Corrosion of Intermetallic Layer and Reliability of Au/Al Bonds

In order to clarify the reliability on the mechanical and electrical properties of the bond between Au wire and Al thin pad, corrosion behavior of Au-Al intermetallic formed at the bond interface in the molding resin were investigated. The bonds were annealed at several elevated temperatures, T (423-573 K). Typical degradation was recognized by a decrease in bond strength as well as a remarkable increase in electrical resistance. The molding resin has great influence on the corrosion. Bi-phenyl (BP) epoxy resin was found to cause the degradation approximately six times faster than o-cresole novolac (OCN) epoxy. Activation energies of the bond failure were 1.5 eV (T>450 K) and 2.0 eV (7<450 K) in BP resin and 2.3 eV in OCN resin. The corroded part was revealed to be Au₄Al intermetallic phase formed in the bond interface. The growth rate of corroded layer was proportional to annealing time t, which indicated the corrosion behavior was not diffusion-controlled. Activation energies of the growth rate of corroded layer in the BP and the OCN resin were 1.6 eV, 2.3 eV, respectively, which were very similar to those of the bond failure. The corrosion reaction of Au₄Al and bromide produced minute Au precipitation (fcc) and Al oxide formation. The Al oxide was identified to be amorphous by EDS and electron diffraction.