Microstructural analysis of lead-free solder alloys
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INTRODUCTION
IN recent years, there has been an increasing need for developing lead-free solder alloys owing to both environmental concerns[1] and technical demand for solders to be used in severe service conditions e.g., high temperatures in automobiles and high current density in microelectronic components. Tin-based eutectic alloys are the suitable substitutes due to their low melting temperatures. A large number of studies have already been carried out on thermodynamics and phase compositions[2,3,4] and mechanical properties of near eutectic tin-based alloys.[4] Near eutectic compositions Sn-3.5Ag and Sn-3.8Ag-0.7Cu are found to be among the most promising alloys as they possess mechanical properties better than many other alloys including lead-tin alloys.[5,6,7] The development of lead-free solder alloys, however, is still hindered by the lack of comprehensive understanding of the dependence of mechanical properties on the microstructure. Predictions of mechanical behaviors and ensuring reliability of a solder joint require an in-depth understanding of structure-property relationships. The current understanding on thermal fatigue of solder alloys is primarily based on studies on Pb-Sn alloys, which have been studied in great detail in the past decades. A comprehensive understanding on the microstructure of lead-free alloys and their dependence on processes, which is required for establishing structure-property relationships, is yet to be achieved. The microstructure of solder alloys is traditionally characterized by using optical or electron microscopy techniques to measure grain sizes, interdendritic spacing, intermetallic (IM) particle sizes, IM phase layer thicknesses, and so forth. However, because the microstructure of solder alloys consists of multiple phases and their morphology and distributions have strong effects on mechanical properties, VINEET KUMAR, Graduate Research Assistant, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. ZHIGANG ZAK FANG, Assistant Professor, is with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] JIN LIANG, Manager, Materials Engineering, and NADER DARIAVACH, Senior Engineer, are with EMC Corporation, Hopkinton, MA 01748. Manuscript submitted March 9, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
understanding crystallographic orientations and orientation relationships between various phases and their dependence on processing conditions is very important for truly understanding their effects on mechanical properties. Orientation imaging microscopy (OIM), also termed as electron backscattered diffraction (EBSD), is a relatively new technique that enables one to investigate not only microstructure feature dimensions but also orientation of phases, misorientation between grains, and orientation relationships between different phases. It uses the electron diffraction patterns formed by the diffraction of backscattered electrons fr
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