It is not always easy to see the actual number because the layers in the PCB are closely connected, but if you look closely at the plate fault, you can tell. If we are careful, we will find a layer or several layers of white material sandwiched between the PCB. In fact, this is the insulation layer between each layer, which is used to ensure that there will be no short circuit problem between different PCB layers. It is understood that the current multi-layer PCB board are using more single or double-sided wiring board, and put a layer of insulation layer between each plate after pressing, the layer number of PCB board represents several layers of independent wiring layer, and the insulation layer between layers has become an intuitive way for us to judge the number of layers of PCB.
Guide hole and blind hole to light method
The “guide hole” on PCB is used to identify the number of PCB layers. Its principle is mainly because the circuit connection of multilayer PCB adopts the guide hole technology. If we want to see how many layers the PCB has, we can tell by looking at the guide holes. On a basic PCB (single-sided motherboard), the parts are concentrated on one side and the wires on the other. If we use multilayer boards, we need to drill holes in the boards so that the pins can pass through the boards to the other side, so the guide holes will penetrate the PCB, so we can see that the pins of the parts are welded to the other side.
For example, the board card uses four layers of board, so it needs to be wired in the first and fourth layers (signal layer), and the other layers have other uses (ground layer and power layer). The purpose of putting the signal layer on both sides of the power layer and the ground layer is to prevent the interference between each other, and facilitate the correction of the signal line. If some board card guide holes appear on the front side of PCB board, but can not be found on the back side, it must be 6/8 layer board. If the same holes can be found on both sides of the PCB board, it is naturally a 4-layer board. However, a lot of board card manufacturers use another way of wiring, is only connected to some of the lines, and in the wiring used buried hole and blind hole technology. Blind holes connect several layers of inner PCB to surface PCB without penetrating the entire board. Buried holes are only connected to the internal PCB, so it is not visible from the surface. Because the blind hole does not need to run through the whole PCB, if it is six layers or more, the board card against the light source to see, the light will not be through.
So there is also a very popular idea: to judge the four and six layers or more PCB by the light leakage through the hole. This method has its reasons, but also has some disadvantages. It is considered that this method can only be used as a reference method.
This is not exactly a method, but an experience. But that’s what we think is accurate. We can judge the number of PCB layers by the wiring of some common PCB board cards and the position of components. Because in the current IT hardware industry, which is changing so quickly, there are not many vendors that can afford to redesign PCBs.
For example, a few years ago a large number of use of 6-layer PCB design of the 9550 graphics card, careful friends can compare it and 9600PRO or 9600XT board type how many different? Some components are omitted, but a high degree of consistency is maintained on the PCB. Back in the 1990s, there was a popular saying that you could tell the number of PCB layers by standing the PCB upright, and many people believed it.
This was later proved to be nonsense, even if the manufacturing process was backward, how can the eye distinguish a distance smaller than a hair? Later this method was continued and modified to evolve into another measurement method. There are many people who believe that it is possible to use sophisticated measuring instruments such as “vernier calipers” to measure the number of layers in a PCB, but we do not agree.
With or without that kind of sophisticated instrumentation, how can we not see that a 12 layer PCB is 3 times thicker than a 4 layer PCB?
Remind everyone, different PCB will use different manufacturing process, there is no uniform standard to measure, how to judge the number of layers according to the thickness? In fact, the number of layers of PCB has a great impact on board cards. For example, why install dual CPUs using at least 6 layer PCB? This allows the PCB to have 3 or 4 signal layers, 1 ground layer, and 1 or 2 power layers. The signal lines can then be kept far enough apart to reduce interference with each other and have sufficient current supply. However, a typical board with a 4-layer PCB design is sufficient, using a 6-layer PCB is too costly, and there is not much performance improvement.
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