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Tin Whiskers: Short Circuit Mitigation Strategies

What is a Tin Whisker?
A whisker is a long, electrically conductive, hair-like structure that grows out of the surface of some metals such as Tin, Zinc, and Cadmium. They can come  in many different shapes and sizes, but in order for a structure to be considered a whisker, it must be at least 0.0004 inches (10 μm) long and 2 times taller than it is wide.
Human hair compared to metal whisker
Tin whiskers can grow long enough to be visible to the naked eye—some have been recorded to be longer than 0.300 inches (8 mm). Tin whiskers are rather sturdy structures. They are not easily dislodged and will merely bend and flex in a gust of wind, shock or vibration. They are also attracted by static electricity and their growth is not impeded by grease.

Tin whisker scale

How does it get there?

One physical property of tin is that the individual atoms can move freely throughout the metal. This is called diffusion. As the atoms diffuse either through bulk diffusion (movement of atoms through grains of the metal) or through grain boundary diffusion (movement of atoms on the border between grains) they can accumulate in certain areas. As more and more atoms build up in these areas, they begin to force the others that came before them out of the surface, creating a whisker. Atoms can continue to congregate at the base of the whisker causing further growth.

Tin whisker growth 02
In this view,  two large whiskers are visible as well as several smaller ones

Nothing about whisker growth is predictable. As of yet no one has been able to predict whether or not whiskers will definitely form on a tin plated part, though for their studies scientists make use of plating baths that have a history of producing  whiskers. Whiskers can begin growing as early as a few minutes after plating to months or even years. They can grow intermittently, experiencing both growth spurts and periods of dormancy. They do not even grow in a predictable direction. Whiskers can change direction as they grow, sometimes appearing to rotate as they grow.
Scientists who have studied tin whiskers have not been able to pinpoint what causes the tin atoms to collect in certain places and begin to grow outwards from the material. The current popular theory is that whiskers are caused by internal stresses within the material. The internal stresses could be caused by the intermetallic layer between the tin plating and the substrate, mechanical surface damage, changes in temperature or humidity, or a number of other potential causes.

How can we inspect for tin whiskers?

tin whisker Westinghouse nuclear
This is a potentiometer found by Westinghouse Nuclear in 2005 covered in metal whiskers, not dust.

To the untrained eye tin whiskers can look like
dust, so inspectors must resist the urge to clean parts before inspection or they may destroy any evidence of whiskers. While some whiskers can grow long enough to be plainly visible to the naked eye, many are much shorter and must be inspected by using a magnification between 3X and 100X.
In addition to magnification when looking for whiskers, the inspector must be able to either manipulate the part or the light source in order to better see if whiskers are present. Simply adjusting the light from one side of a part to the other can either reveal the presence of whiskers or make them disappear from view.

Why is it important to our customers?
Because whiskers are electrically conductive, when they grow long enough they can bridge the distances between electrical leads causing a short circuit. Whiskers are believed to be the root cause of failures of several satellites such as the PanAmSat Galaxy VII, IV, and IIIR, a couple DirecTV satellites and one satellite from Mexico, Solidaridad 1.

Tin whisker short circuit
Whiskers are also named as a cause of failure with the military’s Patriot Missile, the F-15 radar, and the Phoenix Air to Air Missile. They affect the medical industry, as in the occasion when whiskers caused short circuits in heart pacemakers, and the power industry when electrical components regulating nuclear power plants failed due to whiskers. Whiskers are even a potential culprit in the Sudden Acceleration phenomenon faced by Toyota a number of years ago.

Tin whiskers can cause short circuits within critical systems.
Tin whiskers are a concern to the aerospace or medical industries especially because the devices used by them are
expected to be highly reliable and highly durable. A person’s life could depend on a defibrillator and if that piece of equipment is prone to shorting out because of tin whiskers, that life could be lost. When organizations such as NASA send satellites out into space, they expect those satellites to function for many years. If a circuit board on a satellite is plagued by whiskers, it is not a simple matter of going up into space to replace that component. Too much time and money is invested into those satellites to allow them to fail because of whiskers.

Mitigation Strategies

  • The easiest thing to do would be to completely avoid metals—such as tin, cadmium, and zinc—that are prone to producing whiskers. Nickel alloy 770 is one alternative that we can offer to our customers. Alloy 770 has no plating finish and is not known to have any problems with whiskering. It also has the benefit of being more corrosion resistant than tin plated steel. In some cases, however, Alloy 770 may be cost prohibitive, so we must provide other alternatives.
  • When tin whiskers were first discovered back in the 1940’s, researchers found that the addition of lead to tin reduced the appearance of whiskers. Since then lead has been found to cause health problems and as a result been classified as a restricted substance by the RoHS directive of the European Union. Several states, such as California and New Jersey, and other countries such as South Korea and China have their own regulations in the works to control the use of lead. If a customer is concerned about tin whiskers and is not concerned about RoHS compliance or about the presence of lead, using tin-lead plating as a finish would help to mitigate (not eliminate) the growth of tin whiskers.
  • Scientists believe that because a main cause of whiskers is due to internal stresses within the plating. One proposed way to relieve that stress is to heat treat the parts as soon as possible after plating. Within the first 24 hours after plating will produce the best results, but the heat treating must be done no later than 72 hours after plating in order to be effective. Please note that this is only a means of mitigating whisker growth. It may delay the development of whiskers on the tin plating, extending the reliability of the part, but it cannot definitively prevent whisker growth.
  • Another mitigation technique that many customers use is applying a conformal coating to their circuit boards. These coatings can be made from a few different materials—acrylic, polyurethane, silicone and parylene among others—but their basic function is the same. The coatings act as a nonconductive barrier, either capturing whiskers underneath a protective layer of material or preventing whiskers from penetrating through to create a short circuit with a component covered in a conformal coating. While we cannot necessarily cover our shielding components with these coatings as they would impede our product function and solderability, we could recommend this to our customers as an additional protection against whiskers. If they are interested in using a conformal coating, we must also work with the customer to ensure that our product design is such that it will still function after they apply the coating.

Conclusion:
If a part’s finish contains tin or if the part uses some form of a tin solder, then that part is at risk for developing tin whiskers. While there are steps that we can take to reduce the risk of tin whiskers for our customers there is nothing we can do for a part that must contain tin plating or solder that will guarantee no whiskers.

Bibliography
Osterman, Dr. Michael. “Tin Whiskers Failure Risk and Mitigation Strategies.” IPC Workshop Handbook: Morning Session. IPC Tin Whisker Conference. 17 Apr 2012. Ft. Worth: 2012. Print.
Panashchenko, Lyudmyla. “The Art of Metal Whisker Appreciation: A Practical Guide for Electronics Professionals” IPC Workshop Handbook: Afternoon Session. IPC Tin Whisker Conference. 17 Apr 2012. Ft. Worth: 2012. Print.

Photos:
Sampson, Michael, and Dr. Henning Leidecker. NASA. NASA Photo Gallery. 2011. Web. http://nepp.nasa.gov/WHISKER/photos/index.html

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