The cover film is used as a solder resist mask which exposes only the soldering surfaces (soldering points, pads) and component connections while covering the surfaces which are not needed. This keeps them from also being soldered during the subsequent soldering process.
This not only results in a savings on solder (with the wave soldering process) but also prevents short-circuits in finely structured printed circuit boards. Applying the cover film also achieves a higher insulation resistance between neighbouring circuit paths.
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 Beta LAYOUT GmbH Flex-Pool. The exposed areas in the cover film must first be drilled or milled.