2.16 Defect Mechanisms & Fixes

Root Cause

Fix and Control Point

Process Check

Excessive Paste Volume

Reduce stencil aperture width (5–10%) or thickness.

SPI must show Area/Volume below the "Bridge" limit.

Misalignment/Smear

Printer alignment drift or poor gasket seal (solder smears under stencil).

Check Printer Setup for high squeegee pressure or slow separation speed.

Excessive Slump

Paste is too warm, past expiration, or solvent loss (rheology failure).

Check Paste Handling 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.

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 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 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.

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 to ensure contact.

Atmosphere

Excessive oxygen content in the reflow zone accelerates oxidation.

Switch to Nitrogen (N₂) atmosphere to suppress oxide formation.

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 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 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.

Defect Observed

Primary Root Cause Zone

Key Action to Take

Bridging

Printing (Excess Volume)

Reduce stencil aperture area; verify SPI volume C_pk.

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.