This paper, explicitly establishes a modified creep model of a Sn3.8Ag0.7Cu alloy using a physical based micro-mechanical modelling technique. Through experimentation and reformulation steady-state creep behavior, is analyzed with minimum strain rates for different temperatures 35°C, 80°C, and 125°C. The new modified physical creep model is proposed, by understanding the respective precipitate strengthened deformation mechanism, seeing the dependency of the activation energy over the temperature along with stress and fi-nally by integrating the subgrain size dependency λss. The new model is found to accurately modelling the creep behavior of Lead free solder alloy by combining the physical state variables. The features of the creep model can be explored further by changing the physical variable such as subgrain size to establish a struc-ture - property relationship for a better solder joint reliability performance.