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• The eutectic region strength of SAC305 solder decreases with aging by nanoindentation analysis. • The dual-phase RVE CPFEM can simulate aging effect of SAC305 solder. • The simulated strain "hot zones" by DP CPFEM are consistent with SEM-DIC calculation. Because typical solder joints undergo thermal aging during use, models simulating the behaviors of solder under mechanical stress must accurately reflect the changes in properties caused by thermal aging effects. The uniaxial deformation of SAC305 (96.5Sn–3.0Ag–0.5Cu wt.%) solder under different aging conditions indicates that the mechanical properties of solder are weakened with aging. In this study, the nanoindentation method was used to evaluate the effect of aging on the mechanical properties of the β-Sn dendritic regions and eutectic regions of SAC305. Based on this evaluation, a dual-phase (DP) representative volume element (RVE) crystal plasticity finite-element method (CPFEM) model was developed to simulate the thermal aging effects of SAC305 solder. In situ tensile tests performed with scanning electron microscopy (SEM) showed that the tensile deformation was concentrated in the β-Sn dendritic regions of the solder. The DP CPFEM model was established based on the microstructures shown by electron backscatter diffraction (EBSD) analysis performed during the in situ tensile tests. This model was used to verify the accuracy of the DP CPFEM in simulating the deformation localization by comparison with the strain field distribution observed via SEM–digital image correlation (DIC) technology. The developed DP CPFEM model allows the prediction of the potential failure locations of a microscale solder joint. [Display omitted] [ABSTRACT FROM AUTHOR]