PCBs are mainly classified into: single-sided boards, double-sided boards, and multilayer boards.
1. Single-Sided Boards
On the most basic PCB, electronic components are concentrated on one side, while conductive traces are concentrated on the other. Because the traces only appear on one side, this type of PCB is called a single-sided board. Single-sided boards have many strict limitations in circuit design, mainly that traces cannot cross paths and must follow their own routes. Therefore, only early circuits used this type of board, and it is rarely seen now.
2. Double-Sided Boards
These PCBs have traces on both sides. However, to use traces on both sides, there must be appropriate circuit connections between the two sides. These bridges between circuits are called vias. Vias are small holes on the PCB, filled or coated with metal, that connect to traces on both sides. Because double-sided boards have twice the area of single-sided boards, and because the traces can interweave, they are more suitable for more complex circuits than single-sided boards.
3. Multi-Layer Boards
To increase the area available for wiring, multi-layer boards use more single- or double-sided wiring boards. Multi-layer boards use several double-sided boards, with an insulating layer between each board before bonding them together. The number of layers represents the number of independent wiring layers; usually, the number of layers is even and includes the two outermost layers. Most motherboards have a 4-layer structure, but because the layers in a PCB are tightly bonded, the actual number is generally not easy to discern. However, if you observe the motherboard carefully, you might be able to tell, but the chances are slim. Therefore, the common method for distinguishing the number of layers in a PCB is by looking at its thickness and also by feeling its weight.
