The goal is to prevent short circuits with finely structured PCBs.
The application of a solder mask also achieves a higher insulation resistance between adjacent tracks.

A flexible solder mask can also be used in the production of flexible printed circuit boards (PCB Overseas). 
 A pressed-on polyimide film, however, is used in the PCB-POOL® Flex-Pool service. The exposed areas in the cover film must first be drilled or milled.