01
PCB Layout
The first step in PCB production is to organize and check the PCB layout (Layout). The PCB factory receives CAD files from PCB design companies. 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 engineers will check whether the PCB layout is in line with the production process, there are no defects and other issues.
　　02
Fabrication of core plate
Clean the copper clad laminate, if there is dust, it may cause the final circuit short circuit or open circuit.
The following figure is a legend of an 8-layer PCB, which is actually made up of 3 copper clad laminates (core plates) plus 2 copper films, and then bonded with semi-cured sheets. The production sequence is from the middle of the core board (4, 5 layer line), constantly stacked together, and then fixed. The production of 4-layer PCB is similar, except that only one core board and two copper films are used.
　　03
Inner PCB Layout Transfer
First to make the most intermediate core board (Core) of the two layers of the line. After the copper clad laminate is cleaned, a photosensitive film will be covered on the surface. This film will be cured when exposed to light, forming a protective film on the copper foil of the copper clad laminate.
The two-layer PCB layout film and the two-layer copper clad laminate are inserted into the upper PCB layout film to ensure the accurate stacking position of the upper and lower PCB layout films.
The photosensitive film on the copper foil is irradiated by UV lamp. Under the transparent film, the photosensitive film is cured, while under the opaque film, there is still no cured photosensitive film. The copper foil covered under the cured photosensitive film is the required PCB layout circuit, which is equivalent to the function of laser printer ink for manual PCB.
Then, the photosensitive film that is not cured is cleaned with lye, and the required copper foil circuit will be covered by the cured photosensitive film.
The unwanted copper foil is then etched away with a strong base, such as NaOH.
Tear off the cured photosensitive film to expose the required PCB layout circuit copper foil.
　　04
Core plate drilling and inspection
The core board has been made successfully. Then, the alignment holes are punched on the core plate to facilitate the alignment with other raw materials.
Once the core board is pressed together with the PCB of other layers, it cannot be modified, so inspection is very important. The machine will automatically compare with the PCB layout drawing to see the error.
　　05
Lamination
Here we need a new raw material called 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, and also plays a role of insulation.
The lower copper foil and the two layers of prepreg have been fixed in position through the alignment hole and the lower iron plate in advance, and then the prepared core plate is also put into the alignment hole, and finally the two layers of prepreg, one layer of copper foil and one layer of pressure-bearing aluminum plate are sequentially covered on the core plate.
The PCB boards clamped by the iron plates are placed on the bracket and then fed into the vacuum hot press for lamination. The high temperature in the vacuum hot press can melt the epoxy resin in the prepreg and hold the core plates and copper foils together under pressure.
After the lamination is completed, the upper iron plate of the pressed PCB is removed. Then the pressure-bearing aluminum plate is taken away, and the aluminum plate also plays a role in isolating different PCBs and ensuring that the outer copper foil of the PCB is smooth. At this time, both sides of the PCB will be covered by a smooth copper foil.
　　06
Drilling
To connect the four layers of copper foil in the PCB without contact, first drill through holes up and down to get through the PCB, and then metallize the hole wall to conduct electricity.
Use the X-ray drilling machine to position the inner core plate. The machine will automatically find and position the hole on the core plate, and then make a positioning hole for the PCB to ensure that the next drilling is through the center of the hole.
Place a layer of aluminum plate on the punch machine, and then place the PCB on it. In order to improve efficiency, according to the number of PCB layers, 1 to 3 identical PCB boards will be stacked together for perforation. Finally, the top PCB is covered with a layer of aluminum plate, and the upper and lower layers of aluminum plate are used to prevent the copper foil on the PCB from tearing when the drill bit is drilled in and out.
In the previous lamination process, the melted epoxy was squeezed out of the PCB, so it needed to be removed. The master milling machine cuts the periphery of the PCB according to the correct XY coordinates.
　　07
Copper Chemical Precipitation on the Pore Wall
Since almost all PCB designs use perforations to connect different layers of wiring, 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 board.
Therefore, the first step is to accumulate 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.