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What is PCB Solder Mask - Introduction of Solder Mask Updated in 2022

PCBONLINE Team

Mon, Apr 25, 2022

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A solder mask is a protective coating on the top and bottom of printed circuit boards to protect copper traces and PCB materials from oxidation, contamination, shorts, chemicals, moisture, scratches, etc.

This post reveals details of PCB solder mask from the below content:

Part 1: What is a Solder Mask Layer

In PCB design, a solder mask layer is a negative plane and automatically generated on the top and bottom layers. That means, on the top and bottom layers, the visible shapes are PCB pads or holes, and the rest areas that are invisible should be printed or sprayed with solder masks in PCB manufacturing.

PCB solder mask

How is a solder mask formed? On a PCB, solder mask ink is printed or sprayed on the surface of the board and then baked or exposed under UV light. In this way, the ink dries and forms a protective coating, and this coating is the solder mask.

The solder mask permanently covers the surface of the circuit board and insulates the PCB externality from the outside environment.

Almost all PCBs require solder masks for protection and a longer lifespan. Only in some cases for better testing the design, PCBs without solder masks are needed, which are called naked PCBs. On rigid PCBs, the solder mask is also rigid. For flexible PCBs, flexible segments are added to the solder mask ink materials, and the solder mask called overlay is flexible.

Part 2: Solder Mask Colors - Answer to Why is PCB Usually Green

Many beginners may have wondered why PCBs are usually green. This is because the solder mask color is usually green. This is to say, the PCB color is the solder mask color. In fact, solder masks can be in any possible color if only the silk screen text/symbols can be easily distinguished from the solder mask.

The solder mask color has no effect on PCB functions or performances. There are no rules about solder mask colors for PCB uses. But there are some views of solder mask colors below.

Green color: green is the most common solder mask color. It is easy to see the silkscreen on green PCBs. And green is comfortable for the eyes.

green solder mask PCB

Black color: some people think PCBs in the black color look advanced. It may be correct, but aesthetics are subjective. PCBs for power supplies often use a black solder mask.

black solder mask PCB

White color: white solder mask reflect much light shedding on the circuit board. PCBs for LEDs often use white solder masks, but we also have manufactured aluminum PCBs with black solder masks for motor front lamps. It should be noted that we often use white solder mask ink with an 85% of light reflective rate to manufacture LED PCBs.

white solder mask PCB

Red color: PCBs with gold fingers, such as graphics cards, often use a red solder mask.

red PCB gold fingers

Apart from green, black, white, red, PCB solder masks can also be blue, yellow, mink, purple, pink, brown, and transparent.

Part 3: How does a PCB Manufacturer Apply Solder Mask on PCBs

There are two ways to apply solder masks on PCBs - UV light solder mask curing and solder mask exposure development.

UV light solder mask curing

UV curable solder mask, a solvent-free ink, is the earliest solder mask for PCB manufacturing. It is applied on PCBs by screen printing. Then the UV light sheds on the circuit boards, and the solder mask ink gets dry and cures.

UV light solder mask curing is easy but it is not precise. The image tolerance is 0.2mm, and the solder mask is also thick. So it is used only for simple PCBs, such as FR-1 PCBs and XPC paper PCBs.

Solder mask exposure-development

The solder mask ink for exposure development is a solvent-based ink that will cure when exposed under UV light. Under the UV light, the solder mask ink is printed on PCBs through a film in the PCB pads and holes pattern. In this way, the film covers the pads and holes, and the solder mask is only applied and gets dry on the blank areas and copper traces. Then the PCBs need to be baked to totally cure the solder mask.

The solder mask exposure-development process is precise, and the tolerance is 0.05mm. This solder mask applying method is appropriate for PCBs with higher requirements. Especially, a solder mask bridge must be applied in this way. At PCBONLINE, the maximum tolerance of applying black solder masks is 4mil.

For thick-copper PCBs, a solder mask can't be printed but sprayed on PCBs in case the solder mask in the corners is too thick. At PCBONLINE, solder masks are applied on thick-copper PCBs by electrostatic spraying. Electrostatic spraying develops from exposure-development solder mask printing, and it is finished in one-time. After spraying, the circuit boards are also baked. The only downside of electrostatic spraying solder masks is that it consumes plenty of solder masks.

One-Stop Advanced PCB Manufacturing and Assembly - PCBONLINE

If you are looking for high-quality PCB manufacturing, PCBONLINE can provide you with one-stop solutions. PCBONLINE, an advanced PCB manufacturer founded in 1999, has two PCB manufacturing bases and one PCB assembly factory.

In regards to solder mask applying, we have these advantages:

    • Solder mask applying at PCBONLINE is the more precise exposure-development type.
    • Precise control - maximum tolerance of 4mil for black solder masks, maximum tolerance of 3mil for other color solder masks.
    • At PCBONLINE, solder mask electrostatic spraying for thick-copper PCBs is fully automatic and achieves uniform ink thickness on the entire board.
    • Custom solder mask colors on PCBs, such as green, matte green, black, matte black, red, white, blue, yellow, purple, transparent, etc.
    • Provide one-on-one engineering support through your PCB manufacturing project. DFM, DFT, DFX are free.

You can order PCB prototyping and assembly, bulky PCB fabrication and assembly, conformal coating, end-product assembly from PCBONLINE. If you have any questions about solder masks, or if you have any PCB/PCBA manufacturing needs, please feel free to contact PCBONLINE by email at info@pcbonline.com.

Conclusion

A solder mask is a permanent protective coating on the PCB surface against moisture, dust, chemicals, scratches, shorts, and so on. At PCBONLINE, we apply exposure-development solder masks on PCBs with high accuracy. If you are looking for a reliable PCB manufacturer, don't miss the 23-year-old source factory PCBONLINE.