Factors affecting activation energy for pd-coated cu ball bond resistance degradation on Al bond pads in high temperature storage

Copper wire bonds have become a mainstay in many electronics areas, including automotive, where more data is needed regarding the reliability of Cu wire bonds on Al pads in high temperature conditions. Arrhenius models are frequently used to estimate product lifetimes, with the activation energy being the main parameter. Activation energies from 0.46 eV to 1.34 eV have been reported in the literature for similar materials and aging conditions. This apparent inconsistency between the results of different studies indicates one or more failure mechanisms that are not active in all cases, with causes that have not been clearly identified. To obtain uniformly high reliability, it is important to identify and eliminate the causes of rapid degradation observed by some authors. This work investigates the effects of Al pad metallization thickness and the choice of failure criterion on the bond resistance and activation energy for Pd-coated Cu (PCC) ball bonds with epoxy encapsulation. By testing samples at 175 °C, 200 °C, and 225 °C, with pad thicknesses of 800 nm and 3000 nm, we found that at least two mechanisms contributed to resistance changes. The first mechanism has an activation energy of about 0.7 eV, calculated using the times required for a 1 % increase in resistance. The second, calculated from the times until a 10 % resistance increase, has an activation energy of about 0.4 eV. Pad thickness did not have a significant effect on activation energy, but thicker pads did lead to slower increases in resistance at the beginning of aging.