1. Visual inspection

Since the various layers in the PCB are tightly integrated, it is generally not easy to see the actual number, but if you carefully observe the board card fault, you can still distinguish it. If you are more careful, we will find that there is one or several layers of white material in the middle of the PCB. In fact, this is the insulating layer between the layers to ensure that there will be no short circuit problems between different PCB layers. Because the current multi-layer PCB boards use more single or double-sided wiring boards, and a layer of insulating layer is placed between each layer and pressed together, the number of layers of the PCB board means that there are several independent layers. The wiring layer, and the insulating layer between layers has become the most intuitive way for us to judge the number of PCB layers.

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2. Guide hole and blind hole alignment method
The guide hole method uses the "guide hole" on the PCB to identify the number of PCB layers. The principle is mainly due to the via technology used in the circuit connections of the multilayer PCB. If we want to see how many layers the PCB has, we can distinguish by observing the via holes.
On the most basic PCB (single-sided motherboard), the parts are concentrated on one side, and the wires are concentrated on the other side. If you want to use a multi-layer board, you need to punch holes on the board so that the component pins can pass through the board to the other side, so the guide holes will penetrate the PCB board. So we can see that the pins of the part are soldered on the other side.

3. Accumulation method
To be precise, this is not a method, but an experience. We can judge the number of layers of the PCB through the traces of some public PCB boards and the position of the components. Because in the current IT hardware industry that is upgrading so fast, there are not many manufacturers capable of redesigning PCB after all.