OEM and ODM 4 Layer PCB Board Multilayer Circuit Board Assembly What Is 4 Layer PCB Board? A 4-layer PCB is a printed circuit board that contains four conductive copper layers separated by insulating material (typically fiberglass and epoxy resin, like FR4). These layers are laminated together under heat and pressure to form a single, solid board. Structure of a 4-Layer PCB: The typical stack-up is: Top Layer - Signal routing (components often placed here) Internal Layer 1 -