In SMT manufacturing, a detail that often makes or breaks yield is the small gap between adjacent pads. In dense layouts, it can be tempting to draw pads very close together, or even connect them, to gain copper area or ease routing. That shortcut frequently backfires during reflow, causing insufficient solder, solder bridging, and even false calls during AOI (automated optical inspection). This article provides a practical rule set for solder mask coverage between SMT pads and how to apply NSMD (non-solder-mask-defined) pad design correctly.
Rule Overview: Three Hard Rules for Pad-to-Pad Gaps
When designing SMT pads, the following principles must be observed:
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Isolation Principle (Solder Mask Coverage):
The area between two pads of an SMT component—i.e., the region influenced by solder paste—must be covered by solder mask. The solder mask functions as a physical dam, preventing solder paste from merging across the gap during printing and reflow.
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Only exception: For components with pin pitch smaller than 0.4 mm, mask separation between pads may be waived due to PCB fabrication capability limits. This is a process-dependent exception that must be coordinated with manufacturing.
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Connection Principle (NSMD Design):
If a connection between two pads is required—for example to enhance mechanical robustness or thermal conduction—use a routed trace pulled out from a pad rather than a direct copper bridge between pads. The trace width must be less than or equal to the pad width. This maintains paste shape integrity and mitigates reflow defects.
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Special Case for Power Modules and High-Power Parts:
For power modules or other high-current devices, an approach similar to "Sample 3" (pads connected by copper under solder mask) is acceptable, provided that the SMD-type pad size matches the NSMD-type pad size. In other words, do not reduce the pad area simply because a copper tie is present; the effective solderable area must remain consistent.
Why a Solder Mask Dam Is Necessary: Avoiding AOI and Insufficient Solder Traps
1. Prevent AOI False Calls
Modern SMT lines depend on AOI to verify solder joints using image-based analysis of brightness, shape, and edge definition. Without proper solder mask separation and correctly sized mask openings, two common issues arise:
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No mask dam: Solder paste prints as a continuous mass across adjacent pads or slumps during reflow, obscuring the intended boundary. AOI may classify the joint as insufficient solder or even open due to irregular shape or unexpected contour.
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Mask encroachment: If the solder mask infringes into the pad area due to incorrect mask expansion settings, wetting is obstructed and the fillet becomes visually atypical, again triggering a false call. Both conditions inflate false fail rates and re-inspection workload.
2. Prevent Insufficient Solder
Solder mask acts as a mechanical and wetting boundary during reflow. Surface tension drives molten solder to coalesce into the designed footprint; a mask dam ensures that the paste retracts into the pad rather than wandering into the gap. Without a dam, paste can flow away from the pad during heating, reducing the volume available to form a full fillet. The result is an underfilled joint or an inconsistent solder meniscus.
Illustrated Comparison: Three Pad-to-Pad Designs
The following conceptual examples show the design intent and risks for three common approaches:
Sample 1: Unacceptable Pad Design (Bad Design)
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Problem: No solder mask coverage between the two pads, and no routed connection. The paste coverage area spans both pads without a defined boundary.
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Risk: High likelihood of solder bridging during reflow; AOI also struggles to locate a clear joint boundary, leading to false calls and unnecessary rework.
Sample 2: Target Pad-to-Pad Design (Preferred)
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NSMD pad structure: The solder mask opening is larger than the copper pad, so the mask does not define the copper shape. The pad edge is fully exposed, which improves wetting and joint reliability.
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No copper connection between pads: The pads are completely independent, with a solder mask dam between them.
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Trace width ≤ pad width (if a trace is required): When a trace must exit the pad, maintain a width no greater than the pad width to preserve paste geometry and minimize reflow anomalies.
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Where to use: This is the safest, most standard option for the vast majority of SMT components.
Sample 3: Acceptable Design for Power/High-Current
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Feature: The two pads are connected by copper that is covered by solder mask (i.e., a copper tie under the mask).
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Where to use: Power modules and high-power devices where enhanced heat spreading and current-carrying capacity are needed.
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Mandatory requirement: The SMD-type pad size must be consistent with the NSMD-type pad size. Adding a copper tie does not justify reducing the solderable pad area; the effective pad size should remain unchanged to maintain proper wetting and volume.
Practical Implementation: How to Apply These Rules
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Conventional designs (pitch > 0.4 mm):
Use the Sample 2 approach as the default. In your EDA tool, configure solder mask expansion correctly so that there is a visible solder mask dam between pads. Be cautious when using teardrops, polygon pours, or automated copper fills around pads—verify that mask openings do not encroach on pad edges and that the inter-pad mask strip remains intact.
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High-density designs (pitch ≤ 0.4 mm):
Mask separation may be waived after aligning with the PCB fabricator's capability. Clearly note the requirement in the Gerber documentation. Ensure that the solder paste printing process can support this configuration—this typically requires a more precise stencil aperture design and tighter control in printing equipment to avoid bridging and volume loss.
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Power and high-current designs:
When adopting the Sample 3 configuration, confirm that the pad dimensions are identical to your standard NSMD pads. Do not shrink the pad copper or opening to accommodate the copper tie. Maintaining pad size preserves solder volume and joint quality, and helps avoid insufficient solder or poor wetting in reflow.
Summary
In SMT design-for-manufacturability, solder mask is not just a color—it is a critical process control feature. Proper solder mask dams between pads contain the solder paste, promote stable wetting dynamics during reflow, and help AOI correctly identify joints without excessive false calls.
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For standard boards: Maintain solder mask isolation between adjacent pads, except where the pitch is extremely small and process capabilities justify an exception.
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For special cases: If a connection is required, use an out-of-pad trace whose width is ≤ the pad width. For power modules or high-current components using a copper tie under the mask, keep SMD and NSMD pad sizes consistent to preserve solderable area.
Applying these rules yields pads that print and reflow consistently—and joints that AOI can evaluate reliably.
