Real time contact resistance measurement to determine when microwelds start to form during ultrasonic wire bonding

We prove that we can, using contact resistance as a tool, determine the instant when the bonding process starts, i.e. microwelds start to form during ultrasonic bonding. This knowledge permits us to reduce the uncertainty in the estimated bonded area by 5–18%. We proved our claim by combining a real-time contact resistance measurement, aborted ultrasound bonding, and classical SEM analysis of the bonded surfaces. We measured and analyzed, using a 4-wire Kelvin cross setup, the contact resistance of 25 μm by diameter AlSi(1%) wires bonded to a gold pad. The microweld area of 69 bonds was determined. We focused on inferring exactly when do the microwelds start to form. Post hoc analysis showed a linear correlation between the total microweld area and the time elapsed since the initial contact resistance drop. This work may help minimizing the sonication impact which may allow working with thin bond wires and fragile substrates.

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► We used the contact resistance to determine the instant when microwelds start to form during ultrasonic wire bonding.
► We combined a real-time contact resistance measurement, aborted ultrasound bonding, and SEM analysis of the bonded surfaces.
► Analysis showed a linear correlation between the microweld area and the time elapsed since the contact resistance dropped.