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

• Published in: Acta materialia Vol. 224; p. 117513
• Main Authors: Yang, Wenhui, Tran, Xuan Quy, Yamamoto, Tomokazu, Aso, Kohei, Somidin, Flora, Tan, Xin Fu, Kawami, Youichirou, Nogita, Kazuhiro, Matsumura, Syo
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2022
• Subjects: Atomic-resolution XEDS mapping; Long-range ordering; Poisson NLPCA; Structural modulation; η-Cu6Sn5; η-Cu6Sn5; Long-range ordering; Poisson NLPCA; Atomic-resolution XEDS mapping; Structural modulation
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2021.117513

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 ah axis relative to the Ni-dilute case, but anisotropically expands the ch axis in a bimodal fashion; in contrast, the effect of Au appears to be of an isotropically expanding nature. [Display omitted] .