An Introduction to Tin Whiskers
From Pacemakers to Power Plants, Tiny Metal Growths Cause Problems
Nov 29, 2007
Tin has been known for centuries--it is an important component of bronze and pewter. But even though tin is one of the lowliest metals, it plays a huge role in the largest, most sophisticated electronic systems made.
That's because tin mixed with lead becomes solder. Both metals are soft, and can easily be melted with a hot soldering iron. This molten mixture is used to "glue" metal to metal. Solder is found in almost every kind of machine, connecting metal joints. It is also used to attach electronic components and wires onto circuit boards.
But, unfortunately, wherever there is tin, there’s the possibility of whiskers.
Growing Tin Whiskers
Tin whiskers grow slowly, usually in a perpendicular direction, from metallic surfaces. The process can take years. But once the whiskers get long enough to reach across tiny gaps inside electronic devices, they become a potential disaster.
Because the whiskers are metal, they conduct electricity. If they grow close enough to a neighboring device, they cause electricity to flow on a completely different path. This interrupts the electrical signals and causes the circuit to fail.
Tin Whiskers Cause Failures
Although scientists first noticed the whiskers in the 1920s, they still can't explain what makes them grow. Some think that they start near stress points.
But even though no one has figured out what causes tin whiskers, there are plenty of documented cases of the problems they create. According to a white paper on tin whisker experiences, published by the Center for Advanced Life Cycle Engineering at the University of Maryland, tin whiskers have caused major damage to:
- Heart pacemakers;
- Air Force radar systems;
- U.S. Missile Program, including the Patriot Missile;
- Commercial satellites;
- Nuclear power plants; and
- The Space Shuttle.
Preventing Tin Whisker Disasters
For decades, engineers have known that mixing lead with tin prevents whisker growth. But as they learn more about the dangers of lead leaching into the environment from discarded electronic waste, they are trying to eliminate lead in electronics. To do this, engineers are testing different mixtures of metals for different uses. Some are even using pure tin, unaware of the lurking danger.
Whiskers are especially a problem in places where a failure could cause the loss of expensive or critical government systems. If a nuclear power plant experiences a short circuit, the reactor could melt down and leak radioactivity. If a defense missile loses power, a critical military mission could fail. If a satellite falters, important communications and data will be lost.
Getting the Lead out of Tin
Yet, with the push for environmentally sound "un-leaded" electronics, designers must keep up a vigilance against pure tin components in their systems. Even when they specify in a contract that only mixtures of tin must be used, their suppliers may not comply, or may not test components from sub-contractors.
And to make things worse, now that the European Union, along with other regulatory bodies, have decreed that no lead will be allowed in electronic components, designers will find it more and more difficult to find components in the marketplace that are safe to use. The result will be higher costs for custom-made components, and higher costs for critical electronics applications.
Sources
The Center for Advanced Life Cycle Engineering, University of Maryland
Daddona, Patricia, "Reactor Shutdown: Dominion Learns Big Lesson From A Tiny "Tin Whisker," TheDay.com, New London, CT, 7/4/2005.
The NASA website.
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