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May 2014 • The PCB Magazine 15 GRAPHITE-BASED DIRECT METALLIzATION continues ing a positive charge. This is typically accom- plished by the use of a cationic conditioner such as a quaternary amine polymer [1] . The cat- ionic conditioner has a strong attraction to the slightly negatively charged resin and glass ma- terials along the hole wall. But first, one should understand the graphite colloid as depicted in Figure 2. The core of the graphite colloid consists of a highly conductive graphite particle which is plate-like in shape. Surrounding each graph- ite particle is a layer of high molecular weight polymer binder. Certain functionalities attach to the graphite. Negatively charged functional groups extend into the solution as the binder forms a polymer layer. The coating of condi- tioner is not easily removed and can withstand aggressive spray rinsing (Figure 3). Follow- ing conditioning, the printed wiring board is coated with the colloidal graphite. Since the colloidal graphite has a negative charge it is easily attracted to the conditioner and forms a uniform layer of graphite on top. The graph- ite particles themselves have a negative charge. This charge is further enhanced by the use of a binder attached to the graphite, which also has a negative charge. The negative charge is thus attracted to the polymer backbone of the conditioning agent (Figure 4). Besides the elec- trostatic attractive charge of the conditioner, there is a gel formation from the reaction of the graphite binder with conditioner. The purpose of the fixer is to form an ad- herent coating of graphite. The fixer solution is an acidic solution having a fairly high ionic strength that neutralizes the negative charge on the binder. Thus, the fixer is used to set, or fix the graphite coating onto the non-conductive surfaces and remove loosely adherent graphite particles that did not bond with the cleaner- conditioner molecules. The binder gels and so- lidifies, anchoring the tightly bound graphite particles in place. Now the loosely held particles react in such a way that a salt is formed. The loosely adherent materials that have formed as a salt are easily removed with a water rinse with properly controlled solution flow. The remain- ing graphite is tightly adherent to the resin and glass and the fixer actually helps to thin out the Figure 2: core graphite colloid. Figure 3: Positive charges due to the action of the polyelectrolyte in the cleaner/conditioner.