Microstructural evolution in lead-free solder alloys: Part II. Directionally solidified Sn-Ag-Cu, Sn-Cu and Sn-Ag
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Daniel J. Lewis and Robert Schaefer Metallurgy Division, National Institute of Standards and Technology, Materials Science and Engineering Laboratory, Gaithersburg, Maryland 20899 (Received 18 September 2003; accepted 21 January 2004)
The tin–silver–copper eutectic is a three-phase eutectic consisting of Ag3Sn plates and Cu6Sn5 rods in a (Sn) matrix. It was thought that the two phases would coarsen independently. Directionally solidified ternary eutectic and binary eutectic samples were isothermally annealed. Coarsening of the Cu6Sn5 rods in the binary and ternary eutectics had activation energies of 73 ± 3 and 82 ± 4 kJmol−1, respectively. This indicates volume copper diffusion is the rate controlling mechanism in both. The Ag3Sn plates break down and then coarsen. The activation energies for the plate breakdown process were 35 ± 3 and 38 ± 3 kJmol−1 for the binary and ternary samples respectively. This indicates that tin diffusion along the Ag3Sn/(Sn) interfaces is the most likely the rate-controlling mechanism. The rate-controlling mechanisms for Cu6Sn5 coarsening and Ag3Sn plate breakdown are the same in the ternary and binary systems, indicating that the phases evolve microstructurally independently of one another in the ternary eutectic.
I. INTRODUCTION
Quantitative coarsening 1studies of multiphase ternary systems are difficult to carry out, as seen in the companion investigation1 of the eutectic tin-silver-copper system (Sn–3.66 wt% Ag–0.91 wt% Cu2). The ternary eutectic structure for this system consists of rods of Ag3Sn and Cu6Sn5 in a tin solid solution matrix that is usually designated as (Sn).3 To study coarsening in such a system, each phase must be individually tracked over time, which requires the ability to easily discern one phase from another. This is particularly difficult if the scale of the phases is extremely fine, as is often the case. Furthermore, the distribution in alignment of each phase can pose a problem for quantitative stereology. This results from the differing orientations of the eutectic colonies, such that it is frequently impossible to ensure that the eutectic rods being examined fall perpendicular to the observation plane. Finally, it is sometimes not clear if the Commercial equipment and materials are identified in order to adequately specify certain procedures. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. DOI: 10.1557/JMR.2004.0191 J. Mater. Res., Vol. 19, No. 5, May 2004
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coarsening rates of each phase are independent or if the coarsening rate of one phase is dependent on the rate of another. This can make the use of standard coarsening models difficult. In the companion examination of cast ternary specimens,1 it was observed that Cu6Sn5 rods coarsen much more rapidly than Ag3Sn rods. From this observation and the accompanyin
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