Electron microscopy study of tin whisker growth
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The growth of tin whiskers formed on sputtered tin layers deposited on brass was studied using electron microscopy. The occurrence of whiskers appeared to be largely independent of the macroscopic stress state in the film; rather it was microscopic compressive stresses arising from the formation of an intermetallic phase that appeared to be the necessary precursor. Whisker morphology was a result of whether nucleation had occurred on single grains or on multiple grains. In the latter case, the whiskers had a fluted or striated surface. The formation of whiskers on electron transparent samples was demonstrated. These samples showed the whiskers were monocrystalline and defect free, and that the growth direction could be determined. I. INTRODUCTION
Whisker growth in electrical components and micromechanical systems has been shown to cause catastrophic failures due to short circuits and contamination. In high-reliance electric systems, such as those used in aerospace applications and medical components, it is especially crucial that whisker growth does not lead to failure. Whiskers form in several metallic systems including zinc, cadmium, antimony, indium, and tin. In this paper, the formation of tin whiskers is described. The growth of tin whiskers can occur spontaneously and has been observed by many researchers (e.g., Refs. 1–4). The mechanism of whisker growth is not well understood, and the results of some past investigations have been contradictory, partly because crucial variables were not controlled during experimentation due to the lack of understanding. This uncertainty creates problems since, with the push for lead-free electronics, tin appears to be the most feasible alternative for many applications.5,6 If the growth mechanism of tin whiskers were understood, steps could be made to guard against whisker growth and increase the operational lifetime and reliability of electrical devices. The purpose of this present research was to study the growth of tin whiskers using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and to clarify the growth mechanism. Whiskers are defined as fibrous growths with high aspect ratios, often >1000. Whiskers tend to be high in purity and contain very few external or internal defects. Typical whisker diameters range from 1 to 4 m. Whisker lengths are usually only a few millimeters, but can be up to 10 mm.1–3 Several theories have been proposed to account for the growth of tin whiskers. These are summarized as they provide a basis for comparison with the results generated J. Mater. Res., Vol. 18, No. 3, Mar 2003
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in this present study. Eshelby suggested that metallic whiskers may form as a result of surface tension forces acting on a small hump on the metal surface that is “drawn” into a whisker with the aid of a propagating dislocation loop.7 Evidence for such a mechanism would involve the presence of Frank–Read sources at the film surface. Many of the subsequent models for the growth of tin
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