Multiscale modeling of the anisotropic transient creep response of heterogeneous SAC single crystal

This paper provides fundamental mechanistic insights into the significant piece-to-piece variability that many researchers have reported in the creep response of micron-scale high-Sn SAC solder joints in the as-fabricated state, due to coarse-grained microstructure and the anisotropy of Sn. A multiscale mechanistic creep modeling approach is proposed, by combining the individual contributions of the eutectic Sn-Ag phase and the pro-eutectic Sn dendritic phase. The anisotropic transient creep deformation in the eutectic Sn-Ag phase is termed Tier 1 and is modeled with dislocation mechanics. The creep rate of the pure Sn dendritic phase is similarly modeled with dislocation mechanics and combined with that of the eutectic phase, in Tier 2, using an anisotropic load-sharing scheme that utilizes Eshelby methods and Mori-Tanaka homogenization. The creep rate calculations are performed along the dominant slip systems of the Sn grain in a single crystal of SAC solder material, to obtain the transient creep response of a SAC305 single crystal along global loading directions. This model has been calibrated using experimentally obtained transient creep response of a SAC305 single crystal specimen of a particular orientation and then verified against a second SAC305 single crystal specimen of a different orientation. The effect of grain orientation () on the transient creep response of SAC305 single crystal is parametrically demonstrated by varying one of the Euler angles of the grain.