Kirkendall voiding in au ball bond interconnects on al chip metallization in the temperature range from 100 – 200°c after optimized intermetallic coverage

The presentation addresses the reliability of Au ball bonds of different Au wire qualities on Al chip metallizations of different thicknesses and compositions at temperature storage from 100 to 200degC up to 4000 h. In this context the interfacial reactions and intermetallic phase coverage directly after the bonding process was optimized to get the best starting condition for phase growth at elevated temperatures and to avoid critical Kirkendall void growth. This failure mechanism is influenced by numerous factors, such as aging temperature and time, Au wire and Al metallization composition and ratio of mixture as well as the percental area of interconnection formation under the ball. These influences are mainly responsible for ball lift offs under operating conditions. In many cases lift offs already occur at Al metallization thicknesses > 1 mum and temperatures in the range of 175degC, while temperatures up to 150degC or at 200degC are less critical. Investigations include mechanical tests of Au loops and ball contacts as well as microstructure observations of the contacts in correlation to material composition, aging temperature and Al metallization thickness. Au/Al intermetallic phase thicknesses below the Au contacts on Al metallization are typically a few hundred nanometers thick directly after the bonding process, depending on bonding conditions like process parameters and material combination. These phases grow under temperature influence and Kirkendall voiding can occur. A most significant result in this context is that pull and shear lift offs occur if the chip metallization is clearly thicker than 1 mum and intermetallic phase coverage (after bonding) is less than 2/3 of the bottom side ball area. These results will considerably contribute to a better understanding of Kirkendall voiding failure mechanisms.