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Green is the most common circuit-board color because it has become an industry standard.
What I'm interested in is How the traditional "PCB Green" become a standard in the first place?
Were there any interesting historical reasons for the initial choice, or was it just a product of what one particularly successful company was doing that became the de-facto standard?
This is what I have found on the topic so far.
There are a few competing theories for why the solder mask of PCB is commonly green.
Possible explanations:
Digging deeper...
Liquid Photo Imageable Solder Mask (LPISM) technology was developed in the late 1970s and early 1980s to to meet the new application demands placed upon solder masks by the rise in surface mount technology. It seems that modern, green colored PCBs emerged with this technology, and the technology seems to trace back to this patent from 1980.
Consequently, endeavours have been made to produce improved processes for producing a mask image of relatively high resolution for the small-conductor art. It was therefore a relatively obvious step to use photo processes in association with UV (ultra-violet) sensitive photopolymers.
So basically, UV sensitive photopolymers were available and were the first to be used for LPISM. The polymer solution they used in the patent included 3g of dye, but did not describe the color of the dye or why they used it.
When developing an invention for the first time, it seems highly unlikely they would choose the dye or photopolymers because of the military's request or for ergonimic considerations, so we can rule those out. The most plausible explanation is that it was the most accessible, inexpensive and effective materials to be used in fabrication. For whatever reason, the UV sensitive photopolymers that were effective for this invention happened to be green at the time, and this material's proliferation is most likely due to its low cost. Alternatives do exist these days, and PCBs can be virtually any color.
I know this is all speculation, and I wish I could give a more definitive answer. I've read through patents and papers and Electronic Materials and Processes Handbook, but still haven't nailed it down yet. Maybe a PCB process engineer or researcher can help us here.
The following article Soldermask: It’s NOT JUST GREEN Anymore by Jody Williams that I've seen referenced in a few places lists some possible reasons, here's a summary:
However the article states it's based on theories from forum posts and none are proven to be correct. It seems mostly speculative but certainly a few and maybe all would make some sense.
From "The Printed Circuit Board" on W5TXR.net which contains an interesting history of the PCB, the green boards were a result of experimentation with various pigments to yield an acceptable color from the brownish colors obtained from the resin and hardener used in the original mask:
The original masks 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 apparently not very appetizing. They tried adding red pigments but it became a rusty adobe color and using blue simply made it a darker brown. None of them were very appealing colors. Since the laminate materials at the time had a green hue they tried adding more yellow and some blue and ended up with an acceptable green color. It became the standard color we are still using today.
The article may have originated with Advanced Circuits HERE
As a side note, our company like so many others always ordered green solder-mask on PCBs. When the RoHS directive came along, we began ordering our boards for our RoHS compliant products in blue so that they could be easily identified. It's a little ironic that the "lead-free" boards are the ones that are not "green" :)
