Flexible PCB’s are just what the name implies, a PCB with the ability to be flexed/bent. There are two major design types of flex boards and each of these two designs require different materials for manufacturing and also different manufacturing processes.
The first type is the dynamic design. This design is seen in many items like for instance a laptop where a screen needs to be attached to a base that has a hinge, or for joints in a robotic arm. This design is made to be flexed and or bent many many times. Care in manufacturing and material selection is extremely important in this design because this will limit the amount of times the board can flexed/be bent.
The second type, which is the most common design, is a static flex PCB. This design is very common in items like cell phones where the dimension is very limited by design. This design eliminates the use of cables to connect one PCB to another. So in most cases the PCB is bent only once for manufacturing of the item and then stays in the position permanently.
Besides, there is another kind of static flex PCB: Semi-flex PCB. Semi-flex PCB is made from FR4, it is a cheap alternative to polyimide flexible PCB and Rigid-flex PCB. After PCB assembly, the semi-flex PCBs can be bended, but it must be a static using, not dynamic.