A study of thermosonic gold wire bonding onto silver plated copper pad

Abstract

A thermosonic (TS) bonding process for gold wire bonded onto copper pad was successfully developed by plating a thin, silver passivation layer on the copper pad. The copper pad oxidizes easily at elevated temperature during TS wire bonding. The bondability and bonding strength of the gold ball onto copper pads are significantly deteriorated if a Cu-oxide film exists. To overcome this intrinsic drawback of the copper pad, a silver thin film was plated onto it to improve the bondability and bonding strength. Ultrasonic power, bonding time and bonding force are the most important process parameters for a sound bonding. In order to study the effects of these process parameters on bond shear force and the region of bonding, gold wire bonding was performed on a Kulicke&Soffa 1484XQ wire bonder with the silver plated copper pads. This study showed that an appropriate combination of ultrasonic power, bonding force and time is very necessary to obtain optimal bonded bumps. The application of higher ultrasonic power or higher bonding force not only results in a larger bonded area, but also causes excessive deformation of the bump, which may not be acceptable. With an increase in power, the bond shear force also increased and reached a maximal value, but with further increase of power, shear force would decrease. Bonding force showed the similar trend as bonding power, the bond shear force increased with bonding force up to an optimum value, but decreased gradually with further increase of bonding force. In contrast to the effects of ultrasonic power and bonding force on bond shear force, the bond shear force increased with the increase of bonding time monotonously in a certain range of bonding time.