Optimization of Cu–Ag Core–Shell Solderless Interconnect Paste Technology

The use of Cu-Ag core-shell nanoparticles has been proposed as a Pb-free interconnect technology in which upon annealing, the Ag shell dewets the copper surface and forms Ag necks between Cu core particles at particle-particle contacts. Neck formation creates an electrically, thermally, and mechanically stable porous sintered network. To convert this concept into a viable manufacturing technology, optimization of particle characteristics and paste formulation is necessary with respect to sinterability, mechanical strength, and processability for typical die attach and possibly surface mount assembly reflow operations and applications. This paper describes the optimization process carried out using three core-shell powder configurations and over 50 flux formulations. The two best performing pastes consisted of 1-$\mu \text{m}$ diameter core-shell particles with a 9-nm Ag shell and fluxes containing methyl ethyl ketone, isopropanol, methyl pyrrolidone, butyl cellosolve, Terpineol, Teckros D85, Troythix XYZ, and malonic acid. These pastes exhibited the targeted shear thinning behavior necessary for screen printing and assembly processing. From a comparison of the microstructural evolution for paste and dry compacts, the presence of flux did not appear to inhibit neck formation. As the final step, printability optimization using commercial assembly equipment was performed based on squeegee speed and pressure and showed coverage and paste rheology to be comparable with those of commercial Pb-free solder pastes.