How to Judge the Number of PCB Layers?

First, Visual Inspection

Due to the close combination of each layer in PCB, it is not easy to see the actual number, but if you carefully observe the board fault, you can still distinguish it. Be careful, we will find that there are one or several layers of white material in the middle of PCB. In fact, this is the insulation layer between the layers, which is used to ensure that there will be no short circuit between different PCB layers. Because the current multi-layer PCB board uses more single or double-sided wiring board, and put a layer of insulating layer between each layer of PCB board and then press, the number of layers of PCB board represents several independent wiring layers, and the insulating layer between layers becomes the most intuitive way to judge the number of layers of PCB.

Second, Guide Hole and Blind Hole Aiming Method

The guide hole aiming method uses the “guide hole” on PCB to identify the number of PCB layers. Its principle is mainly due to the multi-layer PCB circuit connection adopts the guide hole technology. If we want to see how many layers there are in a PCB, we can identify them by looking at the guide holes.

In the most basic PCB (single panel), parts are concentrated on one side, wires are concentrated on the other side. If you want to use a multilayer board, you need to punch holes in the board so that the component pins can go through the board to the other side, so the guide holes will go through the PCB board. So we can see that the pin of the part is welded on the other side.

For example, if the board uses four layers of boards, it is necessary to route the wires in the first and fourth layers (signal layer), and the other layers have other purposes (ground layer and power layer). The purpose of placing the signal layer on both sides of the power layer and ground layer is to prevent mutual interference and to correct the signal wires. If some card guide holes appear on the front of PCB board, but cannot be found on the back, it must be 6 / 8-layer board. If the same guide holes can be found on both sides of the PCB, it is naturally a 4-layer PCB.

But at present, many card manufacturers use another routing method, that is, only connect some of the lines, and use the buried hole and blind hole technology in the routing. Blind hole is to connect several layers of internal PCB and surface PCB without penetrating the whole circuit board.

The buried hole only connects the internal PCB, so it can’t be seen from the surface. Because the blind hole does not need to run through the whole PCB, if it is six or more layers, the light will not pass through when the board is facing the light source. So there is a very popular saying before: through the hole whether light leakage to judge four and six or more layers of PCB. This method has its own reason, but it is also not suitable, and can be used as a reference method.

Third, Accumulation Method

To be exact, it is not a method, but an experience. But this is what we think is the most accurate. We can judge the number of PCB layers by the routing of some public PCB cards and the position of components. Because in the current IT hardware industry, which updates so fast, there are not many manufacturers who have the ability to redesign PCB.

In fact, the number of layers of PCB has a great influence on the board. For example, why should we use at least 6 layers of PCB to install Dual CPU? In this way, the PCB can have 3 or 4 signal layers, 1 ground layer and 1 or 2 power layers. Then, the signal lines can be far enough apart to reduce mutual interference and have enough current supply. However, it is enough to use 4-layer PCB in general board, but it is too costly to use 6-layer PCB, and there is not much performance improvement.