While PCBs and other circuit boards come in more colors than ever, the majority of them come in a signature green color. PCB is not actually green all the way through. The only green part is the outer covering of resin called the solder masks or solder resist/oil. This is a hardened resin with colored pigments that is applied to the boards in a silkscreen fashion. The purpose of solder mask is to protect the electronic traces underneath from moisture and dust and to control the flow of molten solder.

The actual core of a typical FR-4 circuit board is a plain, dull, yellow color, but the solder mask can come in many colors such as blue, red, yellow, black and white. Even more exotic colors can be found for the extravagant such as orange, pink, purple, matte versions and even mixed color boards. So, we will focus on why PCBs are green in the content below.

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What are the reasons why PCB is green?

Physical advantage of green PCB

As green is proclaimed as the “standard color”, researchers have been analyzing through R&D to make better solder mask oils based on the color  green. The actual chemical pigments used will influence the performance of the resin during PCB application. As a result, the demand for green solder mask increases and it has superior properties as compared to other colors.

Due to the traditional aspect of using green pigments, R&D into making better solder mask oils focused on the standard green color. The actual chemical pigments used influence the performance of the resin during application and in the field. When demand is pushing form factors to their limits, no one cares about having pink boards. As a result, green solder mask now has superior properties compared to other aesthetically orientated colors.

Most prominently, commercial green solder mask is the only available color that can reliably produce solder mask dams of 0.1mm. The most prominent factor of choosing green solder mask is that it is the only available color that can reliably produce solder mask dams of only 0.1mm. Next up is red, yellow and blue that can produce 0.12mm dams and then black and white which can typically only achieve 0.15mm. Solder mask dams are vital for ICs and fine pitch components since they are valuable in preventing solder bridges from forming.

The green in most green soldermask oils comes from chlorine. Chlorine, along with Bromine, which is found in PCB substrates, are halogens which have a severely detrimental impact on health and the environment if disposed of inappropriately. Halogen free soldermask oils and substrates are available. For halogen free solder mask, you could just switch to another color.

Contrast advantage of green PCB

As technology advances, PCB manufacturers nowadays use flying probe techniques and Automatic Optical Inspection (AOI) to perform quality inspections. However, there are other technical reasons why most PCBs manufacturers prefer green PCBs.

In the early days, due to technological restrictions, quality inspections relied on workers manually checking the boards with their bare eyes. Squinting at tiny circuits all days is tiring work, but neurologists and psychologists agree that the wavelength of green light has relaxing effects on the body and can reduce fatigue. Additionally, they have found that the sensors in human eyes, or cones, are most sensitive to green light. Therefore, the contrast is greater between the circuit traces, pads, silkscreen printing and empty spaces. Just by observing the boards from the outside, one can easily identify defects in the outer layers. Compare the below images of green boards to other colors such as blue, yellow or even black and white. With higher contrast, errors are easier to spot.

Neurologists and psychologists studied and agreed that the wavelength of green light has a relaxing effect on the body, enable to reduce human fatigue. In addition, studies have shown that the sensors in human eyes are most sensitive to green light. Thus, the contrast for green is much better to see circuit traces, pads, silkscreen printing and empty spaces. Workers can easily identify defects by observing the boards from the outside.

Of course, nowadays PCB manufacturers use flying probe techniques and Automatic Optical Inspection (AOI) for these tasks, which is very effective in spotting errors. But there is another very important technical reason why most PCBs are green.

Price advantage of green PCB

Solder mask is created using silkscreen technique. This method involves a large blob of oil applied on a screen mesh with the circuit board underneath. After application. the circuit board is then removed to dry up, and the next circuit board will be placed under the mesh.

In an scenario where a company need different colour solder mask, the excess solder mask oil initially used must be removed and the silkscreen must be washed before applying new colour. Another solution to save such hassle will be the installation of another silkscreen station for other colors. However, the chemicals will be wasted if it is not often used. Time & efficiency will also be impaired.

Some factories will not accept certain combinations of PCB features with colors other than the standard green color. Many designs from various orders are grouped together on one panel. This can make the most out of the PCB real estate and reduce waste. However, there are many processes that require the entire panel to be processed as a whole, such as surface finishes and plating for castellated holes. Therefore, all the circuit boards on a particular panel must have the same requirements. By adding more and more specific features and combinations (for instance solder mask color or board thickness) will cause the manufacturers to end up with insufficient designs to fill a panel at a same time.  Cost will also increased significantly. Therefore, PCB factories will keep their costs down by restricting certain combinations of colors and features.

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