Whisker (metallurgy)

Metal whiskers are a crystalline metallurgical phenomenon whereby metal grows tiny, filiform hairs. The effect is primarily seen on elemental metals but also occurs with alloys.

The mechanism behind metal whisker growth is not well understood, but seems to be encouraged by compressive mechanical stresses including:

• residual stresses caused by electroplating;
• mechanically induced stresses;
• stresses induced by diffusion of different metals; and
• thermally induced stresses.

Metal whiskers differ from metallic dendrites in several respects; dendrites are fern-shaped, and grow across the surface of the metal, while metal whiskers are hair like and project at a right-angle to the surface. Dendrite growth requires moisture capable of dissolving the metal into a solution of metal ions which are then redistributed by electromigration in the presence of an electromagnetic field. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does not require either dissolution of the metal or the presence of an electromagnetic field.

Whiskers can cause short circuits and arcing in electrical equipment. The phenomenon was discovered by telephone companies in the late 1940s and was addressed with the addition of lead to tin solder. Elimination of lead in electrical components is driving the development of replacement alloys for pure tin and tin/lead alloys that resist whisker growth. Others have focused on the development of oxygen-barrier coating to prevent whisker formation.

Zinc whiskers have been responsible for increased failure rates in computer machine rooms , satellites and pace-makers. Zinc whiskers grow from galvanized (electroplated) metal surfaces at a rate of up to 1 mm per year with a diameter of a few micrometres. Whiskers can sometimes form on the underside of zinc electroplated floor tiles, and these can become airborne when the tiles are disturbed, usually during maintenance. Whiskers can be small enough to pass through air filters and can settle inside equipment, resulting in short circuits and failure. Whiskers need not necessarily separate to damage sensitive equipment, for example, by connecting two adjacent electronic pathways designed to be insulated from each other a short circuit may occur.

Tin whiskers caused the loss of the Galaxy IV satellite on May 19, 1998.

Silver whiskers are long filaments of silver sulfide formed on the surface of silver electrical contacts operating in an atmosphere rich in hydrogen sulfide and high humidity. Such atmospheres can exist in sewage treatment and paper mills.

References

• NASA metal whisker site
• Fortune article

See also

• Monocrystalline whisker

External links

• Zinc whiskers tangle data centre ops (Computerworld)
• Zinc whisker awareness by NASA (PDF)

• Slashdot: The Tin-Whisker Menace

• Nasa photographs of silver whiskers
• Article on silver sulphide formation (PDF)