Growth kinetics and mechanism of Pd2Zn9 at the interface of Pd substrate with molten Sn-9wt.%Zn solder

A uniform Pd2Zn9 layer was formed at the interface between liquid-state Sn-9wt.%Zn and Pd substrate. Detailed observation revealed a unique cruciform pattern formation of the Pd2Zn9 phase. The Pd2Zn9 layer ruptured at the edges, where no reaction phase was formed. The growth kinetics of Pd2Zn9 was systematically investigated at temperatures of 220-260 °C. In particular, a linear reaction-limited growth was found in the initial reaction stage. While growing to approximately 5 μm thick, the growth rate gradually decreased over the subsequent reaction and obeyed a parabolic relationship. The growth kinetics parameters and the corresponding activation energies were determined. Additionally, the microstructure in the Pd2Zn9 layer was characterized using focused ion beam microscopy. The Pd2Zn9 layer consisted of four distinct sublayers with different grain structures. The growth behavior of Pd2Zn9 was further discussed through a series of observations on microstructural evolutions at different reaction durations.