Solder mask or solder stop mask is a thin lacquer-like layer of polymer that is applied to the copper traces of a circuit board for protection against oxidisation and to prevent solder bridges from forming between closely spaced solder pads.

What different types are there?

Top and Bottom side Masks - A topside solder mask allows the electronic engineer to identify the openings in the green solder mask layer already added to the PCB by one of the epoxy, ink or film techniques. Component pins can then be soldered onto the board using those identified places

Epoxy Liquid Silkscreened onto the PCB pattern, the epoxy liquid is the lowest cost type of solder mask. Epoxy is a thermosetting polymer that has many applications.

Liquid photo imageable solder mask is delivered as an ink formulation. The ink can be silkscreened or sprayed onto the PCB, then exposed to the pattern and developed.

Dry film photo imageable solder mask is applied using vacuum lamination, then exposed and developed. After developing, openings are created in the pattern and parts can be soldered to the copper pads. Copper is layered onto the board inside the holes and on the trace areas using electrochemical processing.