Creep of tin, Sb-solution-strengthened tin, and SbSn-precipitate-strengthened tin
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NTRODUCTION
IN recent decades, the toxicity and mechanical properties of solder alloys have become increasingly important issues. Concerns over the toxicity of lead have resulted in incentives to eliminate the use of solders containing lead. The mechanical properties of solders have become important because of decreasing joint sizes and joint designs that require solders to act as both a mechanical and electrical connection. A prime cause of solder joint failure in the electronic industry is low cycle fatigue resulting from differences in coefficient of thermal expansion between components connected by solder joints.[1] Creep mechanisms are active and play an important role in these fatigue processes because of the high homologous temperatures. Solder alloys that can relax under the cyclic stresses resulting from thermal cycling are considered necessary to prevent early failure of these joints. For other applications, such as optical interconnects, dimensionally stable solders are needed because any creep deformation may result in a device failure.[2] The Au-Sn eutectic solder has been commonly used for these applications. However, there is a desire to replace this solder because of its cost and extreme brittleness. Solder joints that experience extreme temperature environments, such as those found under the hood of automobiles, also require good creep resistance. The present research is concerned with the creep properties of tin-antimony solders. Creep studies were performed on commercially pure tin, tin strengthened by antimony in solution, and tin strengthened by the precipitate SbSn. The RODNEY J. McCABE, Postdoctor, is with the MST-8 Structure/Property Relation Group, Division of Materials Science and Technology, Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: rmccabe@ lanl.gov MORRIS E. FINE, Walter D. Murphy Professor, is with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. Manuscript submitted March 27, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
results of these tests are analyzed in terms of measured microstructural parameters. The tin-antimony phase diagram is shown in Figure 1.[3] Tin-antimony peritectic solders have melting temperatures between that of eutectic Sn-Pb solder (183 ⬚C) and high Pb solders (around 320 ⬚C). For lead-free electronics, Sn-Sb peritectic solders could substitute for high Pb solders in devices required to undergo multiple soldering steps, provided a lead-free solder with sufficiently low melting temperature is used for the subsequent soldering steps. In addition, because of their low melting temperatures, solders are high temperature alloys that are conveniently studied at relatively low temperatures. II. EXPERIMENTAL The alloys tested are shown in Table I. Compositions are given in weight fractions with the reported error varying from 0.1 wt pct for Sn-2.9Sb to 0.25 wt pct for Sn-8.1Sb. The listed volume fractions are for 23 ⬚C; however, the volume fraction for each alloy is expected to remain essentia
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