Intermetallic growth and void formation in Au wire ball bonds to Al pads

In order to clarify the reliability of Au wire bonds to Al pads, void formation and diffusion behavior were investigated using bonds annealed at various temperatures (423-573 K). We investigated the effects of the annealing environments, Al pad thickness, and bonding conditions on void formation. Voids became larger only when Au-Al intermetallics grew non-uniformly, whereas deleterious voids were not observed in the bonds annealed in vacuum. Oxide film on the surface of Al pads acts as a diffusion barrier at the interface. Optimized bonding conditions (applied pressure, ultrasonic energy) broke up the oxide film, resulting in reduction of void formation. Au₅Al₂ phase grew dominantly in the early stage of diffusion, then it transformed into Au₄Al phase because the Al layer was completely consumed. The activation energy Q of transmission velocity at the Au/Au₄Al boundary was 0.85 eV (82 kJ/mol). This is similar to the activation energy of the bond failure by annealing. These results indicate that void formation has a great correlation with the Au₄Al growth. It is predicted that the non-uniform diffusion behavior causes vacancies to pile-up and these vacancies coalesce to form several types of voids in the interface.