Effects of bismuth and nickel on the microstructure evolution of Sn-Ag-Cu (SAC)-based solders

The addition of alloying elements to the SAC alloy composition changes the growth behavior of thick Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) and their influences on mechanical responses. The purpose of this study is to investigate the effects of Bi and Ni on the growth kinetics of interfacial IMCs (Cu6Sn5 and Cu3Sn) and the mechanical strength of the bulk solders during high temperature aging. Reflowed samples of the SAC405 solder balls and the SAC + Bi,Ni solder balls were stored in high-temperature storage (HTS) chamber at 150 °C and 175 °C each for 0 h, 500 hs, 1000 hs, and 2000 hs. The microstructure evolutions of bulk solder and its interfacial areas were evaluated and the associated microhardness values were measured on the solder balls. During aging, the presence of Bi was seen only in the bulk solder region, resulting in increased hardness by a factor of ∼ 2 and resistance to the age softening due to solid solution strengthening. On the other hand, Ni was observed in both the solder ball and IMC regions. After aging at 150 °C, SAC + Bi,Ni solder balls had a thinner Cu3Sn IMC layer at its interface when compared to that in SAC solders with no Ni. At 175 °C, however, Ni addition did not decrease the growth of Cu3Sn. Long aging (2000 h) of the SAC405 sample at 175 °C resulted in the complete consumption of the Cu layer, which was slowed in SAC + Bi,Ni. The presence of Bi and Ni, therefore, show the potential to improve the reliability of the SAC solder ball joint during aging by restricting the solder deformation and the growth of the Cu3Sn layer.