Flexible PCB is a kind of mainly composed of CU (Copper foil) (E.D. or R.A. copper foil), A (Adhesive) (acrylic and epoxy thermosetting adhesive) and PI (Kapton, Polyimide) (polyimide film) circuit board with many advantages such as space saving, weight saving and high flexibility https://www.hemeixinpcb.com/rigid-flex-pcb/flexible-circuits.html