Whisker

Silver whiskers growing out of surface-mount resistors

Metal whiskering or tin whiskers is a phenomenon or fault which occurs in electrical devices. Tin whiskers were noticed and documented in the valve 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 equipment and space flight hardware, which can still use solders which contain lead.

Mechanism

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.

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

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,^[1] 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.^[2]

Zinc whiskers have been responsible for increased system failure rates in computer server rooms.^[3] 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.^[4] 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.^[5] Pacemakers have been recalled due to tin whiskers.^[6] Research has also identified a particular failure mode for tin whiskers in vacuum conditions (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.^[7] 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.^[8]

Silver whiskers are long filaments of elemental silver. They often appear in conjunction with a layer of silver sulphide which forms on the surface of silver electrical contacts operating in an atmosphere rich in hydrogen sulphide 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. ^[9]

See also

• Monocrystalline whisker

References

External links

• Tin Whiskers Eliminated by PCB Design 07
• A Comprehensive Compilation of Articles, Presentations, Video, and Photos Regarding Tin Whisker Issues and Initiatives by LDF Coatings, LLC
• Problems Diagnosing Zinc Needles in High-Tech Environments by Sterile Environment Technologies
• Zinc Contamination in High Technology Facilities
• Zinc Whisker Growing in and on Computer Equipment by Data Clean Corporation (PDF)
• Whiskers in Data Centers are not only on floor tiles...
• WES - Zinc Whisker Reference Document
• Zinc Whiskers break off and become airborne - Video Sample
• NASA Tin Whisker (and Other Metal Whisker) Homepage"
• Fortune article "Tin Whiskers: The next Y2k problem?"
• Teverovsky Gold Whiskers
• Zinc whiskers tangle data centre ops (Computerworld)
• Zinc whisker awareness by NASA (PDF)
• Nasa photographs of silver whiskers
• Article on silver sulphide formation (PDF)
• Tin Whiskers could threaten Shuttle and International Space Station safety (Space.com news article)
• Tackling tin whisker test circuit failures and whisker testing
• Tiny 'tin whiskers' imperil electronics - Associated Press
• iNEMI Tin Whisker Activities
• A History of Tin Whisker Theory: 1946 to 2004, George T. Galyon, IBM eSG Group, SMTAI International conference, September 26-30, 2004 (Chicago, IL).
• Tin Whiskers - calce - University of Maryland
• Tin Whisker growth videos - Brown University
• Zinc Whisker White Paper - 8 Solutions