The role of dissolved hydrogen and other trace impurities on propensity of tin deposits to grow whiskers

Early investigators of tin whiskers concluded that hydrogen dissolved in tin electrodeposits strongly influences the formation of tin whiskers. Early experiments demonstrated that baking immediately after plating mitigated whiskering and this effect was attributed to hydrogen relief. Baking immediately following plating is now widely used to mitigate whiskering, but this effect is routinely attributed to stress relief. A review of recent experimental and theoretical work on tin whiskering, and on the behavior of hydrogen dissolved into metals has been performed, in an effort to discern whether dissolved hydrogen merits renewed consideration. Recent studies suggest that lower cathode efficiency during electroplating, which implies a higher rate of hydrogen co-deposition, also increases whiskering propensity. Other studies show that the hydrogen content of tin electrodeposits can vary by more than a factor of 20. Recent tin whisker growth theories identify the grain boundaries as critical for the whisker phenomenon. Extensive experimental and theoretical work indicate that dissolved hydrogen can have a significant effect on grain boundaries in a wide range of metals. Studies on high-temperature behavior of steels and superalloys indicate that the presence of trace impurities concentrated within grain boundaries have a significant effect on macroscopic properties. Since tin is operating in a high-temperature regime at room temperature, trace impurities should also have a significant impact on macroscopic properties that are dependent upon grain boundaries, such as whiskering. It is recommended that tin whisker investigators consider hydrogen content and the content of other trace contaminants as part of their experimental design.