Rigid flex circuit are a hybrid construction flex circuit consisting of rigid and flexible substrates which are laminated together into a single structure. Each layer of rigid-flex circuit are electrically interconnected by plated through holes (PTH), which is different from a flexible circuit with FR4/PI stiffener on top or bottom of flex circuit only by the means of presensive or thermal senstive adhesive only, no PTH.
Rigid-flex board enjoy the advantage of both rigid FR4 PCB and flexible circuits. In rigid area, components will be soldered on, and whole board can be bendable at the flexible circuit area, so that to save space of assembly, make whole product into a small size but with tight trace designing.
Also people don't need to solder the FPC on PCB, or connect FPC on PCB via connector, furthermore, that plated through hole interconnection is more stable, firm than soldering or connector. So more and more rigid flex board circuit has been widely used in both military and civil area.
Structure of Multi-Layer PCB
Normally rigid circuit board is belonging to multi-layer structure, and layer numbers is in even, such as 4L, 6L, 8L, 10L, etc. Here are some popular stack up of rigid circuit board.
4L Rigid-flex: 
•1L FR4
•2L FPC
•1L FR4
•2L FR4
•2L FPC
6L Rigid-flex: 
•2L FR4
•2L FPC
•2L FR4
•1L FR4
•4L FPC
•1L FR4
8L Rigid-flex:
•2L FR4
•4L FPC
•2L FR4
•1L FR4
•6L FPC
•1L FR4
•2L FPC
•4L FR4
•2L FPC
But sometimes, there're also some special design of rigid flex circuits are used for different application. Here are some examples:
2 layers Rigid-flex circuits
•1L FR4
•1L FPC
3 layers Rigid-flex circuits
•1L FR4
•1L FPC
•1L FR4
•1L FPC
•1L FR4
•1L FPC
