This work investigates the micromechanical properties of Sn96.5Ag3.0Cu (SAC 305) on Immersion Tin (ImSn) surface finished after subjected to high temperature storage (HTS) at 180°C for 200 to 1000 h period. Nanoindentation approach was used to measure the micromechanical properties of the solder. It was observed that the indentation depth and plastic depth were increased and a clear trend of decreasing hardness as opposed to the increasing reduced modulus as the HTS time lengthened. The plasticity-asscociated properties become stronger meanwhile the elasticity-associated properties decreased with the HTS time. These findings indicate that nanoindentation approach can clearly determine the plastic and elastic deformation occurance throughout the test.