High temperature creep response of lead free solders

Lead free solder materials are susceptible to significant creep deformations in harsh high temperature environments including automotive, avionics, military, and oil exploration applications. In addition, dramatic degradations will occur in the creep responses of lead free solder alloys when they are exposed to long term isothermal aging during product applications at high temperatures. Such degradations in the creep compliance of the solder material are universally detrimental to reliability of solder joints in electronic assemblies. In this work, we have characterized the high temperature creep behavior of SAC405 (95.5Sn4.0Ag0.5Cu) lead free solder, which is the most creep resistant of the standard SACN05 alloys. In addition, we have studied the creep behaviors of two doped SAC solders, SAC_Q and Innolot, which have been previously shown to out-perform SAC405 in simple mechanical stress-strain tests at room temperature. Tensile specimens were formed in rectangular cross-section glass tubes using a vacuum suction process, and a water quenched (WQ) solidification profile was utilized to yield fine microstructures and the upper limits of the mechanical properties for each alloy. The samples were then aged for 10 days at room temperature to stabilize their microstructures. After aging, creep testing was performed at two different stress levels (10, 15 MPa) and several different extreme/high testing temperatures (T = 100, 125, 150, 175, and 200 °C). For each set of conditions, the creep performances of the three alloys were compared. The results showed that the doped SAC alloys were more resistant to creep at high temperatures. The creep rates of SAC_Q are roughly 50% of those for SAC405, while the creep rates of Innolot are roughly 33% of those for SAC405. It is likely that the dopants can significantly block the movement of dislocations and thus increase the creep resistance of these solders.