Although modern PCBs can be red, white, blue and many other colours, the vast majority are still a traditional green. If you’ve ever wondered why that is then we have the lowdown…
The outside of PCBs are usually screen printed with a solder mask. This is a thin layer of polymer which protects the copper tracks from damage caused by oxidation, moisture and dust. The mask prevents solder bridges which can form between solder pads which are located close together and cause short circuits.
The solder mask could be any colour, but most are green, and there are various theories why this is.
Glass epoxy is green
One of the most obvious answers is that glass epoxy, which was originally used to make PCB solder masks, is green. Nowadays, green is still the most common colour, and therefore the cheapest to produce, so it is still the default colour used on 21st century printed circuit boards.
American Military
Green was the regulation colour for PCBs ordered by the US military because it withstood the adverse conditions applied to them the best. Manufacturers who supplied the military were therefore well stocked with green solder mask so it was the cheapest option for their non-military customers.
We aren’t sure whether this is the real reason for adopting green as the default colour for PCBs, but use of through-hole plating during World War II for anti-aircraft weapons was subsequently patented with US Army classification. After the war, soldering techniques were pioneered by Moe Abramson and Stanislaus F Danko of the US Army Signal Corps and a subsequent patent was assigned to the army.
Since the US military commissioned significant quantities of PCBs in that post-war era, it’s a reasonable assumption that they were the leaders in their field and others wanted to reproduce the technology, as well as the colours they used.
Ease of inspection
Another practical reason for choosing green is the contrast it provides, as well as the relaxing effect it has when scrutinising boards, looking for faults.
Green is considered the best colour for inspecting printed circuit boards, to avoid fatigue by those who spend their days checking for issues. Inspectors usually use bright lights to highlight any flaws, so anything which can reduce the need to squint is helpful.
These days, automatic testing equipment means that there are fewer manual inspections required in the manufacture of a PCB, but it hasn’t been eliminated completely. Therefore, anything which can be done to minimise the visual impact of manually inspecting boards is worthwhile doing.
Green is cheaper
Cost is a major factor when manufacturing printed circuit boards, and since green is the most popular and common colour, it’s therefore the cheapest. Its base cost is cheaper than other colours but there are also economies of scale. When screen printers are applying the solder mask it’s much more efficient for them to spend their days using a green epoxy coating. If they have to change their setup to use a different colour then there is a delay, and therefore cost, which is passed on to the customer.
Contrast
A white silk screened ident is often printed on the component side of rigid boards. Green is the best contrast for this layer, to allow for human readable text.
In fact, this theory was closest to the mark according to Peter Skippings who was one of the joint founder of Artetch Circuits in the late 1960s. The conventional PCBs that Peter’s team made were screen printed, etched, printed with green solder resist and then roller tinned to the exposed copper pads on the board. The solder resist protected the copper tracks on the PCB from the heat of the roller tinning process. Peter said that green was the only colour available, and it provided the heat resistance needed, which is why that was all they used.
So it seems there isn’t a single reason why PCBs are green, but like most products it’s often the colour used for mass production which proves to be the most cost effective.
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