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Likelihood of Metal Vapor Arc by Tin Whiskers
September 5, 2012 |Estimated reading time: 10 minutes
Editor's Note: This article originally appeared in the August 2012 issue of SMT Magazine.Tin whiskers are hairlike, electrically conductive structures that grow spontaneously and unpredictably from tin and tin alloy surfaces. Tin whiskers present a reliability concern due to their propensity to induce failures in electronic products and systems [1, 2]. The most common concern associated with tin whiskers is unintended electrical shorting due to a whisker bridging adjacent conductive surfaces. Whisker-induced electrical shorts may be intermittent or permanent depending on the electrical circuit created by the whisker [3]. Under elevated voltage potentials with sufficient available electric current, a bridging tin whisker can initiate a destructive metal vapor arc. Metal vapor arcs generate temperatures capable of melting metals and incinerating plastic. Metal vapor arcs can form at ground conditions and in a vacuum. A reduction in atmospheric pressure has been shown to reduce the voltage required to initiate a metal arc [4]. Destructive failures on at least three satellites have been attributed to tin whisker-induced metal vapor arcs [5].
Metal vapor arcs form when the vaporized metal atoms are ionized in the presence of a sufficiently strong electrical field, causing electrons to flow to the cathode and ionized metal atoms to flow to the anode [4]. With regard to metal vapor arcs, the reduction in the number of gas molecules in the arc path increases the likelihood of arc formation. A reduction in pressure results in reduced gas density and promotes the vaporization of tin whiskers.In 1992, Van Westerhuyzen et al. [6] found that 10-A fuses inside a 30-V relay were blown out during a thermal vacuum test due to a metal vapor arc caused by a tin whisker. The failure was later simulated using gold wires with diameters ranging from 18 to 25 µm instead of tin whiskers and established a vapor arc under 0.5 torr. Mason and Eng from Aerospace Corporation [7] showed that a tin wire can generate a metal vapor arc in atmospheric pressure (760 torr) with 28 V. They also demonstrated that the minimum voltage for sustaining a tin whisker vapor arc in a vacuum (0.2 × 10-6 to 2 × 10-6 torr) is 4 V for tin wires 25 to 50 µm in diameter. However, tin and gold wires that were used in previous studies have a larger diameter compared to the majority of real tin whiskers. In addition, several factors such as voltage, pressure, whisker geometry, and conductor gap that can also influence the possibility of establishing a vapor arc have not yet been studied.
To examine the likelihood of metal vapor arc formation being caused by a tin whisker, tests were conducted using harvested test specimens containing tin whiskers. These tests utilized fixed voltages and pressures to determine regions at highest risk for vapor arc formation.
Experiments To simulate a tin whisker bridging two conductive surfaces, a tin whisker was harvested from a tin-finished surface with abundant whisker growth and attached between two tin-plated copper electrodes using conductive silver paint, as depicted in Figure 1. Figure 1: (a) Metal vapor arc test specimen using tin whisker; (b) close-up of detached whisker on tin-plated copper electrodes.
Due to the natural variation in harvested tin whiskers, whisker geometry, including the length and diameter of the whiskers, as well as conductor gap (the spacing between the edges of the tin-plated copper electrodes) were measured by a scanning electron microscope (SEM) prior to each test. The whisker length was measured between points where the whiskers made contact with the conductive silver paint. In addition to the whisker geometry, the electrical resistances of test specimens were measured by a milliohm meter using a four-probe method.
Individual test specimens were placed in under a vacuum jar and connected to a 30A circuit breaker in order to protect the circuit from potential damage resulting from the arc test. Figure 2 shows a schematic of the electric circuit used for the tests. The test specimen was subjected to a positive voltage potential (+V) and a negative voltage potential (-V) using lead-acid batteries. The positive voltage potential was provided from 0 to 37.5 V in 12.5 V steps. The negative voltage potential provided with a single lead-acid battery through a voltage divider that provided voltage from 0 to 12.5 V in 2.4 V steps. The arc tests were conducted at 760 torr (sea level) and 74 torr (approximately 52,000 feet above sea level) pressure conditions. Figure 2: Schematic of the electric circuit for tin whisker arc test.
