PCB design glossary
Soldermask
AKA: solder mask
The (usually green) polymer coating on a PCB that protects copper and exposes only the pads.
Soldermask is the polymer layer applied over copper to prevent oxidation, prevent solder bridges during assembly, and provide insulation. Openings in the soldermask expose pads, fiducials, and test points.
Soldermask color is usually green by default but red, blue, black, white, purple, and yellow are widely available.
What soldermask does
- Protects copper from oxidation and scratches.
- Stops solder from flowing between pads and bridging them during assembly.
- Insulates traces from each other and from things touching the board.
Mask openings, expansion and dams
Every pad needs an opening in the mask. The opening is usually drawn slightly larger than the pad (the mask expansion, often around 0.05 mm, or left to the fab) so small misalignment does not cover the copper. Between fine-pitch pads, the thin strip of mask that remains is called a dam. If the pads are too close for the fab's minimum dam, the pads share one opening and bridging becomes more likely.
Colours and finishes
Green is the default and many fabs quote their finest mask resolution for it. Red, blue, black, white, purple and yellow are common options. Black and white make fine traces harder to inspect. Matte finishes reduce glare for photography and inspection.
See the mask openings the fab will see
Load your Gerbers into the viewer and switch the soldermask layers on and off to check that every pad is open and nothing else is.
Check mask in the viewer