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22 SMT Magazine • February 2015 number of whiskers per unit area; d is whisker diameter; L is the whisker length; and f(L) is a non-linear function of length, that being 1 for a whisker length of 1 micrometer and 500 for a whisker length of 50 micrometers. Using this metric, they demonstrated quantitatively that compressive stresses are a major driving force in tin whisker formation. Consistent with the compressive stress driving force theory, the re- searchers showed that bright tin platings tend to have WIs in the 100,000 range, whereas sim- ply changing to satin bright plating reduces the WI to less than 10. Large grains in the tin plat- ing are associated with low stresses, as is a matte finish, so large grains are another positive indi- cator for mitigating tin whiskers. Xu, et al., also showed that reflowing the tin will minimize compressive stresses. In ad- dition, reflowing can form a CuSn intermetal- lic that will minimize copper diffusion into the tin. These researchers also suggest that a thick tin layer is better because the diffusion of the copper in the tin will take longer to reach the surface, where the copper will cause compres- sive stresses in the tin that will exacerbate whis- kering. The addition of a nickel layer between the copper and tin resulted in WIs close to 0, even when bright tin plating was used (Figure 5). The nickel diffuses more evenly and does not diffuse as aggressively into the tin as does cop- per, hence, minimizing compressive stresses. It is not as widely known that over time this beneficial effect of nickel may diminish. So, for long service life applications, the nickel layer approach may not be adequate. Dimitrovska [12] demonstrated that pulsed composite plating with multiple NiSn layers provided improved performance over a standard nickel layer. It has been shown that alloying the tin plat- ing with small amounts of some metals, espe- cially 2–4% bismuth [13] , significantly reduces the growth of tin whiskers. It has been suggested that bismuth and other alloying metals, includ- ing lead, alter the structure of the tin so that cooper intermetallic growth is more even and reduced, minimizing compressive stress gen- eration in the tin, hence subduing tin whisker growth. The tremendous benefit of alloying tin with lead can be seen in Figure 6. Similar results would be expected with 2% bismuth. Coatings have shown some effectiveness in tin whisker mitigation. Osterman [14] investi- gated tin whisker mitigation effects of machine- sprayed acrylic and silicone, hand-sprayed urethane, and vapor-deposited parylene and ALD-Cap 05TA200. At the end of his studies, he found that only the parylene coating did not have any penetrating whiskers. He also stressed the importance of full and thick coverage of the coating and emphasized that past work on flat rISK aNd MITIGaTION FOr TIN WHISKerS aNd TIN PeST continues Feature Figure 5: The addition of a nickel layer between the copper and tin dramatically reduces the Wi. Figure 6: The dramatic reduction in tin whisker growth as a result of a tin-lead solder dip. Similar results would be expected with 2% bismuth in the tin plating. The image is from the nASA Tin Whisker website.