Atomic insights into the ordered solid solutions of Ni and Au in η-Cu₆Sn₅

The Cu₆Sn₅ intermetallic, which commonly forms at the solder interconnects, is a critical component contributing to the reliability of today's electronic products. It has been established that the structural control of its hexagonal η-Cu₆Sn₅ polymorph can be achieved over a wide temperature range of service conditions via chemical doping with Ni or Au, effectively suppressing the undesirable hexagonal to monoclinic (ƞ → ƞ′) phase transition at 186 °C and its associated volume change. In this study, we further investigate the effects of Ni (26.5 at%) and Au (9 at%), with high doping/alloying contents, on the atomic-scale structure of η-Cu₆Sn₅ using a suite of microscopy techniques including atomic-resolution imaging, chemical mapping, electron diffraction, and in-situ heating, coupled with advanced data informatics. Our study reveals that while Ni occupancy takes place in both the Cu1 and Cu2 sites of η-Cu₆Sn₅ in substantial amounts, Au is mostly substituted at the Cu1 sites of η-Cu₆Sn₅. Most interestingly, characteristic occupational modulations of the Cu₆Sn₅ structure arise with each type of dopants: a three-fold ordered structure for Ni accompanied by a displacive modulation of the constituent atoms, but a two-fold layer-like structure for Au. Moreover, with a high content of Ni, the unit cell of η-Cu₆Sn₅ is found to contract along its hexagonal a_h axis relative to the Ni-dilute case, but anisotropically expands the c_h axis in a bimodal fashion; in contrast, the effect of Au appears to be of an isotropically expanding nature.