ABSTRACT
Metallic whiskers are single crystal eruptions from a metal surface, typically 1-5 Am in diameter and 1-500 Am in length Fig. 1. Whiskers can be straight, kinked, and even curved [1]. Almost always, the whiskers erupt (grow) from metallic films that are electroplated on a substrate material. Metallic-film deposits also demonstrate other types of eruptions that are quite different in appearance from whiskers. These eruptions are referred to as flowers, mounds, extrusions, volcanoes, etc. and have not been as interesting as the considerably longer whisker eruptions (Figs. 2-4). Most, if not all, nonnoble metals grow whiskers under the correct conditions, but the lower-melting-point metals of tin (Sn), zinc (Zn), and cadmium (Cd), along with silver (Ag) in the presence of hydrogen sulfide (H2S) grow whiskers under ambient, or near-ambient conditions. This is a source of concern because of their widespread utilization in electronic hardware applications where whisker growth has caused significant reliability problems. The greatest interest has been focused on Sn, because Sn platings are widely used in electrical hardware on closely spaced lead frames and circuit-board features, where whiskers can readily short one lead or feature to another. Zinc is also of interest because electroplated Zn is a common surface finish on sheet steel used for the underside of raised-floor panels in computer rooms. Cadmium was of interest during, and shortly after, World War II when it was the metal plating of choice for electrical hardware. During the 1950s, Cd was quickly replaced by Sn and lead (Pb)–tin (Sn) alloy
platings. Silver is used in some electronic applications because of its combined properties of low contact resistance and good solderability. But by far, the greatest number of technical publications have dealt with the problems associated with whisker formation in Sn and Sn alloys.
