Whisker Formation in Sn Coatings on Cu
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Whisker Formation in Sn Coatings on Cu Eric Chason, Lucine Reinbold and Sharvan Kumar Division of Engineering, Brown University, Providence, RI 02912
ABSTRACT In the microelectronics industry, Sn is often electroplated as a protective layer on Cu conductors. Pure Sn layers on Cu develop whiskers that can cause component failures and have even been implicated in the loss of several satellites. Alloying Sn with Pb suppresses whisker formation, but the push towards Pb-free processing will make this unacceptable in the future. To understand the driving forces and mechanisms of whisker formation on pure Sn, we are measuring the kinetics of stress evolution and intermetallic formation in Sn/Cu layers. By using thin films of Sn and Cu, we can monitor the stress evolution in real time using wafer-curvature based techniques. Preliminary results of stress evolution in vapor-deposited films are presented showing the evolution of tensile stress in the Cu layers and compressive stress in the Sn layers.
INTRODUCTION Electroplated Sn and Sn alloy finishes are used extensively in the electronic components industry on conductors such as Cu and phosphor bronze. Sn-based coatings provide corrosion resistance and enhanced solderability with the added benefits of excellent thickness control and uniformity (which is especially critical in miniaturized, fine-pitched components) and good electrical conductivity. Since pure Sn coatings are susceptible to failure by the production of Sn whiskers, (i.e., spontaneous hair-like filaments that grow outward from the Sn surface), Pb-Sn alloys are commonly used in high reliability applications since they do not whisker. The European Reduction of Hazardous Substances (RoHS) Directive will require the removal of Pb from electronic components in the near future, which has pushed the electronics industry to eliminate Pb from their products and manufacturing processes. Some manufacturers already are (or will soon be) using pure Sn coatings to replace the Pb-Sn alloys, thus resurrecting the issue of Sn whiskering. This has made an understanding of whisker formation a critical problem. Although the whisker phenomenon was first reported in the 1940’s [1], there is still considerable disagreement about the fundamental processes controlling whisker growth. Determining the factors that control whisker growth kinetics in pure Sn would make it possible to develop alternative (Pb-free) processing approaches or materials systems that are environmentally acceptable. Furthermore, the fundamental understanding developed from such a study could be equally applicable to other situations –for example, to “grow” metallic nanowires in a controlled manner. To develop a fundamental understanding of whisker growth, we are using a combination of techniques on well characterized Sn/Cu layered structures to determine the stress in the layers, the whisker growth kinetics and the formation of the intermetallic produced by the reaction of Sn and Cu. In this work, we present preliminary results of stress in the Sn and
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