Solder mask clearance is the intended opening between the mask image and copper feature; a solder mask dam is the mask web left between adjacent openings. Both must account for the selected mask process, registration, copper geometry, package land pattern and assembly objective. There is no universal 2-mil rule that fits every supplier and pitch.
Key takeaways
- Obtain the fabricator's finished minimum opening, registration and dam capability before final DFM.
- Use non-solder-mask-defined (NSMD) or solder-mask-defined (SMD) pads according to package guidance and process capability.
- When a dam cannot be manufactured reliably, gang/open the mask intentionally rather than leaving fragile slivers.
- Treat vias in pads, via tenting and plugged/capped vias as separate fabrication decisions.
- Solder mask does not control paste volume; stencil design and placement/reflow still drive bridging and tombstoning.
- Verify finished openings and registration on first article for dense packages.
Define the mask rules by feature class
| Feature | Mask decision | Main risk |
|---|---|---|
| Fine-pitch IC pads | Individual openings or controlled gang opening | Encroachment, sliver loss or bridging |
| BGA/µBGA pads | NSMD/SMD per package and assembly guidance | Pad stress, escape geometry and registration |
| Via near pad | Tent, open, plug/cap or via-in-pad | Solder wicking, trapped chemistry or mask fracture |
| Test point | Open diameter and probe access | Mask encroachment or unreliable contact |
| Edge connector | Mask setback and finish boundary | Contamination, peeling or contact interference |
| High-voltage feature | Opening and mask role in spacing | Incorrect assumption that mask is safety insulation |
NSMD versus solder-mask-defined pads
NSMD pads have a mask opening larger than copper, exposing the pad edge. SMD pads have mask overlapping the copper perimeter. BGA suppliers may use either for specific package/board interfaces.
| Option | Benefit | Trade-off |
|---|---|---|
| NSMD | Copper dimension is defined by etch; solder wets sidewall | Needs adequate mask registration and web |
| SMD | Mask can anchor/define small copper feature | Opening registration and mask-to-copper overlap become critical |
Follow the component manufacturer's land-pattern and reliability guidance, then confirm the board fabricator can hold the required structure.
Dam and expansion decision flow
- Start with nominal copper land pattern and fabricator mask-registration capability.
- Apply package-class opening rules, not one global expansion.
- Calculate the resulting finished mask web between adjacent openings.
- Compare it with the supplier's guaranteed finished dam, color and mask-process capability.
- If it is too narrow, choose an intentional gang opening or revise land pattern/assembly strategy.
- Review stencil aperture and paste separation independently.
Via treatment near assembly pads
- Tented via: mask covers the opening on one or both sides; suitability depends on hole size/process.
- Mask plugged via: mask material fills or closes the hole under a controlled process.
- Resin-filled and capped via: conductive surface is planarized for via-in-pad assembly.
- Open via: acceptable where solder wicking, contamination and probing risks are controlled.
Do not use these terms interchangeably. Specify the applicable IPC-4761 type or a clear drawing cross-section, side and acceptance requirement.
Failure-mode matrix
| Defect | Likely contributors | Prevention/evidence |
|---|---|---|
| Mask on pad | Registration, artwork or insufficient expansion | Finished-opening inspection |
| Missing/peeling dam | Dam below process capability, poor adhesion or handling | DFM and visual/adhesion inspection |
| Solder bridge | Paste/placement/reflow plus insufficient separation | Stencil and assembly process review |
| Solder wicking into via | Open/tented via too close to pad | Plug/fill or land-pattern revision |
| Mask blister/delamination | Surface prep, cure, contamination or thermal stress | Process control and IPC-A-600 inspection |
| Exposed unintended copper | Mask registration/artwork or damage | AOI/visual acceptance |
Solder mask RFQ checklist
- Mask material/color, finish and applicable IPC-SM-840 class
- Minimum finished opening, registration and dam requirements
- Package-specific NSMD/SMD/gang-opening rules
- Via tent/plug/fill/cap type, side and acceptance
- Edge connector, test point, fiducial and high-voltage mask keepouts
- ODB++/Gerber, netlist, fabrication and assembly drawings
- First-article measurement/inspection and cosmetic acceptance
- Quantities, panelization, change notification and approved mask alternatives
Use DFM guidelines and submit the controlled package through the PCB quotation page.
Standards and references
- IPC-SM-840 — Qualification and performance of permanent solder mask
- IPC-2221 — Generic printed board design
- IPC-7351 — Generic requirements for surface-mount land pattern design
- IPC-4761 — Design guide for protection of printed board vias
- IPC-A-600 — Acceptability of printed boards
Confirm current revisions and the component/fabricator-specific requirements.
Solder mask clearance FAQ
What is the standard solder mask expansion?
There is no universal value. It must cover copper/mask registration and the selected package while remaining within the fabricator's process capability.
Does a solder mask dam prevent all bridging?
No. Paste volume, stencil aperture, placement, pad design and reflow are major contributors.
What happens if the mask dam is too small?
It may not image or adhere reliably. An intentional gang opening is often better than an uncontrolled mask sliver.
Should BGA pads be NSMD or SMD?
Follow the package supplier's guidance and board/assembly capability. Both structures are used for specific applications.
Is a tented via the same as a filled and capped via?
No. Tenting covers the opening with mask; filling/capping uses a controlled fill and planar copper cap suitable for different assembly needs.