Manufacturing process of driver PCB
The manufacture of PCB is very complicated. Take the four-layer printed circuit board as an example, the manufacturing process mainly includes the steps of PCB layout, core board manufacturing, inner PCB layout transfer, core board punching and inspection, lamination, drilling, copper chemical precipitation on the hole wall, outer PCB layout transfer, outer PCB etching and so on.
1. PCB layout
The first step of PCB manufacture is to sort out and check the Layout of PCB. After our factory receives the CAD file of PCB, because each CAD software has its own unique file format, our factory will convert it into a unified format. Then the engineers in the factory will check whether the PCB layout meets the manufacturing process and whether there are any defects.
2. Manufacture of core board
Clean the copper clad laminate, if there is dust, it may lead to short circuit or open circuit in the end.
An 8-layer PCB board is actually made up of 3 copper-clad plates (core boards) plus 2 copper films, and then bonded together with prepreg. The manufacturing sequence is to start from the middle core board (4, 5-layer circuit), stack them together continuously, and then fix them. The manufacturing of 4-layer PCB is similar, except that only 1 Melanie board and 2 copper films are used.
3. Layout transfer of inner PCB
First, make the two-layer circuit of the middle core board. After the CCL is cleaned, a photosensitive film will cover the surface. This film will cure when exposed to light, forming a protective film on the copper foil of CCL.
Two layers of PCB layout film and double-layer copper clad laminate are finally inserted into the upper layer of PCB layout film, so as to ensure the accurate stacking position of the upper and lower layers of PCB layout film.
The photosensitive machine uses UV lamp to irradiate the photosensitive film on the copper foil. 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 laser printer ink of manual PCB. Then, the uncured photosensitive film is washed away with lye, and the required copper foil circuit will be covered by the cured photosensitive film. Then, the unwanted copper foil is etched away with strong alkali such as NaOH. Tear off the cured photosensitive film to expose the required PCB layout circuit copper foil.
4. Core punching and inspection
Punch alignment holes on the core board to facilitate alignment with other raw materials. Once the board is pressed together with PCB of other layers, it cannot be modified, so inspection is very important. The machine will automatically compare with PCB layout drawings to check for errors.
Here, a new raw material called prepreg is needed, which is the adhesive between the core board and the core board (the number of PCB layers is more than 4), and between the core board and the outer copper foil, and also plays an insulating role.
The copper foil of the lower layer and two layers of prepreg have been fixed in position through the alignment hole and the iron plate of the lower layer in advance, and then the manufactured core board is 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 board. The PCB boards clamped by the iron plate are placed on the bracket, and then sent to the 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 board and copper foil together under pressure.
After lamination, remove the upper iron plate of the pressed PCB. Then take away the aluminum plate under pressure, and the aluminum plate also plays the role of isolating different PCBs and ensuring the smoothness of the copper foil on the outer layer of PCB. At this time, both sides of the PCB will be covered by a layer of smooth copper foil.
In order to connect four layers of non-contact copper foils in the PCB together, we must first drill through holes from top to bottom to get through the PCB, and then metallize the hole walls to conduct electricity.
X-ray drilling machine is used to position the inner core board, and the machine will automatically find and position the hole position on the core board, and then punch the positioning hole on the PCB to ensure that the next hole will pass through the right center of the hole position. Put a layer of aluminum plate on the punching machine, and then put the PCB on it. In order to improve efficiency, according to the number of PCB layers, 1~3 identical PCB boards will be stacked together for perforation. Finally, cover the top PCB with an aluminum plate. The upper and lower aluminum plates are designed to prevent the copper foil on the PCB from being torn when the drill bit is drilled in and out.
In the previous lamination process, the melted epoxy resin was squeezed out of the PCB, so it needed to be cut off. The copying milling machine cuts the periphery of PCB according to its correct XY coordinates.
7. Chemical precipitation of copper on the pore wall
Since almost all PCB designs use vias to connect different layers of circuits, a good connection requires a 25 micron copper film on the hole wall. This thickness of copper film needs electroplating, but the hole wall is composed of non-conductive epoxy resin and glass fiber board.
Therefore, 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. Layout transfer of outer PCB
Next, the PCB layout of the outer layer will be transferred to the copper foil. The process is similar to the principle of the previous PCB layout transfer of the inner core board. The printed film and photosensitive film are used to transfer the PCB layout to the copper foil. The only difference is that the positive film will be used as the board.
(The layout transfer of the inner PCB adopts the subtractive method, and the negative film is used as the board. PCB is covered with cured photosensitive film for circuit. Clean the uncured photosensitive film. After the exposed copper foil is etched, the PCB layout circuit is protected by the cured photosensitive film.)
The layout transfer of the outer PCB adopts the normal method, using positive film as the board. The PCB is covered by the cured photosensitive film as the non-circuit area. Cleaning the uncured photosensitive film and electroplating. Where there is a film, it can’t be electroplated, but 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 tin is removed. The pattern is left on the board because it is protected by tin.
Clamp the PCB with a clip and electroplate the copper. As mentioned earlier, in order to ensure the hole site has good enough conductivity, the copper film electroplated on the hole wall must have a thickness of 25 microns, so the whole system will be automatically controlled by a computer to ensure its accuracy.