Abstract: The interfacial intermetallic compound(IMC) evolution in micro-soldered joints in thermal aging process of flipchip assemblies was investigated.The results show that all (Ni, Cu)₃Sn₄ on the Ni pad interface are transformed into (Cu, Ni)₆Sn₅ after 300 h for thermal aging due to the effect of Cu atoms diffused from the Cu pad interface on the (Ni,Cu)₃Sn₄.On the Cu pad interface,a thin layer of Cu₃Sn forms on the interface between the Cu pad and (Cu,Ni)₆Sn₅ after 100 h for aging, however in the subsequent thermal aging,Cu₃Sn experiences little growth because of the limitation effect of Ni on its growth.The growth rate of (Cu,Ni)₆Sn₅ on the both pad interfaces are fast before 100 h,and after 100 h,it become slower and slower.Furthermore,as the aging time increases,the interface of (Cu, Ni)₆Sn₅ grain inclines to be flat.