Copper-Clad FR-4 Board: Glass Cloth, Epoxy and Foil

FR-4 is the board most printed circuits are made on: woven fibreglass cloth bound with an epoxy resin that is flame resistant (self-extinguishing, typically through bromine compounds). Glass cloth impregnated with resin is called pre-preg. Stacks of pre-preg with copper foil on the outside are laminated in a press with heat and pressure into one inseparable board, the copper-clad laminate that circuits are later etched from.