Materials characterization, conduction development, and curing effects on reliability of isotropically conductive adhesives

Three commercially available, silver filled, snap cure isotropically electrically conductive adhesives for surface mount applications were selected for study. Fundamental material characterizations were conducted on these materials, including thermal analysis [differential scanning calorimetry (DSC), thermo-gravitational analysis (TGA), and thermo-mechanical analysis (TMA)] rheological, and dynamic mechanical analyses. Microstructural investigations [scanning electron microscopy (SEM), transverse electromagnetic (TEM), Auger] were performed to identify the silver flake size, distribution, and contact morphology. These analyses were related to the cure process and electrical conduction mechanisms of isotropically conductive adhesives (ICA´s). The resistivity of these materials was monitored during cure and related to the cure kinetics of the epoxy matrix. The resistivity decreased dramatically (>kΩ cm to mΩ cm) around a specific temperature with ramp cure and over a narrow time range (<10 s) with isothermal cure. Successive heating (25-150°C) and cooling cycles yielded different degrees of consecutive resistivity decreases for these materials which were cured according to the manufacturer´s recommended schedules. Microstructure development during cure was studied with a hot stage in an environmental scanning electron microscope (ESEM) to relate morphological changes with the observed changes in resistance. No significant structural changes and silver flake movements were noticed during cure. The conduction development was accompanied by breakage and decomposition of the tarnish, organic thin layers which cover the silver flake surface, and by the enlargement of the contact area between silver flakes by thermal stress and shrinkage during the epoxy cure. The temperature coefficients of resistance (TCR) were measured for these materials; the TCR is closely related to a conduction mechanism dominated by constraint resistance between the flakes or by the silver flake metallic conduction. The resistivity and interfacial resistance of these materials with bare copper and gold plated pads (with five selected cure schedules) were measured through 85°C/85% RH exposure up to 900 h. The bulk resistivity decreased in the first 100 h of exposure and did not change with humidity; however, the interfacial resistance increased with the copper pads for some materials. This is caused by the oxidation of the copper pads due to moisture attack