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Numerical Analysis of Ultra-High Frequency Wire Bonding
Keywords: ultrasound, wire bonding, finite elements
When the industry moved from the classical 60 kHz ultrasonic transducers to higher frequency transducers operating in the 120 to 140 kHz range, the reactivity at the bond interface during the process increased leading to higher bond quality at lower process temperatures. This is a prerequisit for reliable bonding on many metallized organic substrates including BGAs. With low-k materials increasing the chip performance, Cu bonding wires reducing the material cost, and bonding over active circuitry to reduce chip size, substantial challenges still remain in wire bonding which are addressed by developing “low stress” bonding processes. Moving the ultrasonic frequency up to the next level promises to further reduce bonding stress, address each of the challenges listed, and circumvent current roadblocks. A harmonic frequency response finite elemente model is reported for the first time describing the effect ultrasonic frequency has on the mechanics of the bonding process. The model takes into account capillary material parameters including the damping factor in the ultrasonic range and is applied to a number of capillary designs with various geometrical features typically used in industry. The model is used to analyse the next higher frequency range suitable for wire bonding. This range is found to be between 375 to 425 kHz with the currently used capillaries. In this range, the process is more sensitive to the capillary geometry chosen. About 38% less ultrasonic horn amplitude is required for successful bonding. About 20% less out of plane ultrasonic stress is predicted. This will improve the low stress performance of the bonding process. The results serve as novel guidelines for future designs of ultra-high frequency transducers, capillaries, and bonding processes.
Michael Mayer, Assistant Professor
University of Waterloo
Waterloo, Ontario N2L3G1,

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