Simulation of intergranular crack extension at Cu/Al wire bonding interface

The intermetallic compound layer at the Cu/Al bonding interface is undergoing a dynamic evolution stage with the consumption of the Al pad during the continuous annealing process. This paper establishes a microstructure model of the Cu/Al bonding interface and analyses the crack extension behavior of the bonding interface through finite element simulation under three key aging states: initial bonding, moderately annealed and highly aged. The findings indicated that alterations in the annealed state would result in a modification of the damage mode of the bonding interface, leading to a deviation in the crack path. In the initial stages of the bonding process, crack initiation occurs at the edge of the Al pad and extends inward, resulting in fracture. The occurrence of cracks depends on the extent of annealing, with softer Al pad or Al-rich IMCs, such as CuAl2 layer, exhibiting these defects under moderately annealed conditions. Subsequently, the cracks gradually extend towards the harder and more brittle Cu9Al4 to fracture. In highly aged conditions, cracks initially emerge in the Al-rich CuAl2 layer and subsequently deflect into the CuAl layer. Finally, they deflect once more into the Cu9Al4 layer, eventually leading to fracture. In addition, cracks are affected by the grain orientation of IMCs and form at interfaces that are approximately perpendicular to the loading direction. The simulation results corroborate the fracture modes of the bonding interface in different aging processes from the micro grain perspective, which provides a technical reference for the study of the fracture mechanism of Cu/Al bonding.