Results and Discussion The metal vapor arc was verified by the observation of light generation during the test and the inspection of electrodes for arc damage, such as burn marks or craters (cathode spots). Figure 3 shows a specimen prior to and after the arc test, in which a vapor arc was not initiated. On the surface of the tin-plated electrode, the remains of a melted tin whisker were observed. In this case, current flow did not vaporize the tin whisker before the molten whisker was ruptured by surface tension and pulled downward due to gravity. Typically, if a metal vapor arc is initiated, metal flow and arc damage are observed on the surface of electrodes, as depicted in Figure 4. Figure 3: Specimen in which vapor arc was not initiated: (a) prior to the test; (b) after the test; and (c) close-up of melted whisker after the test on cathode-side electrode.Figure 4: Specimen in which vapor arc was initiated: (a) prior to the test; (b) after the test; (c) close-up of arc damage on anode-side electrode; and (d) close-up of arc damage on cathode-side electrode.
Because the vapor arc event is usually lasts only a few milliseconds, a high-speed camera was used to film arc events at a 180 kHz frame rate. In these tests, two types of vapor arc events were observed. Type 1 was a vapor arc initiated and extinguished within microseconds. In type 2, the vapor arc was initiated and propagated along the gap between tin-plated electrodes. Figure 5 shows a type 1 vapor arc event at 760 torr with 50 V, with the light emission captured by high-speed camera lasting 90.59 µs. In the type 1 event, it appears that the surrounding gas molecules extinguished the initiated arc. Most of the gases found in air (14 eV) have significantly higher ionization potential than tin (7.3 eV). To sustain the arc, a higher voltage and/or current is required to ionize the surrounding quenching additives. Figure 5: Vapor arc test at 50 V in 760 torr: vapor arc initiated and extinguished.
Under reduced pressure conditions (74 torr) with 50 V, type 2 vapor arc events were observed. An example is depicted in Figure 6. Here, the initiated vapor arc was sustained and propagated by supplying additional metal ions from surrounding tin-plated copper electrodes. With the significantly reduced presence of the surrounding gases, the arc was sustained by metal ions cast off from the electrode surfaces that melted under the extreme temperature. In this particular case, both the surface-plated tin and the underlying copper were involved in the vapor arc, which implies that the arc temperature was greater than the melting temperature of copper (1,083°C). Figure 6: Vapor arc test at 50 V in 74 torr: vapor arc sustained and propagated.
Among the physical and electrical parameters, it was observed that the resistance of the test specimen was a good indicator which can determine the likelihood of vapor arcing by tin whiskers. As shown in Figure 7, it can be seen that vapor arc can be initiated if the resistance of the test specimen is less than a certain value which varies depending on the pressure conditions. However, the plot shows an overlap of in resistances that produce vapor arcs and with resistances that do not. From the tests, vapor arcing was not initiated if the resistance of the test specimen are higher than 30 Ω for the tested voltages and pressure conditions. The resistance of the whisker determines the resistance of the test specimen due to its relatively high level of resistance compared to the silver paint and tin-plated copper electrodes in the test specimen. Theoretically, the resistance of a whisker is a function of whisker geometry, which can be estimated using (1):
where Rwhisker is the theoretical whisker resistance, Sn is the resistivity of ρSn(1.09 × 10-7 ohm·m), and Lwhisker and Awhisker is the length and cross-sectional area of the whisker, respectively. Figure 7: Effect of resistance of test specimen on the likelihood of a vapor arc being caused by a tin whisker.
