Void growth in thermosonic copper/gold wire bonding on aluminum pads

Void growth in wire bonds significantly influences the reliability of electronic devices. This paper found that, in the as-bonded state, a few voids nucleate in Au-Al bonds, while they are absent in Cu-Al bonds. Voids grow much faster in Au-Al bonds than in Cu-Al bonds during thermal annealing. It is proposed that void growth in Au-Al bonds due to the oxidation of IMCs and volumetric shrinkage resulted from the growth of Au₈Al₃ and Au₄Al, but not due to the Kirkendall effect. Large voids up to 10μm exist after extended annealing, which is also attributed to the outward diffusion of Au to react with Al in the area beyond the perimeter of the bonds. In Cu-Al bonds, the void growth rate is low, and only a few voids of <;100 nm are present along the alumina remnant after prolonged annealing at a high temperature (e.g. 250°C for 25 h). The formation of void in Cu-Al bonds is probably due to the volumetric shrinkage during the formation of Cu₉Al₄. The oxidation of Cu-Al IMCs and outward diffusion of Cu is insignificant; therefore the void growth rate in Cu-Al bonds is low.