As the name suggests, this is a chemical layer on an unpopulated printed circuit board (PCB) with openings for the areas solder will cover and for masking the rest. The main objective is to prevent solder from accidentally bridging neighboring pads or tracks during assembly, and to protect the board from the environment. Although both are protective coatings, solder mask is not a conformal coating, as the latter protects an assembled board.
Solder mask plays an important role during PCB assembly. Designers place solder mask dams between surface mount pads, and this prevents accidental solder bridging during assembly, greatly reducing the amount of rework for removing shorts. However, not all SMD pads can have solder mask dams between them, especially the fine-pitch components. The spacing between the pads for such components is so small that no solder mask dam is possible.
Solder mask has other uses as well, such as preventing solder starvation. Usually, solder should bind the component lead to its copper pad. However, if solder mask does not cover the track attached to the copper pad, molten solder can flow down the track, starving the pad of adequate solder, thereby creating an unreliable joint. The presence of an uncovered via near a pad also serves to wick molten solder into its hole, again starving the pad of adequate solder. Covering the via with solder mask prevents this wicking.
Continue...
