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22 SMT Magazine • July 2014 also determines the thickness of the coating layer. The organic content, grain size, and sur- face morphology highly depend on the coating chemistry and process parameters, including the type of electrolyte, additive/brighteners, current density, process temperature, and the process control. For instance, high current den- sity allows faster rate of plating, and a faster rate may impede the tin atoms' ability to rearrange to a low-energy state, which contributes to sub- sequent whiskering conditions. Take bright tin as an example, which is re- portedly the most susceptible to tin whisker. Its high susceptibility is largely attributable to the high residual stresses within the tin plating caused by the plating chemistry and process. The added brighteners in making tin bright may serve as nucleation sites and may prevent tin from settling into the low energy state to form large grains. The resulting small grains provide more grain boundaries that in turn offer diffu- sion paths for tin. Plating Process vs. Coating Crystal Structure The effect of microstructure in terms of grain size on whiskers has been observed—equi-axed crystal structure (Type C in Figures 4 and 5) and thin IMC minimizes whiskers [3] . It is hypothe- sized that as grain size reduces below 1 micron, the internal stress and the driving force for re- crystallization will be built up. This condition creates high whisker propensity. Thickness of Tin Coating It is postulated that it takes a proper thick- ness for whiskers to grow. To make a statement on the correlation between the thickness of tin layer and the whisker propensity is indeed over- simplistic. Yet, some results do support that a too thick coating can bury whiskering tendency and a too thin coating can shortchange the materials needed to grow whiskers. The proper thickness also is related to stress distribution ability. Temperature Effect Temperature drives the kinetics of defect dy- namics in the tin layer by affecting stress relax- ation and atomic mobility-related phenomena. For example, high temperature relative to tin's recrystallization temperature is expected to im- pede the continued growth along the protrud- ing direction, resulting in short whiskers. Overall, from the atomic lattice structure standpoint, most of the above sources do not play by itself in the tin coating layer, rather they are intricately interplayed. This is the very challenge imposed to the evaluation of tin whisker propensity based on a set of testing conditions. TIN WHISKERS: CAPSulIzATIoN continues fEATURE figure 4: Tin whiskers—effect of coating crystal structure.