Improvements of microstructure, wettability, tensile and creep strength of eutectic Sn-Ag alloy by doping with rare-earth elements

To improve the properties of the eutectic Sn-Ag lead-free solder alloy, various amounts of mixed rare-earth (RE) elements, mainly Ce and La, were added. The microstructure, wetting properties, melting behavior, mechanical properties, and creep behavior were studied. It was revealed that RE elements can refine the intermetallics and with 0.5% RE addition, the RE-bearing phase can be detected in the microstructure of the slow-cooled alloy. The results of differential scanning calorimetry indicate that the melting points of the RE-doped alloys are slightly lower than that of the Sn-3.5Ag and have a eutectic peak. The wetting property and creep resistance of the Sn-3.5Ag-0.25RE alloy are better than those of the Sn-3.5Ag alloy. The creep properties were studied at the temperatures of 303, 348, and 393 K, at various stress levels between 8 and 34 MPa. The stress exponents of the Sn-3.5Ag and Sn-3.5Ag-0.25RE were obtained at the temperatures. Tensile, creep, and wetting properties were found to improve with the addition of RE elements. The improvement of creep resistance is due to the fine dispersion of intermetallics and the decrease in interface energy between matrix and intermetallics. The wettability improvement is mainly due to the accumulation of RE elements at the solder/flux interface, leading to the reduction of the interfacial tension between solder and flux.