Why are circuit boards green? It’s a matter of practicality. While circuit boards aren’t always green, green provides technical advantages. So that makes green the most common color for printed circuit boards, or PCBs.
The fiberglass the printed circuit board itself is made of tend to naturally be tan. The copper traces and ground planes are, of course, a metallic brown-gold color, unless they get oxidized. Then they turn green. But circuit boards aren’t green because the copper is oxidized for some reason.
The green part comes from the outer layer. That outer layer is called solder mask.
Solder mask is usually green. Solder mask is the protective layer that PCB manufacturers silkscreen onto both sides of the board. It prevents accidental short circuits during the manufacturing process, and protects traces from oxidation in the longer term. Solder mask is made of epoxy resin that cures under UV light. Green PCBs are the industry standard because green solder mask is the most common type in use today. And it has been since sometime in the 1980s.
Why not green?
Green is the optimal color, for reasons we’ll get to in a minute. That said, different colored PCBs do exist. Preproduction boards for R&D purposes often are red, to distinguish them from production. Outside of R&D, sometimes manufacturers use a different color on premium products to make them look distinctive. The downside is they cost more and are harder to work on. Few people care what color their PCB is. They want it to work, and for the cost to be reasonable. Many retro products come on unusual colored PCBs, and when you watch build videos, inevitably the host mentions how much harder it is to find solder bridges, missed solder points, and other mistakes on those distinctive PCB colors.
Delivering reliable devices at a lower cost is more important, ultimately, than distinctive colors on an internal component the majority of consumers will rarely, if ever lay eyes on. Enthusiast-grade PCBs may remain visible because of case windows and lighting, but most consumer products don’t have see-through cases, so the circuit board remains hidden from view.
When circuit boards weren’t green
Early computer circuit boards tended to be tan, reflecting the natural color of the boards themselves. The traces were whatever the natural color of the metal they used.
Brown was another common early color, because early solder mask was brown. A lot of IBM circuit boards from the early 1980s tend to be brown. But it didn’t have enough contrast to make quality control fast, cheap, and easy. If you were IBM, that didn’t necessarily matter as much because the of the high cost and high profit margins. So they didn’t necessarily have much incentive to be the first to change. But if you wanted to beat IBM on price, you needed an advantage. You were probably going to take every advantage you could get.
Eventually IBM started using green too, but it was generally the people who really wanted to beat IBM on price who switched to green first.
Why is solder mask green?
The green color gives a higher contrast between the traces, solder joints, components, and the board. This made quality control and quality inspections easier in the early days when all inspections happened by human eye. It reduced eyestrain and visual fatigue and made defects stand out more easily to bare eyes. The cones in human eyes distinguish between shades of green better than any other color. Neurologists believe this gave humans an evolutionary advantage.
But besides that, the United States military wanted green PCBs to make field repairs under varying conditions as easy and practical as possible.
Not only that, the chemical pigments that go into green solder mask, chlorine and bromine, tend to be more inert than the chemical pigments that go into make other colors like black, which can be conductive. The whole idea behind solder mask was to prevent short circuits, not create them.
It also happens that green solder mask can make smaller solder mask dams. Solder mask dams make it easier to keep components and solder in place, and smaller dams better accommodates SMT manufacturing and ever more tiny circuits as time and technology marches on. So that was a happy accident, but it provides added inertia behind green solder mask and green PCBs.
Quality inspections no longer take place by human eye, at least not on a large scale. But the sensors in modern automatic optical inspection equipment are also tuned to the wavelength of green light, because there was no good reason to change.
Why circuit boards are green: In conclusion
The push for inexpensive yet reliable electronics isn’t going to go away anytime soon. There are times when brand recognition is important enough to make it worthwhile to use a different color, but that is the exception. Even a lot of premium brands of the past, like 3Com, used green circuit boards. Brands that styled themselves as premium while keeping their costs as low as possible, like Sound Blaster, did the same. If all went well, the purchaser only saw the board for a few minutes and didn’t think about the color again after they finished installing it.
Most of the time, consumers just want something that works well at the lowest possible price. And it is much easier and cheaper to distinguish yourself using the outwardly visible components.
The industry is optimized at this point for green solder mask and green circuit boards, so there is little reason to change. Certain companies will buck that trend, but they are the exception, not the rule.