In the manufacture of a conventional printed circuit board, after forming the outer layer conductor pattern, a solder mask protective treatment has to be applied to the conductor pattern so as to prevent oxidation of the conductor pattern or solder connection short circuit.
In a solder mask manufacturing process for the conventional printed circuit board, a solder mask paint is applied to the surface of the circuit board by halftone printing, roller type coating, screen coating or electrostatic spray coating, etc. After pre-baking, drying and cooling, exposure is employed to display images. The method of imaging removes unnecessary paint, and the resin in the remaining paint is completely cured by high-temperature baking in the final step to form a solder mask covered on the surface of the PCB to protect the conductor pattern.
The conventional solder mask manufacturing process has the following disadvantages:
1. Using halftone printing or roller type coating to apply the solder mask paint to the printed circuit board requires multiple halftone printing or roller printing steps in order to accumulate the paint to the desired thickness, which is complicated.
2. During the process of baking the printing circuit board at high temperature, since the resin material in the solder mask paint has an expansion coefficient different from that of the circuit board substrate, internal stress will form between the resin material and the circuit board to cause the bend of the printed circuit board.
3. The thickness of the solder mask paint layer is not easy to control, resulting in electric instability of the circuit board.
4. Air bubbles can easily get in during the process of coating the solder mask paint to result in poor reliability quality of the solder mask and shortened life of the solder mask layer.
5. Coating the circuit board with solder mask paint cannot fill through holes in the circuit board with solder mask paint. Therefore, before coating, insulating fillers are disposed in the through holes respectively and are ground flat before coating the solder mask paint, thereby increasing manufacturing time and cost of the printing circuit board.
6. When coating the circuit board with solder mask paint, coating layer cannot be easily formed on the copper foil conductor pattern, and the bonding therebetween is relatively poor.