Why are Circuit Boards Traditionally Green?

If I ask you to picture the words “digital,” “electronics,” or “cyberspace,” odds are the images that just popped into your head are tinted a bright fluorescent green, like Neo seeing the Matrix for the first time. Indeed, our collective image of digital technology is drenched in shades of green, from the strings of code that make up software to the circuit boards and other hardware that run said software. But why green? In the case of computer code, the answer is simple: early computer monitors were monochrome displays based on a Zinc Silicate phosphor known as P1, which glows bright green. Such displays quickly became emblematic of the early digital age, and association persisted to the present day. When it comes to printed circuit boards, however, the reasons for their iconic green colour are rather more complicated, involving a combination of chemistry, human eyesight, military requirements, and plain old industry inertia.

It is worth pointing out that circuit boards themselves aren’t actually green – at least, not all the way through. Printed circuit boards or PCBs are typically made from a material called phenolic paper, composed of wood fibres mixed with phenol-formaldehyde resin. This material typically ranges in colour from pale tan-green to dark butterscotch brown. To manufacture a PCB, a thin layer of copper is first bonded to the phenolic paper substrate. A resist or mask bearing the desired pattern of electrical traces and component attachment points is then screen-printed onto the copper, and the board immersed in an etching solution like ferric chloride. This solution eats away all the copper not protected by the resist, leaving only the pattern of traces. In older and amateur-built boards, electronic components are attached by drilling holes through the board, passing the component mounting pins through the holes, and soldering the pins to the copper traces on the other side. More modern boards, however, use surface-mount techniques, in which the components are soldered directly to the traces. This method is quicker, less wasteful, and allows components to be more easily mounted to both sides of the board.

In older PCBs, this is where the manufacturing process ended. However, as electronic components grew ever smaller and circuit patterns denser, a number of problems emerged. For example, thinner circuit traces tended to break or oxidize easily, while soldering increasingly tinier component mounting pins often resulted in unwanted solder bridges and short circuits. To correct these problems, starting in the late 1970s circuit board manufacturers began covering PCBs in a protective layer called a solder mask, and it is this layer which gives circuit boards their distinctive green colour. Solder masks are made of pigmented phenolic resin, and like the resist in the PCB printing process are applied using silk-screen techniques. This process coats the entire board except for the component mounting points in a thin layer of resin, which is then exposed to ultraviolet light to cure and harden. This protects the circuit traces from damage and corrosion and creates small circular solder dams that prevent solder from flowing from one mounting point to another.

But this still doesn’t answer the question of why circuit boards are traditionally coloured green. After all, circuit boards can be – and very often are – printed in a wide variety of colours, from green to red, yellow, white, black, and even purple. For example, boards in Apple products are typically printed black, while boards in developmental products are often printed red, only “going green” once a product reaches full-scale production. Similarly, in many computers the motherboard is coloured green to indicate its importance while other boards are colour-coded according to function. Yet the most popular colour for circuit boards is still far and away dark green. There are many reasons for this particular choice, the main one being related to quality control. In the early days, circuit boards were inspected entirely by eye, and a colour was needed which both maximized contrast between the board and the circuit traces and reduced eye strain for the inspectors. The raw resin used in early solder masks was a dark honey brown, and produced poor contrast with the copper circuit traces. Adding red pigments created an even lower-contrast rusty colour, while adding blue simply yielded a darker brown. Finally, manufacturers hit upon a combination of blue and yellow pigments which produced a dark green tint that met both the contrast and eye strain criteria. This finding is consistent with the fact that human vision is most sensitive to – and best able to discern between – shades of green. According to evolutionary biologists, this is likely an evolutionary holdover from our primate ancestors, who needed to be able to discern between multiple shades of green in order to navigate dense forests and jungles.

The widespread adoption of green as the standard colour for circuit boards was further driven by the requirements of the United States military during the Cold War. A series of trials conducted in 1954 at the National Materials and Procurement Center in Cedar Bluffs, Virginia determined that white text on a green background was the most legible colour combination under a wide variety of lighting conditions. Thus, when electronics manufacturers began applying solder masks to circuit boards in the 1970s and 80s, the military demanded they be tinted green. And as the military was often these companies’ largest client, green became standard for civilian circuit boards as well. While today circuit boards are no longer inspected by human eyes, the white-green colour combination nonetheless produces maximum optical contrast for the electronic inspection systems currently in use.

There are other practical reasons why green circuit boards have remained the industry standard. For example, other colours like black and blue are achieved using pigments containing carbon and cobalt which, being slightly conductive, can potentially cause current leakage and short circuits. Green pigments, conversely, typically contain chlorine and bromine, which are far more insulating. More importantly, green solder mask simply works better than other colours. Green is the only colour of solder mask that can consistently produce solder dams as small as 0.1 mm in diameter – compared to red, yellow, and blue at 0.12 mm; or black and white at only 0.15mm. Strangely, however, this superior performance has almost nothing to do with the chemistry of the green pigments themselves. Rather, when manufacturers sought to optimize the performance of solder mask resin, the majority of research and development focused on the most commonly used formulation – which, for the reasons previously mentioned, just so happened to be green. So while green solder mask was originally developed for practical reasons, its absolute dominance of the electronics industry happened more or less by accident.

