Conduction and microstructure development in Ag-filled epoxies

Three commercially available, Ag-filled, electrically conductive adhesives were selected for study. These materials were selected primarily because of their relatively fast cure times, 3 to 10 minutes. They were developed by material suppliers in the last two years specifically for surface mount applications. The resistance of these materials was monitored during cure, and related to the cure kinetics of the epoxy matrix. During cure the resistance decreased dramatically (greater than or equal to k Ohm to m Ohm) around the epoxy gel point and over a narrow time range (<10 seconds). Successive heating cycles yielded consecutively smaller resistances, which eventually stabilized. An attempt was made to correlate the attainment of an invariant room temperature resistance and a linear resistance versus temperature response to the degree of cure. Microstructure development during cure was studied with a hot stage in an Environmental Scanning Electron Microscope (ESEM). This allowed us to compare microstructure development with conduction development. In addition, thermal, rheological and dynamic-mechanical analyses were performed. These analyses were used to determine fundamental materials parameters. These fundamental parameters were then related to the electrical and mechanical property development and then to the reliability performance of these Ag-filled epoxies.