Solder resist fullfils an important function for assembling the PCB. It hinders short-circuits and ensures that the solder only flows where components are applied.
The solder resist department consists of several processes. During the first step, the PCB is fully coated with a photo-sensitive lacquer. Two processes are used at Unimicron Germany: application via spray coating and using a coating curtain. In most cases, the lacquer is green, however other colours such as white, black, blue or red are also available.
During the next step, the PCB is digitally exposed in order to keep the defined contact surfaces for components free of lacquer. When later developed, the pads really are free of solder mask.