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Impact of multi-dimensional vibration trajectories on quality and failure modes in ultrasonic bonding
Keywords: multi-dimensional bonding, wire bonding, failure modes
For decades uni-axial or torsional ultrasonic bonding and welding have been used to create electrical interconnections in electrical devices like battery tabs or the outer connections of modules incorporating power semiconductors like IGBTs (Insulated-Gate Bipolar Transistor), used e. g. for automotive control. Different types of work pieces with varying effective contact areas like wires (<0.5 mm2), ribbons (0.2-2.5 mm2), and control/power terminals (4-16 mm2) are used. With rising contact area, the ultrasonic power required for the bonding process rises. Increased ultrasonic power is realized by increasing ultrasonic vibration amplitude, normal force, or both. This increases the resulting mechanical stress in the substrate during the bonding process and may finally lead to failure modes like cratering and delamination of the substrate structure, [5]. As an alternative ultrasonic system, multi-dimensional ultrasonic bonding has been investigated by several different researchers. Asami et al. [1, 2] presented two multi-dimensional transducer concepts for elliptical and multi-frequency ultrasonic vibration trajectories for ultrasonic bonding; bonding test showed that with multi-dimensional bonding, the bond strength could be increased. Schemmel et al. [5] presented a multi-dimensional transducer concept for elliptical and multi-frequency planar vibration trajectories; the vibration trajectories of the bond tool were measured by laser measurements during ultrasonic bonding of connector pins. The bonding results (shear force values) for multi-frequency bonding were compared to one-dimensionally bonded pins and an increase of shear strength was observed. The laser measurements indicated, that the mechanical stress on the substrate during ultrasonic bonding was less for multi-dimensional and multi-frequency bonding compared to one-dimensional bonding. One aim of multi-dimensional ultrasonic bonding can be to increase the strength and reliability of the bond connection. Another aim is to reduce the oscillating shear stresses during bonding, in order to avoid failure modes which can occur when bonding one-dimensionally with increased ultrasonic power. Whether failures can be avoided in multi-dimensional bonding depends on the parameters of the multi-dimensional vibration trajectory. In [3] a rotation of the bonded connector pin following the circular multi-dimensional ultrasonic excitation trajectory was observed, leading to large penetration of the workpiece into the substrate, micro cracks and cratering. The rotation of the connector pin can be avoided using a multi-frequency vibration locus, [5]. In this contribution, the impact of different multi-dimensional vibration trajectories on the bond quality and failure modes (such as delamination and micro cracks) when bonding connector pins on DCB (direct copper bonded) substrate is investigated experimentally. The bond quality is determined by shear tests and failure modes are analyzed by ultrasonic microscopy and cross sections of the bond connections. Additionally, laser measurements and high speed-camera videos are used to investigate dynamical effects like the rotation of the multi-dimensionally excited workpiece. This work aims to determine the benefits and disadvantages of circular and multi-frequency planar ultrasonic excitation. They are compared to one-dimensional bonding and regarding the intended decrease of mechanical stress in the substrate. This is investigated for an application of bonding connector pins of IGBT modules with an effective contact area of about 3.0 mm2 with ultrasonic power in the range of 100-300 W to sensitive structures like DCB substrates. The benefits of multi-dimensional bonding are transferred to other applications, such as heavy wire bonding and heavy ribbon bonding, where avoiding failure modes caused by high ultrasonic power also is an important task. The presented experimental results show the impact of the ultrasonic trajectories on bond results and yield profound understanding of bond formation as well as identification of unintended side effects of the bonding process when using different multi-dimensional vibration trajectories.
Reinhard Schemmel, Research Assistant
Paderborn University, Chair of Dynamics and Mechatronics
Paderborn, NRW

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