Development of Cu-Ag composite pastes for power IC attachment

Abstract

The combination of excellent electrical conductivity (only 6% less than Ag) and low cost makes copper a good selection for interconnect materials. However, rapid oxidation of Cu nano particles is a fatal demerit. In order to improve the oxidation resistance, Cu-Ag composite particles are developed for the bonding and interconnect applications in this study. To cut the cost and realized mass production, we develop Cu@Ag core-shell submicron particles using commercial copper oxide particles. A low temperature reduction method has been successfully developed, which can transform copper oxide particles to copper particles efficiently (at 250°C for 30 minutes) without aggregation and coarsening. The reduced copper particles are subsequently electroless-plated to form Cu@Ag core-shell particles with continuous Ag shell. TGA analytical results and in situ synchrotron radiation XRD patterns both suggest that Ag coated Cu particles show improved anti-oxidation ability. Mixed with Ag submicron particles obtained from thermal spray pyrolysis, Cu/Ag composite pastes are applied to achieve Cu to Cu bonding. Under a reductive atmosphere, the electrical resistivity of the sintered composite pastes reaches 10.4 μΩ · cm Through adjusting the bonding temperature, load and holding time, the optimal conditions can be suggested. Under the bonding pressure of 10MPa at 275°C for 30 min, the shear strength of the joints reaches 32. 7MPa, and it remains 28.2 MPa as the bonding pressure is reduced to 5 MPa. The results of mechanical tests demonstrate that the joints thus formed had excellent shear strength at high temperatures and excellent reliability subjected to high temperature storage at 250°C and thermal cycling ranged from −65 °C∼150 °C.