Characterization of mechanical properties of metal-coated polymer spheres for anisotropic conductive adhesive

Metal coated small (micron sized) polymer particles are used in developing anisotropic conductive adhesives (ACA). The mechanical properties of polymer particles are of crucial importance both to assembly process and the reliability of ACA. Tn this paper we present a method to determine the mechanical properties of polymer sphere particles by using inverse indentation test - soft elastic sphere against rigid flat. Finite element analyses have been carried out to study the large deformation contact between the sphere particle and a rigid flat. The classical Hertz solution works only for small sphere deformation. A modification has been made to the Hertz contact force-displacement solution and an approximate equation is presented. A capacitance based experimental setup for particle indentation has been built. The proposed method has been applied to determine the elastic properties of typical polymer particles used for conductive adhesives. For the metal plated polymer particles tested, it has been found that a linear elastic model seems to hold for a large range of deformation.