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The standing wave on ultrasonic bonding tips and the tendency of the lower end to fold back upon itself, termed “lug-down,” have been measured under actual bonding conditions. These phenomena have also been modeled on a computer. Lug-down can be a major contributor to bonding failures in designs having an overly slim tip. One definition of a “bonding failure” is the production of a bond between wire and monolithic circuit pad which lifts, when the wire is pulled at 90° to the pad, under a wire tensile force less than 25% of the ultimate tensile strength of the wire. Improper location of nodes can also contribute to bonding failures. A tapered tip has been shown by mathematical analysis to possess a higher resonant frequency and fewer nodes than a tip of uniform cross section. Using the newly developed taper tip, means were found for minimizing lug-down. With judicious trade-offs, it is now possible to optimize tip geometry for a particular bonding application.
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