FR4 Copper Clad Laminate Double Side PCB is one of the most commonly used PCBs in the world. It consists of a glass fiber laminate as a reinforcing material on the core, with copper-clad on both sides.

The core in itself has copper in it along with two layers on top and bottom. Hence the name copper clad. With the addition or removal of the glass fiber epoxy layers, the thickness of the PCB can be controlled. The glass fiber epoxy layers provide the necessary electrical insulation between the two copper foils and also gives the laminate rigidity.

Here, FR in the FR4 stands for fire retardant.

It is widely used for prototyping as the circuit can be designed easily.