While they might seem like minor PCB design and manufacturing decisions, small decisions can have significant ripple effects on the board’s performance and reliability. Look no further than solder mask-defined pads: opening or restricting the aperture during solder deposition changes how the joint forms between the board’s pads and component’s pins. While certain board types may strictly dictate one style of pad opening over the other, both have their place depending on the board’s design intent.
| Solder Mask and Non-Solder Mask Pads, Defined | |
|---|---|
| SMD | NSMD |
|
Some solder mask covers a portion of the underlying pad, i.e., the solder mask forms the extent of the solderable area. + Greater peel strength. + Smaller pad areas can help guide BGA pins during placement and assembly. |
There is a gap between the edge of the pad and the solder mask that ensures complete pad exposure. + Forms a more mechanically robust joint since solder can flow around the sides of the pad. + Greater pad spacing between pads can benefit HDI designs. |
Is There a Clear Winner Between SMD vs. NSMD Pads?
When preparing layout files for assembly, the designer has one of two options for solder mask coverage of the pads: solder mask-defined (SMD) or non-solder mask-defined (NSMD). SMD pads ideally overlap 1:1 with the solderable areas of the footprints, i.e., the solder mask and pads are identical. Since nothing is ever truly perfect in manufacturing, solder mask-defined pads have a solder mask deposit that shrinks the copper area available as a soldering interface. Non-solder mask-defined pads pull back the solder mask opening to ensure the entire surface area of the copper pads is accessible for solder joint formation.
Both methods have benefits and drawbacks, but most applications prefer non-solder mask-defined pads as they enhance reliability. The gap between the pad edge and the solder mask allows the solder to flow down to the sides of the pad/pin, which forms a more robust solder joint than one solely sitting on top of the pad/pin. However, SMD pads have mechanical advantages, too: the distribution of the solder on top of the pad/pin enhances the peel strength of the pad/pin since the joint can’t “grip” the sides, while the mask layer helps prevent pad lift. SMD pads are less susceptible to mechanical or thermal delamination due to single-surface joint localization, with the tradeoff that the joint is more liable to failure. Since it’s generally easier to repair/replace a solder joint than a board, SMD pads are financially beneficial in stressful environments (provided that some board failure is acceptable, i.e., a Class 1 product).
An additional benefit for modern designs (primarily high-density interconnect or HDI) is reducing the board area achievable with NSMD pads. Since NSMD pads allow the solder joint to “sink” into the interstice between the pulled-back solder mask and the pin/pad, the diameter of the solder bead shrinks by approximately a fifth relative to an SMD pad. This reduction is often the deciding factor for BGA layouts with exceptionally dense pinouts or stackups with minimal routing layers/lanes for pin breakout, given the tremendous routing constraint HDI designs face.
The Benefit of Solder Mask-Defined Pads
Beyond HDI designs, how should designers select between SMD and NSMD?
- Prototyping – The reduced strength of the SMD solder joint supports prototyping for a couple of reasons. First, since the design is not final, test engineers can rapidly solder and desolder components when accounting for parasitic contributions, DNI circuitry, and other modular elements. Second, since the prototype is not a field-final design, failure relating to joint strength is almost a complete nonissue outside of stress testing.
- RF – To play fast and loose with electromagnetism, the smaller the conductive surface, the lower the inductance (generally). Recall that NSMD solder beads “sink” into the pad, and the solder ball flattens due to flow around the sides of the pin/pad. SMD pads sit on top of the pin/pad, resembling a mushroom with a thin base and a much wider cap. While this is mechanically less ideal, the smaller cross-sectional area of the solder joint on top of the pad presents a much smaller likelihood for electromagnetic interference (EMI) to/from the solder joint and surrounding pins/traces/conductors.
Like many aspects of PCB manufacturing, SMD and NSMD are not necessarily strict divisions; some designs may necessitate the inclusion of both. While this decision requires careful input from the manufacturer, a BGA can benefit from mixing the two pad types. For example, NSMD pads on power/ground pins allow the pin to artificially “grow” to the mask swell/pull-back size, lowering the inductance to ensure power integrity. At the same time, many SMD pads in the array will help guide the BGA during placement to create a reliable component registration to the land pattern.
Your Contract Manufacturer Analyzes All Aspects of Board DFM
The guiding principle should always be the board’s design intent when deciding between solder mask-defined pads and non-solder mask-defined pads. While an HDI board would benefit from the increased peel strength of SMD pads, the board area reduction from NSMDS pads is too great an advantage. It’s vital to remember that both pad types have their place, and in some cases, a design can utilize both styles for optimal performance. Here at VSE, we’re a team of engineers committed to building electronics for our customers, including a thorough design review and DFM analysis for manufacturing suitability. We’ve been realizing life-saving and life-changing devices for over forty years with our valued manufacturing partners.
