To implement a self-optimization technique for ultrasonic wire bonding machines, a model of the pre-deformation phase is essential. The local material characteristics change abruptly because of the cold work during deformation. Investigations confirm a significant influence on the material properties of the contact members during touchdown. In a first step this paper validates the importance of modeling the pre-deformation experimentally. In a second step, the paper presents a numerical study of the elasto-plastic deformation based on the finite element method. This model includes measured overshoots in the touchdown forces in order to achieve accurate model responses. A validation of the model with the resulting nominal contact area, surface pressure and penetration depth reveals the high model quality.
In power electronics, copper connector pins are e.g. used to connect control boards with power modules. The new chip generation based on SiC and GaN technology increase the power density of semiconductor modules significantly with junction temperatures reaching 200°C. To enable reliable operation at such high temperature, the soldering of these connector pins should be substituted by a multi-dimensional copper-copper bonding technology. A copper pin welded directly on DBC substrate also simplifies the assembly. With this aim, a proper bond tool and a suitable connector pin geometry are designed. This paper presents a two-dimensional trajectory approach for ultrasonic bonding of copper pieces, e.g. connector pins, with the intention to minimize mechanical stresses exposed to the substrate. This is achieved using a multi-dimensional vibration system with multiple transducers known from flip chip bonding. Applying a planar relative motion between the bonding piece and the substrate increases the induced frictional power compared to one-dimensional excitation. The core of this work is the development of a new tool design which enables a reliable and effective transmission of the multidimensional vibration into the contact area between nail-shaped bonding piece and substrate. For this purpose, different bonding tool as well as bonding piece designs are discussed. A proper bonding tool design is selected based on the simulated alternatives. This tool is examined in bonding experiments and the results are presented. In addition, different grades of hardness for bonding piece and substrate are examined as well as different bonding parameters. Optical inspection of the bonded area shows the emergence of initial micro welds in form of a ring which is growing in direction of the interface boundaries with increasing bonding duration.
One of the main cost factors in the wire bonding process are consumables such as bonding tools. The technological transition to copper as wire material causes significant wear on the millimeter size effective contact area of the bonding tool. This wear leads to a reduction of the number of reliable interconnections which can be produced using a single tool by a factor of 30 replacing the wire material from aluminum to copper. To reduce setup time in the production and minimize costs, an increase in bonding tool lifetime is desired. Consequently a better understanding of the wear and recognition of wear pattern is required. This paper presents a method to analyze tool topography change of a heavy wire bonding tool by using a confocal laser scanning microscope. Furthermore, this paper discusses the identification of the main wear indicators by help of the change in topography.
Wear test has been carried out by choosing typical parameters of the production line. This topography change is compared to the topography change caused by increasing the ultrasonic voltage. To evaluate whether the quality requirement of the bond connections made by a single tool cannot be fulfilled, shear tests of the source bond were made in predefined intervals. All worn tools show dominant areas of wear especially for plastic deformation and accordingly abrasion. These wear mechanisms can be referred to the change of main parts of the groove geometry like the rounding of the front and back radius. The most loss in height was identified in the lower part of the tool flanks or rather at the transition from groove flank to the front or back radius. Furthermore the observation of the center of the groove flank shows just a little change. All in all, the identification of the wear indicators will be discussed with the objective of increasing the tool lifetime by optimizing the tool geometry without losses in bond quality and reliability.
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