Stable and Metastable Phase Equilibria Involving the Cu6Sn5 Intermetallic

Cu6Sn5 intermetallic occurs in the form of differently ordered phases eta, eta' and eta ''. In solder joints, this intermetallic can undergo changes in composition and the state of order without or while interacting with excess Cu and excess Sn in the system, potentially giving rise to detrimental changes in the mechanical properties of the solder. In order to study such processes in fundamental detail and to get more detailed information about the metastable and stable phase equilibria, model alloys consisting of Cu3Sn + Cu6Sn5 as well as Cu6Sn5 + Sn-rich melt were heat treated. Powder x-ray diffraction and scanning electron microscopy supplemented by electron backscatter diffraction were used to investigate the structural and microstructural changes. It was shown that Sn-poor eta can increase its Sn content by Cu3Sn precipitation at grain boundaries or by uptake of Sn from the Sn-rich melt. From the kinetics of the former process at 513 K and the grain size of the eta phase, we obtained an interdiffusion coefficient in eta of (3 +/- 1) x 10(-16) m(2) s(-1). Comparison of this value with literature data implies that this value reflects pure volume (inter)diffusion, while Cu6Sn5 growth at low temperature is typically strongly influenced by grain-boundary diffusion. These investigations also confirm that eta '' forming below a composition-dependent transus temperature gradually enriches in Sn content, confirming that Sn-poor eta '' is metastable against decomposition into Cu3Sn and more Sn-rich eta or (at lower temperatures) eta'.

Automated summary

Cu6Sn5 intermetallic compound can undergo composition and order changes in solder joints, affecting the mechanical properties of the solder. Investigations show that Sn-poor eta phase can increase its Sn content through different processes, while Cu6Sn5 growth at low temperature is typically influenced by grain-boundary diffusion.