What is a PCB Solder Mask?

A printed circuit board (PCB) solder mask is a thin lacquer-like layer of polymer applied to the copper traces of a PCB. The solder mask serves several important functions:

1. It protects the copper traces from oxidation and corrosion.
2. It prevents solder bridges from forming between closely spaced solder pads.
3. It provides electrical insulation between conductors.
4. It helps the user identify various components and test points on the board.

The solder mask is applied to the PCB and then selectively removed to expose the copper at the solder pads and contact fingers where components will be soldered or the PCB will be plugged into a connector. Everywhere else, the solder mask covers the copper.

Colors of PCB Solder Masks

While PCB solder masks can theoretically be any color, there are some standard colors that are commonly used. The most popular PCB solder mask colors are:

Color	Pantone Code	RGB Code	Sample
Green	362 C	77, 172, 38
Red	185 C	228, 0, 43
Blue	293 C	0, 61, 165
Black	Black C	45, 41, 38
White	–	255, 255, 255
Yellow	123 C	255, 199, 44
Purple	2685 C	76, 0, 173

Green Solder Mask

Green is by far the most common color for PCB solder masks. The iconic green color has been associated with PCBs for decades. There are a few reasons for the popularity of green:

• Good Contrast: The green provides good contrast for the white silkscreen legends that identify components.
• Ease of Inspection: The green background makes it easy to inspect the PCB for defects. Solder bridges, bad solder joints, and damaged traces tend to stand out against the green.
• Tradition: The green color has been used for so long that it’s become traditional and expected.

Red Solder Mask

Red solder masks are often used for PCBs in the power electronics industry. The red color provides a striking visual indication that the board deals with power. Red boards are common in applications like:

• Switching power supplies
• Motor drivers
• Automotive electronics
• Industrial controls

Blue Solder Mask

Blue solder masks are growing in popularity, particularly in consumer electronics. Many find the blue color to be more aesthetically pleasing than the traditional green. Blue provides good contrast for white silkscreen while giving the PCB a cleaner, more modern look.

Black Solder Mask

Black solder masks are popular for their sleek, professional appearance. The black background really makes the white silkscreen pop. However, black can make it more difficult to inspect the PCB for manufacturing defects. Black is a popular choice for:

• Audio electronics
• High-end consumer devices
• Aerospace applications
• LED lighting

White Solder Mask

White solder masks are less common but can give a unique, clean look to a PCB. The white background provides maximum contrast for black silkscreen legends. However, white shows any dirt or discoloration more readily than other colors. White is sometimes used in:

• Medical devices
• Scientific instrumentation
• Some consumer electronics

Yellow and Purple Solder Masks

Yellow and purple are much less common solder mask colors. They are sometimes used for aesthetic reasons in consumer products or to identify a specific board revision. Purple is occasionally used in the same power electronics applications as red.

Solder Mask Finish Options

In addition to color, there are also different options for the finish and surface texture of the solder mask. The most common finishes are:

Glossy (SMOBC)

A glossy solder mask, also known as Solder Mask Over Bare Copper (SMOBC), is the most commonly used finish. It has a smooth, shiny appearance. The glossy finish provides good protection for the copper traces and good durability.

Matte (LPI)

A matte solder mask, also called Liquid Photo Imageable (LPI), has a non-reflective, textured surface. The matte texture can provide better adhesion for the silkscreen legends and make the PCB more fingerprint resistant. However, the textured surface can make inspection for defects more difficult.

Factors in Choosing a Solder Mask Color

When choosing a solder mask color for your PCB, there are several factors to consider:

Legibility

The solder mask color should provide good contrast with the silkscreen legends to ensure the text is readable. White silkscreen on a green, red, blue, or black background is usually very legible. Black text on a white background also provides excellent contrast. Some specialized PCBs may use different silkscreen colors like yellow on a black background.

Aesthetics

For consumer-facing products, the aesthetics of the PCB may be a consideration. Choosing a color that complements the overall product design can enhance the perceived quality and attention to detail. Blues, blacks, and whites are popular choices for their modern, clean looks.

Application

The PCB application can suggest certain colors. As mentioned, red is often used for power electronics, while black is common in audio devices. Aerospace, medical, and scientific applications may have their own color conventions or requirements.

