PCB News - Detailed explanation of back drilling process in circuit board production

Detailed explanation of back drilling process in circuit board production

1. What is a circuit board back drill?

What is PCB back drilling? In fact, back drilling is a special kind of controlled depth drilling. In the production process of PCB multilayer boards, such as the production of 12-layer circuit boards, we need to connect the first layer to the 9th layer., Usually we drill through holes (one-time drilling), and then sink the copper. 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 column (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 circuit board 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. The local plate thickness becomes smaller;

3. Improve signal integrity;

4. Reduce the use of buried blind holes and reduce the difficulty of PCB circuit board production.

3. What is the function of back drilling?

In fact, the role of back drilling is to drill out the PCB through-hole sections that do not play any role in connection or transmission, to avoid reflection, scattering, delay, etc., which cause high-speed signal transmission, and bring "distortion" to the signal. Research has shown that it affects the signal integrity of the signal system. The main factors of performance are design, PCB board materials, transmission lines, connectors, chip packaging and other factors, and vias have a greater impact on signal integrity.

Fourth, the 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.

5. Back drill production process?

1. First provide a circuit board, the circuit board is provided with positioning holes, using the positioning holes to drill and locate the PCB circuit board and drill a hole;

2. Electroplating the circuit board after a drill hole, and dry film sealing the positioning holes before electroplating;

3. Make outer layer graphics on the electroplated circuit board;

4. Perform pattern electroplating on the PCB circuit board after the outer layer pattern is formed, and perform dry film sealing treatment on the positioning holes before the pattern electroplating;

5. 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;

6. After back drilling, wash the back drilling with water to remove the residual drill chips in the back drilling.

6. If there are holes on the circuit board, how to solve the problem of drilling from the 14th layer to the 12th layer?

1. If the PCB 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, that is, on the circuit, The signal transmission is 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.

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?

7. What are the technical characteristics of the back drilling plate?

1. Most backplanes are hard boards;

2. The thickness is relatively large;

3. The PCB board size is larger;

4. Board thickness: 2.5mm or more;

5. Generally, the minimum aperture of the first drill>=0.3mm;

6. Back drilling depth tolerance: +/-0.05MM;

7. The number of layers of PCB multi-layer boards is generally 8 to 50 layers;

8. Back drilling is usually 0.2mm larger than the hole that needs to be drilled;

9. There are fewer outer lines, and most of them are designed with a square array of crimp holes;

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 0.17 mm.

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.

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