Fine grained Cu film promoting Kirkendall voiding at Cu3Sn/Cu interface

Abstract

Three types of Cu substrates, namely, highly pure (HP) Cu foil, electroplated (EP) Cu film and vacuum sputtered (VS) Cu film were prepared to investigate the effect of surface status on Kirkendall voiding in Sn/Cu joints. It is found that the Kirkendall voids were not obvious in the Sn/HP Cu joint even after isothermally aged at 180 °C for 30 days, while a large number of voids were observed both in the Sn/EP Cu, and the Sn/VS Cu joints after a short aging time. The size of voids formed in the Sn/VS Cu was smaller relative to that in the Sn/EP Cu. Different to the EP Cu, there is no impurity incorporating into the VS Cu, while surface morphology characterization indicates that the VS Cu has the finest grains, the EP Cu follows, and the pure Cu has the coarsest grains. Kinetics analysis upon the growth of intermetallic compound (IMC) layer indicates that diffusion mechanism governs the IMC layer growth, and the finer the grain of film, the bigger the diffusion coefficient. Therefore, the finest grained VS Cu was expected to promote the unbalanced diffusion, generating much bigger vacancy flux, and further induce the Kirkendall voiding.