Stress Relaxation Characterization of Hypoeutectic Sn3.0Ag0.5Cu Pb-free Solder: Experiment and Modeling

This study investigates the time dependent behavior of lead-free hypoeutectic Sn3.0Ag0.5Cu solder using a combination of experimentation and modeling. Experimental data for hypoeutectic Sn3.0Ag0.5Cu (SAC) is obtained from stress relaxation tests at different temperatures and different initial stress levels, and compared with the benchmark eutectic Sn37Pb, using a modified lap shear test. The test specimen uses a modified Iosipescu specimen with a 180 micron wide solder joint between two notched copper platens. The creep modeling uses both primary creep (generalized exponential model) and secondary creep (Garofalo sine hyperbolic model). The model constants are obtained iteratively by matching the test results with viscoplastic finite element models of the test specimen. As expected, the stress relaxation data confirms that eutectic Sn37Pb relaxes faster than hypoeutectic SAC. For example, at a given stress level of 7 MPa and at a given temperature of 75degC, SAC relaxes only by 68% in the time that Sn37Pb relaxes almost completely. Modeling work suggests that both primary and secondary creep behavior must be included in models to achieve reasonable agreement between stress relaxation tests and simulations. Improvements to existing creep modeling strategies to better capture the relaxation response are discussed.