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28 SMT Magazine • July 2014 TIN WHISKERS: CAPSulIzATIoN continues cur at much slower rates and is easily changed by the presence of second phase particles or sol- ute atoms in the structure. The external tem- perature (test temperature) drives the kinetics of defect dynamics in the tin layer by affecting stress relaxation and atomic mobility-related mechanisms. For instance, a high temperature (relative to tin's recrystallization temperature) is expected to impede the continued growth along the protruding direction, resulting in short whiskers. It is also worth noting that tin's recrystallization temperature changes with the level of its purity. In other words, when add- ing elements into tin, tin's behavior in relation to the external temperature (test temperatures) will change. The propensity of a tin deposit to grow whiskers strongly depends on its structure: grain size and the relative crystallographic ori- entation of grains in the deposit. The evidence of recrystallization and grain growth prior to whisker formation is presented for bright tin deposit—large irregular shape grains that are the precursors for whiskers. However, recrys- tallization is only a part of the tin whisker process. Further key points include: • If there is sufficient strain to drive nucleation, whisker grain nuclei may form • If there is sufficient "stored" energy, whisker may grow • To sustain growth, tin material has to be adequately supplied, and tin atoms need to be able to move to a whisker grain through passable paths • Driving forces push the tin from the free surface of the whisker grain outward, resulting in protruding whiskers • The appearance of whiskers in a range of shapes and lengths from rounded mound to long needles depends on relative nuclei sites, stored energy and temperature • But as an aggregate, two points are clear: 1) the driving forces are stress-related, and 2) internal stresses (compressive or tensile) play an important role to both whisker formation and growth • Various tests were performed under temperature cycling and electric field. The lack of harmonious testing results regarding the effects of temperature cycling and electric field on whisker growth suggests the intricate nature of the internal stresses engaged in the process. It is safe to say that tin whiskering is more than a classical recrystallization process and it is more than a classical stress relief phenomenon. I would say that, for a given tin-based material, there is a threshold strain and there is a thresh- old temperature (in lieu of recrystallization temperature) to cause tin whiskering. Concluding Remarks Our effort is to alleviate the uncertainty, ul- timately control tin whiskering propensity. Each of the mitigating tactics has its limi- tations. Combined tactics offer a high level of confidence in preventing tin whisker-related reliability issues. And each of the causes and factors as discussed does not play out by itself. An illustration is the Ni layer approach that has been proven to be effective in most cases. None- theless, a photo in one NASA report [7] reveals that Ni layer did not categorically prevent tin whisker as shown in Figure 6. Some of causes and factors as listed above are intricately interplayed and application-spe- cific. This is the challenge imposed to the evalu- ation of tin whisker propensity based on a set of testing conditions. And this is also the very fEATURE figure 7: Tin whiskers—tin crystal structure.