As the name suggests, flex circuits is a bendable PCB, which is used in some products that require a PCB that can be bent. Similar to the structure of rigid PCB, the flexible board uses a layer of PI (Polyimide) with high temperature resistance and good dielectric layer as the base material, and then uses the adhesive layer of Epoxy to bond the copper foil layer on the top layer. If it is a double sided flexible PCB, the upper and bottom layers of copper foil need to be bonded.
For more PCB substrate properties, please refer to “Introduction To The Material Properties Of PCB CCL”