Suppression of thermomigration in Cu/Sn-3Ag-0.5Cu/Cu solder joints by grain coarsening of Cu6Sn5 at hot end

초록

Background Thermomigration causes directional atomic movement in solder joints subjected to a temperature gradient, resulting in accelerated growth of Cu6Sn5 at the cold end. The accelerated growth of brittle Cu6Sn5 poses a reliability issue, and therefore, suppression of thermomigration becomes an urgent task. Methods In this study, the Cu substrates with three different grain sizes were fabricated using electroplating. The Cu grain size was controlled from a few hundreds of nanometers to tens of micrometers by formulating the plating additives. Sn-3 wt.% Ag-0.5 wt.% Cu (SAC305) solder joint with two Cu substrates at both sides is heated under a temperature gradient of 1753 °C/cm to investigate the thermomigration. Significant Findings Temperature gradient induced thermomigration, resulting in asymmetrical growth of Cu6Sn5 at both ends of the joint. The Cu6Sn5 phase formed at the cold-end interface grew at a much faster rate than that at the hot-end interface as a result of the thermomigration-induced Cu flux from the hot end to cold end. The thickness difference of Cu6Sn5 between both ends was greatly reduced due to the formation of a thicker and coarser Cu6Sn5 phase on the hot-end fine-grained Cu substrate. The phenomenon of asymmetrical growth of Cu6Sn5 could be moderated as the joint sample was pre-heated at 260 °C before thermomigration. The suppression efficacy against thermomigration can be attributed to the thickening and coarsening of Cu6Sn5 at the hot-end interface, serving as a barrier to retard the dissolution of Cu substrate.

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