Do you know how many kinds of PCB?
On the most basic PCB, the parts are concentrated on one side, the wires are concentrated on the other side. Because the wires only appear on one side, the PCB is called single-sided. In general, only the early circuit would use single-sided board, because there are many strict restrictions on the design of such a board .
The circuit board has both sides of the wiring, and there have appropriate circuit connection between the two sides. The "bridge" between such circuits is called via. Vias are small holes filled with metal on the PCB, which can be connected to both sides of the wire. The double-sided board solves the difficulty of interleaving in a single-sided board, because the area of the double-sided board is twice as large as that of the single-sided board. So double-sided board is more suitable for use on complex circuit.
Multi-layer board use more single or double-sided board, through the positioning system and special bonding materials to put them together, and the conductive graphics according to the design requirements. Most of the motherboards are 4 to 8 layers of the structure, but according to the technical theory, Multi-layer board can be done nearly 100 layers of PCB board.