Inspection

The solder mask color can affect the ease of visual inspection for manufacturing defects. Green is popular in part because many defects are readily apparent. Black can be more challenging to inspect. Highly textured matte finishes can also make inspection more difficult regardless of color.

Availability and Cost

Not all PCB manufacturers offer all color options. Some colors like green, red, blue, and black are widely available. More unusual colors like white, yellow, or purple may be harder to find or come with a cost premium. It’s best to check with your manufacturer about their standard color options and any additional costs for non-standard colors.

Silkscreen Legends on Colored Solder Masks

The silkscreen is the text and symbols printed on the PCB that identify components, test points, and other features. The silkscreen is typically printed in white on darker solder masks and black on lighter masks like white. However, other silkscreen colors are sometimes used for aesthetic reasons or to convey special information.

Some examples of alternative silkscreen colors:

• Yellow on a black background for a high-contrast, eye-catching look
• Red on a white background to highlight important warnings or test points
• Gold on a black background for a high-end, luxury appearance

When choosing a non-standard silkscreen color, it’s important to maintain good legibility. Fine pitch text or intricate symbols may not be as readable in some color combinations.

Solder Mask and PCB Functionality

It’s important to note that the solder mask color is purely cosmetic and does not affect the functionality of the PCB. The electrical properties of the solder mask material are the same regardless of the pigment used to color it. A green PCB and a red PCB will function identically if they have the same copper layout and components.

However, the solder mask material itself does have an impact on the PCB’s performance. The solder mask needs to provide adequate insulation between conductors and protection from the environment. It must be durable enough to withstand the soldering process and any expected environmental conditions like heat, humidity, or chemical exposure.

For high-frequency PCBs, the dielectric constant of the solder mask material can affect the impedance of the traces. In these cases, the solder mask material is chosen for its electrical properties first and its color second.

PCB Solder Mask Color Standards and Regulations

There are no universal standards dictating the use of specific solder mask colors. However, some industries or applications may have their own conventions or requirements.

• IPC-A-610: The IPC-A-610 standard for PCB design and manufacture does not specify any requirements for solder mask color. It focuses on the functional aspects of the solder mask like its thickness, coverage, and adhesion.

• MIL-PRF-55110: This military specification for PCBs does not mandate a specific color, but it does require the solder mask to be a contrasting color to the base material and to be compatible with the PCB’s intended use.

• RoHS: The Restriction of Hazardous Substances (RoHS) directive restricts the use of certain hazardous materials in electronics. Some pigments used in solder masks in the past contained restricted substances like lead or cadmium. Modern RoHS-compliant solder masks use safer alternative pigments.

• Automotive: The automotive industry has strict reliability requirements for PCBs due to the harsh environmental conditions in vehicles. Some auto manufacturers may have specific requirements for solder mask color for certain applications, but there is no industry-wide standard.

Ultimately, the choice of solder mask color is up to the PCB designer and the product requirements. It’s always best to consult with your PCB manufacturer about their available color options and any application-specific recommendations they may have.

Frequently Asked Questions

Q: Does the solder mask color affect the PCB’s functionality?

A: No, the solder mask color is purely cosmetic and does not impact the PCB’s electrical functionality. However, the solder mask material itself does have electrical and environmental requirements it must meet.

Q: What’s the most common solder mask color?

A: Green is by far the most widely used solder mask color. It provides good contrast for inspection and has become a traditional color associated with PCBs.

Q: Are there any industry standards for solder mask color?

A: There are no universal standards mandating the use of specific colors. However, some industries may have their own conventions, such as the use of red for power electronics.

Q: Can I get a custom color solder mask for my PCB?

A: It depends on your PCB manufacturer. Most offer a standard selection of colors like green, red, blue, and black. Custom colors may be available but can come with additional costs or minimum order quantities. It’s best to check with your manufacturer about their offerings.

Q: Does the solder mask color affect the assembly process?

A: Not directly, but some colors like black can make visual inspection for defects more difficult. This is one reason green is popular, as many common defects are easily spotted against the green background.

In summary, the choice of solder mask color is a combination of tradition, aesthetics, and application requirements. While color doesn’t affect the PCB’s functionality, it can impact inspection, legibility of silkscreen text, and the overall look of the product. Understanding the pros and cons of different mask colors and finishes can help you choose the best option for your specific PCB project.