Observations of continuous tin whisker growth in NdSn 3 intermetallic compound
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In situ observation of tin whisker growth in NdSn3 compound was carried out by using an optical microscope (OM) and scanning electron microscopy (SEM). The growth rate of ˚ /s) during exposure to Sn-whisker from NdSn3 is shown to be rapid (approximately 8-15 A room ambience, and it is accompanied by formation of a new compound, Nd(OH)3, as was confirmed by x-ray diffraction. This reaction between the Sn-RE compound and trace water in room ambience has significant influence on whisker growth. There is an electron irradiation effect on whisker growth; that is, whiskers stopped growing after being observed in SEM. Therefore, it is suggested that OM be used rather than SEM to observe the continuous whisker growth. In discussion, the driving force per Sn atom for whisker growth is estimated as 1 1014 N in accordance with the whisker growth rate, and its apparent force originates from a chemical potential gradient between the released Sn atoms and the whisker. I. INTRODUCTION
Although the interesting phenomenon of spontaneous growth of tin whiskers from a Sn plating has been reported and studied for more than half a century,1–3 the growth mechanisms of the whiskers have not been fully understood. Galyon4 recently presented an overview of the research in this area, and Osenbach et al.5 published an interesting article entitled “Sn-Whiskers: Truths and Myths” that reviewed the developments in this area and pointed out the interesting “myths” of Sn whisker phenomenon. In these articles, the authors identified two of the most important issues in whisker growth: the growth mechanisms and the driving forces. Thermodynamics considerations generally conclude that the excess energy in Sn plating is the driving force of whisker growth, and whisker growth is a mechanism to reduce the excess energy of the system towards thermodynamic equilibrium. The excess energy in Sn plating could originate from residual compressive stress,6–8 crystal defects such as dislocations9,10 and grain boundaries, as well as impurity contamination including hydrogen,11,12 carbon,13,14 and others. The growth rate of whisker is an important aspect for understanding and explaining the mechanism of whisker growth. For example, Eshelby9 suggested a mechanism of dislocation loop formation and glide under a negative surface energy to explain the way of whisker growth from its root. Using this model, he estimated that the maximum whisker growth rate should be less than a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0334 J. Mater. Res., Vol. 24, No. 9, Sep 2009
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0.001 mm/year, which was significantly less than that of the experimental observation by Fisher et al.8 (approximately 1 mm/year); Furuta and Hamamura15 suggested a model for the whisker growth under the excess volume strain energy, by which the whisker growth rate was estimated to be approximately 1 mm/year; Tu16 proposed a tarnish theory (cracked oxide theory) to explain the mechanism of whisker gro
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