Crystal Structure of Au1-xNixSn4 Intermetallic Alloys.

Abstract Ni coated with a thin layer of Au is commonly used as a metallization in electronics packaging. Such Ni/Au metallizations are alloyed with Sn at temperatures above the Pb–Sn solder liquidus. Thermal treatment of these joints in the solid state, for instance at 150°C, has been shown to result in the formation of Au 1− x Ni x Sn 4 compounds at the solder interface. These compounds are found to be deleterious to joint integrity. In this work, synthesis, microstructural analysis and crystal structure determination of Au 1− x Ni x Sn 4 alloys were performed. The alloys were examined by powder and single crystal X-ray diffraction, metallography, electron microprobe analysis, and thermal analysis. It was found that Ni atoms substitute for Au atoms in the AuSn 4 structure giving compounds with general composition Au 1− x Ni x Sn 4 (with x ≤0.5). The structure of the title compounds is considered isotypical with the AuSn 4 phase (the PdSn 4 structure type), with a symmetry increment from Aba2 to Ccca . Crystal structure refinement from powder data using the Rietveld method was performed for two alloys with x =0.25 and 0.50 yielding Au 0.73 Ni 0.27 Sn 4 (sp. group Ccca , a =6.448 A, b =11.606 A, c =6.441 A) and Au 0.51 Ni 0.49 Sn 4 (sp. group Ccca , a =6.424 A, b =11.522 A, c =6.384 A). A single crystal experiment was performed for compound Au 0.91 Ni 0.09 Sn 4 (sp. group Ccca , a =6.523 A, b =11.676 A, c =6.485 A).