Analysis of low-temperature intermetallic growth in copper-tin diffusion couples
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INTRODUCTION
TIN-bearing solders are widely used to join Cu leads in electronic devices and to protect Cu leads from oxidation during storage prior to assembly (pretinning). The solder bonds strongly to Cu because of the formation of Cu-Sn intermetallics at the wetted interface. However, the intermetallic layer can be a source of mechanical weakness in soldered joints due to brittle cracking of the intermetallic or delamination at the interface and may lead to loss of solderability through excessive growth during storage. Because of their importance to bonding and subsequent properties, many experimental studies have been done on Cu-Sn intermetallic phases and have treated their crystal structures tl] and morphologies, {2-~41 their effects on mechanical properties [~5'~6'17~and solderability, t2-5,1s,~9j and changes in their structure or properties on the addition of other elements into either the copper or the tin-lead solder, t~l'12,~si The intermetallic layer at the solder-Cu interface thickens relatively rapidly during initial soldering or reflow, since the temperature is relatively high and the solder phase is molten. It continues to grow, though much more slowly, during storage and service. This growth in the solid state is of particular technological concern, since it occurs continuously and may cause delayed and unpredicted problems. While a number of empirical relations have been proposed to predict the intermetallic layer thickness as a function of time and temperat u r e , I4'6'7'9"1~ there is less theoretical work of the sort that can generate predictive analytic models. Part of the Z. MEI, Research Engineer, and J.W. MORRIS, Jr., Professor of Metallurgy, are with the Center for Advanced Materials, Lawrence Berkeley Laboratory, and the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. A.J. SUNWOO, Staff Scientist, is with the Lawrence Livermore National Laboratory, Livermore, CA 94550. Manuscript submitted July 23, 1991. METALLURGICAL TRANSACTIONS A
reason for this is the structural complexity of the wetted interface, which, in the common case of eutectic solder on copper, has a two-phase Pb-Sn solder separated from copper by an intermetallic bilayer with e phase (CuaSn) adjacent to the Cu and 7/phase (Cu6Sns) next to the solder. To simplify the problem, we follow Onishi and Fujibuchi t6] in considering interdiffusion at low temperature in a simple Cu-Sn couple and formulate an analytic solution for the planar growth of the intermetallic bilayer. While the results are only qualitatively applicable to intermetallic growth in the solder joint, they allow us to evaluate the interdiffusion coefficients in the intermetallic phases and help to explain important features of intermetallic growth in solder. These include the preponderance of the ,/-phase intermetallic in joints that thicken at relatively low temperature and the apparent shift of the original interface toward the Sn side during growth at higher temperature.
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BACKGROUND
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