A novel insight into Au-Au thermosonic flip chip joint under extreme thermal cycles: Defect characterization and failure analysis

Abstract

Au-Au thermosonic flip chip joints are regarded as a promising chip-substrate interconnect structures and their reliability has been a concern. Compressing the top of Au stud bumps during the coining process would result in plastic deformation of Au; During the bonding process, plastic deformation took place at the interface of Au stud bump and substrate under the impact of ultrasonic vibration, elevated temperature, and bonding pressure; additionally, due to different Coefficient of Thermal Expansion (CTE) between various components of the joint, the mismatch of CTE caused by the thermal cycling process would also result in plastic deformation. For Au element with extraordinary ductility, plastic deformation would inevitably lead to various material defects. This research first performed thermal cycling tests on Au-Au thermosonic flip chip joints, and found that a crack developed after 500 cycles and it propagated along the interface to the entire width of the joint, which seriously injured the joint. Here, for the first time, this research characterized all types of material defects (dislocations, stacking fault, twin, and amorphization) at the bonding interface of the Au-Au flip chip joint which underwent 500 thermal cycles. Their causes and effects on cracking were investigated individually, finally a novel failure mechanism was proposed from the perspective of material defects and a technique to prevent cracking was proposed. This research is anticipated to provide an in-depth study of the microstructure at the bonding interface of Au-Au flip chip joints and the impact of various defects on their reliability.