From the top to the bottom, a rigid-flex PCB is a PCB that has a flexible section and one and above rigid sections that are interconnected.
The flex section refers to the flexible PCB layers in the middle. Its size is the same as the rigid-flex PCB size.
The rigid sections refer to the rigid PCB layers on the top and bottom of the flex section.
As you can see, a rigid-flex PCB basically is a flexible PCB having rigid layers.
So how did engineers come up with such an ingenious design?
Rigid-flex PCBs were developed from aerospace research for reliable wiring in a compact space. High-density wiring, flexibility, limited space, lightweight, and mechanical strength were all the challenges that were met. Rigid-flex PCBs offer a perfect solution to these challenges.
Now, rigid-flex PCBs are not only used in aerospace, they also find applications in medical, consumer electronics, industrial, communication, automotive, defense, and research.