PCB (Printed Circuit Board), the Chinese name is called printed circuit board, also known as printed circuit board, is an important electronic component, is the support body of electronic components. Because it is produced by electronic printing, it is called a “printed” circuit board.
Before PCBS, circuits were made up of point-to-point wiring. The reliability of this method is very low, because as the circuit ages, the rupture of the line will cause the line node to break or short. Wire winding technology is a major advance in circuit technology, which improves the durability and replaceable ability of the line by winding the small diameter wire around the pole at the connection point.
As the electronics industry evolved from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and price of electronic components also declined. Electronic products are increasingly appearing in the consumer sector, prompting manufacturers to look for smaller and more cost-effective solutions. Thus, PCB was born.
PCB manufacturing process
The production of PCB is very complex, taking four-layer printed board as an example, its production process mainly includes PCB layout, core board production, inner PCB layout transfer, core board drilling and inspection, lamination, drilling, hole wall copper chemical precipitation, outer PCB layout transfer, outer PCB etching and other steps.
1, PCB layout
The first step in PCB production is to organize and check the PCB Layout. The PCB manufacturing factory receives CAD files from the PCB design company, and since each CAD software has its own unique file format, the PCB factory translates them into a unified format – Extended Gerber RS-274X or Gerber X2. Then the engineer of the factory will check whether the PCB layout conforms to the production process and whether there are any defects and other problems.
2, core plate production
Clean the copper clad plate, if there is dust, it may lead to the final circuit short circuit or break.
An 8-layer PCB: it is actually made of 3 copper-coated plates (core plates) plus 2 copper films, and then bonded with semi-cured sheets. The production sequence starts from the middle core plate (4 or 5 layers of lines), and is constantly stacked together and then fixed. The production of 4-layer PCB is similar, but only uses 1 core board and 2 copper films.
3, the inner PCB layout transfer
First, the two layers of the most central Core board (Core) are made. After cleaning, the copper-clad plate is covered with a photosensitive film. The film solidifies when exposed to light, forming a protective film over the copper foil of the copper-clad plate.
The two-layer PCB layout film and the double-layer copper clad plate are finally inserted into the upper layer PCB layout film to ensure that the upper and lower layers of PCB layout film are stacked accurately.
The sensitizer irradiates the sensitive film on the copper foil with a UV lamp. Under the transparent film, the sensitive film is cured, and under the opaque film, there is still no cured sensitive film. The copper foil covered under the cured photosensitive film is the required PCB layout line, which is equivalent to the role of laser printer ink for manual PCB.
Then the uncured photosensitive film is cleaned with lye, and the required copper foil line will be covered by the cured photosensitive film.
The unwanted copper foil is then etched away with a strong alkali, such as NaOH.
Tear off the cured photosensitive film to expose the copper foil required for PCB layout lines.
4, core plate drilling and inspection
The core plate has been made successfully. Then punch a matching hole in the core plate to facilitate alignment with other raw materials next
Once the core board is pressed together with other layers of PCB, it cannot be modified, so inspection is very important. The machine will automatically compare with the PCB layout drawings to check for errors.
5. Laminate
Here a new raw material called semi-curing sheet is needed, 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 the role of insulation.
The lower copper foil and two layers of semi-cured sheet have been fixed through the alignment hole and the lower iron plate in advance, and then the made core plate is also placed in the alignment hole, and finally the two layers of semi-cured sheet, a layer of copper foil and a layer of pressurized aluminum plate are covered on the core plate in turn.
The PCB boards that are clamped by iron plates are placed on the bracket, and then sent to the vacuum hot press for lamination. The high temperature of the vacuum hot press melts the epoxy resin in the semi-cured sheet, holding the core plates and the copper foil together under pressure.
After the lamination is complete, remove the top iron plate pressing the PCB. Then the pressurized aluminum plate is taken away, and the aluminum plate also plays the responsibility of isolating different PCBS and ensuring that the copper foil on the PCB outer layer is smooth. At this time, both sides of the PCB taken out will be covered by a layer of smooth copper foil.
6. Drilling
To connect the four layers of non-contact copper foil in the PCB together, first drill a perforation through the top and bottom to open the PCB, and then metalize the hole wall to conduct electricity.
The X-ray drilling machine is used to locate the inner core board, and the machine will automatically find and locate the hole on the core board, and then punch the positioning hole on the PCB to ensure that the next drilling is through the center of the hole.
Place a layer of aluminum sheet on the punch machine and place the PCB on it. In order to improve efficiency, 1 to 3 identical PCB boards will be stacked together for perforation according to the number of PCB layers. Finally, a layer of aluminum plate is covered on the top PCB, and the upper and lower layers of aluminum plate are so that when the drill bit is drilling and drilling out, the copper foil on the PCB will not tear.
In the previous lamination process, the melted epoxy resin was squeezed to the outside of the PCB, so it needed to be removed. The profile milling machine cuts the periphery of the PCB according to the correct XY coordinates.
7. Copper chemical precipitation of 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 fiberglass 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 whole process, such as chemical treatment and cleaning, is controlled by the machine.
Fixed PCB
Clean PCB
Shipping PCB
8, the outer PCB layout transfer
Next, the outer PCB layout will be transferred to the copper foil, and the process is similar to the previous inner core PCB layout transfer principle, which is the use of photocopied film and sensitive film to transfer the PCB layout to the copper foil, 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 PCB is covered by the solidified photographic film for the line, clean the unsolidified photographic film, exposed copper foil is etched, PCB layout line is protected by the solidified photographic film and left.
The outer PCB layout transfer adopts the normal method, and the positive film is used as the board. The PCB is covered by the cured photosensitive film for the non-line area. After cleaning the uncured photosensitive film, electroplating is performed. Where there is a film, it cannot be electroplated, and where there is no film, it is plated with copper and then tin. After the film is removed, alkaline etching is performed, and finally the tin is removed. The line pattern is left on the board because it is protected by tin.
Clamp the PCB and electroplate the copper onto it. As mentioned earlier, in order to ensure that the hole 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.
9, outer PCB etching
The etching process is then completed by a complete automated pipeline. First of all, the cured photosensitive film on the PCB board is cleaned off. It is then washed with a strong alkali to remove the unwanted copper foil covered by it. Then remove the tin coating on the PCB layout copper foil with the detinning solution. After cleaning, the 4-layer PCB layout is complete.