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Some interesting facts about Printed Circuit Boards?

Have you ever wondered why Printed Circuit Boards are commonly coloured Green?

Technology
09/03/2022
Why are Circuit Boards green

Have you ever wondered why Printed Circuit Boards are Green?

When you think of pop culture today and anything related to computers, most people associate computer technology with a bright green colour. When you close your eyes, I am sure you will imagine the falling numbers in the Matrix film series. This colour hasn’t always been the case, as cyan and red are other depictions of future tech and computer-based virtual reality. Like Tron, Ready Player One or Terminator. So why is it that the green seems to fit this narrative better? That is because of printed circuit boards and the original colour of pc monitors that hold a real tangible relationship between fiction and fact. Monochrome monitors were green because they were made to combine strain-free readability with low cost. However, the reason circuit boards are green is slightly more complex.

Circuit board construction

The main reason that circuit boards are commonly green behind your membrane keyboards is a combination of evolution, chemistry, military requirements and industry inertia. Firstly, the circuit boards and PCB’s actual colour isn’t green. Being made from a material called phenolic paper, which uses a combination of wood fibres and phenol-formaldehyde resin, these components’ colours vary between light tan and brown.

First, a layer of copper is applied to the board’s surface. Then a circuit trace mask is printed over this copper. All copper not protected by the ink is then etched away in an acid bath, leaving the circuit diagram intact. Over the years, these trace patterns have become more and more complex, making them delicate and easy to damage. That is why, during the 1970s, PCB manufacturers started adding additional protection. The green colour is a screen-printed protective layer called the solder mask. This solder mask is printed across the circuit board except for the mounting points then exposed to ultra-violet light. The printed resin hardens to protect the circuit traces from damage and corrosion. This screen printed protection creates solder dams that prevent electrical bridging and component failure.

But why green?

Any colour can and has indeed been used over the years in circuit board design. However, the green colour, like with the early pc monitors, was found to have the best colour contrast between board and traces, offering less eye strain for inspection. Research has stated that green is the colour the human eye is most sensitive to and best suited to recognise. Evolutionary scientists claim green eye sensitivity is due to our primate accessors’ need to discern the many differences in dense jungle foliage for survival.

In addition, US military experimentation and testing found that white text on the green background was also the best combination in various extreme lighting conditions. As the military was one of the leading industries requiring electronic components during the cold war, this soon became the industry standard.

Conductive Pigments

A further reason for the solder mask to be green is based on the pigment used in the printing process. Black pigment may contain carbon, and blue may contain cobalt. These elements are more electrically conductive, leading to current leakage and failure. The green ink contains chlorine and bromine that are far more insulating and further protects the component. This naturally adds to the life expectancy of your membrane keypads. Additionally, green has a more exemplary tolerance concerning creating smaller solder dams, not because the pigment is superior, but simply because it has more development over time; green has been the industry standard for decades.

The final point I alluded to was industry inertia. This inertia is a straightforward observation; when preparing the screen-printing machinery for a component that isn’t seen by the end-user, why change the colour. The time it takes to clean out the machinery and prepare a new colour seems like a waste of time. Furthermore, the shelf life of these printable resins is relatively short. So, as green works best, there is no real augment to change the status quo.

Now you understand why circuit boards are screen printed green, all you need to do is contact RH about your next Membrane Switches project, and connect the two!