Direct Bonding Silver to Aluminum Using Eutectic Reaction in Air

Abstract

The high thermal conductivity, light weight, and low cost of aluminum (Al) make it a promising substrate material for high power electronic packaging. Recently, direct bond aluminum (DBA) substrate has received significant attention as a possible alternative to direct bond copper (DBC) substrate which seems to have thermal cycling reliability issues. A main challenge of using aluminum substrates in electronic packaging is the poor bondability. The native aluminum oxide layer prevents aluminum from forming bonding with die-attach materials or metallization layers. Thus, zincating process is required to dissolve the aluminum oxide and deposit a protective layer of zinc, which provides a basis for subsequent metallization. In this research, Ag-Al eutectic bonding has been developed as a novel bonding technique to direct bond Ag to Al substrate. The shear strength of the Ag-Al joints passes military criterion (MIL-STD-883H method 2019.8) with a large margin. SEM and TEM analyses were utilized to study the microstructures in details. The results reveal that eutectic structure of Ag2Al and (Al) phase forms at the Ag/Al bonding interface. A uniform Ag2Al compound layer was observed between the eutectic structure and Ag region, with no Ag3Al compound detected. In the Ag-Al eutectic reaction process, the aluminum oxide layer was broken into pieces and dispersed into the eutectic structure region. To investigate the fracture modes of Ag-Al eutectic joints, the fracture surfaces of Ag-Al joints were evaluated after the shear test. The fracture surfaces correspond to a typical ductile fracture with plastic deformation and drawing matrix. An application of this new technique is to bond Ag foils to Al substrates and make them bondable to die-attach materials such as solders and nano-silver paste. At a more advanced level, device chips can be bonded to the Ag foil on Al substrates at 300 °C using solid state bonding technique. This foil bonding application provides an alternative to the zincating and metallization processes on aluminum substrates. Other potential applications include making Al surfaces easier to blaze to other metals such as brass and copper.