Ultra Highly Conductive Polymer Composites for Lead-free Interconnect

Abstract form only given. Lead, a major component in solder, has long been recognized as a health threat to human beings. One of the proposed lead-free candidates is electrically conductive adhesives (EGAs), which are composed of polymer matrix and conductive fillers. Usually epoxy and silver flakes are used as the polymer matrix and conductive fillers, respectively. Recently, Ag nanoparticles were proposed as conductive fillers in ECAs for the next generation of fine pitch applications. However; for nano isotropic conductive adhesives (n-ICA), the dispersion of Ag nanoparticles in epoxy compared to micron sized silver flakes is much more difficult because of higher viscosity induced by high surface area of nanoparticles. A recent report for nano ICAs demonstrated decent dispersion of nanoparticles in polymers and low resistivity via nanoparticle sintering by nanoparticle surface functionalization. In this study, both silver flakes and surface functionalized silver nanoparticles were incorporated into a polymer matrix and their electrical properties were evaluated and an ultra low resistivity (5 times 10^-6Omegamiddot cm) conductive adhesive was achieved. It was found by differential scanning calorimetry (DSC) that the surface modifiers were debonded from nanoparticle surfaces at certain temperatures. The amounts of bonded modifiers were determined by thermogravimetric analyst (TGA) and characterized by FTIR. By using appropriate surface modifiers, the resistivity of the silver flakes and nanoparticles incorporated adhesives was dramatically reduced to as low as 5 times 10^-6 Omegamiddotcm (silver: 2 times 10^-6 Omegamiddotcm). The ultra low resistivity of conductive adhesives was lower than that of solder joints. The morphology studies showed that the decreased resistivity resulted from the sintering of silver nanoparticles among silver flakes. The sintered particles are believed to fill the gap between the flakes and enhance the inter- - face properties. The contact resistance of the conductive adhesives under 85C/85%RH was significantly stable with respect to aging time. The interfacial properties between the surfactants, flakes and nanoparticles, and the electrical properties of the nanocomposites will be discussed