The gold/lead-free solder system, or Au/SnAgCu is a potential flip chip interconnect solutions for fine-pitch applications. This paper studies the interfacial microstructures and initial isothermal solid-liquid interdiffusion kinetics during the first 3 s of bonding at 230-290 degrees C. As revealed by Scanning Electron Microscopy (SEM), different morphologies of AuSn, AuSn2 and AuSn4 are observed under different bonding conditions. The initial Au-Sn solid/liquid interdiffusion kinetics is discussed with respect to its microstructures. The rate of Au consumption is used as a measure of the rate of intermetallic compound (IMC) formation. The fitted power law relationship reveals kinetically that Au consumption follows the Arrhenius relationship with a time exponent of 0.5. Isothermal aging at temperatures between 125 degrees C and 165 degrees C gives rise to activation energies and the rate of Au consumption in solid-liquid interdiffusion to be two orders of magnitude faster than solid interdiffusion. (c) 2005 Elsevier B.V. All rights reserved.