Effect of Ag Additives on the Consumption of a Cathode Cu Pad in a Cu/Sn3.5Ag/Cu Flip-Chip Structure Under Electromigration

The present work investigated the effect of Ag additives on the consumption of cathode Cu pad under electromigration in a current-stressed Cu/Sn3.5Ag/Cu flip-chip structure. The consumption rate of a cathode Cu pad in a pure Sn system is faster than that of a cathode Cu pad in a Sn3.5Ag system at 80℃ by a factor of 10. Using the kinetics of the consumption equation, the activation energy in the consumption of a cathode Cu pad was calculated. The activation energy (0.65 eV) of the consumption of a cathode Cu pad in a Cu/Sn3.5Ag/Cu flip-chip structure under electromigration (EM) is slightly greater than that (0.62 eV) of the consumption of a cathode Cu pad in a Cu/Sn/Cu flip-chip structure under EM. The dissolution of Cu flux is discussed, and the possible effect of Ag additives on the consumption rate of a cathode Cu pad under EM is proposed. The Ag additives reduced the deficiency of Cu solubility in a solder matrix near the cathode Cu/solder interface, which is the main factor in the reduction of Cu pad consumption. Moreover, the Ag₃Sn phase forming on the cathode Cu₆Sn₅ interface protects Cu₆Sn₅ from dissolving into the Sn3.5Ag solder matrix under EM.