Fracture behavior of isotropically conductive adhesives

The fracture behavior of several commercial, silver-filled epoxies was studied using a combination of fracture mechanics, surface science, and microscopy. Three-point bend tests revealed that the bulk fracture toughness of the silver-filled epoxies fell within a narrow range 1.1-1.3 MPa-m/sup 0.5/. Both electron and optical microscopy studies indicated that crack path deflection due to the silver-particles was the primary micromechanical deformation mechanism. Surprisingly, the interfacial fracture energies between the epoxies and a copper surface ranged from 50 to 900 J/m/sup 2/. Contact angle measurements on the cured epoxies indicated that some epoxy surfaces are more active than others. However, the correlation between thermodynamic work of adhesion and fracture energy is rather weak and suggests only a modest trend. In summary, although the use of contact angles/surface energies to predict adhesion is promising, much more effort is required to make it a reliable screening tool. Fortunately, the use of interfacial fracture mechanics can detect differences in adhesive strength, and should allow packaging engineers to select die attach adhesives with improved adhesion.