Microstructural evolution and the intermetallic compound (IMC) growth kinetics in an Au stud bump were studied via isothermal aging at 120, 150, and $180^{circ}C$ for 300hrs. The $AlAu_4$ phase was observed in an Al pad/Au stud interface, and its thickness was kept constant during the aging treatment. AuSn, $AuSn_2,;and;AuSn_4$ phases formed at interface between the Au stud and Sn. $AuSn_2,;AuSn_2/AuSn_4$, and AuSn phases dominantly grew as the aging time increased at $120^{circ}C,;150^{circ}C,;and;180^{circ}C$, respectively, while $(Au,Cu)_6Sn_5/Cu_3Sn$ phases formed at Sn/Cu interface with a negligible growth rate. Kirkendall voids formed at $AlAu_4/Au$, Au/Au-Sn IMC, and $Cu_3Sn/Cu$ 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.