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Tin whisker phenomena have been observed and its potential mishaps have been recognized for more than six decades in electronic, electrical and industrial applications. For the recent several years, concerns about tin whiskers have been escalating, especially in the sector of high-reliability products. Increasing number of studies and tests has been carried out at various facilities, generating significant amount of quality reports and publications. In my recent tutorial at SMTA International Conference and Exhibition last September there were comments requesting to further discuss the plausible theories behind tin whiskers. When we examine the theories, temperature is one of the key variables that drive the phenomenon, mechanism and how the underlying science operates. So, let’s dedicate this article to the impact of external (testing) temperature and the science behind the impact.
JEDEC Solid State Technology Association (formerly known as the Joint Electron Device Engineering Council) has published several documents that address and/or are related to the testing of tin whiskers. The testing temperature as set in the documents are: ambient temperature storage (30 ±2°C and 60±3% RH); elevated temperature storage (60 ±5°C and 87+3/-2% RH); and temperature cycling are, practically speaking, as good as can be. However, refinements and selection of the testing temperature needs to be made when testing an "unknown" system. The system may possess specificities and contain certain compositions (e.g., active or inert additives [impurities]).
In metal whiskering, the physical metallurgy engaged in the process is complex and intricate—a compositional shift and /or an addition of extraneous elements to a base alloy system, regardless of sources, can change its whisker propensity enormously. As is known, the evidence of recrystallization and grain growth prior to whisker formation is presented for the “bright tin” deposit—large irregular shape grains are the precursors for whiskers. Recrystallization plays a part in tin whiskering process. To complicate further, however, 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 that is made by a given process, there is a “threshold strain” and a “threshold temperature” that cause tin whiskering.
Editor's Note: This article originally appeared in the February issue of SMT Magazine.