Development of Cu-Ag pastes for high temperature sustainable bonding

The combination of excellent electrical conductivity and low cost makes copper a good selection for interconnect materials. However, rapid oxidation of Cu nanoparticles especially at high temperatures is a fatal demerit. To improve the oxidation resistance and realized mass production, Cu@Ag core-shell submicron particles were prepared using commercial copper oxide particles through a low temperature reduction method and subsequent electroless-plating to form Ag shells. Thermal analytical results suggest that Ag coated Cu particles show improved anti-oxidation ability. Mixed with Ag submicron particles obtained from thermal spray pyrolysis, the electrical resistivity of the sintered Cu-Ag composite pastes reaches 10.4 μΩ cm under a reductive atmosphere. Under the bonding pressure of 10 MPa at 275 °C for 30 min, robust Cu to Cu bonding can be achieved with the Cu-Ag composite pastes, for which the shear strength of the joints reaches 32.7 MPa, and it remains 28.2 MPa as the bonding pressure is reduced to 5 MPa. It was also demonstrated that the joints thus formed have superior elevated temperature strength, and excellent reliability subjected to high temperature storage at 250 °C as well as thermal cycling ranged from −65 °C to 150 °C.