Degradation of Bonding Strength (Al Wire‐Au Film) by Kirkendall Voids

Abstract

Degradation in bonding strength of the Al wire‐plated Au film system has been studied. When this bonding system is stored at elevated temperatures, the bonding strength degrades following one of the three degradation modes: (i) A‐mode in which the bonding strength decreases with increasing thermal annealing period until reaching a certain value, (ii) B‐mode in which the strength starts separating into two values after a certain period of annealing, and (iii) C‐mode in which the strength rapidly drops down to zero. The physical mechanisms for the three degradation modes have been well explained by Kirkendall void formation at the gold‐rich intermetallic phase of the aluminum‐gold interface, where the growth rate of the gold‐rich intermetallic phase plays a very important role on deciding the degradation modes. When gold film includes a large amount of Pb or Tl, this rate decreases and C‐mode or B‐mode will take place. Doping of Ag into the gold film sometime results in less degradation in the bonding strength.