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. 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 conformal coatings. Traditionally, lead was added to slow down whisker growth in tin-based solders.
Following the Restriction of Hazardous Substances Directive (RoHS), 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 of whisker formation in lead-free solders.
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,
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 perpendicularly 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.