solder mask
A thin lacquer-like layer of polymer that is usually applied to the copper traces of a printed circuit board for protection against oxidation and to prevent solder bridges from forming between closely spaced solder pads.
solder mask: polymer layer that protects PCB traces and prevents shorts
Solder mask is a thin polymeric coating applied selectively to printed circuit boards (PCBs) to cover copper traces, vias, and most of the board surface, leaving only the solder pads exposed. It serves two critical functions: it prevents oxidation of the underlying copper, which degrades electrical conductivity and solderability over time, and it acts as a physical barrier during soldering to prevent solder from flowing where it should not, a failure mode called solder bridging.
The material is typically an epoxy or acrylic photoresist polymer, applied as a liquid and then UV-cured through a photomask that defines which areas remain exposed. The mask is usually green, but red, blue, white, and black variants are used for identification or aesthetic purposes. Thickness ranges from 15 to 50 micrometers. Some manufacturers use liquid photoimageable (LPI) masks, while others apply dry film photoresists or use a screen-printing process with smaller batch volumes.
During wave soldering or reflow soldering, the exposed solder pads melt and form joints to component leads or pads, while the masked copper underneath stays dry and protected. Without a mask, molten solder would wick across unmasked copper traces, creating shorts between signal lines and power planes. The mask also reduces the risk of dendritic copper growth, a failure mechanism in high-humidity environments where metallic whiskers form and bridge adjacent traces.
Common failure modes and material limits
Mask coverage defects, such as pinholes or incomplete edges, leave copper vulnerable to corrosion or solder bridging. Thermal stress during soldering can cause the mask to blister or crack if adhesion to the substrate is poor. The mask must also survive cleaning processes, whether aqueous or solvent-based. IPC-A-600 defines acceptance criteria for mask coverage, surface cleanliness, and dimensional tolerances; most production boards are inspected for mask defects under magnification at 2x to 10x.
Solder mask sits between the base laminate and the final solder paste or wave process. On multilayer boards, it covers all external surfaces; internal planes remain unmasked because they are never soldered. The choice of mask type and curing method affects turnaround time, cost, and the precision of fine-pitch designs, where spacing between pads may be 0.15 mm or tighter and mask edge definition becomes critical to yield.