Reliability assessment of copper ball bonds by combination of simulation and accelerated mechanical testing

We have investigated the fatigue strength of copper ball bonds attached to silicon chips covered with an aluminum metallization layer. Under service conditions, the chips are exposed to repeated temperature changes leading to thermo-mechanical stresses. In order to predict their lifetime, accelerated mechanical tests were performed at ultrasonic frequency. Thereby, different setups were tested to optimize the method. The stresses resulting from mechanical tests were compared to deviatoric stresses originating from thermal mismatch of the different materials involved. For this purpose, FEM computer simulations were performed. It was found that the stress distributions of the different setups are quite different. Nevertheless, the weakest link of the construction was always the aluminum metallization layer. In consequence, the number of loading cycles to failure could be related to the von Mises stress observed in the aluminum film.