The circuit board system is classified into the following three types: single-sided board, double-sided board, and multi-layer board. Single Panel
As we mentioned earlier, we call this type of PCB a single-sided PCB (Single-sided). Because there are many strict limitations on the design of the circuit on a single-sided board (since there is only one side, the wiring cannot intersect and must follow separate paths), only early circuits used such boards.

Double-sided board
This type of circuit board has wiring on both sides. However, in order to use the wires on both sides, there must be proper circuit connections between the two sides. The "bridges" between these circuits are called vias. Vias are small holes filled or coated with metal on the PCB, which can connect to the wires on both sides. Because the area of a double-sided board is twice that of a single-sided board, and because the wiring can cross each other (can wrap around to the other side), it is more suitable for use in more complex circuits than a single-sided board.

Multilayer board
When dealing with more complex application requirements, the circuits can be arranged in a multi-layer structure and pressed together. Through holes are then laid out between the layers to connect the circuits on each layer.