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EP04.07.32 : Metal Whisker Growth Induced by Localized, High-Intensity DC Electric Fields

5:00 PM–7:00 PM Apr 4, 2018

PCC North, 300 Level, Exhibit Hall C-E

Description
Vamsi Borra1 Osama Oudat1 Daniel Georgiev1 Victor Karpov1 Diana Shvydka1

1, Univ of Toledo, Toledo, Ohio, United States

Electrically conductive hair-like structures, referred to as whiskers, can bridge the gap between densely spaced electronic devices and components. This can lead to current leakage and short circuits, causing significant losses and, in some cases, catastrophic failure in various systems, including such in the automotive, aerospace and other industry. None of the whisker growth models proposed to date is capable of answering consistently and universally why whiskers grow, in the first place. Although there are several generally accepted factors (intermetallic compound formation, tensile / compressive stress, Sn oxide layer presence, coefficient of thermal expansion (CTE) mismatch that influence the whisker growth), the most important factor in whiskering is still a matter of dispute. A recent theory [1], which considers the imperfections (small patches of net positive or negative electric charges) on metal surface, details the quantitative estimates of the metal whisker nucleation along with their growth rates and length distributions. According to this theory, the anomalous electric field (E) formed due to the imperfections will govern the whisker development in those areas. In addition an external electric field, which can be either constant (DC) or varying with time (AC) at high frequencies (including optical), can also contribute to nucleation and promote their growth by means of lowering the free energy of the system.

Here, we present a new way of controlling the growth of whiskers by using localized high-intensity DC electric fields by using an AFM setup and we refer to it as whisker engineering. A current sensing AFM scanner with a conducting cantilever was utilized. The electric field was generated by applying a voltage bias between the sample and the conductive cantilever, which is maintained at a known distance, without causing any dielectric breakdown. SEM examination of samples at the points where the AFM tip was positioned for an extended period was performed before and after electric field application. Minuscule whiskers were observed; whose growth direction matched the direction of the field. The observed whisker formation can be used to design a new non-destructive readily implementable accelerated failure testing procedure as well as in other applications.

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