Crystal Structure of Cu-Sn-In Alloys Around the η-Phase Field Studied by Neutron Diffraction

Study of the Cu-Sn-In ternary system has become quite important in recent years, due to new environmental regulations increasingly restricting use of Pb for bonding technologies in electronic devices. A key relevant issue concerns the intermetallic phases which grow in the bonding zone and strongly affect its quality and performance. In this work, we focus on the eta-phase (Cu2In or Cu6Sn5) that exists in both end binaries and as a ternary phase. We present a neutron diffraction study of the constitution and crystallography of a series of alloys around the 60 at.% Cu composition, and with In contents ranging from 0 at.% to 25 at.%, quenched from 300A degrees C. The alloys were characterized by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and high-resolution neutron diffraction (ND). Rietveld refinement of ND data allowed improvement of the currently available model for site occupancies in the hexagonal eta-phase in the binary Cu-Sn as well as in ternary alloys. For the first time, structural data are reported for the ternary Cu-Sn-In eta-phase as a function of composition, information that is of fundamental technological importance as well as valuable for ongoing modeling of the ternary phase diagram.