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Consumable and Process Improvement for Large Copper Wire Bonding
Keywords: Large copper wire bonding, Consumable lifetime, Wedge bonding process
The Cu wire has a high mechanical strength and higher electrical and thermal conductivities, making it a much more reliable interconnect material in high performance applications. However, the Cu wire is much harder and stiffer than the Al wire and requires 2 to 3 times more bond force and power for bonding. Consumables, especially bond tool, wear much faster than Al wire bonding consumables. This significantly increases the process cost of Cu wire bonding and prevents the industry transition from Al wire to Cu wire. The wear and failure mechanism for Cu wire bonding consumables are investigated. The study focuses more on the bond tool life. Bond tool material, tip geometry, and bond process parameters are primary factors affecting bond tool lifetime. Different from the relatively simple Al wire bonding process, a Cu wire requires sophisticated, multiple bonding phases to achieve a robust bond. A multi-segment bonding parameter was developed to achieve the best result in each phase. Force, power, time, and ramping profile are 4 major variables that affect bond quality as well as consumable lifetime. With an optimized bond parameter, the bond tool life was improved by 2 to 3 times without compromising bond quality. Currently used Cu wire bond tools with a V-groove were copied from Al wire bond tools. Such bond tools rely on friction to couple with wires during bonding and they are designed and optimized for Al wire bonding, not for Cu wire bonding. The high ultrasonic power required by Cu wire bonding generates a large bond tool movement and high tangential force that causes bond tool wear. With the wear of bond tool, the groove volume gets larger and the Cu wire sinks more into the groove, so that the tips of bond tool get closer to the substrate. When the bond tool tip touches the substrate and leaves a mark, called bond tool bottom-out, the ultrasonic energy is dispersed into the substrate and the bond strength is compromised, which indicates the end-of-life for the bond tool. In this study, the V-groove is redesigned and optimized for Cu wire bonding. With an improved groove design, more than 5 times longer bond tool life was achieved. In combining the groove design and bond process improvement, more than 10 times longer bond tool lifetime was achieved without increasing the bond tool cost, since there is no material and manufacturing process change. Besides, we are also studying more durable materials. A wear test setup is developed that allows us to quickly evaluate various materials without making a bond tool. With a more durable material, the bond tool life will be further improved. How to apply a new material tip to an existing bond tool is also being investigated. By optimizing the cut process and introducing different material and heat treatment, the cutter blade lifetime is improved by >10 times. The wire guide lifetime is less concerned because the current wire guide has a longer lifetime than a bond tool or a cutter blade.
Tao Xu,
Kulicke & Soffa Industries, Inc.
Santa Ana, CA

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