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PCB making process!

2022-03-02

PCB making process!

Professional PCB process sharing

PCB fabrication includes PCB layout, core board fabrication, inner PCB layout transfer, core board drilling and inspection, lamination, drilling, copper chemical precipitation of hole wall, outer PCB layout transfer, outer PCB etching and other steps.

PCB layout

The first step in PCB production is to sort out and check the PCB layout. The PCB manufacturing factory receives the CAD files from the PCB design company. Each CAD software has its own unique file format, so the PCB Factory will convert to a unified format - extended Gerber rs-274x or Gerber x2. Then the factory engineer will check whether the PCB layout conforms to the manufacturing process and whether there are any defects.

Fabrication of core plate

Cleaning the copper clad laminate, if there is dust, may cause the final circuit short circuit or open circuit.
The legend of an 8-layer PCB is actually composed of 3 copper clad laminates (core plates) plus 2 copper films, and then bonded with semi cured sheets. The manufacturing sequence is to start from the core board in the middle (4 and 5 layers of lines), stack it together continuously, and then fix it. The fabrication of 4-layer PCB is similar, except that only one core board and two copper films are used.

Inner PCB layout transfer

First, make the two-layer circuit of the middle core. After the copper clad laminate is cleaned, a layer of photosensitive film will be covered on the surface. This film will solidify when exposed to light and form a protective film on the copper foil of copper clad laminate.
Insert two layers of PCB layout film and double-layer copper clad laminate, and finally insert the upper layer of PCB layout film to ensure the accurate stacking position of the upper and lower layers of PCB layout film.
The photosensitive machine irradiates the photosensitive film on the copper foil with a UV lamp. Under the transparent film, the photosensitive film is cured, and there is still no cured photosensitive film under the opaque film. The copper foil covered under the solidified photosensitive film is the required PCB layout circuit, which is equivalent to the role of laser printer ink of manual PCB.
Then clean the uncured photosensitive film with alkaline solution, and the required copper foil circuit will be covered by the cured photosensitive film. Then use strong alkali, such as NaOH, to etch away the unwanted copper foil. Tear off the cured photosensitive film to expose the required PCB layout circuit copper foil.

Core plate drilling and inspection

The core board has been made successfully. Then make alignment holes on the core plate to facilitate 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 check the errors. 

Lamination

A new raw material called semi cured sheet is needed here, which is the adhesive between core board and core board (PCB layers > 4) and between core board and outer copper foil. At the same time, it also plays the role of insulation.
The lower layer of copper foil and two layers of semi cured sheet have been fixed in place through the alignment hole and the lower layer of iron plate in advance, and then the prepared core plate is also put into the alignment hole. Finally, two layers of semi cured sheet, one layer of copper foil and one layer of pressure bearing aluminum plate are covered on the core plate in turn.
Place the PCB boards clamped by the iron plate on the support, and then send them to the vacuum hot press for lamination. The high temperature in the vacuum hot press can melt the epoxy resin in the semi cured sheet and fix the core plates and copper foil together under pressure. After lamination, remove the upper iron plate pressing PCB. Then take away the pressure bearing aluminum plate, which also plays the responsibility of isolating different PCBs and ensuring the smoothness of copper foil on the outer layer of PCB. At this time, both sides of the PCB will be covered with a layer of smooth copper foil.

drill hole

To connect the four layers of non-contact copper foil in the PCB, first drill through holes from top to bottom to get through the PCB, and then metalize the hole wall to conduct electricity. Use the 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 punch the positioning hole on the PCB to ensure that the next drilling passes through the center of the hole position. Put a layer of aluminum plate on the punch machine, and then put the PCB on it. In order to improve efficiency, 1 ~ 3 identical PCB boards will be stacked together for perforation according to the number of layers of PCB. Finally, a layer of aluminum plate is covered on the top PCB. The upper and lower layers of aluminum plate are used to prevent the copper foil on the PCB from being torn when the drill bit is drilled and drilled out. In the previous lamination process, the melted epoxy resin was extruded outside the PCB, so it needs to be cut off. The profiling milling machine cuts the periphery of the PCB according to the correct XY coordinates.
Copper chemical precipitation on pore wall
Since almost all PCB designs are lines of different layers 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 fiberglass 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.

Fixed 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 using the photocopied film and photosensitive film. The only difference is that the positive film will be used as the board. The inner PCB layout transfer adopts the subtraction method, and the negative film is used as the board. The circuit covered by the solidified photosensitive film on the PCB is the circuit. After the uncured photosensitive film is cleaned, the exposed copper foil is etched, and the PCB layout circuit is protected by the solidified photosensitive film.
Layer PCB layout transfer 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 shall be carried out. Where there is a film, it cannot be electroplated. Where there is no film, it shall be plated with copper first and then tin. Alkaline etching shall be carried out after film stripping, and then tin stripping shall be carried out at last. The circuit 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 that the hole has good enough conductivity, the copper film plated on the hole wall must have a thickness of 25 microns, so the whole system will be automatically controlled by computer to ensure its accuracy.

Outer PCB etching

Next, a complete automatic pipeline completes the etching process. First, clean the cured photosensitive film on the PCB. Then clean the unnecessary copper foil covered by it with strong alkali. Then remove the tin coating on the copper foil of PCB layout with tin stripping solution. After cleaning, the 4-layer PCB layout is completed.