The mechanical reliability of sintered silver joints, widely used in power electronics packaging, is critical for long-term applications such as electric vehicle converters. However, conventional homogeneous modeling often oversimplifies internal microstructural variations and limits the accuracy of stress prediction, especially under thermal cycling. In this study, a region-refined modeling framework is proposed to account for epoxy-regulated porosity and mechanical inhomogeneity across the joint. Pressureless die-attach joints were prepared using submicron silver pastes with varying epoxy contents (0∼4 wt%). The joint was divided into five sub-regions from the center to the fillet for localized characterization. Nanoindentation, SEM, and EDS analyses were conducted to assess region-specific mechanical properties and microstructure. Power-law constitutive models were extracted for each region and implemented into finite element simulations of thermal cycling (−55 ∼ 150 °C, 2 cycles/h, 250 h). The sub-region FEM model more accurately captured local stress concentrations and identified failure-prone areas, particularly near the fillet, compared to conventional homogeneous models. Experimental validation confirmed a good correlation between simulated stress zones and observed degradation. This sub-region strategy provides a robust framework for reliability prediction and design optimization of sintered silver joints in high-performance, large-area packaging applications.