Failure mechanisms in TAB inner lead bonding and the relationship between design and reliability

Thermo-compression bonding as an inner lead bonding technique subjects the bonded device to severe temperature and pressure conditions. This is particularly true for the case where gold-plated tape is bonded to gold bumps. One of the principal failure mechanisms for gold-to-gold bonding is related to the development of cracks in the protective inorganic passivation layer propagating into the underlying pad structure. An investigation was undertaken in the AMD TAB (tape automated bonding) laboratory on the prevention of such cracking and of the consequent failure mechanisms. Three different silicon devices fabricated in bipolar and CMOS processes have been used to compare the effects on crack development of the variables of passivation type and geometric configuration of the bump and pad metallization. Cracking at the bump to device interface results in reduced bond strength and failure of the bond at that interface. The failure mechanism is destruction of the titanium/tungsten barrier metallization due to the influx of oxygen and moisture. The incidence of cracking can be reduced or eliminated by appropriate design of the bump-to-pad geometry