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. Many techniques are used to mitigate the problem including
changes to the annealing process, addition of elements like copper and
nickel, and the inclusion of conformal coatings. Traditionally, lead is added
to slow down whisker growth. Following the Restriction of Hazardous
Substances Directive, 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. 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, thermally-induced stresses, and
strain gradients 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. 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,. The
European Restriction of Hazardous Substances Directive, 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 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. Airborne Zinc whiskers have been
responsible for increased system failure rates in computer server rooms. Zinc
whiskers grow from galvanized 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. 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, 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 possible increase in the 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.
Whiskers over 20 µm in length were observed on gold-plated surfaces and
noted in a 2003 NASA internal memorandum.
Mitigation and elimination Several approaches are used to reduce or
eliminate whisker growth with ongoing research in the area.
Conformal coatings 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. Altering plating chemistry
Termination finishes of nickel, gold or palladium have been shown to eliminate
whiskering in controlled trials. Tin whisker examples and incidents
= Galaxy IV= The 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 that space weather contributed to the
failure, but later it was discovered that a conformal coating had been
mis-applied, 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 tradeoff has been an increase in weight,
adding 100 to 200 pounds per payload. = Millstone Nuclear Power Plant=
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.
= Toyota accelerator position sensors false positive=
In September 2011, three NASA investigators claimed that the tin
whiskers they identified on the Accelerator Position Sensors of sampled
models of Toyota Camry could contribute to the "stuck accelerator" crashes
affecting certain Toyota models during 2005-2010. This contradicted an earlier
10-month joint investigation by The National Highway Traffic Safety
Administration and a large group of other NASA researchers that found no
electronic defects. However, in 2012 NHTSA maintained: "We
do not believe that tin whiskers are a plausible explanation for these
incidents...[the likely cause was] pedal misapplication."
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." See also
Monocrystalline whisker Dendrite
Crystal growth Gold-aluminium intermetallic
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
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 Homepage"
Fortune article "Tin Whiskers: The next Y2k problem?"
Teverovsky Gold Whiskers Zinc whiskers tangle data centre ops
Zinc whisker awareness by NASA Nasa photographs of silver whiskers
Article on silver sulphide formation Tin Whiskers could threaten Shuttle and
International Space Station safety 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.
Tin Whiskers - calce - University of Maryland
Tin Whisker growth videos - Brown University
Zinc Whisker White Paper - 8 Solutions