PCB (Printed Circuit Board) is the support body for electronic components. It is called a “printed” circuit board because it is made using electronic printing technology.Before the advent of PCBs, circuits were composed of point-to-point wiring. This solution had low reliability because as circuits aged, line breakages could lead to open or short circuits.As electronic products increasingly appeared in the Internet of Things (IoT) device field, manufacturers sought smaller and more cost-effective solutions, leading to the birth of PCBs.The manufacturing of PCBs is very complex. Taking a four-layer printed board as an example, the manufacturing process mainly includes PCB layout, core board production, inner layer PCB layout transfer, core board drilling and inspection, lamination, drilling, copper chemical deposition on hole walls, outer layer PCB layout transfer, and outer layer PCB etching.1. PCB Layout
The first step in PCB manufacturing is to organize and check the PCB layout. The PCB manufacturing factory receives CAD files from the PCB design company. Since each CAD software has its unique file format, the PCB factory will convert it into a unified format—Extended Gerber RS-274X or Gerber X2. Then, the factory’s engineers will check whether the PCB layout meets the manufacturing process and whether there are any defects.
2. Core Board ProductionClean the copper-clad board, as dust may cause short circuits or open circuits in the final circuit.
The image below is an illustration of an 8-layer PCB, which is actually made up of 3 copper-clad boards (core boards) plus 2 copper films, and then bonded together with prepreg. The production sequence starts from the innermost core board (layers 4 and 5) and continues to stack them together, then fix them. The production of a 4-layer PCB is similar, but only uses 1 core board plus 2 copper films.
3. Inner Layer PCB Layout TransferFirst, the two layers of circuits on the innermost core board (Core) need to be made. After cleaning the copper-clad board, a layer of photosensitive film is applied to the surface. This film solidifies when exposed to light, forming a protective layer on the copper foil of the copper-clad board.
Two layers of PCB layout film and a double-layer copper-clad board are then inserted with the upper layer of PCB layout film to ensure the precise alignment of the upper and lower PCB layout films.
The UV lamp of the exposure machine irradiates the photosensitive film on the copper foil. Under the transparent film, the photosensitive film is solidified, while under the opaque film, the photosensitive film remains unsolidified. The solidified photosensitive film covers the copper foil that represents the required PCB layout circuit, similar to the role of ink in a laser printer for manual PCBs.Then, an alkaline solution is used to wash away the unsolidified photosensitive film, and the required copper foil circuits will be covered by the solidified photosensitive film.
Next, use a strong base, such as NaOH, to etch away the unnecessary copper foil.
Peel off the solidified photosensitive film to reveal the required PCB layout circuit copper foil.
4. Core Board Drilling and InspectionThe core board has been successfully produced. Next, alignment holes are drilled in the core board to facilitate alignment with other materials.
Once the core board is pressed together with other layers of PCB, it cannot be modified, so inspection is very important. Machines will automatically compare with the PCB layout drawings to check for errors.
5. LaminationThis requires a new material called prepreg, which acts as an adhesive between core boards (for PCBs with more than 4 layers) and between core boards and outer copper foils, while also providing insulation.The lower copper foil and two layers of prepreg have already been fixed in position through alignment holes and the lower iron plate, then the prepared core board is placed into the alignment holes, and finally, two layers of prepreg, one layer of copper foil, and one layer of pressure-bearing aluminum plate are sequentially placed on top of the core board.
The PCB, sandwiched between the iron plates, is placed on a rack and sent into a vacuum hot press for lamination. The high temperature in the vacuum hot press melts the epoxy resin in the prepreg, fixing the core boards and copper foils together under pressure.
After lamination, the upper iron plate that pressed the PCB is removed. Then, the pressure-bearing aluminum plate is taken away. The aluminum plate also serves to isolate different PCBs and ensure the smoothness of the outer copper foil of the PCB. At this point, the PCB taken out will be covered on both sides with a layer of smooth copper foil.
6. DrillingTo connect the four layers of non-contact copper foils in the PCB, it is necessary to first drill through-holes that penetrate the PCB, and then metallize the hole walls to make them conductive.Using an X-ray drilling machine, the inner core board is positioned, and the machine will automatically find and position the hole locations on the core board, then mark the positioning holes on the PCB to ensure that the drilling is done through the center of the hole locations.
A layer of aluminum plate is placed on the drilling machine bed, and the PCB is placed on top. To improve efficiency, 1 to 3 identical PCBs may be stacked together for drilling, depending on the number of layers in the PCB. Finally, an aluminum plate is placed on top of the uppermost PCB to prevent tearing of the copper foil on the PCB when the drill enters and exits.
In the previous lamination process, the melted epoxy resin was extruded out of the PCB, so it needs to be trimmed. A contour milling machine will cut along the correct XY coordinates of the PCB.
7. Copper Chemical Deposition on Hole WallsSince almost all PCB designs use through-holes to connect circuits on different layers, a good connection requires a copper film of 25 microns on the hole walls. This thickness of copper film needs to be achieved through electroplating, but the hole walls are made of non-conductive epoxy resin and fiberglass.Therefore, the first step is to accumulate a layer of conductive material on the hole walls, forming a 1-micron copper film on the entire PCB surface, including the hole walls, through a chemical deposition process. The entire process, including chemical treatment and cleaning, is controlled by machines.
Fixing the PCB
Cleaning the PCB
Transporting the PCB
8. Outer Layer PCB Layout TransferNext, the outer layer PCB layout will be transferred onto the copper foil, a process similar to the inner layer core board PCB layout transfer, using photocopy films and photosensitive films to transfer the PCB layout onto the copper foil. The only difference is that positive films will be used for the board.The inner layer PCB layout transfer uses a subtractive method, employing negative films. The solidified photosensitive film on the PCB represents the circuits, while the unexposed photosensitive film is washed away, and the exposed copper foil is etched, leaving the PCB layout circuit protected by the solidified photosensitive film.The outer layer PCB layout transfer uses a normal method, utilizing positive films. The solidified photosensitive film on the PCB covers the non-circuit areas. After washing away the unexposed photosensitive film, electroplating is performed. The areas covered by the film cannot be electroplated, while the areas without film are first plated with copper and then with tin. After stripping the film, alkaline etching is performed, and finally, the tin is stripped. The circuit patterns remain on the board due to the protection of the tin.
The PCB is clamped with clips, and copper is plated onto it. As previously mentioned, to ensure good conductivity of the hole locations, the copper film plated on the hole walls must have a thickness of 25 microns, so the entire system will be automatically controlled by a computer to ensure precision.9. Outer Layer PCB EtchingNext, a complete automated production line will complete the etching process. First, the solidified photosensitive film on the PCB board is washed away. Then, a strong base is used to remove the unwanted copper foil covered by it. Finally, the tin plating on the PCB layout copper foil is stripped away. After cleaning, the 4-layer PCB layout is completed.
Source: Metal ProcessingDisclaimer: The copyright of this article belongs to the original author and does not represent the views of the association. The articles shared by the “Jiangxi Province Electronic Circuit Industry Association” are only for sharing purposes and do not represent the position of this account. If there are any copyright issues, please contact us for removal.
