Use of harsh wire bonding to evaluate various bond pad structures

IC bond pad structures having Al metallization and SiO₂ dielectric have been traditionally designed with full plates in underlying metallization layers, connected by vias. In addition, pads having bond over active circuitry (BOAC) which are much more sensitive to pad cracks, are likely present in the same IC. Cracks in the pad dielectric weaken the bond reliability and may cause electrical leakage or shorts to circuitry under the pad. Cracks are more likely to occur during Cu wire bond due to higher bonding stress as compared to Au alloy wire bonding. Experimental data from bonding with 1mil Au or Cu wires reveals dramatic differences in pad robustness against cracking, depending upon the underlying metal structures and patterns. A “harsh” Au wire bond recipe is also developed to produce the stress effects of Cu wire bond in experiments without having to upgrade older bonding equipment for Cu wire. Cratering test after wire bond is used to evaluate pad cracking. Ball shear testing followed by a cratering test further reveals pad cracking tendencies. Design principles for increased pad robustness to cracking are developed based on the data. Reliability data verifies the effectiveness of the design principles. Proper design of interconnects beneath the pad can greatly increase pad robustness to cracking, allowing much more margin in bonding stress, enabling the option of Au or Cu wire bond on the same IC without pad cracking.