PCBs, also known as printed circuit boards or printed wiring boards, are used in a wide range of applications, no matter how amazing the electronic equipment, including intelligent robots, aerospace and other high-tech equipment can not be separated from the support of PCB products.

Because it is inside the various electronic devices, willing to be the hero behind the scenes, and therefore not known to all, for most people, not to mention its production process.

So are you as curious about its production process as most people are?!

Here we take 8-layer printed circuit board as an example, for the curious you open the mystery of PCB production.

PCB production process mainly includes the layout of the PCB line, the production of the core board, the inner layer of the PCB line transfer, core board drilling and inspection, lamination, drilling, hole wall copper plating, the transfer of the outer layer of the PCB line and etching and other processes.

1、PCB layout

Organise and check the PCB layout (Layout). PCB manufacturers receive various types of CAD files from the PCB design company, the factory engineers will check the PCB layout design to see if it meets the requirements of the factory's production process, and after determining that there are no design flaws and other issues will be converted into a unified format - the Extended Gerber RS-274X or Gerber X2.

2, the production of the core board

Clean the copper-clad board, remove the dust and stains on the top of the board, so as not to cause short-circuits and circuit breakers in the later stage of the circuit board, which will affect the quality of the product.

The figure below is an example of an 8-layer circuit board, which is made of 3 copper-clad boards (core boards) plus 2 copper films, and then bonded together with a semi-cured sheet. The production order is from the middle of the core board (4, 5 layers of lines), constantly stacked together, and then fixed. 4 layers of PCB production is similar, except that only 1 core board plus 2 copper film.

3, the inner PCB line transfer

First of all, we need to make the most intermediate core board (Core) of the two layers of the line. Copper-clad board will be cleaned after the surface covered with a layer of light-sensitive film. This film encounters light will be cured, in the copper foil of the copper-clad board to form a layer of protective film.

Put in the PCB circuit layout film and double-layer copper cladding board, and finally put in the upper layer of PCB layout film, to ensure that the upper and lower layers of PCB layout film laminated position is accurate.

The photographic machine uses UV lamps to irradiate the photographic film on the copper foil, under the light-transparent film, the photographic film is cured, and under the light-impermeable film, the photographic film is still not cured. Cured photographic film covered under the copper foil is the need for PCB layout lines.

Then use alkali solution to clean off the uncured photographic film, the required copper foil line will be covered by the cured photographic film.

Then use strong alkali solvent to etch off the unwanted copper foil.

The cured photographic film will be torn off, revealing the copper foil of the required PCB lines.

4, core board punching and inspection

The core board has been produced successfully. Then punch alignment holes in the core board to facilitate the next and other PCB board alignment.

Once the core board and other layers of PCB pressed together can not be modified, so the inspection is very important. Automatic Optical Inspection (AOI) detects possible defects in the inner layer of the circuit.

05, Lamination

A new substrate called semi-cured sheet is needed here, which is the adhesive between the core board and the core board (number of PCB layers > 4), as well as between the core board and the outer layer of copper foil, and also serves as an insulator.

The lower layer of copper foil and the two layers of semi-cured sheet have been fixed in position in advance through the alignment holes and the lower layer of iron plate, and then the made core board is also put into the alignment holes, and finally the two layers of semi-cured sheet, one layer of copper foil and one layer of pressure-bearing aluminium plate are covered onto the core board in turn.

The PCB boards, which are clamped by the iron plate, are placed on the holder and then sent into the vacuum hot press for lamination. The high temperature in the vacuum hot press melts the epoxy resin in the semi-cured sheet and holds the core board and copper foil together under pressure.

After the lamination is completed, the top layer of the iron plate on which the PCB is pressed is removed. Then the pressure of the aluminium plate to take away, the aluminium plate also plays a role in isolating the different PCB boards and ensure that the PCB outer layer of copper foil smooth. This time to take out the PCB on both sides will be a smooth layer of copper foil covered.

6, drilling

To PCB in the 4 layers of non-contact copper foil connected together, first of all to drill out the top and bottom through the hole through to open the PCB, and then the hole wall metallisation to conduct electricity.

An X-ray drilling machine is used to locate the inner core board. The machine will automatically find and locate the holes in the core board, and then drill the holes for the PCB to ensure that the next hole is drilled right through the centre of the hole.

A layer of aluminium is placed on the hole punch machine and the PCB is placed on top of it. Depending on the number of PCB layers, one to three identical PCBs are stacked on top of each other for efficiency. Finally, the topmost PCB is covered with a layer of aluminium, the top and bottom layers of aluminium are designed so that when the drill bit drills in and out, it will not tear the copper foil on the PCB.

During the previous lamination process, the melted epoxy was extruded to the outside of the PCB, so it needed to be removed. The milling machine against the die cuts the periphery of the PCB according to its correct XY coordinates.

7, hole wall chemical copper sinking

Since almost all PCB circuit boards use through-holes to connect different layers of lines, it is usually necessary to plate a 25 micron copper film on the hole walls.

The first step of electroplating is the first layer of copper plated on the hole wall, through the chemical deposition of the entire PCB surface, including also the hole wall to form a 1 micron copper film.

8, the outer PCB layout transfer and copper plating

Next will be used and the inner core PCB line transfer method to produce the outer line.

Inner layer PCB layout transfer is used to reduce into the method, the negative to do the board. PCB is cured on the photographic film covered by the line, cleaned off the photographic film is not cured, exposed copper foil was etched, PCB layout line is cured photographic film protection and left.

The outer PCB layout transfer adopts the normal method, the positive piece of the board, the PCB is cured photographic film covered by the non-line area. The uncured photographic film is cleaned off and plated. There is a film can not be plated, and there is no film, first plated with copper and then plated with tin. After removing the film, alkaline etching is performed, and finally tin is removed. The line pattern remains on the board because it is protected by tin.

The PCB is clamped in place and the copper is plated on. In order to guarantee a reliable conductivity between the layers, the copper film plated on the hole walls has to be 25 microns thick. So the whole system will be automatically controlled by the computer to ensure the accuracy of the plating thickness.

9、Outer layer PCB etching

Next by a complete automated assembly line to complete the etching process. Firstly, the cured photographic film on the PCB board will be cleaned off. Then use strong alkali to clean off the unwanted copper foil covered by it. Then remove the tin layer on the copper foil of PCB layout with tin-removing liquid. After cleaning the 4-layer PCB layout is completed.

10、Outer layer AOI

Use AOI machine to detect the outer layer of the line.

11、Other processes

After the process is stacked boards, press fit, solder resist, text, surface treatment, testing, packaging and so on.