Study of tin whisker growth accelerated by rare earth phase and the mechanism of tin whisker growth

Alloying with a suitable amount of rare earth (RE) elements in Sn3.8Ag0.7Cu solder alloy has been reported to have beneficial effects on their physical and mechanical properties. However, large sized RE-Sn phases, when adding excessive RE elements, will precipitate in the matrix of the solder. It is interesting to note that RE-Sn phases, when exposed in air, will be oxidized and rapid tin whisker growth will simultaneously appear on the surface of the oxidized RE-Sn phase. Moreover, it can be interpreted that the compressive stress resulting from the oxidation of RE-Sn phases provides the driving force for tin whisker growth, and the tin atoms released from the oxidation of RE-Sn phases becomes the growing source for tin whisker growth. So the mechanism for tin whisker growth on the surface of the oxidized RE-Sn phases can be established as follows: firstly “tin whisker nuclei” are formed, secondly “tin whisker nuclei” are pushed out of the surface to form baby tin whiskers, and finally baby tin whiskers will grow into the tin whiskers. Besides the past-reported rod-like, needle-like, thread-like tin whiskers, some tin whiskers with very special morphology, such as chrysanthemum-shaped whisker; spiral whisker; plate-like whisker; branch-type whisker; joint-type whisker and whiskers with a non-constant cross section were also found in this study. Importantly, the finding of tin whiskers with a non-constant cross section break the consensus of the past research, in which tin whisker should present constant cross section. Because the famous energy equations for tin whisker growth proposed by K.N. Tu could not explain the cross section changing phenomenon with some limitations. By analyzing the growing characteristic of tin whiskers on the surface of the oxidized RE-Sn phases, Tu´s energy equations were modified and one kind of mechanism for cross section changing of tin whisker was proposed as follows: the incoordination between the gr- wth rate and the tin atoms supply results in the cross section changing phenomenon. Finally, based on the study of tin whisker growth mechanism on the surface of the oxidized RE-Sn phase, and combined with recrystallization mechanism and oxide layer rupture mechanism, a “double stress zone” model for tin whisker growth was proposed as follows: the “low stress zone” and the “high stress zone” are required for tin whisker growth. The stress zone located around the root is “low stress zone” and the stress zone connected with the “low stress zone” is the “high stress zone”. The “low stress zone” will provide the driving force for tin whisker growth, and the stress gradient between “low stress zone” and “high stress zone” will provide the tin atoms for tin whisker growth.