How to Prevent Solder Joint Voiding and Cold Solder Defects during the SMT Reflow Process

Analysis of Root Causes
- Too much flux is applied in the solder paste. The flux doesn’t have sufficient time to outgas before the solder has transitioned to a solid state.
- The preheat temperature is too low so any solvent in the flux is difficult to vapour out completely.
- The soak zone time is too short during the reflow process.
- Lead-free solder typically has a 4% shirinkage in volume when it is cooled down to a solid state. It is possible to obtain voids when large pads are cooled down unevenly.
- Paste oxidation occurs.
- Reduce the amount of solder paste deposited.
- For large pads, use small opening grids instead of a large aperture.
- Extend the preheat time so it is long enough.
- Extend the soak zone time so it is long enough.
- Always consider new solder paste for BGA assembly.
Analysis of Root Causes
- The pre-heat time is too long and too high, preventing the flux from activating.
- The solder paste has expired.
- There is not enough heat absorbed by the solder.
- The cooling speed is too slow.
- Adjust the reflow profile according to the manufacturer’s specification.
- Ensure the peak temperature is at least 15°C above the solder’s melting point for more than 45 seconds.
- Change to a new type of solder paste.
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