Electrical conductivity changes in bulk Sn, and eutectic Sn-Ag in bulk and in joints, from aging and thermal shock
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T. Hogan Electrical and Computer Engineering Department, Michigan State University, East Lansing, Michigan 48824-1226
K.N. Subramaniana) Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824-1226 (Received 28 April 2004; accepted 7 October 2004)
Electrical conductivity of electronic interconnects made with Sn-based solders undergo a significant amount of deterioration during service. Several factors, such as anisotropy of Sn, coefficient of thermal expansion mismatches between the entities that make up the joint, and growth of intermetallic compounds present within the solder and solder/substrate interface, may contribute to the damage accumulation during thermal excursions and cause deterioration of properties. This study dealing with effects of aging and thermal shock on electrical conductivity, carried out with bulk Sn, and eutectic Sn–Ag in bulk and joint configurations, is aimed at evaluating the roles of the above factors on the deterioration of electrical conductivity from these thermal excursions. I. INTRODUCTION
Eutectic Sn–Ag solder is considered a basis for identifying the most suitable substitute to the traditional Sn– 37Pb solder due to its nontoxicity, excellent mechanical properties, and comparable wetting characteristics.1–8 However, eutectic Sn–Ag based solders are not as compliant as leaded solders are.9 To improve the comprehensive properties of Sn-based solders, several focused studies have been carried out and various avenues are being pursued to modify their properties to address the needs in service. Such research efforts include studies on microstructural changes that occur during aging and reflow; the roles of the intermetallic compounds (IMCs) that form at the solder/substrate interfaces and within the solder;10–14 and the mechanical behavior of such alloys, especially in joint geometry, at various temperatures and stress levels.15–25 Also, there have been several attempts to improve the mechanical performances of such solders by alloying, or by incorporating reinforcements to produce composite solders.26–32 It also has been documented that repeated reverse straining and Sn anisotropy play significant roles in damage accumulation that results from thermal excursions.33–38
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0064 364
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 23 Mar 2015
In addition to affecting the mechanical reliability of the solder joint, damage resulting from thermal excursions affects the electrical conduction capability of the joint. Changes in electrical conductivity can be measured with great precision. Such a method also facilitates monitoring of progression of changes during further testing with the same specimen. There have been some recent attempts to use this method for characterizing changes in solder joint specimens.39,40 Although these studies have shown that electrical conductivity measurements can be used
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