De-bonding simulation of Cu-Al wire bond intermetallic compound layers

Abstract

Copper wire bonding is being increasingly used as an alternative to gold wire bonding in electronics packaging industry. Copper wire has advantages over gold wire including lower cost, higher electrical and thermal conductivity and also higher mechanical strength, making it a good alternative for the high power interconnection and fine pitch bonding applications. However, introduction of copper wire bonding has also created new sets of challenges including the high susceptibility of copper and Cu-Al intermetallic compound to oxidation. Wire bond reliability especially intermetallic cracking is a predominant failure mode resulting from thermal aging or temperature humidity exposure. In this paper, an IMC grain-level finite element model has been developed to simulate the interfacial de-bonding behavior in order to study the influence of the IMC microstructure characteristics on the mechanical reliability of Cu-Al wire bond. Voronoi tessellations have been used to construct both regular and irregular IMC grain shapes geometry. Intrinsic cohesive zone model has been adapted to model interactions between neighboring grain boundaries including the effect of uniform interfacial strength and Weibull distributed grain interfacial strength. Finally, Cu-Al IMC growth and phase transformation are modeled. Simulation results indicate Cu-Al IMC microstructure characteristics not only influence bond strength but also influences the crack initiation and propagation. Regular-shaped IMC grain provides Cu-Al wire bond with more bond strength while non-uniform grains reduce bond strength. Results also indicate that the increase of IMC thickness makes wire bond less reliable while the crack propagation mode changes with the phase transformation.