Losing By A Whisker: Lead-Free Solder and the Tin Whisker Problem

In 1998, the $250 million Galaxy IV geostationary communications satellite carrying millions of pager signals as well as the broadcast feeds of the CBS and NPR networks failed after only five years of service. Pager service wasn't restored for days and the company operating the satellite suffered considerable financial losses. Engineers determined that the problem was tiny tin whiskers that sprouted from soldered connections in the satellite's primary control processor. Because of a decision made by the European Union to prohibit the use of lead-based solder in electronics, we may see a lot more failures due to tin whiskers in the near future. How did the simple act of choosing electronic components become a complex moral issue? First, you need to understand something about tin whiskers.

When metals such as tin, zinc, and cadmium are under some kind of mechanical stress, one way they tend to relieve this stress is by sprouting tiny threads or sticks of metal called whiskers. They are very thin, much thinner than a human hair, and grow slowly over a period of months or years to a length of a few millimeters. But in the microminiature world of modern electronics, that distance is more than enough to bridge the gap between two terminals that will cause an equipment failure if shorted together. That is exactly what happened to the Galaxy IV satellite in both its primary and backup processor.

The whisker problem was first identified in the late 1940s, and since then engineers have found several ways to mitigate or eliminate it. Adding lead to tin plating or solder typically cures any whisker issues. Until very recently, the standard mixture of solder (the tin/lead alloy used to connect together most electronic components by melting it around terminals to be joined) was 60% tin and 40% lead. This alloy was reasonably inexpensive, had a low melting point, and served the electronics industry well for many decades.

In 2003, the European Union enacted a policy called Reduction of Hazardous Substances (RoHS, for short). This directive said that by July 1, 2006, most electronic products made or sold within the EU could not contain more than a very small amount of lead, cadmium, mercury, and a few other hazardous chemicals. Since the EU is a large market, and it is not practical for the thousands of electronics component manufacturers around the world to maintain two separate production lines, one for RoHS and another for non-RoHS products, this created a huge amount of turmoil in the industry as companies retooled their processes to eliminate lead from their solder, interconnection wires, plating processes, coatings, connectors, and everywhere else it was used. If you look in an electronic parts catalog these days you find "RoHS-compliant" labels on many if not most products, although non-RoHS stuff is still available, including the nasty old lead-bearing solder (which I have used, incidentally, since about the age of ten with no harmful effects). In fairness to the RoHS policy, the concern is not so much that people who use the electronics products are in any immediate danger of exposure, but that both at the manufacturing end and the recycling or disposal end, the lead can cause health problems. And that is an entirely legitimate concern.

But so is the problem of multi-million-dollar systems conking out because of tiny tin whiskers. The only commonly available RoHS-compliant solder, for example, is about 96% tin and 2% silver. Silver is not cheap, and so it costs about 50% more than the lead-bearing solder. It works all right—I've used some—but there is no lead in it to prevent the tin-whisker problem. And apparently there are few if any long-term studies of this new solder formulation that tell us how likely it is that joints soldered with it will need a shave in a few years.

The RoHS directive does exempt certain high-reliability systems such as medical devices from the no-lead requirements. But as some industry spokesmen point out, this is an empty gesture, because pretty soon it will be very hard to find any non-RoHS parts, for the simple reason that the market for them will dry up. NASA, for example, has good reason to be very concerned about the tin-whisker problem, since their satellites, and above all the Space Shuttle, contain electronic systems that are old enough to vote. So far, no life-threatening failure has occurred in the Shuttle due to tin whiskers, but the Shuttle has to keep going another two or three years at least before its commercial replacement may be available.

So what's an engineer to do? Well, the law is the law, and if your company makes or sells anything in the EU, it better comply with RoHS. As for systems that demand high reliability, there are ways around the whisker problem even if you have to use lead-free solder: wax or other impermeable coatings, proper spacing and insulating layers of other kinds, and so on. But many of these techniques are either largely untried or have problems of their own. That is what engineering is all about: solving problems. And the world will be a better place when new electronic products don't carry the burden of toxic heavy metals that they did in the past. But engineers now have to consider a new technical problem introduced by the well-meant, but perhaps technologically immature, RoHS directive. And we'll all be dealing with the consequences, perhaps in unexpected ways.

Sources: The Oct. 8, 2007 Austin American-Statesman carried an AP article by Jordan Robertson on how the high-tech industry is dealing with the challenges of tin whiskers and RoHS. Wikipedia's article "Whiskers (metallurgy)" gives a good description of the phenomenon and problems it can cause. The NASA Tin Whisker Homepage http://nepp.nasa.gov/whisker/ contains several pictures of actual whiskers and articles and presentations about the problem.