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18 The PCB Magazine • May 2014 GRAPHITE-BASED DIRECT METALLIzATION continues 2) the ability of the particles to remain suspend- ed in solution without settling. Each definition of stability involves different factors and will be discussed separately. 1) Consistent Particle Size Colloidal graphite is a hydrophobic col- loid, which means it is not strongly attracted to water. Hydrophobic colloidal particles are repelled from each other in two ways: sterically and electrostatically. Steric repulsion involves forming physical barriers to prevent the par- ticles from aggregating. Electrostatic repulsion involves an electrostatic charge preventing the particles from coming into contact with each other. While both methods of stabilization are involved in keeping colloidal graphite particles from aggregating, electrostatic repulsion is of most interest due to the influence of contami- nants on the electrostatic charges. As previously stated, the graphite particle has a net negative charge. This negative charge is further increased by the addition of a propri- etary binder, which at the operating pH also has a negative charge due to the presence of carboxylate groups (R-COO - ). Surrounding this particle is a cluster of counter-ions, which have a net positive charge. This unequal distribution of electrical charges sets up a potential across the interface and forms an electrical double lay- er. A schematic representation of this electrical double layer is shown in Figure 6. The electrical double layer is critical with respect to the stability of the graphite disper- sion. The binder with its negative charges sub- sequently enhances the attraction of the col- loidal graphite particles to the resin and glass surfaces of the PCB. Of course, the charge den- sity imparted to the non-conductive surfaces (glass, resin) by the cleaner/conditioner action is designed to promote the flocculation of the bound graphite particles to the surfaces. The charge density of the key additives in the poly- electrolyte must be carefully optimized in order to minimize the amount of graphite that coats the non-conductive surfaces. Uniformity of the graphite coating is more important than thick- ness (Figure 7). Graphite occurs as a highly crystalline substance as opposed to carbon black, which is amorphous. The hexagonal lattice of the graphite is documented to be a honeycombed arrangement of carbon atoms. This arrange- ment (and a key electronic property of graph- ite) promotes anisotropy, which is the property of being directionally dependent. An isotropic material conversely implies identical properties in all directions. What this means for through- hole and blind via plating is that the graphite is highly electrically conductive and suitable for metallization. When the resistance through a plated through-hole or blind via is low, the electroplating of copper progresses at a much faster rate (plating propagation) thus enhanc- ing throwing power. Figure 6: electrical double layer surrounding the graphite particle. Figure 7: Final graphite coating showing tightly packed structure and 0.15 µm thick.