Phase Evolution at the Interface between Liquid Solder Sn-Zn-Ag and Cu Substrate Studied by In Situ Heating Scanning Transmission Electron Microscopy

Wetting phenomena concerning Sn-Zn eutectic alloys with Ag addition were studied in contact with Cu substrate at 250 °C and in the presence of ALU33 flux. The chosen wetting time was 5 s. Then in situ STEM isothermal heating at 150 °C lasting for 10, 40 and 70 min was conducted to establish the sequence of phase appearance. The intermetallic phases within the Cu-Zn system were identified. The formation sequence and interface microstructure were determined. It is demonstrated that the ε-Cu(Ag)Zn 4 phase appears first and it is followed by the γ-Cu(Ag) 5 Zn 8 . The resulting Cu saturated solder transforms into γ phase taking scallop-like shape.