A Protective Coating applied to completed printed circuit boards that conforms to the shape of the components and provides complete electrical as well as environmental insulation.is normally applied to the surface of the PCB to prevent damage from the environment in which it will be used. This protective coating can be applied after manufacture and either before or after the electronic components have been soldered on.
The protective coating protects in several different ways. The copper commonly used for the tracks will be corroded by exposure to oxygen in the air, and the protective coating (a passivation layer) puts a barrier between the oxygen and the metal).
If the copper must be accessible, either for soldering (on to pads) or for electrical contact (such as edge connectors off the PCB), then the copper is plated with another metal such as tin or nickel. This additional metal forms a passivation layer that protects the copper from oxidation.
Where the copper need not be accessible, then an electrically insulating protective coating is applied over the metal. This has the additional advantage of preventing dirt and moisture from reducing the insulation resistance between the tracks.
The insulating material used in the substrate (e.g., FR-4) will readily absorb moisture from the air, thereby reducing the electrical properties of the substrate. The protective coating puts a barrier between the substrate and the moisture in the air. protective coating also controls the flow of solder during the soldering process.
This prevents solder from jumping across tracks and causing short circuits. When a protective coating is applied prior to soldering the components onto the board, it is usually referred to as a solder mask. When applied after the components have been soldered, it is usually referred to as a conformal coating.