A Rigid-Flex PCB is a hybrid board that consists of a combination of rigid and flexible printed circuit boards. These boards are designed by combining flexible circuit boards with one or more rigid boards. 

The flexible part of the board is usually used for providing interconnections between rigid boards, which allows narrow conductor lines which take up less space making the boards smaller. Using flexible PCBs for interconnections also eliminates the need for connectors, which are bulky and heavy making rigid-flex printed circuit boards much lighter. Interconnections using flexible PCBs over connectors also provide lower losses and accommodate controlled impedance and eliminate connection problems such as cold joints.

Another major advantage of Rigid-Flex boards is the use of different substrate materials in different sections on the board. Some sections of the board might require high performance substrates which are expensive while others sections might not could use a standard FR-4 substrate. This flexibility in terms of substrate selection can optimize the performance and cost of Rigid-Flex PCBs.

Designers often use Rigid-Flex PCBs as they provide a lot of design flexibility. These boards are also more cost effective than flexible PCB boards.