As a protective layer, the solder mask layer is covered on the traces and substrates that do not need to be soldered to prevent short circuits during soldering. At the same time, it serves to provide a permanent electrical environment with a protective layer that is chemically resistant. The quality of solder mask printing directly affects the appearance quality of the PCB.
Pre-treatment
Before applying the solder mask, the board needs to be pre-treated. This step is to remove oxides, grease, and impurities from the board surface and increase the roughness of the board to enhance bonding force with the ink, making preparation for printing.
Printing solder mask
There are two methods to print solder mask——screen printing or electrostatic spraying, but we more often apply the screen printing method, which is to use an automatic printing machine to evenly leak the photosensitive ink to the board surface through the small holes on the screen under the pressure of a squeegee. The tension and density of the screen, the angle, speed, and strength of the squeegee all have an impact on the printing quality.
Pre-drying
After each printing, the board is pre-dried to cure the solder mask ink. The purpose is to remove the solvent contained in the ink so as to make the ink non-stick. The temperature and time of pre-drying varies according to different ink.
Exposure
Now both sides of the board are coated with a layer of solder mask ink, but the ink is not required on areas that need to be soldered such as the pads and plated through holes. To remove the unwanted solder mask ink, exposure and developing processes are carried out. The solder mask film is first aligned with the holes on the board, and then the aligned board is sent into the exposure machine for exposure. The solder mask ink that is exposed to UV light undergoes a cross-linking reaction and gets hardened to form the solder mask layer. While the ink that is not exposed will be removed by the action of the alkaline solution.
Developing
The ink that is not exposed to UV light will be removed by the action of the alkaline solution to form the required solder mask coating pattern on the board surface. After completing all the processes above, the board is baked in the oven at a high temperature for 40-60 minutes, so that the solder mask ink is completely cured and attached to the board surface to enhance its soldering resistance.
