Green Machine: Why are PCBs always green?

If you're ever lucky enough to visit the ACTRONICS remanufacturing department, you'll notice two very distinct colours. The purple workwear that our staff proudly wear, and the shiny green of the printed circuit boards fitted in almost every electrical component. But have you ever wondered why PCBs are green? We did, and we set off to understand why! 

It's probably a good idea to explain what that familiar green plate actually does. This layer, which is almost always green, is a solder mask. The mask, which is applied ot the copper traces of a PCB, is used ot insulate and protect against oxidation and to prevent solder bridges. What's a solder bridge? That's when an inadvertent electrical connection is made between two conductors caused by soldering tin. 

A solder mask is essential for PCBs that are automatically soldered using reflow or wave soldering techniques, and the various components are then placed on these ready-protected green circuit boards. 

So why green? 

It does seem odd that the overwhelming majority of PCBs are supplied in the same colour. Though there are various theories, here are some of the most likely:

• The colour is the result of old military specifications
• At the time of introductions, most laminates were already green, so a natural choice was green.
• Green provides a great contrast if you need to print white text. 
• The original mix of hardener and base resin resulted in a green colour. Even though technology moved on, the colour remained. 

But which is correct? Or is it an amalgamation of all these stories? The green is said to be the result of various experiments to make circuit boards appealing to the eye. 

The original resin and hardener combination resulted in an unattractive brown, so developers were tasked with coming up with a nicer colour. Testing red and blue pigments didn't much improve the result. Success was found with the addition of yellow and blue pigment to result in a green that closely resembles what we have today. 

Are other colours available?

As manufacturing techniques developed, so did the variety of coloured PCBs available. Especially in the personal computing space, where manufacturers tried to appeal to their target audience with special and unique features, which often include uniquely coloured PCBs.

In terms of automotive applications, units like the Delphi Delco Multec ECU utilise a blue PCB, but the overwhelming majority remain with green boards. Perhaps as much out of familiarity as anything. 

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