Solder mask decides where solder can and cannot go. Colour, finish, thickness and dams are not just cosmetic choices. They affect exposure, assembly and inspection.
| Item | SKPCB published capability | What to check in the design |
|---|---|---|
| Solder mask colour | White, green, black, yellow, red, blue, purple, matte green, matte black, none | Whether fine dams and inspection needs suit the colour |
| Min solder mask dam | 0.1 mm | Whether fine-pitch pads leave room for a dam |
| Solder mask registration | ±0.076 mm | Whether the mask opening clearance covers registration error |
1. Colour: exposure, fine dams and inspection
Solder mask ink is photosensitive. Openings are formed by exposure and development. Colours absorb UV light differently. Dark inks (black, matte black) absorb more, so the exposure window is narrower and fine dams are harder to hold. White ink is highly opaque and also needs its exposure conditions confirmed separately.
Colour also affects inspection. On green mask, copper, opening edges and foreign material show clear contrast for both visual and automated optical inspection. On black or white mask some defects show lower contrast.
Recommendations:
- Use green where fine-pitch pads need the minimum dam.
- If black or white is required for appearance, state the dam requirement in the RFQ so engineering review can confirm it.
- For repeat builds, write the colour in the fab note instead of relying on a default.
2. Finish: glossy or matte
SKPCB offers matte green and matte black. A matte surface reflects less light, so photos and manual inspection show less glare. A glossy surface shows scratches and contamination more readily. The choice mainly follows appearance and inspection practice, and it belongs in the fab note next to the colour.
3. Thickness: thinnest on the trace shoulders
Solder mask is not uniform across the board. Liquid ink flows off the traces as it covers them, so it is thinnest on the trace top and shoulders (corners) and thickest on the laminate between traces. Thicker copper means a higher step and thinner mask on the shoulders.
- Boards with normal copper weights usually do not need a separate mask thickness specification.
- Coverage on heavy copper is assessed separately, see heavy copper solder mask and finish coverage.
- If an insulation or voltage requirement relies on the mask, state it in the RFQ. Do not assume the mask provides insulation by default.
4. Solder mask dams: the lower limit for pad spacing
A solder mask dam is the strip of mask kept between two adjacent pads. During reflow it separates the molten solder on the two pads.
Whether a dam fits depends on three values: pad spacing, mask opening clearance and minimum dam width. Check with:
Pad spacing ≥ minimum dam + 2 × opening clearance per side
The opening clearance has to cover mask registration error, or mask can encroach on the pad. Example: with a clearance per side equal to the ±0.076 mm registration and a 0.1 mm minimum dam, pads need roughly 0.25 mm spacing for a dam. The actual clearance is set in engineering review.
When spacing is too tight there are three options:
- Reduce the opening clearance: accept mask closer to the pad edge.
- Gang the opening (no dam): both pads share one opening, and solder can bridge across the laminate.
- Change the pad or footprint: make room for the dam at the source.
SKPCB raises an engineering query when adjacent SMT pads have no dam. Bare-board electrical test cannot find the bridge, because the two pads are open on the bare board. It shows up only at AOI or functional test after assembly, when the loss is a populated assembly. See the engineering query reference.
5. Limits after curing
- Cured mask does not take solder. Mask or legend ink on a pad stops solder wetting there, giving weak or starved joints. For legend clearance from pads, see PCB legend and silkscreen marking capability.
- Cured mask is comparatively brittle. Flex bend areas must use coverlay, not solder mask ink, or the mask cracks under repeated flexing.
6. Writing the fab note
- Mask colour and finish, for example "matte green"; state each side if they differ.
- Whether dams are required between fine-pitch pads, listing the key reference designators.
- Whether mask openings are 1:1 to Gerber or may be adjusted by the factory.
- Legend colour, and whether legend is clipped or moved when it conflicts with openings.
- Any insulation or voltage requirement that relies on the mask, with its acceptance method.
Mask openings should be planned with the surface finish, because the opening is exactly the area the finish covers. Full process figures are on the capabilities page. When the data package is ready, request a quote.
FAQ
Q: Does solder mask colour affect electrical performance?
A: This article does not cover RF dielectric behaviour. On standard rigid boards, colour mainly affects exposure, fine dams and inspection contrast.
Q: Can black solder mask hold the minimum dam?
A: Dark ink has a narrower exposure window, so fine dams are harder to hold. If you need the minimum dam, state it in the RFQ for engineering review.
Q: What are SKPCB's minimum dam and registration?
A: The published capability table lists a 0.1 mm minimum solder mask dam and ±0.076 mm registration accuracy.
Q: What if there is no room for a dam between pads?
A: Reduce the opening clearance, gang the opening, or change the pad or footprint. A ganged opening carries bridging risk, and SKPCB asks for written confirmation.
Q: Can solder mask be used in a flex bend area?
A: No. Cured mask is comparatively brittle and cracks under repeated flexing. Use coverlay in the bend area.