By a whisker

Before the introduction of the Restriction of Hazardous Substances Directive (RoHS), electronic components were plated with an alloy composed of 80 per cent tin and 20 per cent lead. However, the use of the lead in the alloy was deemed to be environmentally unsound and the directive banned its use.

The lead, however, did serve one crucial purpose, by preventing the growth of what are known as 'tin whiskers' from the surface of the compound.

These are caused by a metallurgical phenomenon and involve the spontaneous growth of tiny, hair-like extensions from the surface of the metal. Although some researchers have reported whisker growth from tin-lead alloys, such whiskers are dramatically smaller than those from pure tin-plated surfaces and are believed to be sufficiently small so as not to pose a significant risk for the geometries of today's microelectronics.

Although the mechanism behind the metal-whisker growth is not well understood, it 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.