Ultrasonic friction power during thermosonic Au and Cu ball bonding

The ultrasonic friction power during thermosonic ball bonding with Au and Cu wires, both 25 µm in diameter, is derived with an improved method from experimental measurements during the bonding process. Experimental data include the current delivered to the ultrasonic transducer and the tangential force measured using piezoresistive microsensors integrated close to the Al bonding pad. The improvement results from a new, more accurate method to derive the mechanical compliance of the ultrasonic system. The method employs a bond process modification in which the ultrasonic current is ramped up sequentially in three steps. In the first two steps, the ultrasonic current is set to levels that are too low to cause sliding. The bonding takes place during the third step, when the current is ramped up to the optimum value required for making good quality bonds. The ultrasonic compliance values are derived from the first two steps and are 8.2 ± 0.5 µm N−1 and 7.7 ± 0.5 µm N−1 for the Au and Cu processes, respectively. These values are determined within an average error estimate of ±6%, substantially lower than the ±10% estimated with a previously reported method. The ultrasonic compliance in the case of Au is 6% higher due to the lower elastic modulus of Au compared with that of Cu. Typical maximum values of relative sliding amplitude of ultrasonic friction at the interface are 655 nm and 766 nm for the Au and Cu processes. These values are 81% of the free-air vibration amplitude of the bonding capillary tip for the respective ultrasonic current settings. Due to bond growth, which damps relative motion between the ball and the pad, the final relative amplitude at the bond interface is reduced to 4% of the equivalent free-air amplitude. Even though the maximum value of relative amplitude is 17% higher in the Cu process compared with the Au process, the average total interfacial sliding is 519 µm in the Cu process, which is 31% lower than that in the Au process (759 µm). The average maximum interfacial friction power is 10.3 mW and 16.9 mW for the Au and Cu ball bonding processes, respectively. The total sliding friction energy delivered to the bond is 48.5 µJ and 49.4 µJ for the Au and Cu ball bonding cases, respectively. These values result in average friction energy densities of 50.3 mJ mm−2 and 54.8 mJ mm−2 for Au and Cu ball bonding, respectively.