The production of printed circuit boards is very complicated. Here, a four-layer printed circuit board is taken as an example to experience how the PCB is made.
laminated
A new raw material is needed here called a prepreg, which is the adhesive between the core board and the core board (PCB layers>4), as well as the core board and the outer copper foil, which also plays a role in insulation.
The lower copper foil and 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 heat 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 take away the pressure-bearing aluminum plate. The aluminum plate also plays 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.
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 punch 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.
Chemical precipitation of copper 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, including the hole wall, by chemical deposition. The entire process, such as chemical treatment and cleaning, is controlled by the machine.
Fixed PCB
Cleaning the PCB
Shipping PCB
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 previous PCB layout transfer principle 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 for 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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