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PCB Tech

Detailed explanation of back drilling process in PCB production
2021-09-25
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Author:Frank

Detailed explanation of back drilling process in PCB production
1. What PCB back drill?
Back drilling is actually a special kind of controlled depth drilling. In the production of multi-layer boards, such as the production of 12-layer boards, we need to connect the first layer to the 9th layer. Usually we drill through holes (one-time drilling) , And then Chen Tong. In this way, the first floor is directly connected to the 12th floor. In fact, we only need the first floor to be connected to the 9th floor. Since the 10th to the 12th floor are not connected by wires, they are like a pillar. This column affects the signal path, which can cause signal integrity problems in the communication signal. So this extra pillar (called STUB in the industry) was drilled out from the reverse side (secondary drilling). So it is called back drill, but it is generally not as clean as the drill, because the subsequent process will electrolyze a little copper, and the drill tip itself is also sharp. Therefore, the PCB manufacturer will leave a small point. The length of this left STUB is called the B value, which is generally in the range of 50-150UM.
2. What are the advantages of back drilling?
1) Reduce noise interference;
2) Improve signal integrity;
3) Local plate thickness becomes smaller;
4) Reduce the use of buried blind holes and reduce the difficulty of PCB production.
3. What is the function of back drilling?

pcb board

The function of back drilling is to drill out the through-hole sections that do not play any role in connection or transmission, to avoid reflection, scattering, delay, etc. of high-speed signal transmission, and to bring "distortion" to the signal. Research has shown that the signal integrity of the signal system is affected. The main factors include design, board materials, transmission lines, connectors, chip packaging and other factors, but the vias have a greater impact on signal integrity.
4. Working principle of back drilling production
Relying on the micro-current generated when the drill tip touches the copper foil of the substrate surface when the drill bit is drilled down to sense the height of the board surface, and then drill down according to the set drill depth, and stop the drill when it reaches the drill depth. As shown in Figure 2, the working diagram is shown
5. Back drill production process?
a. Provide a PCB with positioning holes on the PCB, and use the positioning holes to drill and position the PCB and drill holes;
b. Electroplating the PCB after a drill hole, and dry film sealing the positioning holes before electroplating;
c. Make outer layer graphics on the electroplated PCB;
d. Perform pattern electroplating on the PCB after the outer layer pattern is formed, and perform dry film sealing treatment on the positioning holes before the pattern electroplating;
e. Use the positioning hole used by a drill for back drilling positioning, and use a drill to back drill the electroplated holes that need to be back drilled;
f. After back drilling, wash the back drilling with water to remove the residual drill chips in the back drilling.
6. If there is a hole in the circuit board, how to solve it from the 14th layer to the 12th layer?
1) If the board has a signal line on the 11th layer, there are through holes at both ends of the signal line to connect to the component surface and the solder surface, and the components will be inserted on the component surface, as shown in the figure below, that is, On this line, the signal is transmitted from component A to component B through the signal line on the 11th layer.
2) According to the signal transmission situation described in point 1, the function of the through hole in the transmission line is equivalent to the signal line. If we do not perform back drilling, the signal transmission route is shown in Figure 5.
3) From the figure described in point 2, we can see that in the first good transmission process, the through-hole section from the solder surface to the 11th layer does not actually play any link or transmission function. The existence of this section of through holes is likely to cause reflection, scattering, delay, etc. of signal transmission. Therefore, back drilling is actually to drill out the through hole section that does not play any link or transmission function to avoid reflection, scattering, etc. of signal transmission. Delay, bring distortion to the signal.
Due to certain tolerance control requirements for drilling depth and plate thickness tolerances, we cannot meet the absolute depth requirements of customers 100%. So, should the back drilling depth control be deeper or shallower? Our view of craftsmanship is that it is more shallow than deep, as shown in Figure 6.
7. What are the technical characteristics of the back drilling plate?
1) Most backplanes are hard boards
2) The number of layers is generally 8 to 50 layers
3) Board thickness: 2.5mm or more
4) Thick diameter is relatively large
5) Larger board size
6) Generally, the minimum hole diameter of the first drill>=0.3mm
7) There are fewer outer lines, and most of them are designed with a square array of crimp holes
8) Back drilling is usually 0.2mm larger than the hole that needs to be drilled
9) Tolerance of back drilling depth: +/-0.05MM
10) If the back drilling requires drilling to the M layer, the minimum thickness of the medium from the M layer to the M-1 (the next layer of the M layer) layer is 0.17mm
8. What are the main applications of the back drilling plate?
Backplanes are mainly used in communication equipment, large servers, medical electronics, military, aerospace and other fields. Since military and aerospace are sensitive industries, domestic backplanes are usually provided by military and aerospace systems research institutes, R&D centers, or PCB manufacturers with strong military and aerospace backgrounds. In China, the demand for backplanes mainly comes from the communications industry. The growing field of communication equipment manufacturing.
Realize back drill file output in allegro
1. First select the back drill Net and define the length. Click Edit-Properties in the menu bar to open the dialog Edit property,
2. Click in the menu: Manufacturing→NC→ Backdrill Setup and Analysis, as shown in the figure below:
3. Back drilling can start from the top layer or the bottom layer. Both the connection pins and VIA on high-speed signals need to be back drilled. The settings are as follows:
4. Drilling files are as follows:
5. Pack the back-drilling hole file and the back-drilling hole depth form together and send it to the PCB factory. The back-drilling depth form needs to be filled in manually