Solder mask, also known as "solder resist" is a strong, permanent layer of semi-transparent polymer that protects copper traces and the interfaces between them on printed circuit boards (PCBs). The main function of a solder mask is to prevent conductive solder bridging between different electronic components and causing short circuits. Although traditionally green, it is now available in other colors.
Why is green color soldermask common?
A lot of different theories
It seems there are numerous theories on why a PCB is normally green, ranging from USA Army deciding that it would be their standard, which then became a global norm, through to green being easy on the eye for color contrast during assembly, through to green being environmentally friendly (hardly a deciding factor 100+ years ago?)
The earliest PCB’s date back to the early 1900's, but only really became recognizable to the products manufactured today, when used in early gramophone & valve radio designs of the 1920's. By the 50's & 60's, laminates using different types of resins mixed together were being used, particularly by the defense agencies for new weapons & communications systems.
A mix of different colors
It was at that time that the solder masks being used generally used a base resin that was a brownish yellow and a hardener that was a deeper muddy brown. When they were mixed together, they created a honey brown color that was not particularly attractive. They tried adding red pigments which then turned this to a rusty color and then, using a blue pigment, simply made this darker brown. None of these experiments resulted in very appealing colors. Since the base laminate by that time had a green hue, they tried adding yellow and blue pigments and ended up with an acceptable green, which then became the standard that we see today.
