Green is the colour of the solder mask, which protects the copper circuits printed on the fibre glass core to prevent short circuits and soldering errors. There are a few theories as to why PCBs are green, the most common are:
- 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 printed circuit boards today.
- In the early days, due to technological restrictions, quality inspections relied on workers manually checking the boards with their bare eyes and green is considered the best colour for inspecting printed circuit boards, as the wavelength of green light has relaxing effects on the body and can help reduce fatigue.