The four-layer PCB board is pressed on the basis of the double-sided PCB board (two-layer PCB board), and PP and copper foil are added to both sides of the double-sided PCB board when pressed, and then pressed into a 4 layers PCB board after high temperature and high pressure. To put it simply, the four-layer PCB board has an inner layer. In terms of process, some circuits are etched onto the inner layer and then pressed.
