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Professional engineers and people who enjoy electronics as a hobby have often asked the same question: why are printed circuit boards green? While other colours can be found on occasion, the vast majority are green, leaving users wondering whether there’s a technical reason for this.
The simple answer is that it’s not a mandatory requirement to make green circuit boards. They can be made in a wide variety of standard colours including red, blue, yellow, white and black. More exotic PCBs are available on request, in colours such as pink, orange and purple. Even mixed colour boards can be produced.
Despite these possibilities, almost every PCB on the market is the same shade of green. There are several theories as to why this is the case including industry convention, the colour of the resins used to manufacture PCBs and the army’s role during the 1950s in the development of printed circuit boards.
What are printed circuit boards made of?
Printed circuit boards are used in many electronic devices, including household appliances, phones and medical equipment. They are typically made from non-substrate materials with layers of copper circuits.
While some PCBs contain just one layer of copper circuitry, more complex printed circuit boards may contain 50 or more. Common components of printed circuit boards include resistors to transmit an electric current to produce a voltage and capacitors to hold the electrical charge and then release it when needed.
Inductors also store energy but are commonly used to block signals, such as interference from other electronic devices. Transistors are amplifiers used to control the electronic signals in the PCB. The most common type is a bipolar transistor.
Transformers transfer electrical energy from one circuit to another, while diodes permit the current to flow in only one direction – so it can never flow the wrong way, damaging the circuit board. The sensors detect changes in environmental conditions, generating electrical signals to correspond to the change.
The thin copper layer of the printed circuit board is made from copper foil laminated to the board by heat. A solder mask layer is attached on top of the copper – this is what gives PCBs their green colour. The solder insulates the copper, ensuring it doesn’t come into contact with any other element.
Now read on to find the reasons why printed circuit boards are almost always green…
Superior green solder mask
The green solder mask is physically superior to other colours because the chemical pigments used impact the performance of the resin. Nobody cares about having pink boards when the green solder mask has superior properties!
The commercial green solder mask is the only colour that can produce reliable solder masks of 0.1 mm. Yellow, red and blue solder masks can achieve 0.12 mm, while black and white can typically achieve only 0.15 mm. Fine solder masks are vital to prevent solder bridges from forming.
Making inspections easier
Green can make inspections easier, as it’s a soothing colour that relieves visual fatigue. Due to technological restrictions in the early days of PCBs, quality inspections were completed by workers checking the boards, manually, just by looking at them.
Staring at the tiny circuits continually is tiring, but neurologists believe the colour green has a more relaxing effect on the eyes, reducing fatigue. In addition, research shows our eyes are more sensitive to green light.
Consequently, when checking the boards, it’s easier to see the contrast between the circuit traces, silkscreen printing, pads and empty spaces. This means any defects in the outer layers can be seen more easily when compared with other colours, such as yellow and white, for example, due to the greater contrast.
Today, PCB manufacturers can also use more technical methods of checking for defects, including flying probe techniques and Automatic Optical Inspection, but the trend for green boards remains due to this visual advantage.
Cheaper to stick to one colour
When manufacturing PCBs in bulk, it’s cheaper to stick to a single colour in production terms. Solder mask is usually applied using the silkscreen technique. This involves dragging oil across a screen mesh on top of the PCB.
The board is then removed for curing and the next board goes under the mesh. This is an efficient and cost-effective method of PCB assembly, when one solder colour is used. However, if another solder mask colour is required, the excess solder mask oil must be removed, and the silkscreen washed before the new colour can be applied. The only other option is having a silkscreen station for each different colour.
When it comes to using certain light colours, such as white, another station will be required for black silkscreen ink. If one colour isn’t particularly popular, the oils don’t keep for very long, so the chemicals will be wasted if it isn’t used again for some time.
As green is the most popular colour by default, it is more cost-effective to apply the solder mask in green, instead of wasting time and money switching to other colours that may not be in demand very often.
In today’s challenging economic climate, keeping costs as low as possible, without compromising on quality, is important to everyone.