Abstract: Solder bump was fabricated with Sn-Pb eutectic solder droplet on Au/Ni/Cu pad.The solder/pad was then subject to reflow soldering and aging at 125℃.The IMC evolution at solder/pad interface during this process,especially the formation and distribution of Au-Sn compound were investigated.The results showed that Au-Sn compound forms at solder/pad interface during contact reaction,and Au does not re-act fully with solder droplet.During the subsequent reflow soldering,all Au layer at interface is consumed,Ni layer reacts with solder,which leads to the formation of Ni₃Sn₄ compound at the interface.Acicular AuSn₄ can be found in the solder bulk.AuSn₄ particles redesposites at the interface as a continuously(Au_xNi_1-x) Sn₄ layer during aging at 125℃.The redesposited(Au_xNi_1-x) Sn₄ at solder/pad interface follows decomposition-diffusion mechanism.At the same time,a lead-rich phase emerges with AuSn₄ redeposition at the interface.The shear strength of soldered joint is mainly determined by this evolution and distribution of Au-Sn compound.