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

Abstract A time dependence of the form t 1/ n with n > 3 was observed for the thickening kinetics of the Ni 3 Sn 4 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 Ni 3 Sn 4 scallops and grain boundary diffusion due to the existence of molten channels between the previously formed Ni 3 Sn 4 scallops. Evidence is presented suggesting that highly scattered pores within the Ni 3 P layer play an important role in the seemingly random fluctuations of the Ni 3 Sn 4 thickness in the Sn–58Bi/Ni–P system with respect to reaction temperature and time. By contrast, the thickness of the Ni 3 Sn 4 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) 6 Sn 5 , instead of Ni 3 Sn 4 , 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.