Effects of Physical Aging on the Stress-Strain and Creep Behaviors of Lead Free Solders

Abstract

Solder materials demonstrate evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. Such physical aging effects are exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In this work, measurements of thermal aging induced material behavior evolution have been performed for tin-lead and lead free solders. 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 re-melted in an SMT reflow oven to subject them to a specified/desired temperature profile. Using the developed specimen preparation procedure, isothermal aging effects and viscoplastic material behavior evolution have been characterized for standard 63Sn-37Pb eutectic solder and 95.5Sn-4.0Ag-0.5Cu (SAC405) lead free solder, which is commonly used as the solder ball alloy in lead free BGA components. Samples have been solidified with both reflowed and water quenching temperature profiles, and isothermal aging has been performed at room temperature (20C) and elevated temperatures (+125C and +150C). Stress-strain and creep tests have been performed on non-aged and aged samples, and the changes in mechanical behavior have been recorded for various durations of isothermal exposure. Microstructural changes during aging have also been recorded, and correlated with the observed mechanical behavior changes