PCB is divided into single-sided, double-sided, and multi-layer boards. This classification is based on the number of layers.

　　Do you remember the old-fashioned radio when you were a kid? The PCB inside only has a circuit on one side, and the components are soldered to the circuit on this side. This is a single-sided board.

　　 When the circuit becomes complicated, the wiring density of the circuit increases, and one layer of the same area of the PCB can no longer accommodate so many wires, so it is increased into two or more layers. The conductive hole connection between the layers has achieved the purpose of conduction.

　　 The layer mentioned above actually refers to copper, and a layer of copper constitutes the original layer of PCB. However, what we see on the PCB is not a whole piece of copper, but lines. These lines are the result of chemically etching away the excess part of the original layer of copper, leaving only the lines we want.

　　 Then, the layers are glued with insulating resin, and the holes mentioned above are also made in the resin, so that there are PCBs with various layers