Microstructure and growth mechanism of tin whiskers on RESn3 compounds

An exclusive method was developed to prepare intact tin whiskers as transmission electron microscope specimens, and with this technique in situ observation of tin whisker growth from RESn3 (RE = Nd, La, Ce) film specimen was first achieved. Electron irradiation was discovered to have an effect on the growth of a tin whisker through its root. Large quantities of tin whiskers with diameters from 20 nm to 10 mu m and lengths ranging from 50 nm to 500 mu m were formed at a growth rate of 0.1-1.8 am s(-1) on the surface of RESn3 compounds. Most (>85%) of these tin whiskers have preferred growth directions of < 1 0 0 >, < 0 0 1 >, < 1 0 1 > and < 1 0 3 >, as determined by statistics. This kind of tin whisker is single-crystal beta-Sn even if it has growth striations, steps and kinks, and no dislocations or twin or grain boundaries were observed within the whisker body. RESn3 compounds undergo selective oxidation during whisker growth, and the oxidation provides continuous tin atoms for tin whisker growth until they are exhausted. The driving force for whisker growth is the compressive stress resulting from the restriction of the massive volume expansion (38-43%) during the oxidation by the surface RE(OH)(3) layer. Tin atoms diffuse and flow to feed the continuous growth of tin whiskers under a compressive stress gradient formed from the extrusion of tin atoms/clusters at weak points on the surface RE(OH)(3) layers. A growth model was proposed to discuss the characteristics and growth mechanism of tin whiskers from RESn3 compounds. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.