And the industry has little incentive to change this verdant status quo. In addition to green solder mask simply performing better, most circuit board assembly lines are only equipped with a handful of solder mask application stations. Switching production from green to another colour or circuit board involves the laborious process of shutting down the equipment, scrubbing out all traces of the previous resin colour, and loading in a new resin. This process, along with the short shelf life of solder mask resin, results in significant production delays, materials wastage, and other logistical headaches. It is thus more efficient for manufacturers to stick with a single standard colour, only switching over when absolutely necessary. And since circuit boards are typically hidden away within a product’s casing out of sight of the end user, most clients are more than happy to stick with the ubiquitous green colouring. About the only serious challenge to green’s dominance of the market is concern over environmental toxicity. As previously mentioned, the green tint of solder mask is achieved using chlorine and bromine pigments, which when burned can release toxic fumes. Many companies thus offer “halogen free” circuit boards printed in other colours, which, are, amusingly, both “green” and not green at the same time. However, such products remain only a small part of the market; everywhere else, green still reigns supreme. So, in the end, the answer to the question “why are circuit boards green” is simple: if it ain’t broke, don’t fix it.

Bonus Facts

Circuit boards are far from the only common objects inextricably linked to a particular colour. For example, have you ever wondered why barns, London double-decker buses, and fire hydrants are traditionally red, while pencils and taxis are yellow? Well, wonder no more.

The traditional reddish hue of barns comes from early efforts to prevent the wooden structures from rotting in wet weather. Starting in the 18th Century, farmers experimented with various coatings composed of linseed oil mixed with milk, lime, turpentine, and other additives before hitting upon iron oxide or rust powder, which provided excellent protection against moss, fungi, and other microorganisms. Though more modern and effective wood preservation techniques have since been developed, the tradition of painting barns bright red continues to the present day.

The reason for painting fire hydrants red is even simpler: to make them more visible to firefighters and other first responders. However, it might surprise you to learn that in the United States, the standard colour for fire hydrants is no longer red, but rather yellow. According to the National Fire Protection Agency’ s NFPA 291 code, fire hydrants must be painted factory-standard chrome yellow unless a different colour has already been standardized in the community where the hydrant stands. In addition to being highly visible, chrome yellow also serves to protect the hydrant from corrosion. Surprisingly, the colours painted on the nozzles and caps of fire hydrants also serve a practical purpose. These colours indicate the rated water-flow capacity of the hydrant, with red indicating less than 500 gallons per minute, orange 500-999 gallons per minute, green 1,000-1,499 gallons per minute, and blue 1,500 or more gallons per minute. The more you know…

But few uses of red paint are more iconic than on London’s fleet of double-decker buses. However, unlike the previous examples we have covered, this distinctive livery was chosen not for reasons of corrosion protection or safety, but rather branding. By the early 20th century, London was served by a large number of competing omnibus companies, each with its own distinctive livery. In 1908, the largest of these companies, the London General Omnibus Company, or LGOC, painted its entire fleet bright red in order to stand out from the competition. When LGOC was absorbed into the government-run London Passenger Transport Board in 1933, the red livery was adopted as standard and quickly became a British icon, recognized the world over. The same standard shade of red, Pantone 485 C, is shared by the equally-iconic red telephone boxes and Royal Mail pillar post boxes, though in these cases the colour was primarily chosen for its high visibility and not brand identity. Indeed, prior to 1874, there was no standard colour for British post boxes, with most being painted bronze green to make them unobtrusive. Unfortunately, they proved a little too unobtrusive, for people kept walking straight into them. By 1884, every post box across the UK was repainted bright red, a tradition that persists to this day.

Another vehicle with an iconic paint scheme is the yellow taxicab, a fixture on the streets of New York, Chicago, and other American cities. Like London buses, this distinctive livery originated from a desire to stand out from the competition. In 1908, Albert Rockwell founded a taxicab company in New York City and decided to paint his entire fleet yellow. Why yellow? According to legend, it just so happened to be his wife’s favourite colour. But whatever the reason, the yellow cabs proved highly visible on the streets of New York, and Rockwell’s business prospered, changing its name in 1915 to the Yellow Taxi Cab Company. That same year, car salesman John Hertz, namesake of the modern car rental agency, founded his own taxicab company in Chicago. Also wanting his fleet to stand out in traffic, Hertz consulted a recently-conducted study which revealed that yellow mixed with a bit of red was the most visible colour combination in all lighting conditions. Hertz’s Yellow Cab Company was also an instant success, and his vehicles and their distinctive livery were soon franchised across the United States.

At last, we come to perhaps the most iconic of manufactured objects: the humble pencil. Throughout the 19th Century, most pencils were left unpainted to show off the quality of the wood. Companies that painted their pencils usually did so to cover up imperfections, and thus tended to choose darker colours such as black or maroon. However, at the 1889 Exposition Universelle in Paris, Czech manufacturer Hardtmuth Pencil introduced a line of luxury pencils painted bright yellow. Unlike other manufacturers, Hardtmuth was not trying to cover up the quality of their wood but instead advertise the source of their graphite. At the time, the highest-quality graphite came from China, where the colour yellow has long been associated with royalty, happiness, glory, and wisdom. To further emphasize the luxury of the product, Hardtmuth named the new line of pencils Koh-i-Noor after the famous 105-carat diamond discovered in India in 1849. So successful was this branding exercise that the company soon changed its name to Koh-i-Noor Hardtmuth, and manufacturers around the world began painting their pencils yellow and giving them oriental names to suggest the same level of quality. The ubiquity of yellow pencils eventually resulted in the colour losing its prestige status and becoming associated with cheap, consumer-grade products. Nonetheless, which pencils are today manufactured in a whole rainbow of colours, around 35% are still painted yellow, all thanks to a clever bit of 19th Century marketing.

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