Dynamie simulation for microelectronic packaging of Al pad/underlying pad structure during copper wire bonding

For decades, wire bonding technology has been widely used to interconnect IC chip and the substrate in microelectronic package. In recent years, due to the increasing cost of gold, the copper wires started to be employed in microelectronic package instead, lowing the production cost. The additional benefits include superior performance of copper wires, in terms of electrical and thermal property. Thus, it is likely that, the gold wires will be replaced by the copper gradually in the future. However, greater hardness of the copper wire and weak mechanical strength of low-k dielectric layers lead to higher stress in the Al pad and the underlying pad structure during the thermo-sonic bonding process. An additional concerns arises, as the Al pad is squeezed out by copper ball, which may affect the development of fine pitch. Therefore, it is necessary to redesign the bonding pad, as well as adjust the bonding parameters, such as bonding pressure, ultrasonic energy and bonding time. As finite element software enables dealing with the wire bonding process by a transient nonlinear dynamic analysis, a finite element model for copper wire bonding is developed to investigate the mechanical behavior. The simulation results would focus on the dynamic stress response of wire bonding model and the plastic strain of the Al pad. In order to save computational time and reduce modeling complexity, the copper wire bonding is represented by a simplified 2-D finite element model, whereby the complete wire bonding process mechanism is treated as consisting solely of impact and ultrasonic vibration stages. The calculation of nonlinear transient structural behavior is carried out using an explicit time integration scheme. In the present study, the simulation results indicate that the stress wave would rapidly transfer from the bonding interface to the underlying pad structure when copper ball is in contact with the Al pad. With the increasing contact area, the stress wave path would shift from the inside bonding pad to the outside, resulting in the spread of the stress concentration.