Changes in the phase composition and structure of electrodeposited Sn–Co alloys in Sn–Co/Cu layer systems upon heating

Changes in the phase composition and structure of electrodeposited tin–cobalt (Sn–Co) alloy layers (5–10 μm) containing a limited quantity of Co (0.24, 2.4, 11.0, 17.0 or 26.5 wt.%) were studied using X-ray diffraction (XRD) and optical microscopy. ase composition and coating structures of Sn–Co alloys in Sn–Co/Cu layers underwent considerable changes upon heating at 170 °C for 100–200 h and at 170 °C for 200 h followed by 205 °C for 20 h. The changes were dependent on the Co content in the alloy and the heating temperature. When examining the sample containing 11.0 wt.% cobalt, an unidentified phase containing a Sn–Co compound, CoSn2 and CoSn3 formed upon heating at 170 °C for 100, 140 and 200 h. The structure of the layer surface was composed of spherical crystallites. The formation of the unidentified phase possibly caused the formation of needle-shaped crystals in the structure consisting of spherical crystallites. When the Co content of the Sn–Co layer was increased to 17.0–26.5 wt.% and the sample was heated to 170 °C for 100–200 h, phases of CoSn, CoSn2, Co3Sn2 and CoSn3 were formed. The coating maintained its structure and was composed of spherical crystallites. Sn–Co/Cu system of layers containing 11.0, 17.0 or 26.5 wt.% Co, Sn–Cu intermetallic compounds (Cu6Sn5 and Cu3Sn) did not form after heating at 170 °C for 200 h followed by 205 °C for 20 h.