The creepy strands of “hair” on the metal pictured above are metal whiskers. The appropriate name must have been a no-brainer. However, regardless of the seemingly innocuous name, this kind of “fur-clad” metal is extremely dangerous. It can cause severe damage to electronic and electrical devices and render complete systems useless.
What are metal whiskers?
Metal whiskers are thin crystalline strands of metals that shoot out of metallic objects. Metals like zinc and tin are the most notorious in displaying this phenomenon. Cadmium, silver, gold and some other alloys can exhibit this property too.
Tin is common additive in soldering material. Tin is also used for electroplating copper. Consequently, causing copper based electronic components and circuits to fall victim to the ill-effects of metallic whiskers.
These strands of metal are conductive. They grow out slowly, and their sizes can range from a few micrometers up to 10mm.
Although, note that metal whiskers are different from dendrites. The main point of difference is the fact that whiskers rise perpendicularly from the surface of the object. Whereas dendrites grow along the surface of the object. Another difference is the fact that dendrites have a tree like structure, metal whiskers are single microrod like structures.
How dangerous can metal whiskers be?
Small relays, electronic circuit components, and the occasional household device are some common victims of these metallic strands.
But their power of devastation doesn’t end there. Metal whiskers have brought down an entire computer network at NASA, they have resulted in an emergency shutdown of a nuclear power plant, and they are responsible for the collapse of several missile systems. If compromising such complex systems wasn’t dangerous enough, metal whiskers also have the failure of several satellites attributed to them. Those tiny strands of metal are powerful enough to compromise devices worth millions of dollars and can cripple entire systems.
Take a look at this list compiled by NASA in 2004 recording some of the major recorded failures caused by metal whiskers.
How do metallic whiskers harm a system?
The problem of metallic whiskers is exacerbated by the miniaturization of electronic devices. Since these strands are conducting and grow up to a few millimeters, they can easily cause short-circuits. Multiple short-circuits across a system is enough to decimate it completely.
These protrusions can also vaporize and create highly conductive paths for significant amounts of current to flow. If a bunch of metallic whiskers break free from the surface of origin, they can lodge themselves in different areas and create new uncalled for conducting paths and also cause current leakage.
How are metal whiskers formed?
Despite their potential to render entire operations obsolete, nobody knows the exact reason that causes them to ravage metallic objects. Although. the current prevailing theory cites the metals as the culprits. According to the present understanding of the issue, whiskers are a direct/indirect result of the stresses that a metal experiences.
One of these theories states mechanical stresses as the direct cause. According to Eric J. Mittemeijer at the Max Planck Institute for Metals Research, tin whiskers are a direct result of the mechanical differences between various layers of the tin surface. He and his colleagues arrived at this conclusion after they performed micro-stress measurements on metallic whisker specimens using very fine X-ray beams to achieve a high spatial resolution.
Another theory cites mechanical stress as an indirect cause. A study by Victor Karpov of the University of Toledo in Ohio theorizes that the formation of metal whiskers is due to repulsion of like charges. This repulsion is the result of naturally occurring electric fields in the metal. According to Karpov, mechanical stresses or faults lead to the formation of small patches of localized charges. Subsequently, these patches generate electric fields that repel like charges and cause whiskers to sprout from the metal. His theory was published in the journal Physical Review Applied.
How can we avoid the formation of metal whiskers?
In 2006, members of the European Union undertook the Restriction of Hazardous Substances Directive. According to this directive, the use of lead in electronic devices was banned. That’s good. Lead is harmful to humans. However, the use of lead in solder was a solution to reduce the growth of metal whiskers. Without it, electronic appliances would become safer for humans, albeit with a reduced shelf life.
Currently, the three most important lines of defense against the growth of metal whiskers are conformal coatings, using better methods for plating and avoiding pure tin/zinc. Conformal coatings use materials that don’t let the protrusions pierce through. So they essentially act as barriers.
Many studies are going on to ascertain the reason behind this anomaly among metallic components. Until the root cause remains elusive, a proper mitigation strategy would be hard to create.
- Metal whiskering – Wikipedia
- Restriction of Hazardous Substances Directive
- Why do they grow? Getting to the roots of lethal metal whiskers
- Electrostatic Theory of Metal Whiskers -V. G. Karpov (Physical Review Applied)