The Failure of Sn-Bi-Based Solder Joints Due to Current Stressing

The electrical resistances of Cu/Sn57BiSbNi/Ni solder joints were continuously monitored during current stressing at temperatures between 95 and 125°C, and current densities up to 7kA/cm², for times approaching 3000 h. Scanning electron microscopy was utilized to characterize corresponding changes in solder joint microstructure. The statistics of failure based upon a 20% increase in electrical resistance (correlated with the growth of a continuous layer of Bi at the anode) were analyzed for groups of nominally identical samples, while either current density or temperature was independently varied. At longer times, after prodigious, diffusion-limited growth of a Cu₆Sn₅-based phase, catastrophic failure (an increase in electrical resistance of 20 mΩ) was observed and correlated with crack propagation across the width of the sample. The statistics of catastrophic failure were analyzed during current stressing of these SnBi-based solder joints at a relatively high temperature (125°C) and current density (4kA/cm²).