Improving gold-silver wirebond integrity in plastic packages

A gold-silver wire bond system is widely used as an interconnect between die and die attach paddle to allow stabilization of substrate voltage by connecting it with a capacitor to analog ground. Failure analysis tools and techniques used in determining the characteristics of die paddle ball bond failures are explained. Electrical test screens were evaluated to contain the failures. A comprehensive discussion of various factors influencing the integrity of gold wire ball bonds on silver plated die paddles of plastic packages is presented. Results are based on a series of matrix experiments focused on the effects of critical wire bond process parameters such as ultrasonic power, bond force and ball size ratio on these die paddle bonds prior to and after accelerated stress test. Leadframe conditions such as silver plating thickness and surface finish are also evaluated. Die paddle bond lift failures were found to be due to partial intermixing at the gold/silver interface under conditions of delamination at the molding compound/die paddle interface. Ball size ratio, ultrasonic bond power and silver surface finish were identified to be the most significant factors affecting the die paddle ball bond integrity after temperature cycle stressing. With the correct combination of material and bond process conditions, it was demonstrated that die paddle ball bonds can be made with high reliability