Low-temperature sintering of a nanosilver paste for attaching large-area power chips

A low-temperature silver sintering technology is emerging as a lead-free die-attach solution for high-reliability packaging of power electronics devices and modules. Sintered chips on substrate are reported to have excellent heat dissipation and capability of working at higher junction temperatures. However, one concerning issue with many of the silver sintering die-attach processes is the requirement of large uniaxial stress or pressure, ranging from 10 MPa to 40 MPa, to lower the sintering temperature to about 250°C. In this paper, we report our findings in the evaluation of a nanosilver paste technology developed to eliminate pressure needed for the silver-sintering die-attach process. The nanosilver paste was analyzed by TGA and DSC to show its weight loss and enthalpy characteristics associated with solvent evaporation, binder burn-out, and densification. A custom optical system was used to measure bond-line shrinkage behavior. A fractional factorial design of experiments was carried out to identify the importance and interaction of various processing parameters, such as pressure, temperature and time, on the bond strength and microstructure of sintered nanosilver joints. Based on the findings, a simple die-attach process, consisting of pressure-drying at 180°C under a few MPa pressure, followed by sintering below 260°C with zero pressure was found to produce strong die bonding with strength in excess of 30 MPa.