Stabilization of hexagonal Cu6(Sn,Zn)5 by minor Zn doping of Sn-based solder joints

Cu6Sn5 is the dominant intermetallic compound at the Sn/Cu joint interface. The crystal structure of Cu6Sn5 varies with temperature. After reflow at 250 °C, interfacial Cu6Sn5 revealed a hexagonal structure (η-Cu6Sn5). During aging at 150 °C, hexagonal η-Cu6Sn5 transforms into monoclinic η′-Cu6Sn5. X-ray diffraction and differential scanning calorimetry analyses show that doping small amounts of Zn (0.8–2.1 at.%) into Cu6(Sn,Zn)5 can stabilize the hexagonal structure during the thermal aging process. Thermodynamic calculation also demonstrates that Zn stabilizes the hexagonal Cu6(Sn,Zn)5.