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Nanoscale bonding?
Keywords: bond process, microwelds, nanoscale bonding
We measured microweld expansion during ultrasonic wire bonding. The results show that after the initial slip phase the microweld’s radii expand less than 15 nm per ultrasonic cycle. We used a real-time contact resistance method to determine the instant when the bonding process starts, i.e. microwelds start to form during ultrasonic bonding [1]. After the bonding process microwelds were measured by a scanning electron microscope and each microweld dimensions were measured. Microwelds expansion per bond cycle was determined. We analyzed 85 bonds and 16187 microwelds. The analysis shows that 2 ms after the bonding process the radius of 95% of the microwelds expand less than 15 nm per ultrasonic cycle and after 15 ms bonding process the microwelds radius expand less than 4 nm per ultrasonic cycle. The result indicates that macroscopic slip in the bonding interface is not needed for the microweld development. Instead, microscopic nanometer scale slip may occur without atomic bond breakage in the bonding interface. The ultrasonic bonding mechanism could be as a reversed crack propagation process. This knowledge may be important for nondestructive bond quality measurement as well as for the design of the new bond materials and bonding tools.
Henri Seppänen, Research scientist
FInnish meteorological institute
Helsinki, -

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