Morphology and Growth Kinetics of Straight and Kinked Tin Whiskers
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INTRODUCTION
TIN (Sn) whiskers have become a concern in recent years due to requirements for lead (Pb)-free soldering and surface finishes in commercial electronics. Pure Sn finishes are more prone to whisker growth than their Sn-Pb counterparts and high profile failures have been documented due to whiskers causing short circuits.[1] It is generally understood that stress within the Sn layer, particularly compressive stress, provides the driving force for whisker growth. However, a full explanation of the whisker growth mechanism has yet to be developed. It is unlikely that mechanistic explanations of whisker growth can be fully developed without rigorous, detailed characterization of tin whiskers. The purpose of this work is to develop techniques for accurately measuring and characterizing whisker growth, morphology, length, and growth angle on a statistically significant population of whiskers. Much has been learned about whiskers through scanning electron microscopy (SEM) and related techniques.[2,3] Along with traditional SEM methods, an DONALD SUSAN and W. GRAHAM YELTON, Principal Members of Technical Staff, JOSEPH MICHAEL, Senior Scientist, RICHARD P. GRANT and BONNIE MCKENZIE, Principal Technologist, are with the Sandia National Laboratories, Albuquerque, NM 87185. Contact e-mail: [email protected] Manuscript submitted March 20, 2012. Article published online October 25, 2012 Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration. METALLURGICAL AND MATERIALS TRANSACTIONS A
efficient way to study growth kinetics is through timelapse ‘‘in situ’’ SEM imaging, in which the same whiskers are followed during the growth process.[4–6] The difficulty lies in returning to the exact location of the whiskers after prolonged intervals. This can be accomplished with sophisticated indexed removable sample holders or by simply leaving the sample in the microscope chamber for long periods and using capability to accurately reposition the SEM stage using stored stage coordinates. This approach allows for simultaneous determination of growth kinetics and changes in whisker morphology or growth direction. With in situ SEM imaging, Jadhav et al.[4] showed that the removal of surface oxide was not enough to promote whisker growth, indicating that the underlying grain structure is important for whisker growth. They also studied hillock formation and found significant grain growth and surface grain rotation associated with hillocks. Tu and Li[5] showed SEM time-lapse images that confirmed that whisker growth occurs at the base of the whisker. Reinbold et al.[6] used in situ SEM imaging to observe the growth of whiskers in a bimetallic sample of pure Sn and Sn coated with Cu. They were able to measure the extruded volume of whisker material as a function of time with this technique. Jadhav et al.[4,6] indicated that whisker growth was driven by compressive stre
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