The classification of the multilayer PCB

Single panel（mutilayer PCB)
On the most basic PCB, the parts are concentrated on one side and the wires are concentrated on the other side. Because the wires only appear on one side, this kind of PCB is called single side. Because there are many strict restrictions on the design circuit of single panel (because there is only one side, the wiring can not cross and must go around a separate path), only early circuits used this kind of board.

Double sided board(mutilayer PCB)
This kind of circuit board has wiring on both sides, but to use the wires on both sides, there must be appropriate circuit connection between the two sides. This "bridge" between circuits is called a via. The guide hole is a small hole filled or coated with metal on the PCB, which can be connected with the 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 staggered wiring in the single panel (it can be connected to the other side through the guide hole), which is more suitable for more complex circuits than the single panel.

Multilayer board(mutilayer PCB)

In order to increase the area of wiring, multilayer boards use more single or double-sided wiring boards. A printed circuit board with one double-sided as the inner layer, two single-sided as the outer layer, or two double-sided as the inner layer and two single-sided as the outer layer, which is alternately connected together through the positioning system and insulating bonding materials, and the conductive graphics are interconnected according to the design requirements, becomes a four layer and six layer printed circuit board, also known as multi-layer printed circuit board. The number of layers of the board does not mean that there are several independent wiring layers. In special cases, empty layers will be added to control the board thickness. Usually, the number of layers is even and includes the outermost two layers. Most of the motherboards have a structure of 4 to 8 layers, but technically, they can achieve nearly 100 layers of PCB. Most large supercomputers use multi-layer mainboards, but because such computers can be replaced by clusters of many ordinary computers, super multi-layer boards have gradually been abandoned. Because all layers in the PCB are closely combined, it is generally not easy to see the actual number. However, if you carefully observe the motherboard, you can still see it.