Direct silver to aluminum solid-state bonding processes

The high thermal conductivity, light weight, and low cost of aluminum (Al) make it a promising substrate material for high power electronic packaging. A main challenge of using aluminum in electronic packaging is its poor bondability. The native aluminum oxide (Al2O3) prevents aluminum from bonding to commonly used die-attach materials such as solders. Thus, zincating process is often needed to dissolve the Al2O3 layer and deposit a protective zinc layer which provides a basis for subsequent metallization or soldering processes. In this research, Ag-Al solid-state bonding has been developed as a novel bonding technique to bond Ag directly to Al substrates. No surface treatment was applied on Al substrates to remove the native Al2O3 layer prior to bonding. The shear strength of Ag-Al joints passes the military criterion (MIL-STD-883H method 2019.8) by a large margin. SEM and TEM imaging was utilized to study the microstructures. In the bonding processes conducted at 425 and 450 °C, Ag and Al atoms inter-diffused through the thin Al2O3 to react and form Ag2Al and Ag3Al compounds. To examine the fracture modes of Ag-Al joints, the fracture surfaces after shear tests were evaluated. The effect of bonding temperatures on Ag-Al joint morphology and the fracture behaviors were investigated and discussed. An application of this new technique is to bond thin Ag foils to Al substrates and make them bondable to die-attach materials such as solders and nano-silver paste. This Ag foil bonding method provides an alternative to the zincating process. Other potential applications include making Al surfaces easier to blaze to other metals such as brass, bronze and copper.