The production of PCB is very complex. Taking four layer PCB as an example, the manufacturing process mainly includes PCB layout, core board production, inner PCB layout transfer, core board drilling and inspection, lamination, drilling, copper chemical precipitation on hole wall, outer PCB layout transfer, outer PCB etching and other steps.
1. PCB layout
The first step of PCB production is to arrange and check the PCB layout. PCB Factory receives CAD files from PCB design company. Since each CAD software has its own unique file format, 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, whether there are any defects and other issues.
2. Manufacturing of core board
Clean the copper clad laminate, if there is dust, it may cause the final circuit short circuit or open circuit.
The figure below is an illustration of an 6-layer PCB, which is actually made up of three copper clad laminates (core boards) , and then bonded with PP. The production sequence is to start with the core board (3,4 layers) in the middle, continuously stack them together, and then fix them. The production of 4-layer PCB is similar, but only one core board and two copper foils are used.
3. Layout transfer of inner layer PCB
First, we need to make two layers of wiring for the core. After cleaning, the copper clad laminate will be covered with a layer of photosensitive film. The film will solidify in the light and form a protective film on the copper foil of the copper clad laminate.
The two-layer PCB layout film and double-layer copper clad laminate are inserted into the upper PCB layout film to ensure the accurate stacking position of the upper and lower layers of film.
The exposure machine uses UV lamp to irradiate the photosensitive film on the copper foil. Under the transparent film, the photosensitive film is solidified, while under the opaque film, there is no solidified photosensitive film. The copper foil covered under the solidified photosensitive film is the PCB layout circuit needed, which is equivalent to the role of laser printer ink for manual PCB.
Then clean the non cured film with alkali solution, and the required copper foil circuit will be covered by the solidified film.
The copper foil is then etched with a strong base such as NaOH.
Tear off the solidified photosensitive film to expose the required PCB layout circuit copper foil.
4. Core board drilling and inspection
The core has been made successfully. Then, punch holes in the core plate to facilitate alignment with other raw materials. Once the core board is pressed together with other layers of PCB, it can not be modified, so the inspection is very important. The machine will automatically compare with PCB layout drawing to check the error.
Here we need a new raw material called PP, which is the adhesive between the core board and the core board (PCB layer number > 4), and between the core board and the outer copper foil, which also plays the role of insulation.
The copper foil and two layers of PP in the lower layer have been fixed in position through the alignment hole and the iron boards of the lower layer in advance, and then the core is also put into the alignment hole. Finally, two layers of PP, a layer of copper foil and a layer of pressure bearing aluminum plate are successively covered on the core.
The PCB boards clamped by iron boards are placed on the bracket, and then sent to the vacuum hot press for lamination. The high temperature in the vacuum press can melt the epoxy resin in PP and solidify the core board and copper foil together under pressure.
After laminating, remove the upper iron board of the pressed PCB. Then the pressure bearing steel board is taken away. The steel board also has the responsibility of isolating different PCBs and ensuring the smoothness of the outer copper foil of PCB. At this time, both sides of the PCB taken out will be covered with a layer of smooth copper foil.
In order to connect four layers of copper foil without contact in PCB together, the first step is to drill through the holes through the top and bottom to get through the PCB, and then metalize the hole wall to conduct electricity.
Use X-ray drilling machine to locate the core board in the inner layer. The machine will automatically find and locate the hole position on the core board, and then drill the positioning hole on the PCB to ensure that the next drilling is through the center of the hole position.
Put a layer of aluminum on the punching machine, and then put the PCB on it. In order to improve the efficiency, 1 ~ 3 identical PCB boards will be stacked together to drill holes according to the number of PCB layers. Finally, cover the top PCB with a layer of aluminum plate. The upper and lower layers of aluminum plates are used to prevent the copper foil on the PCB from tearing when the drill bit is drilling and drilling out.
In the previous lamination process, the melted epoxy resin was extruded to the outside of the PCB, so it needs to be removed. The outline of PCB is cut by milling machine according to the correct XY coordinate.
Since almost all PCB designs are made of different layers of wires connected by perforations, a good connection requires a 25 micron copper film on the hole wall. This thickness of copper film needs to be achieved by electroplating, but the hole wall is composed of non-conductive epoxy resin and glass fiber.
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 whole PCB surface, including the hole wall, by chemical deposition. The whole process, such as chemical treatment and cleaning, is controlled by machines.
8.Outer dry film
Next, the outer layer PCB layout will be transferred to the copper foil. The process is similar to the previous inner core PCB layout transfer principle. The PCB layout will be transferred to the copper foil by using photocopied film and photosensitive film. The only difference is that positive film will be used as PCB.
The inner layer PCB layout transfer adopts the subtraction method, and uses the negative as the PCB. The circuit covered by the solidified photosensitive film on the PCB is the circuit. After cleaning the uncured photosensitive film, the exposed copper foil is etched, and the PCB layout circuit is protected by the solidified photosensitive film.
The outer layer PCB layout transfer adopts the addition method, and the positive PCB is used. The printed circuit board is covered by a solidified photosensitive film. After cleaning the uncured photosensitive film, electroplate it. There is a film can not be electroplated, and there is no film, first plating copper and then tin. After stripping the film, alkaline etching was carried out, and finally tin was removed. The circuit pattern is left on the PCB because it is protected by tin.
Clamp the PCB with a clamp and electroplate the copper. As mentioned earlier, in order to ensure that the hole position has enough good conductivity, the copper film on the hole wall must have a thickness of 25 μ m, so the whole system will be automatically controlled by the computer to ensure its accuracy.
9. Outer layer etching
Next, a complete automatic assembly line is used to complete the etching process. First, the photosensitive film cured on the PCB is cleaned. Then clean the unwanted copper foil covered with it with strong alkali. Then, the tin coating on the PCB layout copper foil is removed with tin stripping solution. After cleaning, the PCB layout is completed.