Aging induced evolution of free solder material behavior

Solder materials demonstrate evolving microstructure and mechanical behavior that changes significantly with environmental exposures such as isothermal aging and thermal cycling. These physical aging effects are greatly exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In the current study, mechanical measurements of thermal aging effects and material behavior evolution of lead free solders have been performed. Extreme care has been taken so that the fabricated solder uniaxial test specimens accurately reflect the solder materials present in actual lead free solder joints. A novel specimen preparation procedure has been developed where the solder uniaxial test specimens are formed in high precision rectangular cross- section glass tubes using a vacuum suction process. The tubes are then sent through a SMT reflow to re-melt the solder in the tubes and subject them to any desired temperature profile (i.e. same as actual solder joints). Using specimens fabricated with the developed procedure, isothermal aging effects and viscoplastic material behavior evolution have been characterized for Sn-Ag-Cu lead free solders, which are commonly used as the solder ball alloy in lead free BGAs and other components. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. In our total experimental program, several different SAC alloys have been examined. Samples have been solidified with both re flowed and water quenching temperature profiles, and isothermal aging has been performed at room temperature (25degC) and elevated temperatures (100degC, 125degC and 150degC). In this paper, we have concentrated our efforts on reporting results for 95.5Sn-4.0Ag-0.5Cu (SAC405) subjected to room temperature and elevated temperature aging experiments. Variations of the temperature dependent mechanical properties (elastic modulus, yield stress, ultimate strength, creep compliance, e- - tc.) were observed and modeled as a function of room temperature aging time. Microstructural changes during room temperature aging have also been recorded for the solder alloys and correlated with the observed mechanical behavior changes.