On the initial stages of phase formation at the solid Cu/liquid Sn-based solder interface

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The presence of a thin nanometric η-Cu6Sn5 phase layer at the Cu/liquid eutectic Sn-0.7 wt%Cu solder interface in absence of the ε-Cu3Sn phase is demonstrated experimentally for the first time. The principal aim of this work is to determine experimentally and unambiguously the sequence of phase formation at Cu/liquid solder interface, i.e., to determine which of the two thermodynamically stable intermetallics η-Cu6Sn5 or ε-Cu3Sn forms and grows first at this interface. For this purpose, we have performed specific experiments of ultra rapid dipping of thin Cu foils in a solder bath at 250 °C followed by air cooling of the Cu/solder couple. The contact time between the solid Cu and liquid alloy varies from about 50 ms to 65 ms. Moreover a “long time” experiment of about 1 s of dipping is performed for comparison purposes with previous results reported in the literature. The characterizations of samples by a high-resolution scanning electron microscopy and transmission electron microscopy clearly demonstrate that the only phase that forms and grows at the Cu/liquid solder interface, for contact times less than about 65 ms, is the η-Cu6Sn5 phase. The evolution of the morphology and the average thickness of the single growing η-Cu6Sn5 layer, is also studied. Finally a simple qualitative model of nucleation and growth of the η-Cu6Sn5 phase at the Cu/liquid Sn-based solder is proposed. Thus, this study provides valuable insights into the initial stages of the interactions in the Cu/liquid Sn system of very high technological interest especially in the framework of packaging challenges during miniaturization.