PCB Solder Mask: Types, Colors, and Functions Explained

The solder mask is the colored coating that gives a PCB its distinctive appearance. But it’s far more than cosmetic — it’s a critical functional layer that affects assembly quality, reliability, and even signal performance. This guide covers everything you need to know.

What Does Solder Mask Do?

Prevents solder bridging: During reflow or wave soldering, solder mask between pads prevents molten solder from flowing where it shouldn’t
Protects copper: Prevents oxidation, corrosion, and environmental damage to copper traces
Electrical insulation: Provides additional insulation between closely spaced traces
Contamination barrier: Protects against dust, moisture, and handling damage
Visual contrast: Makes silkscreen text and component outlines easier to read

Solder Mask Types

LPI (Liquid Photo-Imageable) — Industry Standard

The dominant solder mask technology for production PCBs.

Process:

Liquid epoxy ink is applied via screen printing or curtain/spray coating
Pre-baked at 75°C to partially cure
Exposed to UV light through a photomask (or direct imaging)
Developed to remove unexposed areas (revealing pads)
Final cure at 150°C

Advantages: High resolution (minimum dam width 3-4 mil), good adhesion, consistent thickness, cost-effective for mass production.

Dry Film Solder Mask

A pre-formed film laminated onto the board surface.

Advantages: Very uniform thickness, good for thick copper boards. Disadvantages: Lower resolution than LPI, higher cost, not suitable for fine-pitch.

Inkjet Solder Mask

Direct digital printing without photomasks.

Advantages: No tooling cost, good for prototypes, can print different colors on the same board. Disadvantages: Lower throughput, higher per-unit cost.

Solder Mask Colors

Green — The Standard

Why green? Historical reasons: green photoresist was the first to achieve the resolution needed for PCBs, and it provides the best contrast for visual inspection.
Best inspection contrast: Human eyes are most sensitive to green; defects are easiest to spot.
Lowest cost: Default color at all manufacturers.
Best resolution: Green LPI ink achieves the finest feature definition.

Black

Aesthetics: Premium look for consumer products
Heat absorption: Absorbs more heat — consider thermal impact
Inspection difficulty: Harder to visually inspect traces underneath
Higher cost: Slight premium over green

White

LED applications: Maximizes light reflection from LED PCBs
Clean aesthetic: Medical devices, consumer products
Discoloration risk: Can yellow over time with heat exposure
Inspection difficulty: Low contrast with silkscreen

Red

Distinctive appearance: Often used for open-source hardware
Good contrast: Better than black for visual inspection
Moderate cost: Slight premium over green

Blue

Arduino standard: Recognizable for development boards
Good contrast: Visible traces beneath
Moderate cost: Similar to red

Matte Black

Ultra-premium look: Used in high-end consumer electronics
Anti-glare: Reduces surface reflections
Highest cost: Among all color options

Solder Mask Design Rules

Solder Mask Opening (SMO) Types

NSMD (Non-Solder Mask Defined):

Solder mask opening is LARGER than the copper pad
Copper pad edges are exposed
Preferred for SMT: Provides larger solderable area and better solder joint reliability
Typical expansion: 2-3 mil (0.05-0.075mm) per side beyond the copper pad

SMD (Solder Mask Defined):

Solder mask opening is SMALLER than the copper pad
Solder mask overlaps the copper pad edges
Used when: Pad-to-pad spacing is too tight for NSMD, or BGA pad pitch is very fine (< 0.5mm)
Typical restriction: 2-3 mil per side smaller than the copper pad

Solder Mask Dam

The strip of solder mask between two adjacent pad openings.

Minimum Dam Width	Capability Level
3 mil (0.075mm)	Advanced
4 mil (0.1mm)	Standard
5 mil (0.13mm)	Conservative
8 mil (0.2mm)	Basic

If the dam is too narrow: It may wash away during development, leaving no solder mask between pads and increasing the risk of solder bridges.

Solder Mask Clearance on Vias

Tented vias: Solder mask covers the via completely — prevents solder from wicking into the via during reflow. Recommended for vias near SMD pads.
Open vias: Via pad is exposed through the solder mask — allows probing but risks solder wicking.
Plugged and tented: Via is filled with epoxy and covered by solder mask — fully sealed.

Impact on Assembly

Solder Paste Printing

Solder mask thickness affects stencil-to-board gasketing
Typical solder mask thickness: 15-25um over copper, 25-40um over substrate
Inconsistent thickness can cause poor stencil contact and solder paste smearing

Reflow Soldering

Solder mask acts as a solder dam, containing paste to pad areas
Green solder mask has the best solder release properties
Dark colors (black, matte) absorb more IR energy — may need adjusted reflow profiles

Wave Soldering

Solder mask protects bottom-side SMD components from solder wave
Minimum solder mask thickness for wave soldering: 20um

Solder Mask and Signal Integrity

At high frequencies (>1 GHz), solder mask affects impedance:

Solder mask Dk ≈ 3.5-4.0
Adds parasitic capacitance to outer-layer traces
For controlled impedance, the field solver must account for solder mask
Selective solder mask removal over high-speed traces is sometimes used (but adds cost)

Conclusion

Solder mask is a deceptively important PCB layer. Green remains the best default choice for cost, resolution, and inspectability. Choose alternative colors only when aesthetics or specific functional requirements (LED reflection, thermal concerns) demand it. Always specify NSMD openings unless pad spacing forces SMD-defined pads, and work with your manufacturer on minimum dam width capabilities.