A typical recommendation for solder mask expansion is a minimum of 3 mils on all sides. This expansion is crucial for creating non-solder mask defined (NSMD) pads, which are preferred for their enhanced solder joint reliability.
Understanding Solder Mask Expansion
Solder mask expansion refers to the area by which the solder mask opening is larger than the copper pad it surrounds. This small margin ensures that the solder mask material does not encroach upon the copper pad, even with slight misregistration during the manufacturing process.
Why is Solder Mask Expansion Needed?
The primary reason for solder mask expansion is to accommodate potential misregistration between the solder mask layer and the copper layer during PCB fabrication. Factories have tolerances, and layers might not align perfectly. By making the solder mask opening slightly larger than the pad, you guarantee that the entire copper pad surface is exposed for soldering.
According to the provided reference:
- A typical recommendation is a minimum solder mask expansion of 3 mils on all sides.
- This is done to account for up to 2 mils of misregistration.
Solder Mask Defined (SMD) vs. Non-Solder Mask Defined (NSMD) Pads
Solder mask expansion is directly related to creating NSMD pads.
- Non-Solder Mask Defined (NSMD) Pads: In this common approach, the solder mask opening is larger than the copper pad (positive solder mask expansion). The solder joint forms on the exposed copper pad and extends onto the laminate surface around the pad. This technique, enabled by solder mask expansion, is generally recommended as it offers better stress distribution and improved solder joint reliability, especially for fine-pitch components.
- Solder Mask Defined (SMD) Pads: In this less common approach (for typical component pads), the solder mask opening is smaller than the copper pad (negative or zero solder mask expansion). The solder mask partially covers the copper pad edges. The solder joint is confined only within the solder mask opening on the copper pad surface. While sometimes used for large pads like thermal vias under QFNs, it's generally avoided for component lead pads due to potential stress concentration.
| Pad Type | Solder Mask Opening vs. Copper Pad | Solder Mask Expansion | Purpose | Reliability Note |
|---|---|---|---|---|
| NSMD | Larger | Positive (> 0 mils) | Expose entire pad, enable joint on laminate | Generally Higher (Recommended) |
| SMD | Smaller or Equal | Negative (<= 0 mils) | Define pad area via mask (less common) | Generally Lower |
Practical Considerations
While 3 mils on all sides is a common minimum, the optimal expansion can depend on various factors:
- Manufacturer Capabilities: Some fabrication houses can achieve tighter tolerances, potentially allowing for slightly smaller expansions. Always check your manufacturer's design guidelines.
- Component Type: For very fine-pitch components, extremely large expansions might reduce the web of solder mask between pads. A balance is often needed.
- Design Rules: Your specific design rules or company standards may dictate a specific value.
The 3-mil expansion mentioned in the reference provides a good balance, offering sufficient clearance for typical misregistration while maintaining adequate solder mask web between pads.
