Comparison of interfacial reactions of Ni and Ni–P in extended contact with liquid Sn–Bi-based solders

A time dependence of the form t1/n with n > 3 was observed for the thickening kinetics of the Ni3Sn4 scallops formed in both the Sn–58Bi/Ni and Sn–58Bi/Ni–P systems, which can be attributed to the radial growth kinetics of the Ni3Sn4 scallops and grain boundary diffusion due to the existence of molten channels between the previously formed Ni3Sn4 scallops. Evidence is presented suggesting that highly scattered pores within the Ni3P layer play an important role in the seemingly random fluctuations of the Ni3Sn4 thickness in the Sn–58Bi/Ni–P system with respect to reaction temperature and time. By contrast, the thickness of the Ni3Sn4 layer in the Sn–58Bi/Ni system increased with increasing reaction temperature and time. Addition of 1 wt.% Cu into the basic Sn–58Bi solder led to the formation of (Cu,Ni)6Sn5, instead of Ni3Sn4, and hence significantly reduced the consumption rates of both the Ni and Ni–P layers during high-temperature storage. Pure electroplated Ni will not survive appreciably longer than electroless Ni–P when in contact with either molten Sn–58Bi or Sn–Bi–Cu.