Metal whiskering is a phenomenon that occurs in electrical devices when metals form long whisker-like projections over time.Tin whiskers were noticed and documented in thevacuum tube era of electronics early in the 20th century in equipment that used pure, or almost pure, tin solder in their production. It was noticed that small metal hairs or tendrils grew between metal solder pads, causingshort circuits. Metal whiskers form in the presence of compressive stress.Germanium,zinc,cadmium, and evenlead whiskers have been documented.[1] Many techniques are used to mitigate the problem, including changes to theannealing process (heating and cooling), the addition of elements like copper and nickel, and the inclusion ofconformal coatings.[2] Traditionally, lead has been added to slow down whisker growth in tin-based solders.
Following theRestriction of Hazardous Substances Directive (RoHS), theEuropean Union banned the use of lead in most consumer electronic products from 2006 due to health problems associated with lead and the "high-tech trash" problem, leading to a re-focusing on the issue of whisker formation inlead-free solders.
Metal whiskering is acrystallinemetallurgical phenomenon involving the spontaneous growth of tiny,filiform hairs from ametallic surface. The effect is primarily seen onelemental metals but also occurs withalloys.
The mechanism behind metal whisker growth isnot well understood, but seems to be encouraged by compressive mechanicalstresses including:
Metal whiskers differ from metallicdendrites in several respects: dendrites arefern-shaped and grow across the surface of the metal, while metal whiskers are hair-like and projectnormal to the surface. Dendrite growth requires moisture capable of dissolving the metal into a solution of metal ions, which are then redistributed byelectromigration in the presence of anelectromagnetic field. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does not require eitherdissolution of the metal or the presence of an electromagnetic field.
Whiskers can causeshort circuits andarcing in electrical equipment. The phenomenon was discovered by telephone companies in the late 1940s and it was later found that the addition oflead to tinsolder provided mitigation.[6] The EuropeanRestriction of Hazardous Substances Directive (RoHS), which 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 with a focus on preventing whisker formation(see§ Mitigation and elimination). Others have focused on the development of oxygen-barrier coatings to prevent whisker formation.[7]
Airborne zinc whiskers have been responsible for increased system failure rates incomputerserver rooms. Zinc whiskers grow fromgalvanized (electroplated) metal surfaces at a rate of up to a millimeter per year with a diameter of a few micrometers. Whiskers can form on the underside of zincelectroplated floortiles on raised floors. These whiskers can then become airborne within the floorplenum when the tiles are disturbed, usually during maintenance. Whiskers can be small enough to pass through air filters and can settle inside equipment, resulting inshort circuits and system failure.[8]
Tin whiskers do not have to be airborne to damage equipment, as they are typically already growing directly in the environment where they can produce short circuits, i.e., the electronic equipment itself. At frequencies above 6 gigahertz or in fastdigital circuits, tin whiskers can act like miniatureantennas, affecting the circuitimpedance and causing reflections. In computer disk drives they can break off and cause head crashes or bearing failures.[9] Tin whiskers often cause failures inrelays and have been found upon examination of failed relays innuclear power facilities.[10]Pacemakers have been recalled due to tin whiskers.[11] 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.[12] The possible increase in the use of pure tin in electronics due to theRoHS directive drove theJoint Electron Device Engineering Council (JEDEC) andIPC electronic trade association 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.[13]
Silver whiskers often appear in conjunction with a layer ofsilver sulfide, which forms on the surface ofsilverelectrical contacts operating in an atmosphere rich inhydrogen sulfide and highhumidity. Such atmospheres can exist insewage treatment plants andpaper mills.
Whiskers over 20 micrometres (μm) in length were observed ongold-plated surfaces and noted in a 2003 NASA internal memorandum.[14]
The effects of metal whiskering were chronicled onHistory Channel's programEngineering Disasters 19.[15]
Several approaches are used to reduce or eliminate whisker growth, with ongoing research in the area.
Conformal compound coatings stop the whiskers from penetrating a barrier, reaching a nearby termination and forming a short.[12]
Termination finishes of nickel, gold or palladium have been shown to eliminate whisker formation in controlled trials.[16]
Galaxy IV was a telecommunications satellite that was disabled and lost due to short circuits caused by tin whiskers in 1998. It was initially thought thatspace weather contributed to the failure, but later it was discovered that a conformal coating had been misapplied, allowing whiskers formed in the pure tin plating to find their way through a missing coating area, causing a failure of the main control computer. The manufacturer, Hughes, has moved to nickel plating, rather than tin, to reduce the risk of whisker growth. The trade-off has been an increase in weight, adding 50 to 100 kilograms (110 to 220 lb) per payload.[17]
On April 17, 2005, theMillstone Nuclear Power Plant in Connecticut was shut down due to a "false alarm" that indicated an unsafe pressure drop in the reactor's steam system when the steam pressure was actually nominal. The false alarm was caused by a tin whisker that short circuited the logic board responsible for monitoring the steam pressure lines in the power plant.[18]
In September 2011, threeNASA investigators claimed that they identified tin whiskers on the accelerator position sensors[19] of sampledToyota Camry models that could contribute to the "stuck accelerator" crashes affecting certain Toyota models during 2005–2010.[20] This contradicted an earlier 10-month joint investigation by theNational Highway Traffic Safety Administration (NHTSA) and a large group of other NASA researchers that found no electronic defects.[21]
In 2012, NHTSA maintained: "We do not believe that tin whiskers are a plausible explanation for these incidents...[the likely cause was]pedal misapplication."[22]
Toyota also maintains that tin whiskers were not the cause of any stuck accelerator issues: "In the words of U.S. Transportation Secretary Ray LaHood, 'The verdict is in. There is no electronic-based cause for unintended high-speed acceleration in Toyotas. Period.'" According to a Toyota press release, "no data indicates that tin whiskers are more prone to occur in Toyota vehicles than any other vehicle in the marketplace." Toyota also states that "their systems are designed to reduce the risk that tin whiskers will form in the first place."[23]
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