Creep Properties and Microstructure of the Sn-Ag-Cu-Ni-Ge Lead-Free Solder Alloy

A new lead-free solder alloy, Sn-Ag-Cu base adding a small amount of Ni, Ge, has been developed to improve their mechanical properties and prevent oxidation in solder alloys. In this paper, creep properties of two lead-free solder alloys, Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge (abbr. Sn-3.5Ag-0.5Cu-Ni-Ge) solder and Sn-3.0Ag-0.5Cu solder, were investigated at three temperatures ranging from 313K-398K. It was found that the creep strength of the Sn-3.5Ag-0.5Cu-Ni-Ge solder is higher than that of the Sn-3.0Ag-0.5Cu solder. Especially in the low stress region at 398K, the creep rupture time of the Sn-3.5Ag0.5Cu-Ni-Ge solder is about three times as long as that of the Sn-3.0Ag-0.5Cu solder. The microstructure of these solder alloys show that the addition of Ni was found to refine the effective grain size and provide a fine and uniform distribution of Ag₃Sn in the solidified microstructure. The microstructure of the Sn-3.5Ag-0.5Cu-Ni-Ge solder is more stable than that of the Sn-3.0Ag-0.5Cu solder alloy after aging treatment at 398K, 1000 h. TEM observation was also performed, showing that precipitations of (Cu, Ni)₆ Sn₅, the diameter of which are about 0.5 μm, are distributed in the Ag₃Sn/β-Sn phase eutectic area of the Sn-3.5Ag-0.5Cu-Ni-Ge solder after creep test at 398K, 5MPa. It is thought that the precipitations of (Cu, Ni)₆ Sn₅ contribute to creep strength in the Sn-3.5Ag-0.5Cu-Ni-Ge solder alloy.