Accelerated R&D efforts in industry, universities, national laboratories, and government agencies have recently identified several promising lead-free solders to replace lead-containing solders in microelectronic applications. The leading candidates are Sn-3.5Ag, Sn-3.5Ag-0.7Cu, Sn-3.5Ag-4.8Bi, and Sn-.07Cu (in weight percent). These lead-free solders are all tin-rich with a melting temperature between 210 deg C and 27 deg C. They are recommended for such soldering applications as surface-mount technology, plated-through-hole, ball grid arrays, flip chip, and others. Despite the R&D progress and the proliferation of published technical findings on lead-free solders, our knowledge and understanding of these new materials are still at an infancy stage compared to lead-containing solders. Thus, unanswered questions and unresolved issues still persist: Can we make and properly test reliable lead-free solder joints? What are the implications of higher reflow temperatures required for the new solders? Are the new surface finishes needed? What is the lead-free solder candidate for flip-chip applications? Can we maintain the solder hierarchy between the first and second level interconnection? What are the reliability issues of new solder alloys? What are the solidification mechanisms in solder joints? How well are microstructure-property relations understood in lead-free solder alloys and joints? Have interfacial reactions between tin-rich solders and new surface finishes been adequately characterized? What are the influences of microstructural evolution during thermomechanical process? There is much to be learned regarding thermal fatigue mechanisms in solder joints, creep and fatigue interactions, corrosion behavior, etc. to address these challenging and impending issues, TMS has been active over the last several years in organizing a series of symposia on lead-free solders and soldering technologies.
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