Flexible Printed Circuit (FPC) is a highly reliable and excellent flexible printed circuit board made of polyimide or polyester film. It has the characteristics of high wiring density, light weight, thin thickness and good bending performance.

A typical FPC is mainly composed of two materials: substrate and protective film,

Let's first talk about the substrate, which is mainly composed of PI or PET+adhesive+copper,
PI is a "polyimide" insulating resin material, which is characterized by high temperature resistance, good bending performance, and good reliability. It is more than twice the price of PET, and is the main material of FPC,
PET is a "polyester" insulating resin material with the opposite characteristics to PI, which is rarely used in FPC factories.
As we all know, glue and copper are conductive materials. The function of glue is to bond copper with PI or PET,
The final substrate made into FPC, also known as the substrate or baseboard, is equivalent to the PCB substrate.

Let's talk about protective film, which refers to the insulation layer on the surface of FPC,
After completing the circuit on the substrate, a layer of insulation similar to PCB ink function is attached to prevent circuit oxidation and short circuit, which is also composed of adhesive and PI insulation layer.

Extracurricular knowledge: Differences between FPC reinforcement materials FR4 and PI:
They are all additional hard materials added to the local area of the soft board for strengthening the load-bearing component area and facilitating installation.
FR4 is a glass fiber epoxy resin composite reinforcement with low water absorption, good durability, and good bending strength.
PI (polyimide) is expensive, but its flame resistance is good.