The interactions between Au and Cu across a Ni barrier layer have been studied using a wide range of Ni thickness. Above 300 °C, extensive interdiffusion between Au and Cu has been observed, with little involvement of the Ni layer. The interdiffusion between Au and Cu results in a nearly continuous change and uniform distribution in composition for both the parent Au and Cu layers, different from those of the binary Au/Cu system. Reaction mechanisms involved are suggested, and the impact of such interactions on the packaging metallurgy is discussed.