Everything about Whisker Metallurgy totally explained
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 isn't 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's known that whisker formation doesn't require either dissolution of the metal or the presence of an electromagnetic field.
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.
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.
citation needed
Zinc whiskers have been responsible for increased system failure rates in
computer server rooms. 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're typically already growing in an environment where they can produce
short circuits. Tin whiskers caused the failure of the
Galaxy IV satellite in 1998. 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, where in high power components a short circuiting tin whisker is ionized into a plasma that's capable of conducting hundreds of Amps of current, massively increasing the damaging effect of the short circuit.
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 contacts operating in an atmosphere rich in
hydrogen sulfide 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.
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