THE GROWTH OF CU-SN INTERMETALLICS AT A PRETINNED COPPER-SOLDER INTERFACE

This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer. The eta-phase (Cu6Sn5) forms when Cu is wet with eutectic solder at temperatures below 400-degrees-C. The intermetallic layer is essentially unaffected by aging at 70-degrees-C for as long as 13 weeks. On aging a eutectic joint at 170-degrees-C, the eta-phase intermetallic layer thickens and epsilon-phase (Cu3Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the eta-phase, while a Pb-rich boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At 170-degrees-C, the Sn in the eutectic is rapidly consumed to form eta-phase intermetallic, which converts to epsilon-phase. The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into eta-phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk. However, the development of the intermetallic largely ceases when the Sn is consumed; epsilon-phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases.