Metal whisker growth induced by localized, high-intensity DC electric fields
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.374
Metal whisker growth induced by localized, highintensity DC electric fields Vamsi Borra1, Osama Oudat2, Daniel G. Georgiev1, Victor G. Karpov2, Diana Shvydka3
1
Department of EECS, University of Toledo, Toledo, Ohio, U.S.A.
2
Department of Physics and Astronomy, University of Toledo, Toledo, Ohio, U.S.A.
3
Department of Radiation Oncology, University of Toledo, Toledo, Ohio, U.S.A.
ABSTRACT
In this work, a very high, locally applied electric field was used to induce whisker nucleation on an Sn film. The field was generated by using a conductive AFM tip and applying a voltage bias between the sample and the conductive cantilever. The tip-sample separation distance was thus controllable, and any dielectric breakdown could be avoided. At locations where the AFM tip was positioned for an extended period, minuscule whiskers were observed, whose growth direction matched vertical orientation of the field.
INTRODUCTION 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[1], [2]. 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[3]–[8], 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.
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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 ca
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