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

In this paper, the directional growth behavior of Sn whiskers was found to be associated with the interfacial gamma-Ag2Al IMCs at Sn-3.5Ag/Al interface. A 100 mu m thick columnar structured Sn-3.5Ag coating consisted of beta-Sn phases and gamma-Ag2Al phases was fabricated on Al substrate. The as-formed interfacial IMCs between Sn-3.5Ag and Al were confirmed as gamma-Ag2Al and mu-Ag3Al. After aging treatment, the growth direction of Sn whiskers was found to be consistent. mu-Ag3Al phases transformed into gamma-Ag2Al. The newly formed hexagonal gamma-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 gamma-Ag2Al IMCs leading to the formation of in-plane stress. The c-axis of the beta-Sn and gamma-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 degrees. 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.

Automated summary

The growth behavior of Sn whiskers was found to be connected to the interfacial gamma-Ag2Al IMCs at the Sn-3.5Ag/Al interface. Aging treatment showed consistent growth direction of Sn whiskers. Ag atoms diffused into Sn sub-grain boundaries and formed gamma-Ag2Al IMCs, influencing the growth rate.