The production of PCB is very complicated. Taking a four-layer printed board as an example, the production process mainly includes PCB layout, core board production, inner PCB layout transfer, core board punching and inspection, lamination, drilling, and hole wall Copper chemical precipitation, outer PCB layout transfer, outer PCB etching and other steps.
1.PCB layout
The first step in PCB production is to organize and check the PCB layout (Layout). The PCB production factory receives the CAD files from the PCB design company. Since each CAD software has its own unique file format, the PCB factory will convert it into a unified format-Extended Gerber RS-274X or Gerber X2. Then the factory’s engineers will check whether the PCB layout conforms to the manufacturing process, and whether there are any defects and other issues.
2. Production of core board
Clean the copper clad laminate, if there is dust, it may cause the final circuit to be short-circuited or broken.
3. Inner PCB layout transfer
First, make the two-layer circuit of the middle core board (Core). After the copper clad laminate is cleaned, it will be covered with a photosensitive film on the surface. This film will solidify when exposed to light, forming a protective film on the copper foil of the copper clad laminate.
Insert the two-layer PCB layout film and the double-layer copper clad laminate, and finally insert the upper PCB layout film to ensure that the upper and lower PCB layout films are stacked accurately.
The photosensitive machine irradiates the photosensitive film on the copper foil with a UV lamp. The photosensitive film is cured under the light-transmitting film, and there is still no cured photosensitive film under the opaque film. The copper foil covered under the cured photosensitive film is the required PCB layout circuit, which is equivalent to the function of the laser printer ink of the manual PCB.
Then use lye to clean the uncured photosensitive film, and the required copper foil circuit will be covered by the cured photosensitive film.
Then use a strong base, such as NaOH, to etch away the unnecessary copper foil.
Tear off the cured photosensitive film to expose the copper foil of the required PCB layout.
4. Core board punching and inspection
The core board has been successfully produced. Then punch alignment holes on the core board to facilitate alignment with other materials.
Once the core board is pressed together with other layers of PCB, it cannot be modified, so inspection is very important. The machine will automatically compare with the PCB layout drawing to check for errors.
5. Laminate
A new raw material is needed here, called a prepreg, which is the adhesive between the core board and the core board (PCB layer number>4), as well as the core board and the outer copper foil, which also plays a role in insulation.
The lower copper foil and the two layers of prepreg have been fixed in advance through the alignment hole and the lower iron plate, and then the finished core board is also placed in the alignment hole, and finally the two layers of prepreg, a layer of copper foil and A layer of pressure-bearing aluminum plate covers the core plate.
The PCB boards clamped by the iron plates are placed on the support, and then sent to the vacuum heat press for laminating. The high temperature in the vacuum hot press can melt the epoxy resin in the prepreg and fix the core boards and copper foils together under pressure.
After the lamination is completed, remove the upper iron plate that presses the PCB. Then remove the pressure-bearing aluminum plate. The aluminum plate also has the responsibility of isolating different PCBs and ensuring the smoothness of the outer copper foil of the PCB. Both sides of the PCB taken out at this time will be covered by a layer of smooth copper foil.
6.Drilling
To connect 4 layers of non-contact copper foils in the PCB, first drill through the through-hole through the PCB, and then metalize the hole wall to conduct electricity.
Use the X-ray drilling machine to locate the inner core board. The machine will automatically find and locate the holes on the core board, and then punch positioning holes on the PCB to ensure that the next drilling is from the center of the hole. pass.
Put a layer of aluminum plate on the punching machine machine, and then put the PCB on it. In order to improve efficiency, according to the number of PCB layers, 1 to 3 identical PCB boards are stacked together for perforation. Finally, cover the uppermost PCB with a layer of aluminum plate. The upper and lower layers of aluminum plate are used to prevent the copper foil on the PCB from tearing when the drill bit drills in and out.
In the previous lamination process, the molten epoxy was squeezed out of the PCB, so it needed to be cut off. The profiling milling machine cuts its periphery according to the correct XY coordinates of the PCB.
7. Copper chemical precipitation on the hole wall
Since almost all PCB designs use perforations to connect different layers of lines, a good connection requires a 25-micron copper film on the hole wall. The thickness of the copper film needs to be realized by electroplating, but the hole wall is composed of non-conductive epoxy resin and glass fiber board.
So the first step is to deposit a layer of conductive material on the hole wall, and form a 1 micron copper film on the entire PCB surface by chemical deposition, including the hole wall. The entire process such as chemical treatment and cleaning is controlled by the machine.
8. Outer PCB layout transfer
Next, the PCB layout of the outer layer will be transferred to the copper foil. The process is similar to the transfer principle of the previous PCB layout of the inner core board. The PCB layout is transferred to the copper foil by photocopying film and photosensitive film. The only difference is Yes, positive films will be used as the board.
The internal PCB layout transfer uses the subtractive method, and the negative film is used as the board. The PCB is covered by the cured photosensitive film as a circuit, and the uncured photosensitive film is cleaned. After the exposed copper foil is etched, the PCB layout circuit is protected by the cured photosensitive film.
The transfer of the outer PCB layout adopts the normal method, and the positive film is used as the board. The non-circuit area is covered by the cured photosensitive film on the PCB. After cleaning the uncured photosensitive film, electroplating is performed. Where there is a film, it cannot be electroplated, and where there is no film, copper is plated first and then tin is plated. After the film is removed, alkaline etching is performed, and finally the tin is removed. The circuit pattern remains on the board because it is protected by tin.
Clamp the PCB with clamps, and electroplate the copper. As mentioned earlier, in order to ensure that the holes have sufficient conductivity, the copper film plated on the hole walls must have a thickness of 25 microns, so the entire system will be automatically controlled by the computer to ensure its accuracy.
- Outer PCB etching
Next, a complete automated assembly line completes the etching process. First, clean the cured photosensitive film on the PCB. Then use a strong alkali to clean the unnecessary copper foil covered by it. Then use the tin stripping solution to strip the tin plating on the PCB layout copper foil. After cleaning, the 4-layer PCB layout is complete.
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