What is Flex PCB Coverlay? One article will tell

Flex pcb coverlay , also known as protective mask, performs exactly the same function as the solder mask layer of rigid plates. Mainly to protect the circuit board from oxidation and damage. Protect copper foil from exposure to air to avoid oxidation of copper foil; Cover for subsequent surface treatment. Areas that do not need gold-plating are covered with coverlay. In the follow-up SMT, play the role of solder mask layer.
Polyimide short Name:PI; Epoxy Adhesives short Name:AD
The structure of the flex pcb coverlay is divided into three parts, which are PI, AD and Release paper. The used part consists of PI and AD respectively, The release paper only acts as a protective adhesive, which needs to be torn in actual use.
Flex pcb coverlay Main colors: yellow, white, black.

PI use thickness is divided into two types: 12.5um and 25um, special cases can be customized.
AD use thickness is divided into two types: 15um and 25um, special cases can be customized.
Mainly used Flex pcb coverlay size specifications in the market: PI 12.5um/15um AD and PI 25um/25um AD The two stackup methods，A few will use PI 12.5um / 25um AD. You can also customize it in special situations.