ELECTROMECHANICAL CHARACTERIZATION OF METALCOATED POLYMER SPHERES FOR CONDUCTIVE ADHESIVES
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Micron-sized metal-coated polymer sphere(s) (MPS) are a critical component in anisotropic conductive adhesive (ACA) applied in fine pitch electrical interconnect. The MPS are compressed during adhesive bonding, so their electrical and mechanical properties under compression determine their performance in application. A significant lack in the literature has been analysis of single particle properties for ACA applications, as most studies focus on testing bonding components. My major contribution has been to develop experiment techniques to measure electrical and mechanical properties of individual micron-sized MPS. A greater understanding of these properties may guide particle design in the future. The technique of electromechanical nanoindentation, wherein single particles are compressed with a flat punch while a four point electrical measurement is performed, has been pioneered in this work. Resistance characteristics are strongly dependent on the relative deformation of the particles. Particles of different sizes (3-30 μm) and with different metal coatings (Ag, Ni/Au and Au) have been compared. Generally, larger particles give lower measured resistance, which may be attributed to differences in contact area at the same relative deformation. Although the mechanical characteristics show distinctive popins that can be associated to fracture events in the metal coating, the resistance characteristics were not negatively influenced by fracture of the metal coating. Coating fracture in MPS was further investigated using picoindentation, a micromechanical test set-up mounted in a scanning electron microscope (SEM) for in-situ observation. Two fracture trends were observed, a brittle trend for metal coatings containing Ni and ductile trend for other (Ag and Au) metals. Fracture lines always extended parallel to the direction of compression, which explains why fracture did not influence the resistance characteristics. To evaluate the resistivity of the metal coatings, I conceived a novel application of the van der Pauw method to spherical thin films. Greek cross structures were fabricated on the surface of the spherical, thin film metal coatings, and four microactuated probes in a SEM were used to perform four-point measurements. It was found that the coating resistivity was highly dependent on the coating thickness, and the deviated from bulk resistivity values was significant. Finally, some tests were made to compare single particle resistance measurements with a test chip where particles were compressed in adhesive. There is a significant discrepancy between the resistance measured using electromechanical nanoindentation and that in adhesive. Not only are the absolute values different, but whereas Ag coated particles have lower resistance in electromechanical nanoindentation than NiAu coated particles, the opposite is true in adhesive. These observations mean that the single particle tests do not have a direct correlation to resistive properties in adhesive.