Growing Tin and Other Dangerous Whiskers


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Tin (Sn) whiskers are extremely thin metal filaments, 1 to 3 µm in diameter, that grow from tin metal surfaces as straight, kinked, or even spiral single crystals of Sn. Field failures due to tin whisker growth have occurred in the highest reliability applications, as well as high-volume consumer ones. Lead-free solder and finer pitches are increasing the risk of failures caused by tin whiskers.

On June 25, 1993, the Galaxy 4 communications satellite was launched into space by rocket from Kourou, French Guiana. Its mission was to provide coverage to the United States and the Caribbean basin. On May 19, 1998 the satellite’s controller failed, shutting down telephone connections, pagers, automated teller machines, credit card terminals and some television broadcasts across the U.S.1 According to Fortune magazine,2 one NASA engineer called this satellite “a doorstop in space.” After many months of investigation, it was determined that the failure was due to whisker formation in a switch containing metallic tin, which was designed to control the processor.

In the 1980s, the FDA recalled pacemakers made by one company when they linked the high rate of failure to tin whiskers bridging between the crystal component and the case, causing complete loss of output from the pacemaker.

In 2005, there was a major shutdown at the Millstone Nuclear Generating Station in Connecticut. A single tin whisker caused a short circuit between a diode and the output trace on a circuit card. This caused the system to malfunction, signaling an unsafe drop in pressure, which resulted in the system shutting down.3

In 2003, the Swatch Watch Company began developing a lead-free soldering process in anticipation of the European Union (EU) legislation on Restriction on Hazardous Substances (RoHS). This legislation, which became law in July 2006, required lead-free electronics. In 2005, Swatch began full production of their lead-free watches and by January 2006, they discovered a serious problem. Thirty percent of the quartz crystal oscillators in the watches had tin whiskers and 5% of their product had experienced failure. They immediately applied for RoHS exemption and received it.

Tin Whiskers

Tin whiskers are extremely thin metal filaments, 1 to 3 µm in diameter, that grow from tin metal surfaces as straight, kinked, or even spiral single crystals of Sn. Their length is usually less than a millimeter but they can grow to as much as 10 mm in some cases. If they bridge the gap between conductors, they create a short circuit that can be catastrophic. This becomes more of a problem as our present and future electronics involve finer-spaced circuitry and are made with lead-free solder. The presence of lead in solder, even in small amounts, will prevent Sn whisker formation.

There are many theories on the cause of whisker formation. Studies have shown that they can grow under a variety of conditions of temperature and humidity, in air or vacuum. As metals, they carry a current and have low electrical resistivity; thus posing a threat to electronics.

In electronics, Sn is used to plate component terminations. It has been found that bright, electroplated tin finishes are more susceptible to Sn whiskers than matte finishes. Residual stresses from the plating process and from the diffusion process used to make intermetallic compounds (e.g. Cu6Sn5) are one problem; mechanical stresses such as bending the lead after plating are another. Tin-plated brass especially should be avoided. The addition of a barrier layer such as nickel (Ni) between the copper (Cu) and Sn is helpful but an added expense. Annealing the plated Sn has been shown to significantly reduce whisker formation. Reflowed tin is even better. Researchers have shown that certain crystal orientations of the surface tin grains also provide an advantage.

Lead-free solders such as tin/silver/copper (SnAgCu — SAC) and SnCu alloys are not immune to whisker formation. In one study of 8 lead-free alloys, all alloys showed whisker growth at some point in the testing. Accelerating soak conditions of 60°C and 87% relative humidity (RH) created some whiskers at 1000 hours, and all lead-free alloys showed whiskers at 3000 hours. The study also concluded, as others have also shown, that whiskers will grow under tension as well as compression.

A recent Japanese study evaluated the effect of soldering flux halide content with air versus nitrogen reflow. Quad flat packs (QFPs) are particularly susceptible to whisker growth because the leads are either tin-plated copper or tin-plated alloy 42. In this study, researchers determined that the combination of high-halogen flux content reflowed in an air environment was the most susceptible to whisker formation, while the nitrogen-reflowed parts produced no whiskers.

More articles on tin whiskers:Applied Nanotech Selected to Develop Tin Whiskers Remediation TechniquesManufacturing challenges with plastic overmolded packages Scientists Research Tin Pest and WhiskersPart 9: Lead-free System Reliability

Other Whiskers

Tin is not the only metal that grows whiskers. Zinc (Zn) whiskers from electroplated floor grates can grow as long as 1 mm/year. These floor tiles are used on raised floors in computer server rooms. When the whiskers are dislodged, they are light enough to become airborne and small enough to pass through air filters and settle inside the electronics, shorting out some circuits. NASA engineers have brought portions of the grates to various meetings and conferences and they look like pin cushions or porcupines because the whiskers are so numerous and long.

Conclusion

In this lead-free world, there are many challenges facing technologists. Whisker formation is a critical one because it can lead to catastrophic results. Will you feel safe flying in a plane that has all lead-free electronics?

ACKNOWLEDGEMENTS

NASA’s valuable tin whiskers information is available at http://nepp.nasa.gov/whisker/failures/index.htm.

REFERENCES:

1. Galaxy 4 - S. Silverstein, "Reasons for Failure Lost with Galaxy 4", Space News, Aug. 17-23, 1998, pp. 19-202. Ivan Amato, “Tin Whiskers:  The Next Y2K Problem?, Fortune, January 10, 2005.3. Millstone Nuclear Generator - P. Daddona, "Reactor Shutdown: Dominion Learns Big Lesson from a Tiny Tin Whisker", The Day (New London, CT), July 4, 2005, http://nepp.nasa.gov/whisker/reference/tech_papers/2005-dadonna-nuclear-reactor-shutdown.pdf.

Laura J. Turbini, Ph.D., is an SMT Advisory Board Member, an adjunct faculty member at the Universities of Toronto and Waterloo, and Chemistry Lab Manager and Principal Scientist at Research in Motion. She also serves on the Board of Directors at the SMTA. Contact her at (519) 888-7465, ext. 77744; lturbini@rim.com.

Read Dr. Turbini's recent columns:Our Relationship with the Environment Challenges of Lead-free Electronics The Integrated Circuit Turns 50 Halogen: The Latest Green Initiative Sulfur Meets Silver

SMT, February 2010

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