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?

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    Traditional boards are brown :-p Actually I don't know why green, but there is glass fiber in the ones I think you are talking about and if you look at a sheet of glass from the side, it colors green too. Maybe that has something to do with it? Therefore my vote at this moment goes to impurities in the glass. – jippie Sep 18 '13 at 8:38
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    The board itself isn't green (at least, not usually), it's the solder mask that's green. Scrape it off or look at the cut-off edge of the board and it's a brownish white. – Johnny Sep 18 '13 at 12:13
  • I guess it is not entirely clear what is being asked here. Wonder how it feels to put a +22 question on hold ;o) – jippie Sep 18 '13 at 13:52
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    @MichaelKjörling Newly protected questions means my mentor is around. And of course I was just kidding about closure/hold, hence the ;o) – jippie Sep 18 '13 at 16:30

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:

  1. The US military required PCBs to be green
  2. When mixing the base resin and the hardener together, they turn green
  3. It is an ergonomic choice due to the human eyes ability to detect green, and the contrast of green with white text
  4. Some combination of the above

Source: Thefreelibrary

Source: Quora

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:

  • Based on old military requirements.
  • The most popular resin and hardener at the time came out that colour and the laminates at the time were green as well making it easy to accept.
  • Green has the highest human eye response so gives the best contrast. One of those mentions a study by the US military but I haven't been able to find a definitive reference to the study.

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.

  • The color and contrast point is actually the major one. The most obvious thing is: it needs to contrast not only for the human eye, but also against the color of bare pads and non-soldermasked areas. Polycarbonate being grey-yellowish, copper and gold being yellow-brown and solder being silvery, green had the best contrast. – user36129 Sep 18 '13 at 8:54
  • The "natural colour of the most effective resin" theory gets weight from the table of exposure guidelines. It looks like a common UV wavelength of lamp is ( uvp.com/elseries.html ) about 250um. Twice that is about 500um, which looks green. Therefore green may be more sensitive to UV exposure lamps, resulting in better printing. – pjc50 Sep 18 '13 at 9:01
  • Regardless of the origin, likely the reason for the prevalence now is the lower cost, in relation to other colors, that is probably due to legacy supply/capacity to produce being better than the others. – scld Sep 18 '13 at 12:29

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" :)

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    Why is that ironic? Lead is grey, and it turns paint white. If green PCBs were in standard use long before RoHS standards existed, parts with lead in them would be more common on green boards, especially old ones. – Ace Frahm Sep 18 '13 at 15:42
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    It's just a play on words ... "green" is a common term for "environmentally friendly" – Tut Sep 18 '13 at 15:44
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    Honey brown is not appetizing? Let's play some free word association: electrical engineer -> beer -> honey brown lager. Hello? – Kaz Sep 18 '13 at 23:22
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    @Kaz I'm with you ... just quoting the source. They must have been Chemical Engineers :-) ... If it had been us, maybe the standard color would be brown. I just bottled some Honey Nut-brown Ale! – Tut Sep 19 '13 at 0:54

protected by Kortuk Sep 18 '13 at 13:51

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