Understanding the movements of metal whiskers

Abstract

Metal whiskers often grow across leads of electric equipment and electronic package causing current leakage or short circuits and raising significant reliability issues. The nature of metal whiskers remains a mystery after several decades of research. In addition, metal whiskers exhibit a rather unusual dynamic property of relatively high amplitude movements under gentle air flow or, according to some testimonies, without obvious stimuli. Understanding the physics behind that motion would give additional insights into the nature of metal whiskers. Here, we quantitatively analyze several possible mechanisms potentially responsible for the observed movements: (1) minute air currents, (2) Brownian motion due to random bombardments with the air molecules, (3) mechanically caused movements, such as (a) externally transmitted vibrations of the sample, and (b) torque exerted due to material propagation along curved whiskers responsible for the whisker growth (similar to the known garden hose oscillations), (4) time dependent electric fields due to diffusion of ions across the metal surface, and (5) nonequilibrium electric field configurations making it possible for {\it some} whiskers to move. For all these scenarios we provide numerical estimates. Our conclusion is that the observed movements are likely due to the minor air currents, intentional or ill-controlled, and that external mechanical vibrations could force such movements in a rather harsh environment or/and for whiskers with severe constrictions. We argue that under non-steady state conditions, such as caused by changes in the external light intensity, some whiskers can exercise spontaneous oscillations.