Microstructural transformation and mechanical properties of Cu-sputtered alumina ceramic/Zn5Al/AA2024 ultrasonic soldering joints

Abstract

Cu films were deposited on the surfaces of alumina (Al2O3) ceramics via magnetron sputtering, and the ceramics were then joined to AA2024 by using a Zn5Al solder assisted by ultrasonic soldering. Increasing the duration of Cu sputtering led to the continued diffusion of Al from the Al alloy substrate into the solder. This led to the growth of the ZnAl eutectic phase within the solder joint, with the precipitation of CuZn5 diffusing into the ZnAl eutectic and η–Zn phases. The increased formation of the ZnAl eutectic phase enhanced the mechanical strength and stability of the solder. The CuZn5 phase formation suppressed solder shrinkage during cooling, improving the coefficient of thermal expansion to match those of the metallised ceramic and AA2024 alloy. This reduced the residual stress at the joint interfaces in the metallised Al2O3 ceramic/Zn5Al/AA2024 joint. A maximum average shear strength of 65.835 MPa was achieved with 2 h of Cu sputtering, representing a 224.95% increase compared to the joints without Cu. Fractures occurred at the ceramic–solder interface, combining adhesive, cohesive, and mixed failures.