Metal Whiskers – A Brief History

The Discovery Of Metal Whiskers

In WWII many airborne radios became essentially unusable due to static whenever the tuning knobs were turned. The cause of the failure was not known until H.L. Cobb documented the ‘Cadmium Whiskers’ to the American Electroplaters Society in 1946 [1]. These hairs were apparently growing from one metal surface and crossing to a nearby metal surface in the meshing metal plates of the tuning capacitors. It turned out that Cadmium used as corrosion protecting plating had this strange tendency to grow these hairs. These hairs were called whiskers because they mostly stand straight out from a surface like the whiskers on a man’s face. Metal whiskers had been discovered and named.

Bell Laboratories experienced similar failures in 1948 on channel filters which caused them to change over to pure tin electroplating. They quickly discovered that pure tin experienced whisker problems very similar to those exhibited by Cadmium. These findings were reported by Compton, Mendizza and Arnold of Bell Laboratories in 1951 [2].

metal whiskers on connector shell
Metal whiskers forming on plated electrical connector, photo courtesy Bill Rollins

A photo library of these whiskers on air spaced variable capacitors may be found here: http://nepp.nasa.gov/whisker/photos/index.html#aircap

The First Solution to Metal Whiskers

For the metal surfaces that were plated with tin, the solution was to add a little lead to the tin, and for reasons unknown the metal whiskers tended then not to grow. This of course is the tin-lead alloy solder that we formerly used. Electronic component leads were plated in it and solder was made of it. That pretty much solved the problem, as it (usually but not always) took years for the whiskers to grow on pure alloys, and even then growth was slow relative to the spacing of electronic components.

RoHS. European Legislation Re-Introduces Tin whiskers

As electronics got smaller, the size of electronic components got smaller, and when transistors shrank into integrated circuits the leads were spaced an eighth to a sixteenth of an inch apart. Despite the fact that the distances between conductors were shrinking rapidly, the metal whisker resistant tin-lead alloy solder gave us reliable electronics. All the leads were plated with the tin-lead alloy, so that they would not corrode and could easily be joined to printed circuit boards by controlled melting of a small amount of that same tin-lead solder to make the electrical connections.  The sizes of these “integrated circuits” continued to shrink, until today the leads on their packages may only be spaced a hundredth of an inch apart.

So, everything worked fine until a certain politically-inspired “environmentalist” activity in Europe became a law. It is called RoHS (Restriction on Hazardous Substances), and it banned importation of any electronics containing lead. This legislation was adopted in February 2003, and took effect on 1st July 2006. The electronic component manufacturers and product manufacturers changed from traditional tin-lead plated parts to RoHS compliant pure tin plated parts. This legislation driven change was not resisted as the lessons of three generations ago had been lost; that pure tin plated surfaces grow whiskers.

This legislation, whilst law only in Europe, affects essentially all electronic components worldwide now. As Europe is a massive purchasing group, manufacturers specify to meet the requirements of the strictest market and all markets thus get sold RoHS compliant electronics.

Why does your TV fail? Check the RoHS sticker

With component leads now so close, there might be a 30% chance of failure in about five years from a tin whisker growing from one lead to another. The “consumer” probably doesn’t care, and may not even have noticed; having been trained to toss the old appliance when something new and shiny was offered by manufacturers that needed to maintain their cash flow. If most people don’t own anything with electronics very long, long term reliability is less important. If you have noticed that electronics do not last as long as they used to, you may have noticed the effect of metal whiskers.

With products containing electronics being upgraded so fast, the consumer might have something only a few years before buying a newer or better one. It is thus easy to not notice the rare failures for no understandable reason. Mass produced consumer products failing in only a few years has became common, but with costs so low, it has became “normal” to replace and not to repair. Furthermore, as the tin whiskers often made only erratic contact, the “failures” were only seen intermittently. Thus, for the consumer there was not a lot of impact, aside from some Toyota automobiles whose accelerator-control-modules were found afterwards to contain tin whiskers.

