Surface Mount Technology (SMT) is widely used in the PCB assembly process, but like any complex manufacturing technology, it is not immune to defects. Whether you’re dealing with solder balls, bridging, tombstoning, or unmelted solder paste, troubleshooting is a key part of ensuring high-quality SMT production.

In SMT, defects may arise due to various factors like improper solder paste application, reflow profile issues, or equipment calibration. This guide covers the most common SMT soldering problems and their solutions to help maintain an efficient production process.

Common SMT Troubleshooting Issues

1. Solder Balls
2. Solder Beading
3. Bridging
4. Tombstoning
5. Open Joints / Insufficient Solder
6. Unmelted Solder Paste
7. Excessive Filleting
8. Slumping
9. Dewetting
10. Disturbed Joints
11. Cellulite

1. Solder Balls: Problems & Solutions

Possible Causes:

• Solder Paste Smeared on Stencil: Solder paste could spill from the stencil and cause balls.
• Incorrect Squeegee Pressure: Too much or too little pressure can cause inconsistent paste deposits.
• Stencil Cleaning Issues: Residual solvent after cleaning may affect paste application.
• Alignment Issues: Misalignment of stencil and PCB.

Solutions:

• Check squeegee pressure: Ensure it’s uniform and correct.
• Check stencil alignment: Confirm proper alignment with PCB pads.
• Verify stencil cleaning process: Ensure solvents evaporate completely before printing.

2. Solder Beading: Problems & Solutions

Possible Causes:

• Slow Reflow Profile Ramp-Up: If the profile ramps too slowly, capillary action may draw the solder paste away from pads.
• Excessive Solder Paste: Too much paste on component pads can cause beading under the component.
• Paste Contamination on Stencil: Contaminants from the stencil can lead to improper paste distribution.

Solutions:

• Speed Up the Ramp-Up: Ensure the reflow profile ramps at 1.5°C to 2.5°C per second.
• Reduce Solder Paste on Pads: Use a smaller stencil aperture or adjust the dispenser settings.
• Clean Stencil Properly: Remove contaminants and ensure correct squeegee pressure and stencil alignment.

3. Bridging: Problems & Solutions

Possible Causes:

• Cold Collapse: After printing, the solder paste spreads too much, causing reduced height and increased surface area.
• Hot Collapse: During reflow, paste spreads too much due to improper profile settings.
• Excessive Paste on Pads: Over-depositing paste can cause bridging between component leads.
• Paste on Stencil Bottom: Solder paste may spread outside pad areas, leading to bridging between leads.

Solutions:

• Adjust Paste Viscosity: Use the correct viscosity and adjust printing speed.
• Optimize Reflow Profile: Reduce ramp-up time in the reflow profile to avoid excessive paste spreading.
• Control Solder Paste Amount: Reduce the solder paste volume to prevent excessive deposit.
• Check Stencil Alignment: Make sure stencil alignment is correct to prevent paste leakage.

4. Insufficient Solder: Problems & Solutions

Possible Causes:

• Scooping during Printing: Excessive scraper pressure can cause paste to be scraped off pads.
• Clogged Stencil Apertures: Dried solder paste can clog stencil openings.
• Foreign Material on Pads: If solder mask is printed over pads, it can obstruct paste deposition.

Solutions:

• Reduce Scraper Pressure: Use softer or metal scrapers to avoid scooping.
• Unblock Stencil Apertures: Clean stencil and unclog any paste obstructions.
• Check Solder Mask: Ensure solder mask isn’t covering pads.

5. Tombstoning: Problems & Solutions

Possible Causes:

• Uneven Heat Distribution: Imbalanced heat during the reflow process can cause components to lift on one side.
• Solder Paste Imbalance: Excessive solder paste on one side of the component can create an imbalance during reflow.

Solutions:

• Adjust Reflow Profile: Optimize the reflow profile to ensure even heat distribution.
• Balance Solder Paste Amount: Ensure equal solder paste distribution on both pads.

6. Unmelted Solder Paste: Problems & Solutions

Possible Causes:

• Incorrect Reflow Profile: A slow temperature ramp-up can result in insufficient heat for solder paste melting.
• Poor Paste Handling: Solder paste may have dried out or been improperly stored.

Solutions:

• Optimize Reflow Profile: Ensure the profile has a proper ramp-up rate and peak temperature.
• Use Fresh Solder Paste: Always use properly stored, fresh solder paste.

7. Dewetting: Problems & Solutions

Possible Causes:

• Improper Pad Finish: Certain pad finishes (e.g., HASL) can cause dewetting if not compatible with the solder paste.
• Incorrect Reflow Profile: A fast or high ramp-up in the reflow profile can cause insufficient solder wetting.

Solutions:

• Use Compatible Pad Finishes: Choose finishes that are more compatible with solder paste.
• Adjust Reflow Profile: Slow down ramp-up to allow proper wetting.

8. Excessive Filleting: Problems & Solutions

Possible Causes:

• Excess Solder Paste Deposited: If too much solder paste is deposited, it can cause excessive fillet formation around the component.
• Improper Reflow Profile: Rapid reflow can cause excessive filleting.

Solutions:

• Adjust Solder Paste Amount: Reduce paste amount for components with smaller pads.
• Optimize Reflow Profile: Slow down reflow ramp rates to prevent excessive fillet formation.

SMT troubleshooting is an essential part of ensuring high-quality PCB assembly. By understanding common defects like solder balls, bridging, tombstoning, and insufficient solder, and knowing the root causes and effective solutions, manufacturers can resolve issues efficiently and ensure a smoother production process.

Always focus on optimizing key aspects such as solder paste application, reflow profile settings, stencil cleanliness, and component placement to minimize errors. Additionally, ensure proper calibration of equipment like screen printers and reflow ovens to maintain consistent quality and avoid costly defects.