3.5 Defect Mechanisms & Fixes
Reflow soldering defects are the consequence of failed controls upstream: paste, stencil, placement, or profile. Correct troubleshooting requires disciplined analysis of the defect mechanism to identify the root cause in the process chain. The failure to distinguish a print-related defect (volume/alignment) from a thermal-related defect (wetting/balance) guarantees continuous, costly rework.
3.5.1 Solder Bridging (Shorts)
Mechanism: Excessive solder paste volume connects neighboring pads, usually on fine-pitch components (QFP, BGA outer rows). The molten solder's surface tension is overcome by the excessive volume, allowing coalescence between pads.
Root Cause | Fix and Control Point | Process Check |
Excessive Paste Volume | Reduce stencil aperture width (5–10%) or thickness. | SPI (Chapter 1.6) must show Area/Volume below the "Bridge" limit. |
Misalignment/Smear | Printer alignment drift or poor gasket seal (solder smears under stencil). | Check Printer Setup (Chapter 1.5) for high squeegee pressure or slow separation speed. |
Excessive Slump | Paste is too warm, past expiration, or solvent loss (rheology failure). | Check Paste Handling (Chapter 1.2) for proper thaw/open time protocol. |
Too Long TAL | Excessive time above liquidus allows solder to flow too far. | Shorten TAL with faster belt speed (Chapter 3.2). |
3.5.2 Tombstoning (Manhattan Effect)
Mechanism: Unequal wetting forces on a component's opposite pads cause it to stand on one end, creating an open circuit. This occurs when the solder on one pad melts and pulls the component upright while the solder on the opposite pad is still solid or lagging. This is most common on small, light passives (0402, 0201).
Root Cause | Fix and Control Point | Process Check |
Uneven Heating (∆T) | One pad is connected to a thermal mass (plane/via), the other is isolated. | Thermal Profile (Chapter 3.2) must achieve minimal ∆T across the board (Soak profile preferred). |
Unequal Paste Volume | One pad has significantly more paste (higher surface tension). | SPI must confirm volume symmetry (±5%) on both pads. Use Home-Plate Apertures (Chapter 1.4) to balance forces. |
Fast Wetting | The transition to liquidus is too quick, maximizing the surface tension torque. | Slow down Ramp Rate (1–2˚C/sec) and ensure adequate preheat time. |
Placement Skew | Component placed with a significant offset, causing one end to contact the paste poorly. | Verify Placement Accuracy and Coplanarity during First Article (Chapter 2.5). |
3.5.3 Head-in-Pillow (HIP)
Mechanism: The solder ball on the BGA component and the solder paste on the PCB pad melt but fail to fully coalesce, forming a weak mechanical interface (a latent defect that often passes electrical test but fails in the field under stress). This is almost exclusively due to warpage and oxidation.
Root Cause | Fix and Control Point | Process Check |
Package/PCB Warpage | Component or board lifts during the high-heat zones, allowing the ball to oxidize. | Implement PCB/BGA flatness screening at incoming. Use lower peak temperature (if possible) to minimize warpage. |
Oxide Layer | Oxide forms on the lifted BGA ball, and the paste's flux is too weak/spent to clean it upon recontact. | Use High-Activity Flux paste. Ensure TAL is sufficient to maximize flux activation and joint collapse. |
Insufficient Volume/Contact | Starved deposit at the BGA corners (where warpage is worst). | Use Aperture Reduction with volume biasing on outer rows/corners (Chapter 1.4) to ensure contact. |
Atmosphere | Excessive oxygen content in the reflow zone accelerates oxidation. | Switch to Nitrogen (N2) atmosphere (Chapter 3.3) to suppress oxide formation. |
3.5.4 Voiding and Cold Joints
Mechanism: Voids are gas bubbles trapped within the solidifying solder, compromising the joint's thermal and electrical conductivity. Cold joints are those where the solder paste did not reach the minimum liquidus temperature (Incomplete Reflow).
Defect | Root Cause | Fix and Control Point | Process Check |
Voiding (Thermal Pads) | Volatiles (from flux/solvents) are trapped by the large molten solder mass. | Stencil Design: Use Window-Pane Apertures with Chimneys (Chapter 1.4) to create vent paths. | AXI (X-ray Inspection) must confirm voiding is below the acceptable limit (e.g., ≤25% total area). |
Solder Balls | Rapid heating (too fast ramp) or insufficient preheat time vaporizes solvents, causing solder to splatter. | Extend Preheat/Soak time (Chapter 3.2) to allow solvents to evaporate gradually before reflow. | Reflow Profile must show a controlled, gradual ramp rate (≤3˚C/sec). |
Cold Joint/Poor Wetting | Insufficient TAL or peak temperature for the alloy, or poor pad solderability. | Increase Belt Speed (to extend TAL) or Zone Temp (to raise peak). | Profile Plot must confirm minimum TAL/Peak targets were hit. |
Final Checklist: Troubleshooting Reflow Defects
Defect Observed | Primary Root Cause Zone | Key Action to Take |
Bridging | Printing (Excess Volume) | Reduce stencil aperture area; verify SPI volume Cpk. |
Tombstoning | Profile (Thermal Imbalance) | Tune profile for longer soak; check SPI for volume symmetry. |
HIP | Materials/Profile (Warpage/Oxide) | Use N₂ atmosphere; confirm adequate BGA TAL; check BGA flatness. |
Voiding | Design/Paste (Gas Entrapment) | Revise stencil to add vent paths; monitor results with AXI. |
Cold Joint | Profile (Insufficient Heat) | Increase TAL/Peak until Profile Plot confirms targets are met. |