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Cu 6 Sn 5   intermetallic occurs in the form of differently ordered phases η, η′ and η′′. 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 Cu 3 Sn + Cu 6 Sn 5  as well as Cu 6 Sn 5  + 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 η can increase its Sn content by Cu 3 Sn 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 η phase, we obtained an interdiffusion coefficient in η of (3 ± 1) × 10 −16  m 2  s −1 . Comparison of this value with literature data implies that this value reflects pure volume (inter)diffusion, while Cu 6 Sn 5  growth at low temperature is typically strongly influenced by grain-boundary diffusion. These investigations also confirm that η′′ forming below a composition-dependent  transus  temperature gradually enriches in Sn content, confirming that Sn-poor η′′ is metastable against decomposition into Cu 3 Sn and more Sn-rich η or (at lower temperatures) η′. Graphic Abstract

Stable and Metastable Phase Equilibria Involving the Cu6Sn5 Intermetallic