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16 The PCB Magazine • May 2014 GRAPHITE-BASED DIRECT METALLIzATION continues coating and remove some of the graphite from the copper surfaces (Figure 5). Following fixer, the graphite coating is dried to provide proper adhesion. The panel is then passed through a micro-etchant. The purpose of this step is to remove graphite from the cop- per surface and copper interconnections along the hole wall. This provides a good copper-to- copper bond during electroplating. As a final, optional step, an anti-tarnish coating may be applied to protect the copper surface during storage prior to resist application or electroplat- ing. At this point the circuit board is ready ei- ther for primary imaging followed by electro- plating of copper or panel plating. Science of Colloidal Graphite Dispersions What makes such a system function for to- day's highly complex PCB designs? First, as de- scribed above there must be a robust process. Secondly the chemistry of the system must be completely understood. This starts with a dis- cussion of the consistency of colloidal graphite dispersion and the stability of the colloid. Sta- bility is defined as 1) the ability to maintain a consistent particle size without aggregation and Figure 4: Graphite particles tightly bound to the substrate while other particles are loosely held. Figure 5: after fixer and rinse, a thin layer of con- ductive graphite has been tightly bound to resin and glass, and excess graphite has been removed.