Soldermask

Soldermask, also called solder resist, is a thin polymer layer applied to printed circuit boards (PCBs) to insulate copper traces and restrict solder wetting during assembly. By covering copper areas except where solder joints are intended, the mask reduces the risk of short circuits and solder bridges, protects copper from oxidation, and provides mechanical protection. The openings in the mask expose pads, vias, and edges where soldering or testing is required.

Most boards use photosensitive solder mask, typically epoxy- or acrylic-based polymers. Two common forms are liquid photoimageable (LPI) masks, which are spun or coated and then UV-exposed through a mask, and screen-printed masks, which are applied as a paste and cured. Mask colors vary, with green being most common, but red, blue, black, white, and yellow are also used.

Mask openings, or apertures, define where solder will contact copper. Pads can be mask-defined (mask covers part of the pad) or non-mask-defined (NSMD), where the copper pad is larger and the mask clears around it; NSMD is often preferred for fine-pitch assemblies. Mask relief around vias, castellations, and test points should balance solderability and masking. Adequate clearance and avoidance of mask slivers help reduce defects and bridging.

Manufacturing involves applying the mask as a liquid or paste, aligning with the PCB pattern, exposing/curing, and developing to remove masked areas. The mask must withstand soldering temperatures and flux exposure; rework may require masking steps or tenting vias. Solder mask is a standard feature in modern PCB fabrication and is specified during design.

Limitations include adhesion issues and potential delamination under thermal cycling, as well as possible contamination of pads if openings are misdefined. Industry standards and fabrication notes guide mask material selection, thickness, and process tolerances.