Increasing bondability and bonding strength of gold bmps thermosonic bonding to flex substrates by depositing a nickel layer

To improve the bondability and bonding strength of gold bump thermosonic bonding to copper electrodes over the flex substrates, the nickel layer was deposited on the surface of copper electrodes to strengthen their stiffness. A silver layer was then deposited on the nickel layer to prevent copper electrodes from oxidizing during thermosonic (wire) bonding process. The bondability and bonding strength from gold bumps and copper electrodes are thus improved. This nickel layer was expected to enhance the rigidity of copper electrodes over flex substrates, increasing the efficiency of ultrasonic power from gold bumps to copper electrodes. The bondability and bonding strength of gold bumps thermosonic bonding to copper electrodes increased with the thickness of nickel layer under the same bonding parameters. One hundred percent bondability and high bonding strength were achieved when gold bumps thermosonic bonding to copper electrodes with depositing 0.5μm-thick nickel layer. The bonding strength was higher than that stated in JEDEC standards. The bonding morphology of the gold bumps exhibited an extensive bonded area with aligned traces for gold bumps onto copper electrodes containing the nickel layer, while gold bump bonding to copper electrodes without depositing nickel layer resulted in a blank surface morphology. This experimental result can be used to explain the efficiency of the nickel layer in promoting rigidity of copper electrodes, and in increasing the bonding efficiency of ultrasonic power at bonding interface between gold bumps and copper electrodes. With deposition of the nickel layer onto copper electrodes, a layer attached to the gold ball can be observed at the fracture morphology of gold bumps, implying the bonding strength of gold bumps bonded on copper electrodes was even higher than the adhesive strength of the layers deposited on the copper electrodes. Deposition of 0.5μm-nickel layer on copper electrodes over the flex substrates, the rigidity of copper electrodes was improved, and the ultrasonic power can be efficiently transferred to the bonding interface between gold bumps and copper electrodes, which results in increasing the bondability and bonding strength.