigid flex construction, where the FPC is situated on the outer layer of the rigid construction. Containing three or more layers with plated through holes.
Multilayer rigid flex construction with buried / blind via (microvia) as part of the rigid construction. 2 layers of microvia are achievable. Construction may also include two rigid structures as part of a homogeneous build. Capability is 2+n+2 HDI structure
Book-binder and air-gap build – a complex build. With spaces between layers of flex to allow for improved flexing of the FPC. Flex layers may be of different lengths on the book-binder constructions to minimize compression of inner flex layers within bend radius.