Intermetallics-induced directional growth of Sn whiskers in Sn-3.5Ag coating on Al substrate

Highlights: • Directional growth behavior of Sn whiskers is associated with the interfacial γ-Ag₂Al IMCs. • A transitional area forms between the whisker and underneath grain. • A mathematical model is proposed to calculate the distribution of Ag atoms. • Diffusion mode for whisker growth is pipe diffusion. In this paper, the directional growth behavior of Sn whiskers was found to be associated with the interfacial γ-Ag₂Al IMCs at Sn-3.5Ag/Al interface. A 100 μm thick columnar structured Sn-3.5Ag coating consisted of β-Sn phases and γ-Ag₂Al phases was fabricated on Al substrate. The as-formed interfacial IMCs between Sn-3.5Ag and Al were confirmed as γ-Ag₂Al and μ-Ag₃Al. After aging treatment, the growth direction of Sn whiskers was found to be consistent. μ-Ag₃Al phases transformed into γ-Ag₂Al. The newly formed hexagonal γ-Ag₂Al grew out of the IMCs layer and provided the out-of-plane stress for whisker growth. Ag atoms diffused into Sn sub-grain boundaries and formed γ-Ag₂Al IMCs leading to the formation of in-plane stress. The c-axis of the β-Sn and γ-Ag₂Al is parallel, leading to the similar growth direction of the whiskers. The misorientation between the whisker and the underneath grain was lower than 2°. A transitional area formed between the whisker and underneath grain, which consists of extra half-planes of atoms. A mathematical model was proposed to calculate the heterogeneous distribution of Ag atoms. By comparing the calculated growth rate with the measured growth rate of whisker, the Sn atomic transport mode was pipe diffusion.