Single panel
On the most basic PCB, parts are concentrated on one side, while wires are concentrated on the other side. Because the wires only appear on one side, this type of PCB is called a single sided. Because there are many strict restrictions on the design of single panel circuits (as there is only one side and the wiring cannot cross and must follow separate paths), only early circuits used this type of board.
Double sided board
This type of circuit board has wiring on both sides, but to use wires on both sides, appropriate circuit connections must be made between them. The "bridge" between these circuits is called a via. A through-hole is a small hole filled or coated with metal on a PCB, which can be connected to wires on both sides. Because the area of the double-sided panel is twice as large as that of the single panel, the double-sided panel solves the difficulty of staggered wiring in the single panel (which can be connected to the other side through vias), making it more suitable for use in more complex circuits than the single panel.
Multi-layer board
In order to increase the area that can be wired, multi-layer boards use more single or double-sided wiring boards. A printed circuit board with one double-sided inner layer, two single-sided outer layers, or two double-sided inner layers and two single-sided outer layers, is called a four layer or six layer printed circuit board, also known as a multi-layer printed circuit board. The printed circuit board is interconnected by a positioning system and insulation bonding material alternately, and the conductive pattern is designed according to the requirements. The number of layers on a board does not necessarily mean there are several independent wiring layers. In special cases, empty layers may be added to control the board thickness. Usually, the number of layers is even and includes the outermost two layers. Most motherboards have a structure of 4 to 8 layers, but technically, it is theoretically possible to achieve a PCB board with nearly 100 layers. Most large supercomputers use quite a few layers of motherboards, but because these types of computers can already be replaced by clusters of many ordinary computers, super multilayer boards are gradually no longer being used. Because each layer in the PCB is tightly integrated, it is generally not easy to see the actual number, but if you carefully observe the motherboard, you can still tell.