Microscopy Study of Tin Whiskers
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FEATURE
Microscopy Study of Tin Whiskers Robert F. Champaign • Robert R. Ogden
Submitted: 28 July 2010 / Published online: 14 September 2010 Ó ASM International 2010
Abstract Tin whiskers are electrically conductive crystalline structures that may spontaneously grow from pure tin-plated surfaces. Since pure tin-plating is used as an electronic component lead finish, there is an inherent possibility that a short circuit may be formed between the component’s leads by tin whisker growth. The focus of this article is to share tin whisker observations that the authors have encountered over years of tin whisker research. Keywords Tin whiskers Microscopy Scanning electron microscope (SEM) Electron microscopy Failure analysis Tin whiskers have gained notoriety as an issue since the European Union passed legislation to restrict the use of several hazardous substances. One significant point of the legislation was to restrict lead, which had been extensively used in electronic products for applications such as solder, component surface finishes, printed wiring board surface finishes, etc. The legislation forced the commercial electronics industry to begin a search for compliant, environment-friendly materials for use in manufacturing electronic products. Available lead-free alternatives for component surface finishes include nickel/palladium/gold, nickel/gold, and pure tin. Due to its low cost compared to the other options, numerous companies selected pure tin. The use of pure tin surface finish poses a reliability risk for one R. F. Champaign (&) Raytheon Failure Analysis Lab, NCS Division, 2501 West University, McKinney, TX 75071, USA e-mail: [email protected] R. R. Ogden Raytheon, Space and Airborne Systems, 2501 West University, McKinney, TX 75071, USA
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very good reason. Lead in tin-based solders and alloys suppresses tin whisker growth, while pure tin surface finishes have the potential to grow whiskers [1–6]. Tin whiskers are electrically conductive tin crystal structures that can spontaneously grow from tin-plated surfaces. The main reliability risk associated with tin whiskers is that their presence among electronics creates the potential for transient or long-term electrical short circuits. With certain environmental and electrical conditions, metal vapor arcing also occurs. Tin whiskers can also easily break loose from components and assemblies, generating conductive debris [5, 6]. The most compelling theory for tin whisker growth is that it occurs due to a compressive stress relief mechanism in the tin plating. Sources of stress in tin include plating residual stress, mechanical loading, scratches in the plating surface, plating surface oxidation, intermetallic formation, and mismatches in the coefficient of thermal expansion between the plating and the underlying substrate [4–7]. Tin whisker growth can be a serious reliability concern, especially in space and military equipment where reliability is a key performance parameter. There are many documented cases in which tin whiskers have resulted
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