Enhancements of bondability and die-shear force of chips thermosonically bonded to flex substrates by depositing a nickel layer

Abstract

The purpose of this study was to investigate the influence of the nickel layer on the bondability and die-shear force of chips and flex substrates they were assembled using thermosonic flip-chip process. The copper electrodes over the flex substrate that were electroplated with a 0.5 μm-thick nickel layer on the surface of copper film and the silver film was then deposited on the nickel layer to be as the bonding layer. This nickel layer was expected to improve the bondability and die-shear force of the assembly of chips that were thermosonically flip-chip bonded on the flex substrates. The thermosonicially flip-chip bonded experiments were conducted using an automatic thermosonic flip-chip bonder developed by ITRI. After chips bonded on the flex substrates, a subsequent die-shear test was performed to evaluate the bonding quality according to the JESD22-B116 standard. Chips were successfully thermosonically flip-chip bonded on the copper electrodes that deposited with a 0.5 μm-thick nickel layer using the adequate parameters. 100% bondability can be obtained and the die-shear force was higher than the minimum required values stated in the JEDEC standards for the assembly of chips they were thermosonically flip-bonded on copper electrodes with depositing a 0.5 μm-thick nickel layer. A poor bondability and low die-shear force were existed when chips thermosonically flip-bonded on the copper electrodes that the nickel layer without depositing on them. According to observation on the surface morphology of the copper electrodes that were deposited with the nickel layer, a clear bonding trace can be observed, indicating the nickel layer was effective to improve propagation of the ultrasonic power to the bonding interface between the copper electrodes and the gold bumps. Enhancing the bonding quality of the assembly of chips and substrates during thermosonically flip-chip bonded process. Both bonability and the die-shear force are thus improved. In this study, the adequate bonding parameters of chips they were thermosonically flip-bonded to the flex substrates were an ultrasonic power of 20.66 W, a bonding force of 625 gf, a bonding time of 0.3 s and a bonding temperature of 200°C. Deposition of the 0.5 μm-thick nickel layer on the copper electrodes over flex substrates is effective to improve the bondability and die-shear forces of the assembly of chips they were thermosonically flip-bonded on the flex substrates.