Solder mask
Solder mask is the protective polymer coating applied to a PCB to insulate exposed copper traces, prevent solder bridges during assembly, and protect against oxidation and contamination. It is what gives most boards their characteristic green colour.
What it is
Solder mask is a liquid photo-imageable (LPI) lacquer applied to both sides of a finished PCB. After application, the mask is exposed and developed so that openings appear over the pads where components will be soldered. The remaining mask insulates traces, vias, and copper fills from short circuits and protects the underlying copper from environmental exposure.
Green is the industry standard, but black, white, blue, red, yellow, and matte variants are widely available. Colour choice is mostly cosmetic, though some colours (matte black, white) can affect optical inspection and silkscreen contrast during assembly. The mask material itself is functionally identical across colours.
Standard solder mask conforms to IPC-SM-840, which defines performance requirements including dielectric strength, adhesion, hardness, and resistance to flux residues. Mask thickness typically falls between 12 and 75 µm, with 20-30 µm common for standard applications. IPC-SM-840 specifies a minimum dielectric strength of 500 VDC per 25 µm rather than a fixed thickness, so the exact thickness depends on copper weight and the fabricator's process.
When it matters
Solder mask design affects manufacturability and reliability. Solder mask defined (SMD) pads use the mask opening to define the pad shape; non-solder-mask defined (NSMD) pads expose the copper around the pad and are preferred for BGAs above 0.5 mm pitch because they accommodate plating tolerance better. For BGAs at 0.4 mm pitch and below, SMD pads are often preferred to prevent bridging. Solder mask slivers — narrow strips of mask between adjacent features — have a minimum width (typically 100 µm / 4 mil) below which the mask cannot be reliably imaged. Designs that violate this minimum will be flagged in DFM review and may require pad adjustments.
At Nordic PCB
Standard green solder mask is included in our default quote. Alternative colours (black, white, blue, red, yellow, matte) are available on request — usually at no additional cost for standard production runs, though small-batch colour changes may add nominal setup fees. Solder mask geometry is reviewed during DFM to flag slivers, registration issues, and pad-style problems before tooling.
Related terms
- Silkscreen / legend
Silkscreen (also called legend) is the printed text, symbols, and outlines on a PCB used to identify components, connectors, and orientation marks. Typically white or black epoxy ink, it sits on top of the solder mask. Minimum text height is usually around 0.8 mm (32 mil) for legibility.
- Solder mask colours and types
Standard solder mask is green liquid photo-imageable (LPI) lacquer, but black, white, blue, red, yellow, and matte variants are widely available. Colour choice is cosmetic and does not affect electrical performance; matte and dark colours can affect optical inspection contrast during assembly.
- DFM
DFM (Design for Manufacturability) is a structured review of your PCB design against a fabricator's process limits — trace widths, drill sizes, annular rings, solder mask clearances, and stack-up choices — to catch issues before tooling starts. A good DFM review prevents rework, scrap, and missed delivery dates.
- IPC-A-600
IPC-A-600 is the IPC standard that defines visual acceptance criteria for bare printed circuit boards. It specifies what surface, dimensional, and structural conditions are acceptable across three class levels, and is the reference document fabricators use to inspect boards before shipment.