Models for Initiation and Growth of Microwelds in Ultrasonic Bonding

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Ultrasonic joining technology is important for the manufacturing of microelectronics, automotive and aerospace components, speciality clothing, and ultrasonic additive manufacturing. Ultrasonic joining is a low temperature and low cost process and suitable for joining parts made from dissimilar materials. Ultrasonic joining is successfully used in large and small dimensions e.g. for sheet metal joining and microelectronic wire bonding, respectively. There is no fundamental limit to improvements of ultrasonic joining towards even larger (automotive) and smaller (nanotechnology) domains. However, ultrasonic joining is facing practical limitations and challenges. In automotive for example, development has been focusing on proving ultrasonic joining's feasibility for thicker sheets, harder materials, and combinations of dissimilar lightweight materials such as Ti, Al, Mg, or composite materials. Challenges not yet overcome for ultrasonic joining in microelectronic wire bonding arise due to miniaturization towards the nano domain, the use of low cost materials, and thin die packages with reduced stiffness.The proposed research aims at furthering the knowledge of ultrasonic joining by discovering predictive theoretical models. Such models can be used to improve and accelerate the designs of ultrasonic joining processes that overcome existing challenges. Currently, only with severely simplifying assumptions is it possible to describe the process physics of ultrasonic joining. Unfortunately, this results in low accuracy predictions of joint quality. Novel approaches in this research will describe the effect microwelds have on the bond quality. Microwelds nucleate in the bond zone during ultrasonic joining and lead to successful growth of strong bonds. However, the exact mechanisms how microwelds growth into strong bonds is currently unknown. The proposed research will address this lack of knowledge with a methodolgy based on fundamental physics. This will shed light on microweld formation mechanisms and lead to improved models and more accurate predictions. The experimental work consists of ultrasonic joining processes to be developed on microelectronic wire bonding (low power, 130 kHz ultrasound) and die attach (high power, 20 kHz ultrasound) machines available to the research group at the University. Process outcomes are measured and used to determine parameters of the theoretical models which will be developed in theoretical work. Students trained in this research project will be highly skilled in electronics assembly technologies, ultrasonic welding and microwelding technologies, analytical methods, project planning, leadership, and communication.