Models for Initiation and Growth of Microwelds in Ultrasonic Bonding

******超声波连接技术对于微电子、汽车和航空航天部件、特种服装和超声波增材制造的制造非常重要。超声波连接是一种低温、低成本的连接工艺，适用于异种材料零件的连接。超声波连接成功地应用于大尺寸和小尺寸，例如钣金连接和微电子线连接。超声波连接在更大（汽车）和更小（纳米技术）领域的改进没有根本的限制。然而，超声波连接在实际应用中面临着诸多限制和挑战。以汽车为例，研发人员一直致力于证明超声波连接在较厚板材、较硬材料以及不同轻质材料（如Ti、Al、Mg或复合材料）组合上的可行性。微电子线键合中超声连接尚未克服的挑战是由于向纳米领域的小型化，低成本材料的使用以及降低刚度的薄模封装。*********提出的研究旨在通过发现预测理论模型来进一步了解超声波连接。这些模型可以用来改进和加速超声波连接工艺的设计，克服现有的挑战。目前，只有严格简化假设，才有可能描述超声连接的过程物理。不幸的是，这导致对关节质量的预测精度较低。*********本研究的新方法将描述微焊对键合质量的影响。在超声连接过程中，微焊缝在键合区成核，导致强键的成功生长。然而，微焊点生长成强键的确切机制目前尚不清楚。拟议的研究将用一种基于基础物理学的方法来解决这一知识的缺乏。这将揭示微焊形成机制，并导致改进的模型和更准确的预测。*********实验工作包括超声波连接工艺，将在大学研究小组可用的微电子线键合（低功率，130千赫超声波）和模具连接（高功率，20千赫超声波）机器上开发。过程结果被测量并用于确定理论模型的参数，这些理论模型将在理论工作中发展。参与本研究项目的学生将具备电子组装技术、超声波焊接和微焊接技术、分析方法、项目规划、领导能力和沟通能力。

******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.