According to the circuit layer classification: divided into single panel, double-sided board and multilayer board. The common multi-layer board is generally 4-layer board or 6-layer board, and the complex multi-layer board can reach dozens of layers. There are three main types of PCB
Single sided boards are the most basic PCB. The parts are concentrated on one side, and the wires are concentrated on the other side (when there are patch elements, they are the same side as the wires, and the plug-in devices are on the other side). Because wires only appear on one side, this kind of PCB is called single sided. Because there are many strict restrictions on the design circuit of a single panel (because there is only one side, the wiring can not cross, but must go around its own path), only the early circuits used this kind of board.
Double sided boards have wiring on both sides of this kind of circuit board, but to use wires on both sides, it is necessary to have proper circuit connection between the two sides. The bridge between these circuits is called via. A pilot hole is a small hole in a PCB filled with or coated with metal, which can be connected with wires on both sides. Because the area of the double-sided board is twice as large as that of the single panel, the double panel solves the difficulty of wiring interleaving in the single panel (it can be conducted to the other side through the hole), and it is more suitable for more complex circuits than the single panel.
In order to increase the area of multi-layer boards that can be wired, multi-layer boards use more single or double-sided wiring boards. Printed circuit boards with one double-sided inner layer, two single-sided outer layer, two double-sided inner layer and two single-sided outer layer are alternately connected by positioning system and insulating bonding materials, and the conductive patterns are interconnected according to the design requirements. They are also called multilayer printed circuit boards. The number of layers of a board does not mean that there are several independent wiring layers. In special cases, an empty layer is added to control the thickness of the board. Generally, the number of layers is even and includes the outermost two layers. Most of the motherboards are 4 to 8-layer structure, but technically, the theory can achieve nearly 100 layers of PCB board. Most large supercomputers use multi-layer motherboard, but because this kind of computer can be replaced by many ordinary computer clusters, the multi-layer motherboard has been gradually not used. Because each layer of PCB is closely combined, it is not easy to see the actual number, but if you carefully observe the motherboard, you can still see it.