Cu6Sn5 intermetallic: Reconciling composition and crystal structure

Cu6Sn5 is the most prominent intermetallic compound developing upon soldering Cu-containing base materials with typical Sn-based lead-free solders. The order-disorder transition occurring at 430–490 K and the possibly associated build-up of stresses is regarded as relevant for reliability of solder joints. In the present work, contradictory reports on the crystal structures and actual composition of the Cu6Sn5 intermetallic are explained by composition-dependent occurrence of the “classical” commensurately ordered η' phase and of an incommensurately ordered, more Cu-rich η phase. The previously reported η8-Cu5Sn4 and η4+1-Cu46Sn37 structures with different unit cells and symmetries are actually approximants of the η phase.