Facilitating intermetallic formation in wire bonding by applying a pre-ultrasonic energy

It is conventionally believed that wire bonding initiates at the periphery of the contact area and no bonding occurs in the central area. However, this paper demonstrated that two bonding patterns exist, and are determined by bonding processes. If a selected pre-ultrasonic energy is applied, intermetallic compounds initiate in both peripheral and central area of bonds. However, if a pre-ultrasonic energy is absent, intermetallic compounds are only present at the peripheral area as conventionally reported. The application of the pre-ultrasonic energy significantly improves bonding strength, from 66.8 to 94.5 MPa for 20 μm Au wire bonds, due to the intermetallic compounds of greater structured integrity. Two different mechanisms are respectively proposed to account for the intermetallic formation in the center and periphery of the bond interface.