Ask Me Anything about SMT Assembly

Events Ask Me Anything
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SMT assembly questions keeping you up at night? Wondering how far you can push miniaturization before your yields push back?

We’re opening up the floor for a no-filter AMA on all things SMT—from solder paste quirks and stencil design to tombstoning, reflow profiles, and pick-and-place tricks of the trade.

Post your questions now in this thread!

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Quick question about fine-pitch SMT components, how do you prevent those annoying solder bridges?

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I’m very interested in any special recommendations you might have for DFA of sensitive MEMS devices, such as piezo abs atm pressure sensors and IMUs. Some that I’ve already found are:

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I’m always debating about custom footprints for SMT. Is it ever really worth veering from IPC standards for a performance gain or is that just asking for trouble down the line?

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To prevent solder bridging on fine-pitch components, create an accurate paste mask layer that results in precisely sized stencil apertures. This makes sure each pad gets just the right amount of solder, enough for a good connection but not so much that it causes bridging between pins.

In addition, ask your manufacturer to:

• Use solder paste specifically formulated for fine-pitch applications.
• Set optimal stencil printing parameters (pressure, speed, snap-off).
• Ensure accurate component placement and alignment.
• Fine-tune the reflow profile for proper solder wetting and controlled flow.

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When assembling MEMS devices, we follow protocols tailored to their unique mechanical sensitivity. Most pick-and-place machines use pneumatic suction nozzles, which must be carefully controlled for MEMS. Excessive nozzle pressure can introduce stress or damage the device’s internal structure.

Stick to these DFA guidelines to ensure a reliable MEMS assembly process:

  1. Provide adequate clearance around MEMS packages so the pick-and-place nozzle can access them without interference from nearby tall or bulky components.
  2. Use datasheet-recommended land patterns and prefer NSMD pads to relieve stress during soldering.
  3. Stick to nozzle pick-zone guidelines provided by the OEM, and define no-placement zones around vent holes or moving elements (e.g., pressure sensor diaphragms).
  4. Avoid high-stress zones like PCB corners, mounting holes, or vibration-prone areas. Use mechanical isolation slots or flexible sub-assemblies if needed.
  5. Clearly indicate MEMS orientation in the assembly drawing to avoid placement issues that could affect calibration. Additionally, include handling notes such as:
  • Avoid hard tool contact near the MEMS.
  • The pick-and-place machine should trigger alarms for incorrect pick height or false picks.
  1. Where possible, recommend low-force suction settings to reduce stress during the pick-up phase.
  2. Place MEMS away from heat sources (e.g., PMICs, processors).
  3. Verify if the MEMS thermal specs align with your reflow profile. Check the moisture sensitivity level (MSL) and communicate floor life and baking requirements to your board assembler.
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IPC footprints are a safer choice. Custom footprints can be worth it if there’s a clear performance goal. Here are a few cases where it makes sense:

High-frequency / RF designs
If you’re working on RF-critical paths (like antennas, filters, or transmission lines), small tweaks to pad shapes, such as tapering or rounding, can help with impedance matching and reduce parasitic effects. These subtle changes can make a measurable difference in signal integrity.

Thermal management
For heat-sensitive components (like QFNs or power MOSFETs), slightly enlarging thermal pads or adjusting solder mask openings can improve heat dissipation. IPC footprints are intentionally conservative, so minor customizations here can enhance thermal performance without hurting reliability.

When should you stick to IPC standards?

For most general-purpose designs, IPC-recommended footprints are the safest choice. They’re carefully designed to work well with standard manufacturing processes like pick-and-place, stencil printing, and reflow soldering.

For common components, such as resistors, capacitors, and typical ICs, IPC footprints are already optimized for consistent performance and soldering quality.

Using custom footprints without proper validation can lead to issues like tombstoning, misaligned parts, or weak solder joints.
If you’re unsure, it’s best to start with IPC footprints and always check with your fabricator before making custom changes.

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How does a Non-SolderMask-Defined pad relieve stress? For that matter, what difference does SMD vs NSMD make at all in fabrication or assembly?

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When choosing between SMD and NSMD pads, understanding how they handle mechanical stress can help you make the right design choice. Here are the main differences:

• In NSMD pads, the solder mask opening is larger than the copper pad. This allows the solder joint to form directly on the copper and slightly over the adjacent solder mask. Since copper is more flexible than solder masks, it can better absorb mechanical stress caused by thermal expansion during fabrication.

• SMD pads in contrast have solder masks overlapping the edges of the copper pad. This creates a rigid boundary that can concentrate mechanical stress at the joint leading to potential cracking or long-term reliability issues.

• NSMD pads distribute stress more evenly across the joint by allowing the copper to flex. This is particularly beneficial in applications where long-term reliability matters, such as in BGAs where joint integrity is critical.

• SMD pads may provide better anchoring in some mechanical applications, but their rigidity can be a drawback in environments with movement, heat cycling, or vibration.

• NSMD pads are generally preferred for components like BGAs due to their superior stress handling and more consistent solder joint formation. Unless there’s a specific mechanical requirement that favors SMD, NSMD is the more reliable choice for most SMT designs.

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Thank you so much for taking the time to write it in brief.

Thank you so much for taking the time to write it in brief.

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Thanks a lot