Accelerated lifetime estimation of thermosonic Cu ball bonds on Al metallization

Abstract

In this study a fast mechanical shear fatigue test technique for the quality assessment of thermosonic ball bonded interconnects was developed to estimate their lifetime behavior. The micro-interconnects were subjected to cyclic shear stress using a special specimen set-up in combination with an ultrasonic resonance fatigue system at a frequency of 20kHz and a laser Doppler vibrometer for velocity measurements. Two different types of power semiconductor devices comprising single and multiple Cu ball bonds on Al pads of the same dimension and bonding quality were tested to validate the proposed testing technique. Due to the dimensional restrictions of both specimen types, suitable sample preparation techniques were designed to induce cyclic shear stresses in the interface of the ball bonded interconnects. The obtained high cycle fatigue life curves could be related to the specimen design and the resulting loading modes. Using FEM simulations the loading modes and fatigue response of both set-ups were analyzed. Life-time prediction curves based on a Basquin type relationship (von Mises stress - loading cycles) were obtained in which dissimilar geometries and loading modes could be equated. Additional SEM investigations of the fractured areas were compared and discussed to understand the microstructural failure mechanisms occurring during fatigue loading. It can be concluded that this new fatigue testing technique accompanied by FEM simulations and subsequent inspection of the fractured area is a suitable time saving and economic testing method for the assessment of the reliability and fatigue life response of small micro-interconnects.