Bond-pad damage in ultrasonic wedge bonding

Abstract

Utilizing SEM/EDX analysis, microscale fracture at the bond-pad is detected when Cu wire is wedge bonded to a Cu or Al substrate. It is found that when a bond is fractured by pulling the wire similar to a pull test, a bulge on the wire and a cavity in the substrate are formed and fracture occurs in the original substrate. Using a 3D optical profiler, it is revealed that for a constant bond force and bond power, cavity's depth, radius, and surface area increase with bond time. These metrics are proposed as new factors for optimizing the wedge bonding process. A suitable combination of bonding parameters should maximize the cavity's surface area (as it pertains to bond's pull force) while minimizing the cavity's depth relative to the substrate's thickness to prevent damage in the substrate. Furthermore, employing Molecular Dynamics simulations, a possible plastic deformation mechanism for bond-pad damage is proposed. The mechanism points to the potential benefits of using a small-grain-sized substrate, a low transducer's vibration amplitude, and a high transducer's frequency for minimizing the cavity's depth.