Definition

Solder mask is a polymer coating applied over the PCB copper to protect traces from oxidation, prevent solder bridges, and provide electrical insulation. It's typically epoxy-based, applied by screen printing, curtain coating, or spray, then imaged to create openings over pads and vias. Standard color is green, but blue, red, black, white, and matte finishes are available. Solder mask thickness affects impedance and must be considered in controlled-impedance designs. The mask-defined pad technique uses solder mask to define pad size.