The definition of a flexible circuit board is a single-layer, double-layer, or multi-layer printed circuit board made of flexible substrates, with or without a cover layer (solder mask), in other words, it also calls FPC or Flexible PCB.

There are 4 types of FPC basically,

Single-layer flex circuit board

The cost of a single-sided flexible board is the lowest. If a printed board that does not require high electrical performance, a single-sided flexible board should be used for single-sided wiring. The conductive pattern layer is generally rolled copper foil. The insulating substrate can be polyimide, polyethylene terephthalate, aramid fiber ester, and polyvinyl chloride

Double-sides

The double-sided flexible board is a conductive pattern made by etching a layer on both sides of the insulating base film, the plated through holes connect the patterns on both sides of the conductive copper layer, this is to meet the flexible design and use the function.

Multilayer

Laminate 3 or more single-sided or double-sided flexible circuits together, you can get a multilayers flex circuit board, But multilayer circuits have huge functional differences in terms of higher reliability, better thermal conductivity, and more convenient assembly performance.

Flex-Rigid PCB

Laminate a flex circuit board during the lamination of rigid PCB, you can make a flex-rigid PCB, this kind of PCB can meet the demand of some electronics because you may need a PCB which have a rigid board to support the weight of parts, in addition, you also need the flex circuit board to get a flexible turning, like the foldable devices.