Microstructural evolution and the intermetallic compound (IMC) growth kinetics in an Au stud bump were studied via isothermal aging at 120, 150, and 180˚C for 300hrs. The AlAu4 phase was observed in an Al pad/Au stud interface, and its thickness was kept constant during the aging treatment. AuSn, AuSn2, and AuSn4 phases formed at interface between the Au stud and Sn. AuSn2, AuSn2/AuSn4, and AuSn phases dominantly grew as the aging time increased at 120˚C, 150˚C, and 180˚C, respectively, while (Au,Cu)6Sn5/Cu3Sn phases formed at Sn/Cu interface with a negligible growth rate. Kirkendall voids formed at AlAu4/Au, Au/Au-Sn IMC, and Cu3Sn/Cu interfaces and propagated continuously as the time increased. The apparent activation energy for the overall growth of the Au-Sn IMC was estimated to be 1.04 eV.