A printed circuit board (PCB) is the board of most modern electronic devices that has lines and pads that connect the various points together. Even if it is a small board, its production process is very complicated and delicate. Here will be a step-by-step introduction to the PCB manufacturing process with pictures and videos.
Step 1. PCB CAD file
The first step in PCB production is to organize and check the PCB layout. PCB manufacturers take CAD files from PCB design companies and convert them into a unified format - Extended Gerber RS-274X or Gerber X2, as each CAD software has its own unique file format. The electronic engineer will then check whether the PCB layout conforms to the manufacturing process, whether there are defects, etc.
When making PCBs at home, the PCB layout can be printed on paper with a laser printer and then transferred to a copper clad laminate. During the printing process, because the printer is prone to lack of ink and breakpoints, it is necessary to manually replenish ink with an oil-based pen.
However, factories typically use photocopying to print the PCB layout on film. If it is a multi-layer PCB, the printed layout film of each layer will be arranged in order.
Alignment holes will then be punched in the film. Aligning the holes is very important, and it is essential to align the layers of the PCB material.
Step 2. Plate making
Clean the copper plate. If there is dust, it may cause a short or open circuit in the final circuit.
The picture below is an example of an 8-layer PCB, which is actually composed of 3 copper clad laminates plus 2 copper films, which are then bonded together with prepreg. The production sequence starts with the intermediate boards (layer 4 and 5 circuits), stacked together in succession, and then fixed. The production of 4-layer PCB is similar, including a core board and two copper films.
Step 3. PCB Inner Layers
First make the two-layer circuit of the middle core board. After the CCL is cleaned, the surface will be covered with a photosensitive film. The film is cured by light to form a protective film on the copper foil.
Insert the two-layer PCB trace film and the double-layer copper clad laminate into the upper PCB trace film to ensure accurate stacking of the upper and lower PCB trace films.
The machine irradiates the photosensitive film on the copper foil with a UV lamp. The clear film is cured in the light and there is still no cured photosensitive film. The copper foil covered under the cured film is required for PCB layout, which is equivalent to the role of laser printer ink for manual PCB. In addition, the copper foil covered by the black film will be corroded, and the cured transparent film will be preserved.
Rinse the uncured photosensitive film with lye, the cured film will cover the desired copper foil circuit.
Then use a strong base, such as NaOH, to etch away the unwanted copper foil.
Peel off the cured photosensitive film to expose the copper foil for the desired PCB layout.
Step 4. Board and Check
The core board has been successfully produced. Then punch alignment holes in it for easy access to other materials.
Once the core board is laminated with others, it cannot be modified. So PCB inspection is very important. The machine will automatically compare with the PCB layout to find errors.
The first two layers of PCB boards have been fabricated.
Step 5. Lamination
A new raw material called Prepreg is introduced here, which is an adhesive between the core board (PCB layers > 4) and between the core board and the outer copper foil, and also acts as an insulation.
The lower copper foil and two layers of prepreg have been pre-fixed through the wiring hole and the lower iron plate, and then the finished core board is also placed in the wiring hole, and finally two layers of prepreg, one layer of copper foil and one layer of bearing Pressed aluminum plate covers the core plate.
To improve work efficiency, the factory stacks three different PCB boards together before fixing them. The upper iron plate is magnetically attracted for easy alignment with the lower iron plate. After the two layers of iron plates are successfully aligned by inserting the positioning pins, the machine compresses the space between the iron plates as much as possible, and then fixes them with nails.
The PCB board clamped by the iron plate is placed on the holder, and then sent to the vacuum heat press for lamination. The high temperature melts the epoxy in the prepreg and holds the core and copper foil together under pressure.
After bonding, remove the upper iron plate pressing the PCB. Then remove the pressure-bearing aluminum plate. The aluminum plate also plays the role of isolating different PCBs to ensure the flatness of the copper foil on the outer layer of the PCB. Finally, the PCB taken out at this time will be covered with a smooth copper foil.
Step 6. Drill the holes
So how to connect the 4 layers of copper foils in the PCB that are not in contact with each other? First through holes are made in the PCB, then the walls of the holes are metallized to conduct electricity.
Lay a layer of aluminum on the puncher and put the PCB on it. Since drilling is a relatively slow process, to improve efficiency, 1 to 3 identical boards are stacked on top of each other to drill holes, depending on the number of layers of the PCB. Finally, cover the topmost PCB with a layer of aluminum. The upper and lower aluminum plates are used to prevent the copper foil on the PCB from tearing when drilling.
Next, you just need to select the correct drilling program on the computer, the rest is done automatically by the drilling machine. The drill bit is driven by air pressure, and the maximum speed can reach 150,000 rpm. Because such a high speed is enough to ensure the flatness of the hole wall.
The replacement of the drill bit is also done automatically by the machine according to the program. The smallest drill bits can reach a diameter of 100 microns, while the diameter of a human hair is 150 microns.
In the previous process, molten epoxy was extruded out of the PCB, so it needed to be cut off. Here, the copy milling machine cuts the periphery of the PCB according to its correct XY coordinates.
Step 7. Copper Chemical Precipitation on Holes
Since almost all PCB designs use through-holes to connect lines on different layers, a good connection requires a 25-micron copper film on the walls of the holes. The thickness of the copper film needs to be achieved by electroplating, but the walls of the holes are composed of non-conductive epoxy resin and fiberglass board. So the first step is to deposit a layer of conductive material on the wall of the hole to form a 1-micron copper film on the entire PCB surface by chemical deposition. The whole process of chemical treatment, cleaning, etc. is controlled by the machine.
Next, transfer the outer layer of the PCB to the copper foil. The process is similar to the transfer principle of the previous PCB inner core board. The PCB layout is transferred to the copper foil by means of photoprinting film and photosensitive film. The only difference is that the positives will be used as boards.
The transfer of the internal PCB layout described above uses subtraction, with the negative as the board. The PCB is covered by the cured photosensitive film as a circuit, and the uncured film is cleaned. After the exposed copper foil is etched, the PCB layout circuit is protected by the cured film. The transfer of the outer layer PCB layout adopts the normal method, and the positive film is the board. The non-circuit area is covered by a photosensitive film cured on the PCB. After cleaning the uncured film, electroplating is performed. Places with films cannot be electroplated, and places without films are plated with copper and then tinned. After the film is removed, an alkaline etch is performed and the tin is finally removed.
Put the cleaned PCB on both sides of the copper foil into the laminator, and press the photosensitive mold on the copper foil.
The printed upper and lower PCB layout films are fixed through the holes, and the PCB board is placed in the middle. Then the photosensitive film under the light-transmitting film is cured by the irradiation of the UV lamp, which is the circuit that needs to be reserved.
After cleaning the unwanted and uncured photosensitive film, inspect the PCB board.
The PCB is clamped with clips and copper plated. As mentioned earlier, in order to ensure sufficient conductivity of the hole, the copper film plated on the hole wall must have a thickness of 25 microns, so the entire system will be automatically controlled by the computer to ensure its accuracy.
After the copper film is electroplated, the computer gives instructions to electroplate a thin layer of tin. Then, check to make sure the copper and tin plating is the correct thickness.
Next, a complete automated assembly line completes the etching process. Then, clean the cured photosensitive film on the PCB.
Then use a strong alkali to clean the unwanted copper foil it covers.
Finally, peel off the tinned layer on the copper foil of the PCB layout with tin stripping liquid. After cleaning, the 4-layer PCB layout is complete.