Whisker (metallurgy)

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Metal whiskers is 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 it was later found that the addition of lead to tin solder provided mitigation. Restriction of Hazardous Substances in electrical and electronic equipment 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 coatings to prevent whisker formation.

Zinc whiskers have been responsible for increased system failure rates in computer machine rooms.^[1] 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 form on the underside of zinc electroplated floor tiles on raised floors due to stresses applied when walking over them; and these whiskers can then become airborne within the floor plenum 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 system failure.

Tin whiskers don't have to be airborne to damage equipment, as they are typically already growing in an environment where they can produce short circuits. Tin whiskers caused the failure of the Galaxy IV satellite in 1998^[2]. At frequencies above 6 GHz or in fast digital circuits, tin whiskers can act like miniature antennas, affecting the circuit impedance and causing reflections. In computer disk drives they can break off and cause head crashes or bearing failures. Tin whiskers often cause failures in relays, and have been found upon examination of failed relays in nuclear power facilities.^[3] Pacemakers have been recalled due to tin whiskers ^[4].

Silver whiskers are long filaments of elemental silver. They often appear in conjunction with a layer of silver sulfide which forms 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.

Gold whiskers are thin filaments of elemental gold. Whiskers over 20 µm in length were observed on gold-plated surfaces and noted in a 2003 NASA internal memorandum. ^[5]

References

1. ^ http://www.era.co.uk/news/rfa_feature_06.asp
2. ^ Felps, Bruce. "'Whiskers' Caused Satellite Failure: Galaxy IV Outage Blamed On Interstellar Phenomenon", Wireless Week, 1999-05-17. 
3. ^ http://www.nrc.gov/reading-rm/doc-collections/event-status/event/1999/19990712en.html
4. ^ http://www.fda.gov/ora/inspect_ref/itg/itg42.html
5. ^ http://nepp.nasa.gov/whisker/other_whisker/gold/2003-teverovsky-gold-whiskers.pdf

See also

• Monocrystalline whisker