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The reliability of microelectronic interconnections depends on hot deformation of solders. In this work, we studied the localized stress relaxation of Sn3.5Ag eutectic alloy using the impression testing in the temperature range of 393–488 K. By incorporating the effect of internal stress in the analysis, we obtained the strain rate-stress exponent of 6.59. The activation energy for the stress relaxation is in the range from 38.6 to 43.8 kJ/mol, which compares well with the estimated activation energy of dislocation pipe diffusion, 46 kJ/mol, in pure tin. This suggests that a single mechanism of dislocation climb limited by dislocation pipe diffusion might be the controlling mechanism for the localized stress relaxation of the Sn3.5Ag eutectic alloy.

I. INTRODUCTION

Because of a growing concern about the hazards of Pb-based solders to human health and the environment, Pb–Sn based solders will be completely replaced by Pbfree solders in the future. Thus, developing viable alternative Pb-free solders for microelectronic assembly and packaging is of importance worldwide. It is known that the time-dependent plastic deformation of solders is crucial for accurate prediction of the performance and reliability of microelectronic interconnects. Current knowledge of the time-dependent plastic deformation of Pb-free solders is limited and generally based on the characterization of bulk materials. In studying the hot deformation behavior of Pb-free solder, tensile and impression tests1–5 have been used with the focus on the creep behavior of Sn3.5Ag eutectic alloy. In general, the steady-state creep rate ⑀˙ of Sn3.5Ag eutectic alloy can be related to the tensile stress and temperature by using the power law, ⑀˙ ⳱ A␴n˜ exp(−Q/ RT), where A is a constant, ␴ is the tensile flow stress, n˜ is the stress exponent, Q is the activation energy, R is the gas constant, and T is the absolute temperature. Stress relaxation is one of several methods of indirectly studying time-dependent plastic deformation of materials at elevated temperatures. This method is valid only under the assumption that the microstructure of specimen does not change during stress relaxation.6 Both recovery and aging must be avoided.7 Using the impression technique,8–22 localized stress relaxation has been

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0335 J. Mater. Res., Vol. 21, No. 10, Oct 2006

http://journals.cambridge.org

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used to characterize the stress relaxation of LiF 6 and Pb–Sn eutectic alloy.17 The advantages of using the impression stress relaxation include the ability to use only a small amount of material and, possibly, to evaluate local mechanical behavior of inhomogeneous materials. Up to now, there has been little study on the stress relaxation of Pb-free solder. To avoid the use of tens of samples needed for the usual tensile relaxation tests and to avoid sample-to-sample microstructural variations, we evaluated the localized stress relaxation of Sn3.5Ag eutectic alloy