PCB Classification by Layers

It's divided into a single sided PCB, double sided PCB and multilayer PCB. The Common multilayer board is generally 4 layers or 6 layers, complex multilayer board up to dozens of layers.

I. Single Sided PCB
Single Sided Boards are the most basic parts of PCB. Components are concentrated on one side, the wire track is focused on the other side (SMD components and wires for the same side, then the other side of the through-hole components). Because the wire appears only in one side, so this kind of PCB is called single sided.
It has many strict restrictions in the design on the line (because of only one side, each wire must not cross and goes around its own path), so this type of circuit board is used at the early stage of circuit.

II. Double sided PCB
Both sides of the circuit board are wired, but to use the two sides of the wire, we must have the proper circuit connection between the two sides. The "bridge" between the circuits is called the pilot hole (via). The via is filled with or coated with a small hole in the PCB. It can be connected to the two sides of the wire. Because double sided PCB is twice larger in area than a single sided PCB, it solves the difficulty that wiring staggered in single side PCB (through hole conduction to the other side). It is more suitable for use in circuits that are more complex than a single sided board.

III. Multilayer PCB
In order to increase the area that can be wired, multilayer board uses more single or double sided wiring board.

PCB using a double side board as inner layer and two pieces of single layer as the outer layer or two double-sided printed circuit board as the inner layer and two pieces of single layer as the outer layer, through the insulation and bonding materials to press each layer together, and the conductive pattern are as per design requirements for interconnection becomes the 4 layers of printed circuit board, 6 layers of printed circuit board, also known as multilayer printed circuit board.

The number of layers of the board does not represent a separate layer of wiring. In particular, the air layer will be joined to control the thickness of the board. Usually the number of layers is even, and contains the outermost 2 layers. Most of the host mainboard is 4 to 8 layers of structure, but the theory can be technically nearly 100 layers of PCB boards. Large supercomputers use quite a lot of layers of mainframe board, but because this kind of computer can be replaced with a lot of ordinary computer clusters, super multilayer board has gradually not been used. Because all of the layers in the PCB are closely combined, it is generally not easy to see the actual number, but if you look carefully at the motherboard, you can also see it.

PCB can be also classified by its softness. There’re rigid PCB, flexible PCB and Rigid-flex PCB. Metal core PCB and high frequency PCB are classified by its material.