PCB Technical - Skills and methods to see the PCB layer and judge the thickness

1. Visual inspection

Considering that the various layers in the PCB circuit board are closely integrated, it is generally not easy to determine the actual number, but if you carefully observe the board card fault, you can still distinguish it. Carefully we will see that there is one or more layers of white material between the PCB circuit boards. In fact, this is the insulating layer between the layers to ensure that there will be no short circuit problems between different PCB circuit board layers. . This is because the multi-layer PCB circuit board at this stage uses 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 circuit board is only It means that there are multiple independent wiring layers, and the insulating layer between the layers has become the most intuitive way for us to judge the number of layers of the PCB circuit board.

2. Guide hole and blind hole alignment method

The light guide hole method uses the "guide hole" on the PCB circuit board to identify the number of layers of the PCB circuit board. The principle is mainly considering that the circuit connection of the multilayer PCB circuit board adopts the via technology. If we want to determine how many layers the PCB circuit board has, we can identify it by observing the via holes.

On the most basic PCB circuit board (single-sided motherboard), the parts are concentrated on one side, and the wires are concentrated on the other side. If a multi-layer board is to be used, then it is necessary to punch holes on the board, so that the component pins can pass through the board to the other side, and the via holes will penetrate the PCB circuit board. So we can see that the pins of the part are soldered on the other side.

For example, if the board uses a 4-layer board, it needs to be routed on the 1st and 4th layers (signal layer). Other layers have other uses (ground layer and power layer). Place the signal layer on the power layer and the power layer. The purpose of both sides of the ground layer is to prevent mutual interference and facilitate correction of signal lines. If some board card guide holes appear on the front of the PCB circuit board but cannot be found on the back, then it must be a 6/8-layer board. If the same guide holes can be found on both sides of the PCB circuit board, it will naturally be a 4-layer board.

However, at this stage, many board card manufacturers have adopted another type of routing method, which is to connect only some of the lines, and use buried vias and blind vias in the routing. Blind vias connect the multi-layer internal PCB circuit board with the surface PCB circuit board without penetrating the entire circuit board.

Buried vias only connect the internal PCB circuit board, so they are basically invisible from the surface layer. Taking into account that the blind hole does not need to penetrate the entire PCB circuit board, if it is six layers or more, look at the board against the light source, and the light will not pass through. Therefore, there was also a very popular saying in the past: judging four-layer and six-layer or above PCB circuit boards based on whether the vias can leak light. This method has its reasons, and there are also some areas where it is not suitable, and it can be used as a reference method.

3. Accumulation method

To be precise, this is not a type of method, but a type of practical experience. But this is what we think is the most accurate. We can judge the number of layers of the PCB circuit board based on the traces and component positions of some public PCB circuit boards. This is because in the IT hardware industry that is upgrading so fast at this stage, there are not many manufacturers who have the professional ability to redesign PCB circuit boards.