Microstructure and creep properties of Sn-Ag-Cu lead-free solders bearing minor amounts of the rare earth cerium

Purpose - The purpose of this paper is to investigate the effects of minor addition of the rare earth (RE) element cerium, Cc, on the microstructures and creep properties of Sn-Ag-Cu solder alloys. Design/methodology/approach - The pure Sn, Sn-Cu alloy, Sn-Ag alloy and Cu-Ce alloy were used as raw materials. Sn-Ag-Cu alloys with different contents of RE Cc were chosen to compare with Sn-Ag-Cu. The raw materials of Sn, Sn-Cu alloy, Sn-Ag alloy, Cu-Ce alloy were melted in a ceramic crucible, and were melted at 550 degrees C +/- 1 degrees C for 40 minutes. To homogenize the solder alloy, mechanical stirring was performed every ten minutes using a glass rod. During the melting, KG + LiCl (1.11), were used over the surface of liquid solder to prevent oxidation. The melted solder was chill cast into a rod. Findings - It is found that the microstructure exhibits smaller grains and the A0n/CU6Sn5 intermetallic compound (IMC) phases are modified in matrix with the addition of Cc. In particular, the addition of 0.03 wt.% Cc to the Sn-Ag-Cu solder can refine the microstructures and decrease the thickness of the IMC layers of Sn-Ag-Cu solder alloys. Meanwhile, thermodynamic analysis showed that these phenomena could be attributed to the reduction of the driving force for Cu-Sn IMC formation due to the addition of Cc. Results calculated using the thermodynamic method are close to the above experimental data. Thus, the optimum content of Cc in Sn-Ag-Cu solder alloys should be about 0.030 percent. Additionally, the effect of Cc on the creep rupture life of Sn-Ag-Cu soldered joints was studied. It was found that the creep rupture life may be increased up to 7.5 times more than that of the original Sn-Ag-Cu alloy, when Cc accounts for 0.030 percent. Originality/value - This paper usefully investigates the effects of the RE cerium (Cc), on the microstructures and creep properties of Sn-Ag-Cu solder alloys, optimizing the quantity of Cc in the Sn-Ag-Cu solder alloy through a thermodynamic method and by creep-rupture life testing.