Packaging factors affecting the fatigue life of power transistor die bonds

This paper presents an overview of die bond fatigue and a study of packaging and assembly factors and their influence on power transistor cyclic life. Power transistors with soft solder die bonds will fail by catastrophic failure, under cyclic conditions, as fatigue cracks develop and propagate through the die bond. Susceptibility to catastrophic failure increases over the life of the device, as crack growth through the die bond destroys the capability of the device to transfer heat. Power cycling tests followed by failure analysis show that die bond thickness has the most significant effect on catastrophic failure, followed by die bond thickness variation. Analysis shows, that for a given device, the die bond is not uniform and that the nonuniformity or die tilt (ratio of average to minimum thickness) influences device life by affecting strain concentration and crack growth. Oxygen content in the package, also influences device life to a lesser extent, as indicated by a statistical analysis of residual gas analysis results compared to cyclic life. This can be explained by crack closure effects during the development of die bond fatigue cracks. These findings further the understanding of die bond physics of failure and underscore the importance of optimizing die bond thickness in design and limiting variations and oxygen content in hermetic metal packages, during device manufacturing and assembly.