Interdiffusion and Reaction in Bimetallic Cu-Sn Thin Films

A multiphase diffusion model was constructed and used to analyze the growth of the e- and η-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements of intermetallic layer growth were used to compute the interdiffusivities in thee andη phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the e phase(≈De) is well fit by an Arrhenius expression with D0 = 5.48 × 10−9 m2/s andQ = 61.9 kJ/mole, while that in the η phase (≈Dη) has D0= 1.84 × 10−9 m2/s andQ = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in theη phase has the consequence that theη phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in theη phase has the result that thee phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20 °C, the predicted e-phase thickness is ≈10 pct of that ofη, while at 200 °C, its thickness is 66 pct of that ofη. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within theη-phase layer after diffusion. It lies near the midpoint of theη-phase layer at higher temperature (220 °C) and, hence, appears to shift toward the Sn side of the couple. The results are compared to experimental observations on intermetallic growth at solder-Cu interfaces.

Analysis of low-temperature intermetallic growth in copper-tin diffusion couples

Reaction-Diffusion in the Cu–Sn System

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 η phase (Cu6Sn5) forms when Cu is wet with eutectic solder at temperatures below 400 °C The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and e phase (Cu3Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η 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 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to e 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 η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk How ever, the development of the intermetallic largely ceases when the Sn is consumed; e 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

The growth of Cu-Sn intermetallics at a pretinned copper-solder interface

A phase transformation has been reported to occur at approximately 460 K in the compound Cu6Sn5, which has the NiAs (B81 structure. Single-crystal and polycrystalline specimens of this compound were heat treated to produce the high-temperature Ŋ-phase and the low-tem-perature Ŋ’-phase. X-ray diffraction determinations confirmed that the transformation was of the type: Simple Superlattice Ŋ Long-Period Superlattice Ŋ’ An analysis proposed by Pauling for phases with the NiAs structure indicated that the nature of bonding in Cu6Sn5 is predominantly metallic. The heat of formation and the energy effect associated with the Ŋ-Ŋ’ transformation were measured by liquid metal solution calorimetry with tin as solvent. The heat of formation of the Ŋ’-phase is approximately —1.68 kcal per g-at., which also suggests metallic bonding. The energy effect of the Ŋ -Ŋ ’ transformation is approximately -0.06 kcal per g-at. and is consistent with the nature of the transformation of a simple superlattice to a long-period superlattice.

P.L Brooks

Papers

The ϵ-phase in the CuSn system