sheet metal
What is needed here is a new raw material called prepreg, which is core board and core board (PCB layers >；; 4), and the adhesive between the core plate and the outer copper foil also play an insulating role.
Fix the lower copper foil and two layers of prepreg in place through the positioning holes and the lower iron plate in advance, then put the manufactured core plate into the positioning holes, and finally cover the core plate with two layers of prepreg, one layer of copper foil and one layer of pressure-bearing aluminum plate in turn.
The PCB clamped by the iron plate is placed on the bracket and then sent to a vacuum hot press for lamination. The high temperature in the vacuum hot press can melt the epoxy resin in the prepreg and fix the core plate and copper foil together under pressure.
After lamination, remove the upper iron plate that presses the PCB. Then the pressed aluminum plate is taken away, and the aluminum plate also plays the role of isolating different PCB plates and ensuring the smoothness of the copper foil on the outer layer of PCB plates. At this time, both sides of the PCB board will be covered with a smooth copper foil.
bit
To connect the four layers of copper foil in PCB without contact, first, drill a through hole to get through the PCB, and then metallize the hole wall to conduct electricity.
Use X-ray drilling machine to locate the inner core board, and the machine will automatically find and locate the hole on the core board, and then punch the locating hole on the PCB board to ensure that the next drilling will pass through the center of the hole.
Put a layer of aluminum plate on the press, and then put the PCB on it. In order to improve efficiency, 1~3 identical PCB boards will be stacked and punched according to the number of PCB layers. Finally, cover the top PCB with aluminum plate. The upper and lower aluminum plates are designed 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 molten epoxy resin was squeezed out of the PCB, so it needs to be removed. The copying milling machine cuts the periphery of PCB according to the correct XY coordinates.
Chemical precipitation of copper on the hole wall
Because almost all PCB designs connect wires of different layers through holes, a good connection requires a 25 micron copper film on the hole wall. The copper film of this thickness needs 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 substance on the hole wall, and form a layer of 1 micron copper film on the whole PCB surface including the hole wall by chemical deposition. The whole process of chemical treatment and cleaning is controlled by machines.
Fixed PCB
Clean PCB
Transport PCB
External PCB layout transfer
Next, transfer the PCB layout of the outer layer to the copper foil. The process is similar to the PCB layout transfer of the previous inner core board. Photocopying film and photosensitive film are used to transfer PCB layout to copper foil. The only difference is that the positive film will be used as a board.
The layout transfer of inner PCB adopts reduction method, with negative film as the board. The PCB is covered by the cured photosensitive film, and the uncured photosensitive film is washed away. After the exposed copper foil is etched, the PCB layout lines are protected by the cured photosensitive film.
The layout transfer of the outer PCB adopts the conventional method, using the positive film as the board. The PCB is covered by a cured photosensitive film as a non-circuit area. Cleaning the uncured photosensitive film and electroplating. Where there is a film, it can't be electroplated. Where there is no film, it should be plated with copper and then tinned. After stripping, alkaline etching is carried out, and finally tin is removed. The wiring diagram is left on the board because it is protected by tin.
Clamping PCB and electroplating copper. As mentioned above, in order to ensure that the hole has good enough conductivity, the thickness of the copper film electroplated on the hole wall must reach 25 microns, so the whole system will be automatically controlled by the computer to ensure its accuracy.
External PCB etching
Next, the etching process is completed by a complete automatic production line. First, clean the cured photosensitive film on the PCB. Then use strong alkali to clean the unnecessary copper foil covered by it. Then the tin coating on the copper foil of PCB layout is removed by tin stripping solution. After cleaning, the layout of the 4-layer PCB is completed.