Evaluation of creep behavior of near-eutectic Sn–Ag solders containing small amount of alloy additions

Abstract

Understanding and quantification of creep behavior of lead-free solder joints are essential for lifetime prediction of electronic systems. This is especially true for circuits with surface mount and chip components that are subjected to severe environments and higher temperatures. Creep deformation behavior of Sn–4Ag–0.5Cu, Sn–3.5Ag–0.5Ni and Sn–2Ag–1Cu–1Ni solder alloys was determined at room temperature (25 °C) and at elevated temperature (85 °C) using miniature single shear lap joint specimens that are comparable in size to actual solder joints used in electronic packaging. Various creep parameters such as global creep strain, secondary creep rates as well as the strain for the onset of tertiary creep in the solder joint were determined. The effects of Cu and Ni alloy additions on the creep properties of eutectic Sn–3.5Ag solder joints were studied by comparing with the creep deformation behavior of eutectic Sn–3.5Ag solder joints that were used as the baseline. General findings in this study revealed that the creep resistance of Sn–4Ag–0.5Cu solder joints is comparable to but slightly higher than that of eutectic Sn–3.5Ag solder joints at both room and elevated testing temperatures, particularly at lower stresses. The Sn–3.5Ag–0.5Ni solder joints have comparable creep resistance to Sn–4Ag–0.5Cu and eutectic Sn–3.5Ag solder joints at 85 °C, but much better creep resistance at room temperature. The Sn–2Ag–1Cu–1Ni solder joints were two orders of magnitude less creep resistant than solder joints made with other solder materials at 85 °C. However, the shear strains for the onset of tertiary creep in Sn–2Ag–1Cu–1Ni solder joints were found to be the highest at 85 °C. Microstructural analysis showed significant creep deformation along Sn grain boundaries.