Characterization of intermetallic compounds in Cu-Al ball bonds: Mechanical properties, delamination strength and thermal conductivity

In high power automotive electronics copper wire bonding is regarded as most promising alternative for gold wire bonding in 1^st level interconnects and therefore subjected to severe functional requirements. In the Cu-Al ball bond interface the growth of intermetallic compounds may deteriorate the electrical, thermal and mechanical properties. The layer growth and properties of these intermetallic compounds are crucial in the prediction of the long term behavior. To mimic the growth of intermetallic compounds during and after copper ball bonding, diffusion couples of aluminium and copper were annealed at 225-500°C and chemically analyzed by SEM/EDS. Also five separate intermetallic compounds were melted together from the pure elements and aged in evacuated quartz ampoules for 240 hours at 500°C. In this work values for the indentation Young's modulus, load independent hardness, indentation fracture toughness, volumetric densities, interface delamination and thermal conductivity are presented. It can be concluded that the Cu-rich intermetallics Cu₉Al₄ and Cu₃Al₂ are less sensitive to fracture and have lower average densities than the other intermetallic compounds. The volumetric decrease during formation causes internal stress. Interfacial delamination initiates in the Al-rich intermetallics (CuAl, CuAl₂) and propagates easily into other intermetallic layers. The Cu₉Al₄ - Cu s.s. is also found to be susceptible for delamination fracture. The thermal conductivity is much lower than for pure copper or aluminium and in the range of 26-87 W/m^-1*K^-1, where Cu₃Al₂ layer has the lowest thermal conductivity (26-33 W/m^-1*K^-1).