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

Abstract

In high power automotive electronics copper wire bonding is regarded as the most promising alternative for gold wire bonding in 1st level interconnects and therefore subjected to severe functional requirements. In the Cu–Al ball bond interface the growth of intermetallic compounds may deteriorate the physical 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 240h 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. interface is also found to be susceptible for delamination fracture. The thermal conductivity for 3 intermetallics is much lower than for pure copper or aluminium and in the range of 26–87W/m−1K−1, where Cu3Al2 layer has the lowest thermal conductivity (26–33W/m−1K−1).