Even small-lot PCB manufacturing relies on automated solder deposition processes to expedite the assembly process while providing greater consistency in solder application/joint reliability. Solder masking is necessary to prevent unintentional bridging of exposed copper and enhance the reliability of copper features that do not require atmospheric exposure. Using a carefully cut stencil, manufacturers apply an even coating to the areas of the board they wish to cover that leaves openings on the pads for solder joint formation. A 1:1 pad size-to-solder mask stencil opening would be ideal, but manufacturers use solder mask expansion to provide extra tolerance for component placement and assembly.
How Solder Mask Expansion Improves Reliability
Copper of the bare board intended for assembly requires an opening in the solder mask stencil to allow the copper to remain solderable. The openings of the solder mask result in two additional clearance constraints:
- The solder mask swell is the clearance from the copper to the edge of the opening.
- The mask dam is the clearance/air gap between adjacent solder mask openings.
Consider three cases of stencil opening:
- The stencil opening is smaller than the pad.
- The stencil opening is identical to the dimensions of the pad.
- The stencil opening is larger than the pad.
Case 1 results in a solder mask-defined (SMD) pad, i.e., the dimensions of the solder mask opening become the solderable space during assembly. Cases 2 and 3 produce a non-solder mask-defined (NSMD) pad, i.e., the extent of the pad is equal to or greater than the copper pad. Why differentiate between Cases 2 and 3 if both produce NSMDs? Case 2 is an ideal outcome that assumes a perfect stencil-cutting process to match the pad defined in the artwork. Any issues in stencil production or stencil-board alignment mean the stencil opening and pad are no longer 1:1, thus producing an SMD. In that sense, the solder mask expansion covers some manufacturing “slop” to account for misregistration errors between the stencil and bare board.
In general, NSMDs are preferable due to their greater reliability, which reduces stress on the solder joint. In an NSMD pad, solder can flow in the planar gap between the solder mask and pad/lead junction. However, SMD pads restrict solder flow to only the site above the pad vertically, producing a smaller and less mechanically robust joint. However, some manufacturers and applications may prefer SMD pads due to greater peel strength as the solder mask envelops some of the pad area.
| Comparing SMD vs. NSMD | ||
|---|---|---|
| Advantages | Disadvantages | |
| SMD |
|
|
| NSMD |
|
|
Case 3 is the safest choice for maximizing solderability and allotting tolerance to registration, placement, and soldering processes. When a designer does not request a tolerance for the solder mask expansion (either with a +/- range or minimum solder mask sliver), the fabricator may adjust the stencil as needed to bypass misregistration errors. This adjustment may or may not be an issue for designers; it’s worthwhile to ask the following:
- Is the layout significantly dense, and can changes to the solder mask opening undermine assembly? For many layouts, the lead size relative to the pad supports a reasonable amount of misregistration without imperiling the joint reliability. Here, changes to the solder mask expansion are unlikely to cause any solderability issues.
- Are you familiar with the manufacturers’ process? Do they indicate whether they will inform any solder mask concerns during design reviews, or will they perform the minimum expansion necessary?
From the manufacturers’ side, they can shrink the width of the pad just enough to meet the minimum solder mask requirements (effectively, a solder mask expansion) or group neighboring pads together in a single stencil opening. Depending on pad width and pitch, individual solder mask openings may not be reliably machinable on the stencil; fabricators use a block (or gang) relief for these pads. The block relief does not have a solder mask dam between individual pads, as is usually the case. In other words, solder-bridging becomes far more possible.
Your Contract Manufacturer Is Expanding Possibilities in Electronics
Ultimately, the designer and manufacturer must collaborate to determine the best solder mask expansion solution that provides the best performance and device reliability. Whether incorporating SMD or NSMD pads, designers can explicitly instruct the manufacturer in the fabrication notes regarding their acceptable level of alteration to the solder mask shown in the drawings and Gerber files. When the solder mask expansion lacks clarity, designers should confirm with the manufacturer as to their resolution process. At VSE, our engineers are committed to building electronics for our customers, including a fully transparent process for DFM. We’ve been realizing life-changing and life-saving devices for over forty years with our valued manufacturing partners.