While the resistance of the test specimen appears to be the strongest factor in determining the likelihood of vapor arc formation, the electrical resistance value does not consider the role of voltage in arc formation. Therefore, some combination of the test parameters may be required to determined metal vapor arc formation. To simultaneously consider multiple parameters, we created the “arc current metric” as a metric for determining metal vapor arc formation. The arc current metric is the theoretical maximum current and is defined as (2):
where Vapplied is the bias voltage, and Rspecimen and Rtest_circuit are the measured resistances of the arc test specimen and the test circuit with wires, respectively. The measured resistance of the test circuit was 82.83 mΩ. By examining metal vapor arc formation and the arc current metric, a threshold value can be established. The vapor arc possibility depending on the arc current metric is shown in Figure 8. At 760 torr, the tin whiskers initiated vapor arcs when the arc current metric was higher than 3 A. At 74 torr, the metal vapor arcs initiated when the arc current metric was greater than 2 A. Figure 8: Effects of arc current metric on the likelihood of the vapor arc by a tin whisker.
Conclusion
The metal vapor arc behavior of tin whiskers was investigated under various pressure conditions using test specimens constructed with real tin whiskers. Experimental results showed that the electrical resistance of the test specimen with the tin whisker appears to be the strongest indicator of whether a metal vapor arc will occur. Based on the experimental evidence, a metric, defined as the ratio of bias voltage to initial theoretical circuit resistance, was proposed to characterize the potential for vapor arc formation by tin whiskers. Test data indicated that arc formation is highly probable above a threshold value of the arc current metric. As a result, the arc current metric may be useful in designing circuits that minimize the vapor arc propensity due to tin whiskers.
Acknowledgments The authors would like to thank the companies and organizations that support research activities at the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland, College Park. The authors would also like to thank the members of the Electronics Products and System Consortium (EPSC) at CALCE for their support of this work.
References:1. S. Ganesan and M. Pecht, “Lead-free Electronics,” Hoboken, NJ, Wiley-Interscience, 2006. 2. G. Galyon, “Annotated tin whisker bibliography and anthology,” IEEE Transactions on Electronics Packaging Manufacturing, Vol. 28, pp. 94-122, 2005.3. D. Pinsky, M. Osterman and S. Ganesan, “Tin whiskering risk factors,” IEEE Transactions on Components and Packaging Technologies, Vol. 27, pp. 427-31, 2004. 4. “HS601 Satellite Failures,” Satellite News Digest.5. P. Slade, “Electrical Contacts: Principles and Applications,” New York: Dekker, 1999. 6. D. Van Westerhuyzen, P. Backes, J. Linder, S. Merrell, and R. Poeschel, “Tin whisker induced failure in vacuum,” Proceedings of International Symposium for Testing and Failure Analysis, pp. 407-412, 1992.7. M. S. Mason and G. Eng, “Understanding tin plasmas in vacuum: A new approach to tin whisker risk assessment,” Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, Vol. 25, pp. 1562-1566, 2007.Sungwon Han received the B.S. and M.S. degrees in Mechanical Engineering from Chung-Ang University, Korea. He is currently a Ph.D. candidate and a graduate research assistant at the Center for Advanced Life Cycle Engineering (CALCE) in the Mechanical Engineering department of the University of Maryland, College Park. His research areas include reliability of electronic products, mechanical and electrical characterization of IC packages, and risk analysis, and characterization of failures caused by tin whiskers.Michael Osterman is a senior research scientist and the director of the CALCE Electronic Products and System Consortium for the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland. He heads the development of simulation assisted reliability assessment software for CALCE which provides life expectancy analysis of PCBs and components. He served as a subject matter expert on phase I and II of the Lead-free Manhattan Project sponsored by Office of Naval Research in conjunction with the Joint Defense Manufacturing Technical Panel (JDMTP). He has conducted multiple experiments on temperature cycling, vibration, and mechanical bend of electronic assemblies. He is a member of ASME, IEEE IMAPS, and SMTA. Professor Michael Pecht has an M.S. in Electrical Engineering and an M.S. and Ph.D. in Engineering Mechanics from the University of Wisconsin at Madison. He is a Professional Engineer, an IEEE Fellow, an ASME Fellow, an SAE Fellow, and an IMAPS Fellow. He is the founder of Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland, which is funded by over 150 of the world’s leading electronics companies. He is also a Chair Professor in Mechanical Engineering and a Professor in Applied Mathematics at the University of Maryland.