Fundamental Investigation into the Mechanism of Ultrasonic Wedge-Wedge Bonding through Change of Topography
Final Report Abstract
The main objective of the project has been achieved. The oxide removal process has been completely revealed in detail. It consists of four steps including the occurrence of cracks, the detachment of oxides, the milling of these oxides into small particles and the transportation of these particles to the peripheral region. Four mechanisms drive the transportation of the oxide particles: metal penetration, oxide flow, pushing and metal splash. Metal penetration and oxide flow play the most significant role in the transportation process. During the transportation process, some oxide particles agglomerate together into larger particles. Microweld formation and breakage were studied via molecular dynamics simulations. They showed that microwelds can be formed and broken in an extremely short time. The formation and breakage of microwelds take place continuously during the bonding process. The local positions where old microwelds were broken still offer the opportunity for the formation of new microwelds, even with the same asperities. The horizontal movement of the wire significantly changes the surface topographies. Under the same conditions, a high stiffness of the materials has a negative effect on the microweld growth. The surface topography, especially the tall asperities, significantly affects the change of microwelds. A larger approaching distance or a larger deformation of asperities significantly helps the microweld growth. A large vibration amplitude makes the microweld changes faster while it does not necessarily increase the shear stress and the microweld area. A sensor array based on the dice-and-fill method was accomplished containing 12 sensors in total. Due to their size an interposer chip had to be used to electrically contact each sensor of the array individually. During the development of the manufacturing process various challenges have been overcome. First, suitable parameters for mechanically structuring the brittle PZT with precision dicing were found. Chipping on the ceramic surface can be reduced by using a rotational speed of 30,000 rpm and a dicing blade with the least grain size. By setting the feed rate to 0.5 mm/s no cracks are initiated during dicing. For filling the diced kerfs, different materials were tested. Polyimide turned out to be unsuitable due to layer tension as a consequence of shrinkage during the curing process. With epoxy based fillers Vitralit® UD 2018 and Vitralit® 2020 the highest bonding strength can be obtained with the former showing a smaller coupling effect among sensor columns Thermocompression bonding of gold and tin proved to be a suitable technology for contacting the sensor chip to the interposer chip. An average shear strength of more than 20 MPa is achieved with a parameter set using 230 °C, 200 s and 7,8 MPa, respectively. An upper electrode Au showed a much higher bondability compared to Al. As the thickness increases, the bonding strength but also the coupling effect among the columns increase. The local tangential forces have been measured in-situ by the manufactured sensor array and the underlying mechanisms were derived. The two central columns undergo the largest tangential force, the 4 corner columns provide the smallest force signals, and the 6 peripheral columns undergo the largest increase of contact area. In the first approx. 5 ms, a fast increase (sticking) – plateau (sliding friction dominating) – fast increase (increasing significance of sticking contact behavior) of the force was detected on all sensor columns. After this stage, a decrease of force was observed on the central columns due to reduced local normal force. As more microwelds were formed, the force amplitude became constant. For peripheral and corner columns, a slow increasing force amplitude appeared due to the expansion of contact area and the increase of the local normal force. Compared to central columns, the signal at peripheral and corner columns indicated a more sliding friction. An energy flow model was further quantified according to the detected relative motions. The majority of the US energy flows to the vibration induced friction at the wire/substrate and the wire/tool interfaces, and the vibration induced microwelds formation, deformation and breakage. Even though the rest items are significant to the process, only a little amount of energy is delivered to them. A good coupling of the normal force and the US power could guide more energy to the wire/substrate interface for microwelds formation. If the two factors are not well coupled, even though more energy can be provided to the wire/substrate interface, the breakage of microwelds will take up a big part. Different surface structures have been tested for wire bonding and it is found that the deposited strips and straight ditches facilitate oxide removal and can enlarge process window. For the deposited strip structure, the edges of strips helped break the oxide scale. The oxide transportation occurred earlier and more oxides can be removed, compared to bonding on smooth surfaces. Thus, a relatively high bonding strength can be achieved. As for bonding on ditch structures, aluminum and aluminum oxide were continuously cut from the wire, accumulated in the ditches, pressed and finally squeezed to the outside of the ditches to form long chips. Accompanying with the growing of such chips, metal splashes were generated which further enhanced the oxide removal process. Under the same driving current, the bonding strength on straight ditches was up to 4 times higher than that on smooth surfaces.
Publications
- 2018. Revealing of ultrasonic wire bonding mechanisms via metal-glass bonding. Materials Science and Engineering: B, 236, pp.189-196
Long, Y., Dencker, F., Isaak, A., Li, C., Schneider, F., Hermsdorf, J., Wurz, M., Twiefel, J. and Wallaschek, J.
(See online at https://doi.org/10.1016/j.mseb.2018.11.010) - 2018. Self-cleaning mechanisms in ultrasonic bonding of Al wire. Journal of Materials Processing Technology, 258, pp.58-66
Long, Y., Dencker, F., Isaak, A., Hermsdorf, J., Wurz, M. and Twiefel, J.
(See online at https://doi.org/10.1016/j.jmatprotec.2018.03.016) - 2019. Quantification of the energy flows during ultrasonic wire bonding under different process parameters. International Journal of Precision Engineering and Manufacturing- Green Technology, 6(3), pp.449-463
Long, Y., Schneider, F., Li, C., Hermsdorf, J., Twiefel, J. and Wallaschek, J.
(See online at https://doi.org/10.1007/s40684-019-00061-0) - 2020. Contact mechanics and friction processes in ultrasonic wire bonding-Basic theories and experimental investigations. Journal of Sound and Vibration, 468, p.115021
Long, Y., Twiefel, J. and Wallaschek, J.
(See online at https://doi.org/10.1016/j.jsv.2019.115021) - 2020. Investigations on the mechanism of microweld changes during ultrasonic wire bonding by molecular dynamics simulation. Materials & Design, 192, p.108718
Long, Y., He, B., Cui, W., Ji, Y., Zhuang, X. and Twiefel, J.
(See online at https://doi.org/10.1016/j.matdes.2020.108718) - 2022. Impact of surface texture on ultrasonic wire bonding process. Journal of Materials Research and Technology, 20, pp. 1828-1838
Long, Y., Arndt, M., Sun, C., Dencker, F., Wurz, M.C., Twiefel, J.and Wallaschek, J.
(See online at https://doi.org/10.1016/j.jmrt.2022.07.187)
