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PCB-Jan2017

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34 The PCB Magazine • January 2017 by Michael Carano RBP CHEMICAL TECHNOLOGY Introduction Electroplating a printed circuit board is by no means a trivial task. Higher layer counts, smaller-diameter vias (through-hole and blind) as well as higher-performance material sets contribute to the greater degree of difficulty with today's technology. So, process engineers pay close attention to the "softer" issues such as cathode current density, solution chemistry (copper sulfate and sulfuric acid concentration) and addition agent control (well, sometime!). The concern here is that acid copper pat- tern plating of a printed circuit board has many more critical aspects that must be diligently controlled for optimum performance. These include solution agitation and filtration, anode length and placement, current distribution ef- fects, reducing electrical resistance in the plat- ing cell, photoresist development and con- trolled organic contamination in the plating solutions. These are the subjects of this article. Solution Agitation For uniform surface plating distribution, a homogeneous mixing of the electrolyte is nec- essary to avoid overplating the surface while the through-hole or blind via is being plated from the bottom up. The engineer should adjust flow rates so as to not create an excessively turbulent solution movement. Solution agitation of the copper plating electrolyte maybe accomplished with air agita- tion, eductors, solution impingement and/or cathode bar movement. The main purposes of agitation have been stated many times and in- clude: • Elimination of solution stagnation and dispersal of reaction products • Increase of deposition rates by mass transfer enhancement • Dissipation of heat at electrode/solution interfaces While the use of air agitation (supplied by a blower) was the standard method for agitating FEATURE

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