The purpose of solder mask is to physically and electrically insulate those portions of the circuit to which no solder or soldering is required. Increasing density and surface mount technology have increased the need for solder mask to the point that, with the exception of "pads only" designs, nearly all parts require it. Manufacturers have had some autonomy in selecting masks. Many specifications do not call out a specific product or product type and this has allowed the manufacturer to choose masks based on processing as well as performance issues.
Three basic types of masks are commonly applied: thermally cured screen-printed masks, dry film, and screen-applied (printing is done with a phototool) or curtain-coated liquid photoimageable (LPI). Thermal masks have predominated for decades but are gradually being replaced by LPI, despite being the lowest cost alternative. Screen printing has gradually fallen out of favor due to the difficulty of holding the increasingly tight tolerances with this method. Dry film has some specific advantages, such as ease of application, but its use seems poised to decline as well in the face of improving LPI formulations.