Fatigue Life of Joint-Scale SAC305 Solder Specimens in Tensile and Shear Mode

In functional applications, solder joints in printed wiring assemblies experience multiaxial stress state, including shear and peeling stresses. Limited failure data suggests that durability of Tin-rich solder joints may be dependent on the triaxiality of the stress state, due to their coarse-grained structure. To investigate this dependence, the fatigue durability of Sn-3.0Ag-0.SCu (SAC305) grain-scale solder joints is experimentally investigated in both tensile and shear modes, at room temperature, using joint-scale solder specimens. Cyclic mechanical tests are performed using a Thermo-Mechanical-Microscale (TMM) test system, which has both tensile and shear testing capabilities. Custom-made joint-scale few-grained specimens are used, based on a modified Iosipescu design for cyclic shear tests and a dog-bone design for cyclic tensile tests. The cyclic durability behavior is described using Coffin-Manson relation, in terms of equivalent strains that are measured using Digital Image Correlation (DIC). The cyclic mechanical test results exhibit reduced durability of SAC305 solder joints in tensile mode compared to that in shear mode, indicating that fatigue durability is affected not just by equivalent strain but also by stress triaxiality. Failure analysis was conducted using Environmental Scanning Electron Microscopy (ESEM) technique to understand the failure modes in both specimens. Although, test data is limited, the findings of this study, clearly shed light on the different influences of tensile and shear stress state on the fatigue life of grain-scale solder joints.