PACKAGING FACTORS AFFECTING THE FATIGUE LIFE OF POWER TRANSISTOR DIE BONDS

This paper presents an overview of die bond fatigue and a study of pac kaging and assembly factors and their influence on power transistor cy clic life, Power transistors with soft solder die bonds will fail by c atastrophic failure, under cyclic conditions, as fatigue cracks develo p and propagate through the die bond. Susceptibility to catastrophic f ailure increases over the life of the device, as crack growth through the die bond destroys the capability of the device to transfer heat. P ower cycling tests followed by failure analysis show that die bond thi ckness has the most significant effect on catastrophic failure, follow ed by die bond thickness variation, Analysis shows, that for a given d evice, the die bond is not uniform and that the nonuniformity or die t ilt (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 t o cyclic life. This can be explained by crack closure effects during t he development of die bond fatigue cracks, These findings further the understanding of die bond physics of failure and underscore the import ance of optimizing die bond thickness in design and limiting variation s and oxygen content in hermetic metal packages, during device manufac turing and assembly.