UC Irvine

A major challenge in today’s high-temperature power electronics is the lack of reliable packaging materials with good performance. Silver (Ag) has the advantages of high electric and thermal conductivity, high melting point, and excellent mechanical properties, making it a promising material for wire bonding and die-attachment in microelectronic and power electronic packages. On the other hand, aluminum (Al) is often used as a conductive metal layer in semiconductor devices, such as chip metallizations and thick film substrates. This research aims to evaluate the material properties of the Ag-Al intermetallic compounds, and develop new Ag-Al bonding processes for high-temperature applications.

First, Ag-Al alloys and Ag-Al joints are produced to study the intermetallic phases in Ag-Al alloys and at Ag/Al interface. The deformation and fracture behaviors of Ag2Al and Ag3Al are evaluated using micro-indentation technique. The results show that Ag3Al exhibits high hardness and low fracture toughness with brittle fracture mode, while Ag2Al shows ductile deformation with no crack formation. Moreover, a corrosion experiment is designed to measure the corrosion rates of Ag, Al, and Ag3Al and Ag2Al compounds in epoxy molding compounds under humidity test. The results show that the corrosion rate of Ag3Al is twice of that of Ag2Al, and no corrosion occurs on Ag or Al. The results above clearly suggest that Ag3Al is likely the weak link and should be avoided. Finally, this research further develops a novel Ag foil bonding technique to bond Ag foils directly to Al substrates. Two Ag-Al bonding processes are developed: solid-state and eutectic reaction. Using the Ag foil as a bonding medium, Si chips are bonded to the Ag-cladded Al substrates using solid-state process at 300 °C. In Ag-Al soli-state bondings, Ag and Al atoms inter-diffused through the thin Al2O3 to form Ag2Al and Ag3Al compounds. In Ag-Al eutectic bondings, Ag2Al+(Al) eutectic structure forms at bonding interface without Ag3Al compound formation. The native Al2O3 layer, a potential fracture path, is broken into pieces during eutectic reaction and possibly dispersed into the eutectic structure. Shear test results of Si/Ag/Al joints far exceed the military criterion. The fracture mechanisms are studied in detail.