High Profile Tin Whisker Failures

The users-of-electronics that WERE impacted were the operators of nuclear power plants, among them the Millstone nuclear reactor in Millstone, Connecticut. This reactor was unexpectedly shut down by a tin whisker; that ruined the day for a lot of folks. Also concerned were the owners and operators of satellites that failed prematurely due to tin whiskers. The average person noticed one of those when all the pagers in America suddenly went dead.

A history of these documented tin-whisker-caused failures can be found at www.nepp.nasa.gov.

Government Agencies Fight against Metal Whiskers

The Defence companies and the U. S. Department of Defence however were very concerned. Their radios, vehicles, ships, planes and weapons of any sort need to be absolutely reliable when called upon to perform in time of conflict. Furthermore 100% reliability is required of missile and monitoring systems during peace time. The most senior in responsibility of these agencies was the American Missile Defence Agency (MDA), because their missile-intercepting missiles needed to be absolutely reliable. The price of one failure in an MDA missile might be the loss of one city.

Since the tendency of the tin plated electronic components to grow tin whiskers had become very well known in the last twelve years, pure tin finished parts were thus absolutely forbidden in the manufacture of any of its products, as well as in the manufacture of just about anything used by any Defence agency. However, the pure tin (RoHS compliant) finished parts have become so pervasive and Defence use is such a minuscule part of the entire worldwide manufacturing activity that it was impossible to guarantee they would not slip in here and there, which they did, and in an entirely unacceptable amount.

Something now very urgently needed to be done about this, and things came to a head in about 2003. Having realised that it was impossible to control the problem by specifying materials to be used in manufacture a different approach was decided upon. A project to design a solution to control the problem of Metal Whiskers after manufacture was started.

Tin Whisker Remediation

Given the problems that lead free RoHS compliant solder was causing, the MDA in conjunction with its aerospace partners  decided to fund research to develop a coating that would not be punctured by tin whiskers.

Conformal Coatings

There are coatings for electronic circuit boards called conformal coatings, because they are thin and conform to the shape of the components across the board. Normally, they protect against condensation, dirt and dust that might cause electrical short circuits. The combined American Defence industry initiated testing of many kinds of conformal coatings for their ability to contain the tin whiskers that inevitably and unpredictably grew on electronic assemblies. All of the coatings failed; the whiskers punctured them and grew right through them.

There was a huge knowledge base for the thin hard coatings used in making microcircuits. These coatings were not applicable for designing a new conformal coating that would captivate tin whiskers under the coating. That was the beginning of understanding the problem.

A team of scientists was funded by the MDA to work this science out, and develop from that science a circuit board coating that would contain the tin whiskers whenever they might grow. This coating would have to resist puncture by those ‘stronger than steel’ tin whiskers.

One of the first discoveries was that the science of how thin flexible elastic films could contain tin whiskers, or how they were punctured by them, was actually not a well known science. We all know a pin punctures a balloon, yet the physics of failure as a function of the properties of the elastic materials in the balloon was just not a mature science in 2008.

After many years and some millions of dollars they eventually succeeded, and the tin whisker risk is now in the process of being addressed with a specialised coating designed for electronic circuit boards.

Business-case for tin-whisker-containment with conformal coatings

Tin whisker captivation by conformal coatings

The Whisker-Tough™ coating to encapsulate tin whiskers is bound by export restrictions, and can not be obtained outside of the United States of America. The materials science that was generated by that MDA funded team has now been applied to the problem of zinc whiskers. The product resulting from that is available worldwide. You can read the zinc whisker remediation story here.

References

[1] H.L. Cobb, “Cadmium Whiskers,” Monthly Rev. Am. Electroplaters Soc., 33 (28): pp. 28-30, Jan. 1946.

[2] K.G. Compton, A. Mendizza, and S.M. Arnold, “Filamentary Growths on Metal Surfaces – Whiskers,” Corrosion 7(10): pp. 327-334, October 1951.