Whisker (metallurgy)

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Silver whiskers growing out of surface-mount resistors

Metal whiskering is a phenomenon which occurs in electrical devices. Tin whiskers were noticed and documented in the vacuum 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 or tendrils grew between metal solder pads causing short circuits. Metal whiskers form in the presence of compressive stress. Zinc, cadmium, and even lead whiskers have been documented.[1] Many techniques are used to mitigate the problem including changes to the annealing process (heating and cooling), addition of elements like copper and nickel, and the inclusion of conformational coatings.[2] Traditionally, lead is added to slow down whisker growth.

The European Union banned the use of lead in most consumer electronic products in the early 21st century due to health problems associated with lead and the “high-tech trash” problem, leading to a re-focusing on the issue, see Restriction of Hazardous Substances Directive (RoHS).

Mechanism[edit]

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:

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.

Effects[edit]

Several mm long zinc whiskers on zinc coated steel

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,[4] 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.[5]

Zinc whiskers have been responsible for increased system failure rates in computer server rooms.[6] 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.[7] 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.[8] Pacemakers have been recalled due to tin whiskers.[9] 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.[10] 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.[11]

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.

Whiskers over 20 µm in length were observed on gold-plated surfaces and noted in a 2003 NASA internal memorandum.[12]

Mitigation and elimination[edit]

Several approaches are used to reduce or eliminate whisker growth with ongoing research in the area.

Conformal coatings[edit]

Conformal compound coatings stop the whiskers from penetrating a barrier and reaching a nearby termination and forming a short. These include barriers made of a ceramic or polymeric compound. Polymeric compounds tend to deflect the whisker away while ceramic chemistries prevent puncturing of the coating.[13]

Altering plating chemistry[edit]

Termination finishes of nickel, gold or palladium have been shown to eliminate whiskering in controlled trials.[13][14]

Tin whisker examples and incidents[edit]

Galaxy IV[edit]

The Galaxy IV was a telecommunications satellite that was disabled and lost due to short circuits caused by tin whiskers. It was initially thought that space weather contributed to the failure, but later it was discovered that whiskers had formed in the pure tin plating and broke through a wax-like conformal coating 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 tradeoff has been an increase in weight, adding 100 to 200 pounds per payload.[15]

Millstone Nuclear Power Plant[edit]

On April 17, 2005, the Millstone 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 which was responsible for monitoring the steam pressure lines in the power plant.[16]

Toyota accelerator position sensors false positive[edit]

In 2011, NASA investigators found that the tin whiskers they identified on the Accelerator Position Sensors[17] of sampled models of Toyota Camry did not contribute to the so-called "stuck accelerator" crashes affecting certain Toyota models during 2005-2010.[18] 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.” Indeed, no data indicates that tin whiskers are more prone to occur in Toyota vehicles than any other vehicle in the marketplace. To the contrary, Toyota's systems are designed to reduce the risk that tin whiskers will form in the first place." [19]

See also[edit]

References[edit]

  1. ^ Lyudmyla Panashchenko. "Whisker Resistant Metal Coatings". NEPP NASA. Retrieved 23 October 2013. 
  2. ^ Craig Hillman, Gregg Kittlesen, and Randy Schueller. "A New (Better) Approach to Tin Whisker Mitigation". DFR Solutions. Retrieved 23 October 2013. 
  3. ^ http://dx.doi.org/10.1016/j.scriptamat.2011.05.007
  4. ^ George T. Galyon. "A History of Tin Whisker Theory: 1946 to 2004". iNEMI. Retrieved 21 December 2012. 
  5. ^ "Whisker Effect". INELCO. Retrieved 5 January 2011. 
  6. ^ "Zinc whisker induced failures in electronic systems". ERA Technology. Retrieved 21 December 2012. 
  7. ^ Felps, Bruce (1999-05-17). "'Whiskers' Caused Satellite Failure: Galaxy IV Outage Blamed On Interstellar Phenomenon". Wireless Week. Archived from the original on 2009-03-03. 
  8. ^ "Event Notification Report for July 12, 1999". U.S. Nuclear Regulatory Commission. Retrieved 21 December 2012. 
  9. ^ "ITG SUBJECT: TIN WHISKERS - PROBLEMS, CAUSES, AND SOLUTIONS". Food and Drug Administration. 1986-03-14. Retrieved 21 December 2012. 
  10. ^ Jay Brusse, Dr. Henning Leidecker, Lyudmyla Panashchenko (December 5, 2007). "Metal Whiskers: Failure Modes and Mitigation Strategies". NASA. Retrieved 21 December 2012. 
  11. ^ "JEDEC and IPC Release Tin Whisker Acceptance Testing Standard and Mitigation Practices Guideline". JEDEC.org. 4 May 2006. Retrieved 5 January 2011. 
  12. ^ Alexander Teverovsky (April 2003). "INTRODUCING A NEW MEMBER TO THE FAMILY: GOLD WHISKERS". NASA. Retrieved 21 December 2012. 
  13. ^ a b John Burke (September 2010). "Tin Whiskers Eliminated". 
  14. ^ Keun-Soo Kim, Suk-Sik Kim, Seong-Jun Kim, Katusaki Suganuma, ISIR, Osaka University, Masanobu Tsujimoto, Isamu Yanad, C. Uyemura & Co., Ltd., Prevention of Sn whisker formation by surface treatment of Sn plating Part II, TMS Annual Meeting, 2008
  15. ^ Bruce Felps. "'Whiskers' Caused Satellite Failure: Galaxy IV Outage Blamed On Interstellar Phenomenon". Retrieved 25 October 2013. 
  16. ^ Reactor Shutdown: Dominion Learns Big Lesson From A Tiny ‘tin Whisker'
  17. ^ http://nepp.nasa.gov/WHISKER/reference/tech_papers/2011-NASA-GSFC-whisker-failure-app-sensor.pdf
  18. ^ http://www.nytimes.com/2010/01/22/business/22toyota.html?_r=0
  19. ^ http://toyotanews.pressroom.toyota.com/releases/tin+whiskers+other+discredited+unintended+acceleration+theories.htm

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