Plastic deformation processes in Cu/Sn bimetallic films
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Although the driving force for the growth of Sn whiskers from the surface of Sn coatings on copper is thought to be internally generated stress due to the formation of Cu6Sn5 at the Cu/Sn interface, little is known about the nature of this internal stress and how it cracks the surface Sn oxide (an important precursor to whisker formation). Arguments based on elasticity alone do not appear to be sufficient and suggest an important role for plastic deformation. Direct observations, made by transmission electron microscopy of cross-sectioned bimetallic Cu/Sn thin-film specimens, confirm plastic deformation of the Sn grains due to the formation of Cu6Sn5. Dislocation motion and pile-up at the surface Sn oxide, rotation associated with subgrain boundary formation, interaction of the subgrain boundaries with the Sn surface, and diffusional processes are various mechanisms that can produce stress at the Sn surface and crack the Sn oxide.
I. INTRODUCTION
The formation of single-crystal whiskers on the surface of metals such as Sn, Zn, Cd, and Al has been studied over the past six decades,1–6 and in the 1950s, significant effort was expended in understanding the mechanism of whisker growth.7–10 The particular case of the formation of whiskers from the Sn surface when it is electroplated on copper has been of significant interest to the microelectronics industry.2,11–15 Such Sn whiskers are usually a couple of micrometers in diameter but can grow to several millimeters in length over time while an electronic component is in service. These whiskers can bridge the gaps between closely spaced interconnects and can create short circuits, leading to system failure. Numerous examples of such failures have been documented in the literature and include pacemakers, satellites, and missiles.13,16–20 The problem of Sn whiskering had been circumvented by adding Pb to Sn, but recent international regulations that require the elimination of Pb from electronic components have led to a re-emergence of significant scientific and technological interest and a flurry of research activity in this area.11,14,15,21
a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www. mrs.org/jmr_policy b) Present address: Raytheon Company IDS, Maritime Mission Center, Portsmouth, RI 02871. DOI: 10.1557/JMR.2008.0351 2916
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J. Mater. Res., Vol. 23, No. 11, Nov 2008 Downloaded: 18 Mar 2015
Various driving forces have been suggested for whisker formation, including recrystallization,1,22–25 oxidation,7,8,26 and stress in the layers.5,11,12,21,27–40 Fisher et al.33 in 1954 first demonstrated that compressive stress in the Sn layer can serve as a driving force for whisker growth by applying ring clamps to Sn-plated steel specimens and measuring whisker growth rates as a function of clamping pressure and time. The current con
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