Random Change of Vibration Modes in Thermosonic Bonding

Abstract

Thermosonic bonding is being widely used for wire bonding and tape automated bonding (TAB). It is also being developed for flip-chip assemblies. The bonding process is sensitive to system parameters such as ultrasonic frequency, bonding tool length, and friction. One of the process control challenges is to prevent the random change of vibration modes. The change from an axial to a bending mode excitation may reduce the ultrasonic vibration amplitude at the bonding interface and result in poor bond quality. In this work, we have studied the random mode change effected by tool length, friction, and the frequency window selected for the power generator. In one case, the mode change was not observed when the tool end extended 2.32 cm below the longitudinal axis of the bonding horn. In the other case, however, the resonant frequency switched between 58.4 and 60.0 kHz when the tool moved 0.31 cm downward with all other system parameters held constant. Such a frequency shift resulted in a change of vibration amplitudes between 1.63 and 0.55 μm. Different random mode changes measured were explained by our theoretical models consisting of a modal analysis for the system and a vibration model for the tool. The models generated a guideline to prevent mode change by selecting the right system parameters.