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Exploring tiny structures with big consequences

Probing the rapid mass transport mechanisms of tin whiskers.

Even when not shaving, Edmund B. Webb III and Yibo Wang worry about whiskers – tin whiskers, to be specific, that cause significant material destruction.

Webb, associate professor of mechanical engineering and mechanics, and Wang, a graduate student, study tin thin films, which are used in electronic circuits as coatings on contacts and solders for joining components. The films are found in everything from cell phones to satellites.

It’s widely known that the films grow “whiskers,” or thin crystalline structures, about 1/100th the thickness of human hair and long enough, sometimes, to form circuits between contacts and cause damage and loss.

NASA reports tin whiskers have caused system failures on Earth and in space. In a handful of incidents, whiskers have caused complete loss of satellite functionality. Typically, says Webb, whiskers are not a problem for products like cell phones or computers, which most people upgrade every few years. But they can become a concern in components and systems that sit idle for years and then must be restarted.

Lead had been used to solve this problem for more than 50 years, but its use today is precluded by health concerns. Webb and Wang are collaborating with Sandia National Laboratories to understand the rapid mass transport mechanisms of whisker growth.

“Most metal films are composed of microscopic crystallites, or grains, that grow together and merge during the formation process,” says Webb. “Regions where grains merge are called grain boundaries, and we suspect these regions control the formation of whiskers. We are studying the influence of atomic transport along grain boundaries to determine where on a surface a whisker will form. We also want to understand how stress in a thin film – known to play a role in driving whisker formation – influences grain boundary transport.”

Their models are revealing for the first time that anomalously rapid atomic transport for some boundaries directly determines where a whisker forms. If this is true, then preventing whiskers requires preventing the formation of the anomalous boundaries.

This phenomenon may explain why adding lead mitigates whisker formation since lead atoms are expected to reside in grain boundaries. The next step will be to use models to suggest non-lead-based processing strategies for achieving this same effect.