Tin whisker growth from titanium-tin intermetallic and the mechanism

doi:10.1016/j.jmst.2022.04.034

Abstract

Abstract:

Spontaneous growth of metal whiskers, represented by tin whiskers, has haunted tin-based platings and solder joints for decades and caused huge losses to the electronics industry. Despite numerous efforts, the underlying growth mechanism has been resisting interpretation, and the whiskering phenomenon even continues to expand its territory. Here, we report the growth of tin whiskers from a Ti₆Sn₅ intermetallic. These tin whiskers share similar characteristics with those found on the platings or solder joints, but grow more and faster, with finer diameters. After tin whisker growth, Ti₆Sn₅ retains its crystal structure, implying a dealloying process. Combining experimental and first-principles calculation results, we analyzed the growth mechanism of tin whiskers in detail, and proposed a diffusion-based growth model. The strain energy stored in Ti₆Sn₅ during deformation provides a driving force for whisker growth, and the short-circuit diffusion paths generated by such deformation accelerate whisker growth. These findings identify the critical role of intermetallic substrate in the whiskering phenomenon, shedding new light for comprehensively understanding the whisker growth mechanisms. Furthermore, the plenty and rapid growth of tin whiskers also means a new method for the preparation of one-dimensional metallic materials.

Key words: Tin whisker, Intermetallics, Vacancy, DFT

Zhihua Tian, Peigen Zhang, Yan Zhang, Jingwen Tang, Yushuang Liu, Jian Liu, ZhengMing Sun. Tin whisker growth from titanium-tin intermetallic and the mechanism[J]. J. Mater. Sci. Technol., 2022, 129: 79-86.

Figures/Tables 11

Fig. 1. Characterization of the as-synthesized Ti6Sn5 sample: (a) SEM image and EDS results (inset); (b) XRD pattern; (c) DSC curve.

Fig. 2. XRD patterns of Ti6Sn5 samples. (a) Freshly ball-milled (6 h) sample; (b) after being stored at ambient conditions (20 ± 2 °C and a humidity of 60% ± 5%) for 48 h.

Fig. 3. SEM images of the ball-milled Ti6Sn5 sample: (a) whiskers growing on the particle surfaces, and the striation characteristic of the whiskers shown in the inset; (b) a high magnification image of the selected area of (a).

Fig. 4. STEM characterization of the whiskers: (a) HAADF morphology and element mapping; (b) TEM image of the whisker in (a) and its SAED pattern (the inset) showing it is a β-Sn single crystal; (c) high-resolution TEM (HRTEM) image of the square area of the Sn whisker in (b), showing that the Sn crystal whisker has a very thin oxide scale; (d) EDS results of whisker and its substrate, respectively.

Fig. 5. SEM images showing the morphological characteristics of whiskers. (a) Whisker with a “cap”; (b) root morphology of another whisker; (c) the magnified image of the rectangular area in (b).

Fig. 6. TEM characterization of the whisker and the whisker/substrate interface. (a) bright field (BF) image and the element mapping; (b) magnified image of the area near the whisker/substrate interface, and the corresponding SAED patterns (insets); (c) HRTEM image of the rectangular area in (b); (d) HRTEM image of the small substrate particle to the right of the whisker in (a).

Table. 1. Site occupation (chemical environment) of atoms in Ti6Sn5 (P63/mmc) [38].

Inequivalent site	Bonding geometry	Bonding with atoms
Ti₁	7-coordinate	4Ti and 7Sn
Ti₂	12-coordinate	6Ti and 6Sn
Sn₁	6-coordinate	6Ti and 2Sn
Sn₂	8-coordinate	8Ti
Sn₃	9-coordinate	9Ti

Fig. 7. (a) Configuration of a Ti6Sn5 cell with one Ti vacancy or Sn vacancy. (b) DFT calculation results for single vacancy formation energy in Ti6Sn5.

Fig. 8. Poor wettability between molten Sn and Ti6Sn5.

Fig. 9. Schematic diagram of Sn whisker growth from the ball-milled Ti6Sn5 substrate.

Fig. 10. Rapid and massive growth of Sn whiskers from Ti6Sn5 sample (a) and the typical morphology of harvested whiskers (b).

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