Solder mask

Solder mask is the thin protective layer that gives most circuit boards their color. It covers the copper traces and leaves openings only at the pads where parts get soldered. That stops solder from bridging between neighboring traces and protects the copper from scratches, dust and corrosion.

Most solder mask is a liquid photoimageable ink with three key ingredients. Epoxy resin makes it tough and able to survive soldering heat. Acrylate resin makes it react to light: where ultraviolet light hits, the acrylate links up and becomes insoluble. Green pigment gives the classic color. The ink is coated over the whole printed circuit board by screen printing or spraying, then dried. A film or a laser exposes it to UV light everywhere except over the pads. A mild alkaline wash removes the unexposed ink, opening up the pads, and a final bake cures the epoxy hard. The finished layer is a few tens of micrometers thick at most.

Why green? It's partly tradition and partly practical: green gives good contrast for inspecting boards by eye or camera, and green formulas are the most refined. Solder mask also comes in red, blue, black, white and other colors.