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C. Kru¨ger Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany

U. Welzela) and J.Y. Wang Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany

E.J. Mittemeijer Max Planck Institute for Metals Research, D-70569 Stuttgart, Germany; and Institute for Materials Science, University of Stuttgart, D-70569 Stuttgart, Germany

W. Hu¨gel Robert Bosch GmbH, D-72770 Reutlingen, Germany (Received 30 March 2010; accepted 19 July 2010)

After sputter deposition of Sn (layer thickness of 350 nm) on Cu substrates and during subsequent aging at room temperature, Cu and Sn reacted to form the intermetallic phase Cu6Sn5 in the Sn layer at the Cu/Sn interface, which led within a few hours of aging to the development of a compressive stress parallel to the Cu/Sn interface in the Sn layer. One day after aging at room-temperature whisker formation occurred on the surface of the Sn layer. It was shown that whisker growth is associated with long-range Sn diffusion parallel to the Cu/Sn interface. Sn layers of the same thickness sputter deposited on pure Si substrates exhibited throughout the same aging time at room temperature a tensile stress parallel to the Cu/Sn interface (no intermetallic phase formation took place) and whisker formation did not occur. The interrelationship of intermetallic compound formation, stress development, and whisker growth is discussed.

I. INTRODUCTION

Pure Sn is currently the material of choice for use as “surface finish” (it provides surface passivation and solder reactions with the leadframe) of electronic components, as application of the until recently commonly used Sn–Pb alloys (introduced in industrial manufacturing in the middle 1960s1) has become unacceptable according to legislative directives (RoHS, Restrictions of Hazardous Substances) for “green manufacturing” (e.g., Pb-free soldering) in the electronics industry.2 However, for more than 50 years it has been well known that pure Sn thin films deposited on Cu are prone to the formation of Sn whiskers3 (lengths of several millimeters4 can be achieved), which can lead to short-circuit failure of the electronic equipment5 and thus the whisker-growth phenomenon recently attained great technological and scientific interest. Countermeasures such as “postbake” (a postplating annealing treatment) and application of diffusion barriers (e.g., Ni) were proposed.5 a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0276

2166

http://journals.cambridge.org

J. Mater. Res., Vol. 25, No. 11, Nov 2010 Downloaded: 13 Apr 2015

˚ /s6–8 and Whiskers can exhibit growth rates of about 1 A ˚ up to 10000 A/s in the presence of externally applied mechanical stresses.9 Whiskers are nearly perfect singlecrystalline filamentary structures9–11 (with diameters of about 1–10 mm) growing through continuous addition of material to their base12 on the surface of thin metal films with a low melting point (e.g., Cd, Zn, and Sn).3 The morphologies of whiskers can be diverse; they can be st