A student from the University of South Carolina has pinpointed the mechanism behind the formation of a mass murderer of many an electronic device – the infamous ‘tin whisker’.
The whiskering phenomenon was noticed and documented as far back as the early 20th century during the era of valve technology. It was noted that when pure, or nearly pure, tin solder was used, small (often microscopic) metal strands or whiskers grew between solder pads, sometimes causing short circuits. It was this discovery that led to the introduction of lead into solder compounds, which effectively eliminated whiskering.
Fast-forward to the early 21st century and the killer gets released on a technicality, as the widespread ban against lead in consumer electronic products, combined with the perpetual trend toward miniaturisation, has led (pun intended) to a re-emergence of the old problem. Some fear mongers have gone so far as to predict that it is only a matter of time before miniature devices built after the ban start failing en masse.
A number of suspects have been proposed for the mechanism behind tin whisker formation, but Yong Sun, a mechanical engineering doctoral student at the USC’s College of Engineering and Computing, has obtained real forensic evidence against the culprit.
Sun used scanning electron microscopy (SEM) and a process called digital image correlation (DIC) to track the deformation of the surfaces and was able to prove that the growth of whiskers is caused by high-strain gradient built up inside the device. So what does this mean exactly?
While his research paper goes into some detail, most of it would best be understood by a materials scientist. Perhaps the most salient passage that gives an overview for the electronic engineer is this:
“From the DIC results, it is proposed that strain or stress gradient, instead of an overall compressive stress field is the key for whisker growth. Results from SEM and DIC analysis also indicate that the whisker growth is a continuously dynamic process, during which the subsequent whisker is triggered by the redistribution of strain or stress field after local strain relaxation. The findings have advanced the understanding of whisker growth mechanisms and may provide insight for developing whisker mitigation technology for lead-free solder alloys.”
Sun’s findings were published in the Scripta Materialia, a materials science journal, and he won the prestigious international Acta Student Award, one of only six to receive the honour.
The importance of Sun’s work goes well beyond extending the operating life of consumer electronics; NASA has verified multiple commercial satellite failures it attributes to tin whiskers. Missile systems, nuclear power stations and heart pacemakers have also fallen victim to tin whiskers over the past several decades and they are also considered a suspect in reported brake failures in Toyota vehicles.
This discovery is likely to prompt manufacturers of lead-free products to seek ways to diffuse and mitigate the particular stresses identified by the research.
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