What is Soldermask and Why Use White for LED PCBs?

Soldermask, also known as solder resist or solder stop mask, is the coating applied to the copper traces of a PCB. It serves several important functions:

• Protects the copper traces from oxidation and corrosion
• Prevents solder bridges from forming between closely spaced solder pads
• Provides electrical insulation
• Identifies component designators, test points, and other markings

For LED PCBs, the soldermask color plays an additional role in maximizing the efficiency and brightness of the mounted LEDs. White soldermask is often preferred because it has high reflectivity, returning more of the LED light output and improving overall luminous efficacy.

Reflectivity Comparison of Soldermask Colors

Soldermask Color	Reflectivity
White	80-90%
Black	5-10%
Green	30-35%
Red	40-50%
Blue	20-25%

As the table shows, white soldermask reflects a much higher percentage of light compared to other common soldermask colors. This makes it an ideal choice when brightness and efficiency are priorities, as is often the case with LED lighting applications.

Benefits of White Soldermask for LED PCBs

Increased Brightness and Efficiency

The high reflectivity of white soldermask enables mounted LEDs to achieve greater brightness levels and luminous efficacy (lumens per watt). By reflecting more of the emitted light rather than absorbing it, white soldermask essentially acts like a secondary optic, directing more useful lumens out of the LED device.

This optical efficiency boost translates to several benefits:

• LEDs appear brighter for a given power input
• Lower power LEDs can be used to achieve the same brightness
• Fewer LEDs may be needed to meet a total lumen requirement
• Lower system power consumption and heat generation

Improved Color Rendering and Mixing

For LED applications involving color mixing, such as RGB LED modules or tunable white luminaires, white soldermask helps maximize color rendering performance. By reflecting all wavelengths equally without any tint, white soldermask ensures the color output of the LEDs is accurately delivered.

In contrast, using a colored soldermask would absorb certain wavelengths more than others, skewing the chromaticity coordinates of the mixed light output. This is especially problematic for achieving good color rendition across the visible spectrum in tunable white lighting.

Better Heat Dissipation

White soldermask also has some thermal advantages compared to darker soldermask colors. Its high reflectivity means it absorbs less energy from the LED light, reducing PCB heat buildup.

Additionally, many white soldermasks have higher emissivity values than glossy colored resists. This allows the PCB to more effectively radiate heat away to the ambient environment.

While thermal management of high power LED PCBs often requires more substantial heatsinking, every bit of cooling optimization helps in maintaining LED performance and longevity.

Manufacturing Considerations for White Soldermask

Material Selection and Application

Achieving a high quality white soldermask finish requires careful selection of resist material and application parameters. There are a few common challenges that must be addressed:

• Yellowing: Some white soldermask formulations are prone to yellowing over time, especially in high heat and UV exposure environments common to LED applications. This degrades reflectance and efficiency. Selecting a resist with strong yellowing resistance is critical.

• Coverage: Due to the high pigment loading needed for opacity and reflectivity, white soldermasks are inherently more viscous than other colors. This can make it difficult to achieve even, consistent coverage without pinholes or voids that could compromise insulation resistance. The material and application process must be optimized to ensure good flow and leveling.

• Adhesion: The high solids content of white resists can also impact adhesion to the PCB substrate if not properly formulated and cured. Inadequate adhesion may lead to delamination and degraded protection. Proper surface preparation and cure cycles are essential.

Partnering with an experienced PCB fabricator is recommended to ensure the right white soldermask material and processes are used for your specific LED application needs.

Legend Printing

Applying component designators, logos, and other markings on white soldermask requires a high contrast legend ink, usually black or dark gray. Pad printed epoxy or UV cured inks are commonly used.

The combination of silkscreen background color and legend ink should be carefully considered for best readability. Where very fine pitch legends are needed, a black soldermask with white legend ink may be preferred for crisper resolution.

Compatibility with Other PCB Finishes

For LED PCB designs requiring other surface finishes like ENIG (Electroless Nickel Immersion Gold) or HASL (Hot Air Solder Leveling), it’s important to confirm compatibility with the chosen white soldermask. The different thermal expansion coefficients and mechanical properties of these finishes can impact soldermask adhesion and stability if not properly matched.

FAQ on White Soldermask for LED PCBs

Q1: Are there any downsides to using white soldermask for LED PCBs?

A1: The main potential downsides are related to manufacturability – achieving good coverage, adhesion, and yellowing resistance as described earlier. This may require more specialized materials and processes compared to standard green or black soldermasks. White also shows flux residues and handling marks more readily, so cleanliness is critical.

Q2: Is white soldermask more expensive than other colors?

A2: In general, yes white soldermask is more costly than standard green due to the higher pigment loading and more stringent quality requirements. However, the efficiency gains enabled often make it a worthwhile investment for LED applications. The exact cost differential depends on the specific material grade and supplier.

Q3: Does white soldermask impact the solderability of the PCB pads?

A3: White soldermask should have similar solderability performance to any other color when properly applied and cured. The main concern is ensuring the white resist fully develops and does not leave residues on the exposed copper pads that could interfere with solder wetting. Selection of compatible flux chemistry is also important.

Q4: Can white soldermask be used for other types of PCBs besides LED boards?

A4: Certainly, white soldermask can be a good aesthetic choice for many consumer electronics where a bright, clean appearance is desired. It’s commonly used for things like IoT devices, wearables, and medical equipment. The same manufacturability considerations apply as with LED PCBs.

Q5: Are there any industry standards or certifications to look for when specifying white soldermask?

A5: The key standards to reference are the IPC-SM-840 Qualification and Performance of Permanent Solder Mask and the IPC-A-600 Acceptability of Printed Boards. These define criteria like minimum thickness, adhesion strength, electrical resistance, and more. UL (Underwriters Laboratories) also maintains a registry of recognized soldermask materials that meet certain performance and safety requirements. It’s a good idea to call out compliance to these standards in your PCB fabrication documentation.

Conclusion

White soldermask offers significant optical, thermal, and aesthetic benefits for LED PCB applications. By maximizing brightness, efficiency, and color rendering while minimizing heat buildup, white soldermask helps optimize the performance and reliability of LED lighting systems.

As with any PCB design decision, it’s important to weigh the trade-offs and partner with an experienced manufacturer to ensure the right material selection and processes for your application. With proper implementation, white soldermask can be a powerful tool in advancing LED technology.