Time-dependent deformation behavior of near-eutectic 60Sn-40Pb solder
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I. INTRODUCTION
THE near-eutectic 60Sn-40Pb solder alloy is extensively used in the electronics industry as an electrical, and mechanical, interconnect material. There is a long history of the use of this alloy for electronic applications that dates back almost 100 years. The near-eutectic Sn-Pb alloy has a number of processing parameters and physical properties, which make it attractive for the assembly of electronic packages. The alloy has a melting temperature of 183 8C that is compatible with the materials used in electronic packages and has excellent wetting behavior on metal surfaces such as Cu, Au, and Ag. The environment that electronic packages encounter in service is severe for the materials involved. The packages can operate under humid conditions, shock and vibration (for automotive and aerospace applications), and thermal cycling environments. The thermal fluctuations include cooldown from soldering temperatures (typically 220 8C down to room temperature), ambient temperature cycles (that range from a low of 255 8C to highs in excess of 125 8C), and temperature cycles created by turning the electronics on and off (Joule heating that occurs over a range of room temperature up to 150 8C). Strain is imparted to the solder during thermal cycling because the electronic package is composed of materials with different coefficients of thermal expansion. The joined components are usually much stiffer than the solder so the thermally induced strain is taken up primarily by the solder. Depending on geometry, this strain can easily exceed 10 pct and is imparted very slowly due to the relatively slow changes in temperature. These strain J.J. STEPHENS and D.R. FREAR, Principal Members of Technical Staff, are with Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted February 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
conditions result in temperature-induced time-dependent deformation. The reliability of solder interconnects is critical as they are often the life limiting feature in many electronic packaging applications. The failure of a single joint could render an entire electronic assembly inoperable. Therefore, the rateand time-dependent deformation properties must be characterized. The time-dependent deformation is also used in predicting the behavior of solder joints under conditions of thermomechanical fatigue.[1,2,3] In these lifetime prediction methodologies, the time-dependent deformation behavior of the solder is described in the form of a constitutive model used to calculate stress and strain in finite element analysis calculations. However, the relations do not cover all the temperature regimes of interest in electronic packaging: typically, only data from elevated temperature tests (23 8C to 150 8C) are available. Also, the constitutive relations have been derived as a mathematical best fit to the test data with little mechanistic interpretation or accounting for microstructure. Near-eutectic Sn-Pb solder possesses a structure of Pb-rich and Sn-rich phases that form colo
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