Characterization of Micron-size Polymer Particles by Nanoindentation

Mono-sized and metal coated conductive polymer particles with diameter from 500nm to 10μm are increasingly used in electronics and microsystem. Until now, the focus has been on metal coated particles for use in Anisotropic Conductive Adhesive, but also applications as spacers for Liquid Crystal Displays. In these applications, the characterization, analysis and control of mechanical properties of the particles are of crucial importance. Due to the inherent complexity of the spherical geometry, characterization of the nanostructured polymer particles is different from traditional material characterization and possesses great challenges. A flat-punch based nanoindentation method has been recently developed for testing the mechanical properties of polymer particles with deformation up to 80%. A series of polymer particles with various diameters has been tested. The polymer particles show peculiar rate-dependent, yielding, hardening and fracture behavior. It has been found that for same deformation rate the resulting contact stress-strain curve is strongly dependent on particle diameter, and traditional material models are of limited accuracy. Specific constitutive models that include viscoelastic response, size effect, large deformation, and nanoscale adhesion are greatly needed to understand the mechanical behavior of polymer particles.