Low Temperature Die-Attach Bonding Using Copper Particle Free Inks

The paper presents the potential of a particle-free copper ink, based on a complexed metal salt, with regard to increasing performance requirements, particularly in terms of thermal conductivity and lower bondline thicknesses. The copper ink is applied on ENEPIG and copper substrates. An integral aspect of the research is the designed predrying process, in which nanoparticles are generated in Situ using formic acid-enriched nitrogen. As optimal conditions a predrying for 10 min at 130°C was investigated, to have still sufficient tack for the chip placing and already during predrying generated nanoparticles. The formation of Cu nanoparticles caused by the thermal decomposition of the metal salt, both during predrying and sintering, is another important aspect. The predrying process is critical to achieve a homogeneous surface thickness on which the chip is sintered. In the following process, the chips are sintered under nitrogen under a pressure of 15 or 30 MPa for 5 or 15 min at 250°C. The results show the successful realization of a homogeneous, thin copper sintered layer (6 μm) by sintering for 15 min at 30 MPa and 250°C under nitrogen. For this process conditions an average shear strength of 53 ± 9 MPa for a copper-copper bond was achieved. With slightly different sintering conditions of 225 °C, 15 minutes and 30 MPa, shear forces of 42 ±3 MPa were achieved. The tests with ENEPIG boards and chips also show comparable shear values, with 42 ± 2 MPa under optimum sintering conditions. The fracture from shearing was detected within the interconnect. With further increasing the homogeneous layer thickness, the applied sinter pressure can be reduced.