Whiskers

WHISKERS

Metal whiskering is a phenomenon which implies the formation of needle-like protrusions on the surface of a metal. Those whiskers are capable of causingshort circuits or arcing in electrical circuits.
The length is up to 10mm ( typically <1mm).
The diameter is from 0,006 to 10 µm.
It´s really unknown how whiskersappear and grow. In many cases, whiskers never appear; but in others, whiskers appear several years after the component has been fabricated.
The National Electronicsmanufacturing initiative says that whiskers, once started, grow very quickly but not indefinitely.

It seems to be encouraged by compressive mechanical stresses like:-Residual stresses caused by electroplating.
-Thermally induces stresses.
-Diffusion metals stresses.

This stress can increase with time, probably due to the growth ofintermetallic layers at the film interface.

Other theories contend that growth may be attributable to recrystallization.

This phenomenon is more common inelemental metals than in alloys. The most known metal whiskering is the tin one. But there are some other metals that may form whiskers include some tin alloys, zinc, cadmium,indium, antimony, silver among others. 

The risks associated with tin whiskers are:

-Stable short circuits in low voltage, high impedances circuit.
-Transientshort circuits.
-Metal vapor arc. If a tin whisker initiates a short circuit in a high voltage circuit, then the metal vapor arc phenomenon happens. The whisker isvaporized into a plasma highly conductive metal ions. This arc can carry hundreds of amperes and it can be sustained for a long duration (several seconds).
-Contamination.