Effect of heating rate on bonding strength of pressure-free sintered nanosilver joint

Chip-bonding by sintering silver particles (micron-scale or nanometer-scale) is widely believed to replace soldering for manufacturing high-performance power semiconductor devices and modules because sintered silver joints are better for heat dissipation and more reliable in temperature-cycling and power-cycling tests than soldered joints. Common raw materials used for the silver sintering process are in the form of paste consisting of silver particles mixed in an organic system of binders, surfactants, and solvents. In our recent studies, we developed a mathematical model based on diffusion of solvent molecules and viscous-flow mechanics of a silver paste to show that drying of the paste in the bonding process is a critical step in determining the bond-line microstructural and mechanical quality. Our modeling results showed that stresses and strains generated in the shrinking silver paste were responsible for observed delamination and cracking in the sintered bond-line. In this study, we extended the modeling analysis to investigate effect of heating rate on the bond-line quality. A numerical simulation algorithm of the model was developed to determine the time-dependent physical properties of the silver paste as the material being dried at different heating rates. The simulation results showed a strong dependence of the relative density of the sintered bond-line on heating rate. By lowering the heating rate, the relative density of the sintered silver could be increased. Higher sintered density would mean stronger bonding strength, and this was verified by our experimental data. The findings of this study can be used to optimize the manufacturing process that uses sintering of silver paste for bonding power semiconductor chips.