Microstructurally Adaptive Model for Evolution of Creep Due to Aging in SnAgCu Solder Alloys

The creep response of SnAgCu alloys used in microelectronic assemblies evolves with aging and is of significant practical importance. In this paper, a microstructurally adaptive creep model is developed that captures the effect of isothermal aging on primary and secondary creep behavior of SnAgCu alloys in the dislocation glide-climb regime. The model development relies on stereological observations of Ag3Sn precipitate evolution in Sn3.0Ag0.5Cu alloy that is isothermally aged at temperatures varying from 25 degrees C to 125 degrees C , with aging times in the range of 15-90 days. An exponential time primary-cum-secondary creep model is used to determine the effect of aging on creep behavior. The creep model stress exponent and the threshold stress are quantitatively related to two carefully selected microstructural descriptors, namely, Ag3Sn intermetallic compound precipitate size and primary-Sn dendrite size. A total of 48 creep experiments on the Sn3.0Ag0.5Cu alloy were performed to develop the relationship between the microstructural parameters and the creep behavior. The developed model was then validated against new data at different aging conditions that were previously not used to build the model. The model was also shown to be accurate when used on microstructures resulting from a different alloy composition with an altogether different substrate. The creep behavior predicted by the model based solely on the observations of microstructural state agrees well with the experimentally measured creep behavior over a wide range of applied stress, test temperature, and aging conditions.