Improved creep resistance and thermal behavior of Ni-doped Sn-3.0Ag-0.5Cu lead-free solder

To develop lead-free solders for advance electrical components, a series of Sn-3.0Ag-0.5Cu (SAC 305) solders containing small amounts of Ni have been investigated. Results showed that the addition of Ni not only decreased the amount of undercooling by about 7.9-8.5 °C, but also reduced the solidus temperature of SAC(305) solder from 219.9 to 216.2 °C. Microstructure analysis revealed that Ni could replace the Cu atoms in the Cu6Sn5 phase and generates a new η-(Cu, Ni)6Sn5 IMC phase containing large amount of Ni after 0.5%Ni addition. The high solubility of Ni in Cu6Sn5 increased the substitutional defects and generated inter-atomic stress around Ni atoms, which in turn impeded the dislocation movements in different crystal directions, resulting in an increase in the creep resistance and total fracture time of 0.5%Ni-doped SAC(305) solder. In addition, the formation of fine fiber-like Ag3Sn and finer dot-shaped precipitates at the surface of β-Sn matrix could provide more obstacles for dislocation pile up in the adjacent grains and enhanced the creep resistance. However, when the concentration of Ni exceeded 0.5 wt%, the benefits of creep behavior and fracture time were reduced due to the formation of small amount of abrasive Ag3Sn and coarsening of (Cu, Ni)6Sn5 IMCs in the eutectic colony.