Stacking-disordered CoSn3 and tetragonally stacked CoSn4 formed during solid-state interdiffusion of Co and Sn

The reactive interaction of Sn-rich solders with transition metals at temperatures close or above the melting point of the Sn leads to formation of stable or metastable intermetallic phases. Adequate microstructure analysis of solder joints relies on correct identification of the phases and their microstructures. The present work, thereby, focusses on the intermetallic forming from Co and Sn under solid-state conditions (150°C–190 °C) in model diffusion couples. The crystal structures and microstructures having developed were characterized using by X-ray diffraction (XRD) and scanning electron microscopy) supplemented by electron-dispersive X-ray spectroscopy and electron-backscatter diffraction (EBSD). While at 190 °C virtually exclusively a CoSn3 layer develops, at 170 °C and 150 °C an additional CoSn4 layer is formed. As evidenced by XRD, both the CoSn3 and CoSn4 having formed here show crystal structures different from previous works. The CoSn3 shows a random stacking of Co2Sn6 double layers, intermediate between the known equilibrium polytypes α-CoSn3 and β-CoSn3. The CoSn4 has the tetragonal β-IrSn4 structure (also I41/acd) with a largely regular stacking of CoSn4 layers (referred to as β-CoSn4), whereas until now a different polytype (orthorhombic α-CoSn4) has been reported. Including also results from first-principles calculations on the stable polytypes, criteria for accurate phase identification by XRD and EBSD methods have been elaborated, involving strategies to reliably distinguish the polytypes of CoSn3 or CoSn4, but also to distinguish (whatever form of) CoSn3 from (whatever form of) CoSn4.