Stress behavior of electroplated Sn films during thermal cycling
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The mechanical behavior of electroplated Sn thin films was investigated using thermal-expansion induced strain. For stress above a threshold value, the stress relaxation observed during the thermal cycles is well-described by a power law creep mechanism with exponents similar to those of the bulk material. However, the stress relaxation showed significant thickness dependence so that the relaxation in thicker films is faster than thinner films. The surface oxide was also shown to have a considerable effect on retarding the relaxation by inhibiting diffusion to the surface. The relevance of the stress relaxation to whisker formation in Sn-based coatings is discussed. I. INTRODUCTION
Sn-based coatings play a major role in electronics manufacturing, for example, to enhance solderability, prevent oxidation, provide wear resistance, and so on. Recently, the drive toward Pb-free manufacturing has lead to the substitution of pure Sn coatings to replace Pb-Sn alloys in compliance with the EU’s RoHS (The Reduction of Hazardous Substances) directive. This has raised multiple concerns based on the different mechanical and thermal properties of the new coatings relative to the traditional ones. The use of Pb-free Sn coatings can cause severe problems such as mechanical failure of solder joints and long whisker growth; these problems are believed to be related to the stress relaxation in the material. The higher melting points and yield stresses of Pb-free alloys can induce larger thermal stresses during fabrication.1–3 According to recent research,4–6 compressive stress in the Sn layers induced by Cu-Sn intermetallic compound formation is the driving force for the growth of whiskers. Therefore, understanding the processes controlling stress in Sn-coatings is essential to determining their reliability and improving their performance. In the current study, we present measurements of stress relaxation in pure Sn coatings. Strain was induced by heating due to the thermal expansion mismatch between the coatings and the substrate. The resulting stress was measured in real time using wafer curvature techniques, similar to the approach used in other thin film materials, for example, Cu and Al.7–12 We measured the stress evolution under various conditions of temperature and stress in films of different thickness. During heating, the thin films of Sn were found to attain higher stresses a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0172
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 4, Apr 2009 Downloaded: 19 Mar 2015
than the yield stress found in bulk materials. A power law creep mechanism was found to agree quantitatively with the observed relaxation kinetics with an exponent similar to bulk materials. In addition, the relaxation kinetics were found to depend on film thickness (or grain size). The presence of the surface oxide was found to significantly retard the relaxation kinetics by inhibiting diffusion to the surface; removal of the oxide by light acid etc
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