Assessment of Electrical Shorting and Metal Vapor Arcing Potential of Tin Whiskers

Tin whiskers are conductive crystal growths that form unpredictively from tin and tin alloy surfaces. The growth of tin whiskers presents a reliability concern in electronic equipment due to their potential to create electrical shorts and metal vapor arcs. Concern with tin whiskers is increasing due to the ever tightening conductor spacing in smaller electronic products and the increased use of pure tin and lead-free tin alloys. While tin whiskers present a failure risk for electronics, a tin whisker mechanical bridging between two differently electrically biased conductors doesn't guaranteed electrical shorts due to surface films on tin whisker and conductors. The voltage must exceed a threshold level in order to produce the current flow through the tin whisker. However, the influence of contact force and presence of surface contaminations on breakdown voltage of tin whiskers has not been adequately investigated. Furthermore, whisker-induced electrical shorts can initiate destructive metal vapor arcs. The potential for metal vapor arc formation is affected by several factors, including whisker geometry, bias voltage and pressure. Previous studies demonstrated metal vapor arc formation using gold- and tin-wires; however, material and geometry differences between these test articles and actual tin whiskers have not been examined. Further, a practical guide for assessing the potential for tin whisker-induced metal vapor arc formation has not been provided. This dissertation provides characteristics and assessment of tin whisker-induced electrical shorts and metal vapor arcs. The breakdown voltage of tin whisker was measured using gold- and tin-coated probes to characterize the influence of two different contact materials on breakdown voltage. As a part of this effort, the effect of contact force on breakdown voltage and its current-voltage characteristics related with the failure mode and the possibility of electrical shorting by tin whiskers were also investigated. With regards to tin whisker-induced metal vapor arc formation, the effect of whisker geometry, bias voltage and pressure was investigated. Based on the experimental evidence, a metric defined as a function of bias voltage and resistance was proposed and the logistic regression model that can assess the likelihood of tin whisker-induced metal vapor arc formation was developed

Evaluation of Environmental Tests for Tin Whisker Assessment

Tin whiskers are electrically conductive crystalline structures of tin that over time may grow outward from tin-rich surfaces and present a reliability hazard to electronic systems. While the problem has been known for decades, no satisfactory explanation of whisker growth mechanisms exists, leaving the industry to create whisker-assessment tests based on empirical data gathered under various environmental storage conditions controlled for temperature, humidity and temperature cycling. The long-term predictability of these environmental storage tests has not been addressed and the accuracy of these tests in foreseeing whisker growth is unclear. In this thesis, different tin finishes are assessed for whisker growth in accordance with existing environmental test standards and compared to growth seen in ambient storage conditions. The results indicate that environmental tests may over-predict, under-predict, or show little distinguishable growth as compared to ambient-stored tin finishes. In conclusion, environmental tests are not a reliable method of assessing future whisker growth

Effect of Cu Substrate Roughness and Sn Layer Thickness on Whisker Development from Sn Thin-Films

The effect of copper substrate roughness and tin layer thickness were investigated on whisker development in the case of Sn thin-films. Sn was vacuum-evaporated onto unpolished and mechanically polished Cu substrates with 1 µm and 2 μm average layer thicknesses. The samples were stored in room conditions for 60 days. The considerable stress – developed by the rapid intermetallic layer formation – resulted in intensive whisker formation, even in some days after of the layer deposition. The developed whiskers and the layer structure underneath them were investigated with both scanning electron microscopy and ion microscopy. The Sn thin-film deposited onto unpolished Cu substrate produced less but longer whiskers than that deposited onto polished Cu substrate. This phenomenon might be explained by the dependence of IML formation on the surface roughness of substrates. The formation of IML wedges is more likely on rougher Cu substrates than on polished ones. Furthermore, it was found that with the decrease of layer thickness, the development of nodule type whiskers increases due to the easier diffusion of other atoms into the whisker bodies