Virtual lifetime estimation is growing in importance, as replacing physical tests by simulations leads to cost reductions in the development of microelectronics assemblies. However, the predictions made by fatigue models often differ significantly from the lifetimes recorded in physical tests. Tuning these models is not straightforward, and results are often accurate only in specific test cases. Deviations may arise from manufacturing tolerances in the soldering process which can lead to deviations in the solder joint geometry. These include variations in the size of the copper pad area or in the volume of solder material. These factors, which have impacts on estimated lifetimes, are not fully understood. This paper assesses the impact of solder geometry in parallel with that of thermal cycling properties on estimated lifetimes. It is demonstrated that the shape and thermocycling properties of the solder joint significantly affect the thermomechanical lifetimes of surface-mounted resistors.