Whisker growth in the crack region of the cathode interface during current stressing process in lead-free solder joints

Whisker formation mechanism has been widely investigated for recent decades since its great influence on degrading the reliability of the electronic devices. In our work, effect of current stressing on whisker growth in Cu/Sn3.8Ag0.7Cu/Cu and Cu/Sn58Bi/Cu solder joints was investigated. Results show that after current stressing for a long time, many whiskers are observed in some local regions at the cathode interface. What is most important thing is that the whisker growth occurred at the cathode interface in the crack region. These whiskers are all with filament-type morphology, and no column-type is formed. In general, they can reach beyond 10μm in length. EDX reveals that these whiskers are mixtures of Sn and Bi for Sn58Bi solder and single Sn crystal for Sn3.8Ag0.7Cu solder. However, as the stressing time increases, the whiskers seem to stop growing any more. In conclusion, this filament-type whisker growth at the cathode interface may be responsible for the current density accumulation at the crack region, leading to much Joule heat aggregation that triggers the whisker formation.