PCB Fabrication Processes And Procedures

PCB, 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 in the interior of a variety of electronic equipment, willing to be the hero behind the scenes, so it is not known to all, for most people, not to mention its production process.

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

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

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

1、PCB Layout

Organize and check the PCB layout (Layout). PCB manufacturers in the receipt of various types of CAD files from the PCB design company, the factory engineers will check the PCB layout design is in line with the factory's production process requirements, in determining that there are no design flaws and other issues will be converted to a uniform format - the Extended Gerber RS-274X or Gerber X2.

2、Make the core board

Clean the copper-clad board, remove the dust and stains on top of the board to avoid causing short circuit and circuit breakage of the circuit board in the later stage, which will affect the quality of the product.

An 8-layer circuit board is made of 3 sheets of copper-clad laminate (core board) plus 2 sheets of copper film, 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-layer 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 middle core board (Core) of the two-layer 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 on the laminate to form a 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 stacking position is accurate.

Photoreceptor with UV lamps on the copper foil photographic film for irradiation, light-transparent film, the photographic film is cured, opaque film is still not cured under the photographic film. Cured photographic film covered under the copper foil is the need for PCB layout lines. Then use a strong alkali solvent to etch off the unwanted copper foil.

Then use the alkali will not be cured photographic film cleaning off, the need for copper foil lines will be cured photographic film covered by the cured photographic film will be cured photographic film torn off, revealing the need for the PCB line copper foil.

4, core board punching and inspection

The core board has been produced successfully. Then in the core plate on the alignment holes, easy to follow 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. It is the adhesive between the core board and the core board (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, a layer of copper foil and a layer of pressurized aluminum plate are covered to 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 heat press melts the epoxy resin in the semi-cured sheets, holding 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 take away the pressure of the aluminum plate, aluminum plate also plays a role in isolating the different PCB boards as well as ensure that the role of the PCB outer layer of copper foil smooth. At this point take out both sides of the PCB will be covered by a layer of smooth copper foil.

6. Drilling

To connect the 4 layers of non-contacting copper foils in the PCB, the first step is to drill holes through the PCB at the top and bottom, and then metallize the walls of the holes to conduct electricity.

Using an X-ray drilling machine to locate the inner core board, the machine will automatically find and locate the holes in the core board, and then punch the positioning holes for the PCB to ensure that the next drilling is from the center of the holes through. A layer of aluminum is placed on the punch press 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 top PCB is covered with a layer of aluminum plate. The top and bottom layers of aluminum plate 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 die leaning milling machine cuts the periphery of the PCB according to its correct XY coordinates.

7, hole wall chemical copper sinking

As almost all PCB circuit boards are through-hole to connect the different layers of the line, usually need to be plated on the hole wall of 25 microns of copper film.

The first step of electroplating is to plate a layer of copper on the hole wall first, through the chemical deposition of the entire PCB surface, also including 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 is the normal method, the positive piece of the board, the PCB is cured photographic film covered by the non-line area. After cleaning off the uncured photographic film, plating is performed. 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 with clips and copper is plated on. In order to guarantee a reliable conductivity between the layers, the copper film plated on the hole walls must 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. First of all, the cured photographic film on the PCB board is 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 to stack the board, press together, solder resist, text, surface treatment, testing, packaging and so on.

In this way, a qualified circuit board is born.