Before PCB, 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 lead to the break or short circuit of the line node. Winding is a major advance in circuit technology, which improves the durability and rechangeability of the circuit by winding the small-diameter wire around the column at the connection point.

As the electronics industry moved from vacuum tubes and relays to silicon semiconductors and integrated circuits, the size and price of electronic components fell. The increasingly frequent presence of electronic products in the consumer sector has prompted manufacturers to look for smaller and more cost-effective solutions. Thus, the PCB was born. The manufacturing process of PCB is very complex. Taking four-layer PCB as an example, the manufacturing process mainly includes PCB layout, core board production, 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.

1. The PCB layout

The first step of PCB production is to organize and check the PCB Layout. The PCB fabrication plant receives the CAD files from the PCB design company. Since each CAD software has its own unique file format, the PCB plant converts them to 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, whether there are any defects and other problems.

2. Core plate production

Clean the copper clad plate, if dust may cause the final circuit short circuit or break. Figure 1 is an illustration of an 8-layer PCB, which is actually made up of 3 copper-clad plates (core boards) plus 2 copper films and then glued together with semi-cured sheets. The production sequence starts from the core board (four or five layers of lines) in the middle, and is continuously stacked together before being fixed. The 4-layer PCB is made similarly, but with only one core plate and two copper films.

3. Make intermediate core board circuit

The layout transfer of inner PCB should first make the two-layer circuit of the most middle Core board (Core). After the copper-clad plate is cleaned, the surface 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. Insert two layers of PCB layout film and two layers of copper clad board, and finally insert the upper layer of PCB layout film to ensure that the upper and lower layers of PCB layout film stacking position is accurate. Photosensitizer uses UV lamp to irradiate the photosensitive film on copper foil. The photosensitive film is solidified under the transparent film, and the photosensitive film is not solidified under the opaque film. The copper foil covered by solidified photosensitive film is the PCB layout line needed, equivalent to the role of laser printer ink of manual PCB. The uncured film is then washed away with lye and the required copper foil circuit is covered by the cured 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 copper foil needed for PCB layout circuit.

4. Core plate drilling and inspection

The core plate has been made successfully. Then make the opposite hole in the core plate for easy alignment with other raw materials. Once the core board is pressed with other layers of PCB, it cannot be modified, so it is very important to check. The machine will automatically compare with PCB layout drawings to check errors.

5. Laminated

Here we need a new raw material called semi-cured sheet, which is the core board and the core board (PCB layer & GT; 4), and the adhesive between the core plate and the outer copper foil, but also plays a role in insulation. The lower layer of copper foil and two layers of semi-solidified sheet have been in advance through the positioning hole and the lower iron plate fixed position, and then the good core plate is also put into the positioning hole, and finally in turn two layers of semi-solidified sheet, a layer of copper foil and a layer of pressure aluminum plate covered on the core plate. PCB board clamped by iron plate is placed on the support, and then into the vacuum hot press for lamination. The heat in the vacuum hot press melts the epoxy resin in the semi-cured sheet, holding the core and copper foil together under pressure. After laminating, remove the top iron plate that presses the PCB. Then the pressurized aluminum plate is removed. The aluminum plate also plays a role in isolating different PCBS and ensuring the smooth copper foil on the outer layer of THE PCB. Both sides of the PCB are covered with a layer of smooth copper foil.

6. Drilling

To connect four layers of copper foil that do not touch each other in a PCB, first drill holes through the PCB, then metallize the hole walls to conduct electricity. The X-ray drilling machine is used to locate the core board of the inner layer. The machine will automatically find and locate the hole position on the core board, and then make positioning holes for the PCB to ensure that the following drilling is through the center of the hole position. Place a sheet of aluminum on the punch machine and then place the PCB on top. To improve efficiency, one to three identical PCB boards are stacked together for perforation according to the number of PCB layers. Finally, the top PCB is covered with a layer of aluminum, the top and bottom layers of aluminum so that when the drill drills in and out, the copper foil on the PCB will not tear. In the previous laminating process, the melted epoxy was extruded to the outside of the PCB, so it needed to be removed. The die milling machine cuts the periphery of the PCB according to the correct XY coordinates.

7. Chemical precipitation of copper on pore wall

Since almost all PCB designs use perforations to connect different layers of lines, a good connection requires a 25 micron copper film on the hole wall. This thickness of copper film is achieved by electroplating, but the hole wall is made 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, through chemical deposition. The whole process, such as chemical treatment and cleaning, is controlled by machines.

8. Transfer the layout of outer PCB

Next, the layout of the outer PCB will be transferred to the copper foil. The process is similar to that of the PCB layout of the inner core board, which is transferred to the copper foil using photocopied film and photosensitive film. The only difference is that the positive plate will be used as the board. The inner PCB layout transfer adopts the subtraction method and adopts the negative plate as the board. PCB covered by solidified photosensitive film is circuit, clean the unsolidified photosensitive film, exposed copper foil is etched, PCB layout circuit is protected by solidified photosensitive film. The outer PCB layout is transferred by the normal method, and the positive plate is used as the board. The area covered by a cured film on a PCB is a non – line area. After cleaning the uncured film, electroplating is carried out. There is no film can be electroplated, and there is no film, first copper and then tin plating. After the film is removed, alkaline etching is carried out, and finally tin is removed. The circuit pattern is left on the board because it is protected by tin. Clamp the PCB and electroplate the copper. As mentioned before, in order to ensure that the hole has good electrical 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 computer to ensure its accuracy.

9. Outer PCB etching

Next, a complete automated assembly line completes the etching process. First, clean off the cured film on the PCB board. A strong alkali is then used to clean off unwanted copper foil that is covered by it. Then the tin coating on the copper foil of PCB layout is removed with tin stripping solution. After cleaning, 4 layers PCB layout is completed.