PCB Multilayer Circuit Board Production Process (2)

B. Decontamination and Copper Sinking
Purpose: Metallize the through hole
1. The substrate of the circuit board is composed of copper foil, glass fiber, and epoxy resin. In the manufacturing process, the cross-section of the base material after drilling is composed of the above three parts of materials.
2. Hole metallization is to solve the problem of covering the cross section with a uniform layer of metal copper with heat shock resistance.
3. The process is divided into three parts: de-drilling, electroless copper deposition, and thickened copper (copper plating on the whole board).

C. Immersion Copper and Thick Copper
The metallization of the hole involves a concept of capability-the ratio of thickness to diameter. The thickness-to-diameter ratio refers to the ratio of the plate thickness to the hole diameter. When the plate becomes thicker and the hole diameter decreases, it becomes more and more difficult for chemicals to enter the depth of the drill hole. Although the electroplating equipment uses vibration and pressure to allow the liquid to enter the drill hole center, but the center plating layer caused by the concentration difference is still unavoidable. At this time, there will be a slight open circuit of the drilling layer. When the voltage increases and the board is impacted under various harsh conditions, the defects are completely exposed, causing the board to fail. The line is short-circuited and the specified work cannot be completed.
Therefore, designers need to know the process capability of the board manufacturer in time, otherwise the designed PCB will be difficult to realize in production. It should be noted that the thickness-to-diameter ratio parameter not only needs to be considered when designing through holes, but also when designing blind and buried holes.

D. Outer dry film and pattern plating
The principle of outer layer pattern transfer is similar to that of inner layer pattern transfer. Both use photosensitive dry film and photographing methods to print circuit patterns on the board. The difference between the outer layer dry film and the inner layer dry film is:
If the subtractive method is used, the outer dry film is the same as the inner dry film, and the negative film is used as the board. The cured dry film part of the board is used as a circuit, the uncured film is removed, and the film is retreated after acid etching. The circuit pattern remains on the board because of the protection of the film.