Reliability of TCT and HH/HT test performed in chips and flex substrates assembled by thermosonic flip-chip bonding process

With the adequate bonding parameters, a bonding load of 930 gf, an ultrasonic power of 20.66 W, a bonding temperature of 200 °C and a bonding time of 0.3 s, a sound bond with sufficient die–shear force was achieved in chips thermosonically bonded directly onto copper electrodes with a 0.5 μm-thick nickel layer. After chips thermosonically bonded onto copper electrodes, the reliability of temperature cycling test (TCT) and high humidity/high temperature (HH/HT) test was evaluated. The reliability of TCT and HH/HT test demonstrates that gold bumps were firmly bonded onto copper electrodes over the flex substrates for subjected various test durations. Therefore, the reliability of TCT and HH/HT test for gold bumps thermosonically bonded directly onto copper electrodes should not be an important issue. During TCT, thermal stress at the interface between the gold bumps and bond pads of silicon chips caused crack at the interface. That is, the TCT degraded the die–shear forces of chips and flex substrate assemblies. The reliability of the TCT in chips thermosonically bonded onto flex substrates was dependent on the severity of crack. Similarly, crack and blisters that occurred at the bonding interface between gold bumps and bond pads of silicon chips after HH/HT tests of varying durations deteriorated the die–shear forces. Moistures penetrated into deposited layers of bond pads, deposited layers lost their adhesion, and cracks and blisters were formed as increasing the test durations. The reliability of TCT and HH/HT test must be improved to prevent crack at the bonding interface between the gold bumps and bond pads and to prevent moisture from penetrating into the deposited layers of bond pads, respectively.