Why are PCBs green? This is one of the questions I get asked when people ask me what I do for a living, especially after I explain that my company supplies the little green things that one will find in pretty much anything and everything you purchase today that you cannot eat or drink.
So…why are they green? I did not know, so I resorted to Google. My favourite answer was…for the same reason that jeans are blue and NY taxi cabs are yellow. Which actually is not strictly true.
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 colour contrast during assembly, through to green being environmentally friendly (hardly a deciding factor 100+ years ago?).
The earliest PCBs date back to the early 1900s, but only really became recognisable to the products manufactured today when used in early gramophone and valve radio designs of the 1920s. By the ’50s and ’60s, laminates containing different types of resins mixed together were being used, particularly by the defence agencies for new weapons & communications systems.
A mix of different colors
It was at that time that the solder masks being used generally contained 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 colour that was not particularly attractive. They tried adding red pigments which then turned this to a rusty colour. Adding a blue pigment, simply made this darker brown. None of these experiments resulted in very appealing colours. 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.
So, it seems, this is the most logical reason why PCBs are green, and perhaps you will also conclude, a little bit of a dull reason.
However, much more interesting…
Why are jeans normally blue?
Natural indigo dye binds externally to the cloth’s threads, and with each washing, some of these dye molecules are stripped away in a process which softens rough fabrics and individualises the colour. That, combined with the robustness of denim (originally a silk-wool blend from France, Serge de Nimes, which was shortened to De Nimes) sold like hot cakes in the 1870s to gold rush prospectors requiring a durable uniform whilst seeking their fortunes. And thanks to this, jeans rapidly came to symbolise the character of the wild American west.