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High Temperature and Element Alloying Influences on Kirkendall Voiding in Au Ball Bond Interconnects on Al Chip Metallization
Keywords: Au ball bonding, Au/Al intermetallic phases, Kirkendall voiding
The presentation addresses the reliability of Au ball bond interconnects on Al chip metallizations of different thicknesses and compositions. In this context the interfacial reactions, intermetallic phase and Kirkendall void growth are being discussed. The findings contribute to a better un-derstanding of failure mechanisms caused by interdiffusion and Kirkendall voiding. 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 load. In many cases lift offs already occur with Al metallization thicknesses > 1 m and temperatures in the range of 175C, while temperatures of 150C or 200C are not so critical. Therefore interpolation or extrapolation of aging temperature results is not longer allowed. 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 met-allization are typically a few hundred nanometers thick directly after the bonding process, de-pending 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 of pure Al (not alloyed with Si and Cu) and clearly thicker than 1 m and temperature is in the range of 175C. These results will considerably contribute to a better understanding of Kirkendall voiding failure mechanisms.
Dr. Martin Schneider-Ramelow,
Fraunhofer Institute for Reliability and Microintegration (IZM)
Berlin 13355,

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