The growing importance of Pb-free Sn-xAg solders and high cost of Ag has attracted the attention of researchers to perform experiment with solders with varied content of Ag element. In this study, Sn-xAg(0,1,2,3.5) solder balls on Cu substrate have been reflowed at 250/275/300 degrees C for 60 s and air cooled. Finite element method has been utilized to simulate the concentration and temperature during cooling. The experimental results suggest that the Cu flux through grain boundary diffusion is the dominant mechanism for Cu6Sn5 intermetallic compound growth in Pure Sn and Sn-1Ag solders. When the Ag content is increased, as in case of Sn-3Ag and Sn-3.5Ag alloys; the shielding effect of Ag3Sn nanoparticles is the determinant factor for affecting the thickness of Cu6Sn5 IMC. Both Cu influx rate and shielding effect of Ag3Sn nanoparticles are enhanced with the raise in pre-cooled isothermal reflow temperature. Sn-1Ag solder is observed to have the thickest Cu6Sn5 IMC.