Why is the solder mask on PCBs typically green in color?

At Advanced Circuits, we often get asked as to why the solder mask on almost every printed circuit board (PCB) is green in color. There seems to be any number of unsubstantiated reasons for the green PCB color ranging from the involvement of the US government in developing and using the first circuit boards to the how easy it is was on the eyes during the long hours of hand assembling the first circuit boards.

The most believable explanation we’ve heard is that the original solder mask inks used a base resin that was brownish yellow in color and a hardener that was a deeper muddy brown. When these colors were mixed together they created a yellowish brown color that apparently was not very appealing .

To enhance the PCB color, red pigments were then added but the the rusty adobe color was still found to be undesirable. The next step in this color evolution was to add blue pigments but this only make the solder mask a darker brown. It was then determined that since the laminate materials at that time had a green hue, a suitable combination of yellow and blue pigments were added, eventually ending up with an acceptable shade of green that became the precursor to the standard color we use today.