Nickel-stabilized hexagonal (Cu, Ni)6Sn5 in Sn–Cu–Ni lead-free solder alloys

Cu6Sn5 is an important intermetallic compound (IMC) in lead-free solder alloys. Cu6Sn5 exists in two crystal structures with an allotropic transformation from monoclinic η′-Cu6Sn5 at temperatures lower than 186 °C to hexagonal η-Cu6Sn5. A detailed analysis by transmission electron microscopy (TEM) in Sn–0.7 wt.% Cu–0.06 wt.% Ni reveals that when the Ni content in (Cu, Ni)6Sn5 is ∼9 at.% Ni, the hexagonal allotrope of (Cu, Ni)6Sn5 becomes stable at room temperature.