Gold to Gold Thermosonic Flip Chip Bonding on Ceramic Substrate

In this study, a thermosonic flip chip process has been developed to assemble gold bumped die to an Al₂O₃ ceramic substrate. The effects of bonding conditions, such as ultrasonic power, temperature, bonding force and bonding time, on the reliability of the interconnections were investigated. It was found that ultrasonic power and bonding force were the major factors influencing the reliability. High ultrasonic power and bonding force resulted in high reliability. The thermal cycling test results (from −55 °C to 125 °C) showed that the assemblies passed 1250 cycles with no change in resistance. The pressure cooker test results (15 psig pressure and 100% RH at temperature of 121 °C) showed that the assemblies passed 168 hours with no change in resistance. The shear force test results showed that the shear force at failure ranged from 10 to 40 grams per bump depending on bonding parameters. The interconnect cross-sections and the fracture surfaces on selected dies and substrates were observed with scanning electron microscope (SEM). It was found that the shear failure usually occurred between a gold pad and ceramic substrate, indicating a strong bonding interface between the gold bump and pad on the substrate. The performance of the assemblies fell into the reliable region as long as the bonding force, temperature, and ultrasonic power were high enough. The failure mechanisms consisted of separation between Au bump and Au pad, separation between Au pad and substrate and cracking inside Au pad or trace. The ultrasonic power drove relative movement between Au bumps and Au pads, causing friction and heat in the interface to result in micro-welding.