Models for Initiation and Growth of Microwelds in Ultrasonic Bonding

超声连接技术在微电子、汽车和航空航天零部件、特种服装和超声波添加剂制造中具有重要意义。超声波连接是一种温度低、成本低的工艺，适用于异种材料零件的连接。超声波连接在大尺寸和小尺寸上都得到了成功的应用，例如分别用于板材连接和微电子引线键合。超声连接向更大(汽车)和更小(纳米技术)领域的改进没有根本的限制。然而，超声连接在实际应用中面临着局限性和挑战。例如，在汽车领域，开发的重点是证明超声波连接对于更厚的板材、更硬的材料以及不同轻质材料的组合(如钛、铝、镁或复合材料)的可行性。微电子引线键合中的超声连接由于纳米领域的小型化、低成本材料的使用以及较薄的芯片封装降低了硬度而出现了尚未克服的挑战。本研究旨在通过发现可预测的理论模型来加深对超声连接的认识。这样的模型可以用来改进和加速超声连接过程的设计，以克服现有的挑战。目前，只有在极大地简化假设的情况下，才有可能描述超声连接的过程物理。不幸的是，这导致了对接头质量的低精度预测。本研究中的新方法将描述微焊接对粘接质量的影响。在超声连接过程中，微焊缝在结合区形核，并导致强结合的成功生长。然而，目前尚不清楚微焊接如何将生长转化为牢固的键的确切机制。这项拟议的研究将用一种基于基础物理学的方法论来解决这种知识缺乏的问题。这将有助于阐明微焊接的形成机制，并导致改进的模型和更准确的预测。实验工作包括将在微电子引线键合(低功率，130 kHz超声波)和芯片贴合(高功率，20 kHz超声波)机器上开发的超声波连接工艺，这些机器可供大学的研究小组使用。过程结果被测量并用于确定将在理论工作中发展的理论模型的参数。在这个研究项目中接受培训的学生将在电子组装技术、超声波焊接和微焊接技术、分析方法、项目规划、领导和沟通方面具有很高的技能。

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.