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

Abstract

In high power automotive electronics copper wire bonding is regarded as most promising alternative for gold wire bonding in 1" 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 inter metallic compounds during and aft er copper ball bonding, diffusion couples of aluminium and copper were annealed at 22S-S00°C and chemically analyzed by SEMIEDS. Also five separate intermetallic compounds were melted together from the pure elements and aged in evacuated quartz ampoules for 240 hours at SOo°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 AI-rich intermetallics (CuAI, CuAI2) 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 Wlm-i *Ki, where Cu�12 layer has the lowest thermal conductivity (26-33 Wlm-i*Ki).