Ensuring the structural integrity of electronic assemblies exposed to severe random vibrations is a critical aspect of space systems engineering. In such assemblies, cracks typically initiate and propagate at the solder joint–Printed Circuit Board (PCB) interface due to the mismatch in material stiffness, thermal expansion coefficients, and the presence of complex dynamic stresses. Interfacial failure of solder joints not only reduces the mechanical reliability of the system but may also lead to functional failure of electronic modules. Therefore, developing an accurate and physics-based predictive model for interfacial fracture is of major significance for enhancing the durability and reliability of space borne electronic systems. In this study, a new interfacial fracture criterion is developed to predict crack initiation and propagation in PCB–solder interfaces subjected to mixed-mode I/II loading. The proposed model is formulated based on the Maximum Strain Energy Release Rate (MSERR) criterion, with explicit incorporation of the T-stress term, which accounts for the non-singular component of the near-tip stress field. The PCB is modeled as an orthotropic material, while the solder joint is treated as isotropic. The orthotropic response of the PCB is represented through a reinforced isotropic solid modeling approach, enabling an efficient yet accurate description of anisotropic stiffness characteristics. Parametric analyses demonstrate that the T-stress plays a decisive role in controlling the crack initiation angle, mode mixity, and overall failure envelope. Neglecting this term leads to notable deviations in predicting interfacial fracture under mixed-mode conditions. Incorporating the T-stress enhances the model’s ability to capture the interaction between normal and shear components of stress, thereby improving the accuracy of predicted failure paths. Finally, the proposed criterion is validated against available experimental data and published analytical results. The comparison confirms strong agreement in predicting crack trajectory, critical energy release rate, and ultimate load capacity. The developed criterion provides valuable insights for the design and reliability assessment of microelectronic and aerospace structures where complex mixed-mode loading conditions prevail. This study highlights the essential role of the T-stress in advanced fracture mechanics formulations for predicting interfacial failure with higher fidelity.