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Metal whiskering or tin whiskers is a phenomenon or fault which occurs in electrical devices. Tin whiskers were noticed and documented in the valve (tube) era of electronics early in the 20th century, in equipment which used pure, or almost pure tin solder in their production. It was noticed that small metal hairs grew between metal solder pads causing short circuits. The problem was solved with the addition of lead which prevents the growth of the hairs. The European Union banned the use of lead in most consumer products in the early 21st century due to health problems associated with lead, leading to a re-emergence of the problem. New laws exclude some medical and military equipment and space flight hardware, which can still use solders which contain lead.
Metal whiskering is a crystalline metallurgical phenomenon involving the spontaneous growth of tiny, filiform hairs from a metallic surface. 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.
Recently research at the University of South Carolina has found whiskers are a result of strain gradient in materials  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, but the European Restriction of Hazardous Substances Directive (RoHS), that took effect on July 1, 2006, restricted the use of lead in various types of electronic and electrical equipment. This has driven the use of lead-free alloys. 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 server rooms. 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 (accelerated by autocatalytic "tin pest" expansion) caused the failure of the Galaxy IV satellite in 1998. 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. Pacemakers have been recalled due to tin whiskers. Research has also identified a particular failure mode for tin whiskers in vacuum (such as in space), where in high-power components a short-circuiting tin whisker is ionized into a plasma that is capable of conducting hundreds of amperes of current, massively increasing the damaging effect of the short circuit. The increase in use of pure tin in electronics due to the RoHS directive drove JEDEC and IPC to release a tin whisker acceptance testing standard and mitigation practices guideline intended to help manufacturers reduce the risk of tin whiskers in lead-free products.
Silver whiskers 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.
|Wikimedia Commons has media related to: Whiskers (metal)|
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- Teverovsky Gold Whiskers
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- Nasa photographs of silver whiskers
- Article on silver sulphide formation (PDF)
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- Tiny 'tin whiskers' imperil electronics - Associated Press
- iNEMI Tin Whisker Activities
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- Tin Whiskers - calce - University of Maryland
- Tin Whisker growth videos - Brown University
- Zinc Whisker White Paper - 8 Solutions