The influence of porosity on whisker growth in electroplated tin films
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The influence of porosity on whisker growth in electroplated tin films J.P. Wintersteina) and M.G. Norton School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164 (Received 9 June 2006; accepted 2 August 2006)
Tin whisker growth has been observed since the 1950s and has become of more interest in the past 15 to 20 years due to the desire to use lead-free solders, pure tin being a good lead-free candidate. In the same time period, failure of satellites and other devices using pure tin solders has been blamed on tin whisker growth. The accepted driving force for whisker growth is compressive stresses in films. This article reports a microstructure-control method of limiting whisker growth through the introduction of pores that permit an alternate means of stress relief.
Tin whiskers are single-crystal filaments that grow on platings. Satellites, pacemakers, and military missile and radar equipment are among the systems that have failed due to whisker growth creating short circuits or enabling plasma arcing.1 Alloying with lead can prevent whisker growth,2 but lead is an environmental hazard and has recently been banned from certain microelectronic devices in the European Union. Pure tin has good solderability, compatibility with other solder alloys, and desirable plating characteristics, making it a candidate to replace lead-containing solders, but pure tin platings are prone to whisker growth.3 Though a comprehensive understanding of microstructure-whisker growth relationships is lacking, a number of researchers have studied microstructural influences. Kakeshita et al.,4 for example, reported whisker growth was limited on films with large grains (1–8 m in diameter) that were well polygonized. Research by Schetty,5 however, has contradicted this claim. Schetty also noted reduced whisker growth on films with a (220) texture relative to films with a (211) texture.6 Toward the goal of a more complete understanding of microstructure influences on whisker growth, this paper describes the effect of film porosity. The mechanism of whisker growth is not fully understood. However, analysis of growth directions and
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0368 J. Mater. Res., Vol. 21, No. 12, Dec 2006
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whisker grain orientations suggest a recrystallization mechanism (see Refs. 2, 7–9). Lee and Lee and Choi et al. reported that grains from which whiskers grew (the whisker grains) had different orientations than the film texture. A whisker-grain orientation of (210) on a film with a preferred orientation of (321) was reported by Choi et al.8 For films with a (200) preferred orientation, whisker-grain orientations of (220), (420), and (620) and for films with a (220) texture, whisker-grain orientations of (420), (501), and (321) were observed by Lee and Lee.9 The fact that whiskers grow from grains with orientations different from the film texture suggests whisker grains
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