Fundamental Investigation into the Mechanism of Ultrasonic Wedge-Wedge Bonding through Change of Topography

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Even since the ultrasonic wire bonding technique has been widely applied for more than half a century, the underlying mechanisms are still not completely understood, which prevents further improvement. The proposed project will be dedicated to investigate the unrevealed mechanisms as well as the influences of the bonding parameters on them. Specifically the friction and the softening phases are the focus points of the project. These phases play a significant role in oxides removal and microwelds formation and determine the bonding speed and strength.The oxide layer on the surface of wire or substrate is the main obstacle for the bonding formation. Both friction and softening phases affect the oxides removal process with regard to the detachment of oxides from pure metal surface and the transportation of the detached oxides. Due to the extremely tiny dimension, the natural oxide layer and the discretized oxides during the bonding process are hard to be observed, especially for real-time observation. In the proposed project, micro-particles and intransparent layers will be utilized for emulating the natural oxides. Through the step-wise and real-time observations, the removal paths of the oxides will be deduced. The substrate asperities provide locations for microwelds formation. The softening effect on these asperities, however, is unclear. In this project via structuring the substrate, the deformation of asperities at different locations will be analyzed so that the softening effect can be deduced. Furthermore, the microwelds area growth during the bonding process and the average strength of microwelds will be studied to obtain more information on the bonding strength.The oxide-metal contact regions, the metal-metal contact regions and microwelds regions are always changing within the bonding process. The complex dynamic bonding interface places an obstacle to the understanding of the bonding speed and strength. In order to real-time study the complex interface, a novel high resolution distributed sensor array will be developed and embedded in the substrate. By using this, the local tangential and normal forces at different locations of the interface will be measured. The output of the sensor array will provide large amount of information on the local changes, especially the local microwelds formation and breakage.Through the whole project, design of experiment (DOE) will be applied to analyze the influence of the process parameters. Finally, an empirical model will be established for predicting the changes at the interfaces over time, including the oxides distribution, the substrate roughness, the local strengths, and global strength. Based on the above analysis, new topographies on the substrate as well as on the wire will be proposed and evaluated so that a promotion of the bonding speed and quality will be gained. All of the results will provide a fundamental insight into the bonding mechanisms and close gaps in this field.