Whisker (metallurgy)
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2008-07-16T20:27:14Z
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[[Image:Matte-tin-plated-IC-02.jpg|thumb|right|300px|Tin whiskers growing on a MATTE tin-plated copper leadframe commonly used in the manufacture of 28 pin small outline integrated circuit (SOIC) leadframe after 3 years of ambient storage.]]
[[Image:Tin whiskers.jpg|thumb|right|300px|Solder dip vs. plated whisker growth.]]
'''Metal whiskering''' is a [[crystal]]line [[metallurgy|metallurgical]] phenomenon involving the spontaneous growth of tiny, [[crystal habit|filiform]] hairs from a [[metal]]lic surface. The effect is primarily seen on [[chemical element|element]]al metals (pure single element metals, not alloys) but also occurs with [[alloy]]s.
The mechanism behind metal whisker growth is not well understood, but seems to be encouraged by compressive mechanical [[stress (physics)|stress]]es including:
*[[residual stress]]es caused by [[electroplating]],
*mechanically induced stresses,
*stresses induced by [[diffusion]] of different metals, and
*thermally induced stresses.
Metal whiskers differ from metallic [[Dendrite (crystal)|dendrite]]s 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 circuit]]s and [[electric arc|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{{Fact|date=June 2008}}. [[RoHS|Restriction of Hazardous Substances]] (RoHS) in 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.{{Fact|date=June 2008}}
'''[[Zinc]] whiskers''' have been responsible for increased system failure rates in [[computer]] [[server room]]s.<ref>[http://www.era.co.uk/news/rfa_feature_06.asp ERA Technology | Zinc whisker Induced failures in electronic systems<!-- Bot generated title -->]</ref> Zinc whiskers grow from [[Galvanization|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 [[electroplating|electroplated]] floor [[tile]]s 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 circuit]]s 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 circuit]]s. Tin whiskers caused the failure of the [[Galaxy IV]] [[satellite]] in 1998<ref>{{cite news|url=http://www.wirelessweek.com/article.aspx?id=104090|last=Felps|first=Bruce|date=[[1999-05-17]]|title='Whiskers' Caused Satellite Failure: Galaxy IV Outage Blamed On Interstellar Phenomenon|publisher=Wireless Week}}</ref>. At frequencies above 6 GHz or in fast digital circuits, tin whiskers can act like miniature [[Antenna (radio)|antennas]], affecting the circuit [[Characteristic impedance|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 [[relay]]s, and have been found upon examination of failed relays in [[nuclear power]] facilities.<ref>[http://www.nrc.gov/reading-rm/doc-collections/event-status/event/1999/19990712en.html NRC: Event Notification Report for July 12, 1999<!-- Bot generated title -->]</ref> Pacemakers have been recalled due to tin whiskers <ref>[http://www.fda.gov/ora/inspect_ref/itg/itg42.html FDA itg Page 1<!-- Bot generated title -->]</ref>. Research has also identified a particular failure mode for tin whiskers, 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.<ref>[http://nepp.nasa.gov/whisker/reference/tech_papers/2007-brusse-metal-whiskers.pdf Metal Whiskers: Failure Modes and Mitigation Strategies, Jay Brusse, Dr. Henning Leidecker, Lyudmyla Panashchenko, NASA, December 5, 2007, page 10]</ref>
'''[[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 contact]]s operating in an atmosphere rich in [[hydrogen sulfide]] and high [[humidity]]. Such atmospheres can exist in [[sewage treatment]] and [[paper mill]]s.
'''[[Gold]] whiskers''' are thin filaments of [[chemical element|elemental]] gold. Whiskers over 20 µm in length were observed on [[gold plating|gold-plated]] surfaces and noted in a 2003 NASA internal memorandum.
<ref>http://nepp.nasa.gov/whisker/other_whisker/gold/2003-teverovsky-gold-whiskers.pdf</ref>
== References ==
{{Reflist}}
== See also==
* [[Monocrystalline whisker]]
==External links==
* [http://money.cnn.com/magazines/fortune/fortune_archive/2005/01/10/8230971/index.htm Fortune article "Tin Whiskers: The next Y2k problem?"]
* [http://nepp.nasa.gov/whisker/other_whisker/gold/2003-teverovsky-gold-whiskers.pdf Teverovsky Gold Whiskers]
*[http://www.computerworld.com.au/index.php/id;464368043;relcomp;1 Zinc whiskers tangle data centre ops (Computerworld)]
*[http://nepp.nasa.gov/whisker/reference/tech_papers/Brusse2003-Zinc-Whisker-Awareness.pdf Zinc whisker awareness] by [[NASA]] (PDF)
*[http://nepp.nasa.gov/whisker/other_whisker/silver/index.htm Nasa photographs of silver whiskers]
*[http://www.koaeurope.de/at/koa/downloads/ag2s_210.pdf Article on silver sulphide formation] (PDF)
*[http://www.space.com/missionlaunches/ft_060427_sts121_shuttlefix.html Tin Whiskers could threaten Shuttle and International Space Station safety] (Space.com news article)
*[http://www.reed-electronics.com/tmworld/article/CA6251552.html Tackling tin whisker test] circuit failures and whisker testing
*[http://www.salon.com/wires/ap/scitech/2007/10/08/D8S562QG0_tin_whiskers/index.html Tin Whiskers Imperil Electronics] - [[Associated Press]]
*[http://www.inemi.org/cms/projects/ese/tin_whisker_activities.html iNEMI Tin Whisker Activities]
*[http://thor.inemi.org/webdownload/newsroom/Presentations/SMTAI-04_tin_whiskers.pdf A History of Tin Whisker Theory: 1946 to 2004, George T. Galyon, IBM eSG Group, SMTAI International conference, September 26-30, 2004 (Chicago, IL).]
*[http://www.calce.umd.edu/tin-whiskers/ Tin Whiskers - calce - University of Maryland]
[[Category:Electronic engineering]]
[[Category:Metallurgy]]
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