This study investigates the microstructural characteristics of 3.0Ag-0.5Cu-Sn (SAC305) solder joint as in assembled state, and their evolution with aging treatment at 100 and 150°C for various hours. Specific aim of the study is (1) to understand thermodynamic stability of asreflowed microstructure (beta-Sn dendrite, inner dendrite Cu6Sn5 intermetallic (IMC) compound, and the eutectic structure), (2) to find direction of their change with aging, and (3) to relate interfacial reaction to such evolution in microstructures. This investigation is spurred by the findings made in a separate study where fatigue resistance of solder joint and failure location shows markedly keen dependence on aging treatment of the joint. The findings made in the present investigation reveals supporting evidences that the change in fatigue reliability is closely related to the change of mechanical properties induced by evolution of microstructure that are not uniform within solder matrix and interfaces.;There are three main findings made in this investigation bearing scientific and engineering importance. The first is the fact that the as-reflowed SAC305 solder joint undergoes non-equilibrium solidification due to rapid cooling during solder assembly and large super-cooling of beta-Sn, resulting in a larger fraction of unstable beta-Sn dendrite than equilibrium and near binary Ag3Sn-Sn eutectic (not ternary eutectic). This, non-equilibrium solidification, results in solder microstructure that is thermodynamically unstable. Therefore, several unusual changes are found to occur by aging treatment. Among many, the most remarkable change is the disappearance of Cu-rich phase from the eutectic pool. It is believed that such a change is resulted because the eutectic formed at as-reflowed state is not true ternary eutectic (due to consumption of Cu by growing Sn-dendrite). Coarsening of Ag3Sn and Sn phases within eutectic pool is observed. Also found is the fact that beta-Sn dendrite collapse with aging, growing to one grain. This result, coarsened Sn grain structure, is especially note-worthy because it may be one of the main sources that reduce the fatigue reliability with aging.;The second finding made in this investigation is the fact that as-reflowed SAC 305 is enriched with Cu due to entrance of Cu from Cu pad during interfacial reaction at reflow temperature. The addition of Cu is found to be sufficient amount to change the solidification sequence of the alloy. Specifically, bulk alloy of SAC305 is known to show solidification sequence of beta-Sn, Ag3Sn and finally ternary eutectic. In the current case, it is found that the alloy solidifies with sequence of Cu3Sn, beta-Sn and eutectic. The enrichment of Cu is especially significant at the interface between solder and Cu pad, probably because Cu pad acts as a source of Cu.;The third finding is related to the growth of inner dendrite Cu 6Sn5 IMC. It is found that aging treatment makes solder to be continuously enriched with Cu through solid state diffusion of Cu through Cu/solder interface. On the other hard, the Cu enrichment, seen as increase in Cu6Sn5 volume fraction, at Ni interface side is found to be minimal or even absent. This result suggests that the solder near at Cu pad interface would continuously strengthened with Cu addition (precipitation hardening), while such strengthening is absent in the solder near at Ni interface. This may explain the fatigue failure location change with aging, which is found to occur initially at Cu/solder interface (due to a large strain singularity) but change to Ni/solder interface after aging treatment.;While more study is necessary in order to better understand the unique metallurgical mechanisms of microstructural evolution found in the present study, the results and understanding gained so far provide sufficient evidence that conventional approach to solder reliability assessment, which is based largely on interface IMC microstructure, needs to be changed.
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