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.

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