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May 2015 • The PCB Magazine 13 OPTIMIzATION OF ACID COPPEr ELECTrODEPOSITION PrOCESSES continues FEaturE ions in solution. Of course, one cannot reduce the copper ions in such a way that the limiting current density will be quite low. This is where mass transfer becomes increasingly important. And mass transfer is dependent on diffusion- the movement of ions and additives through the plating electrolyte. Organic additives will also affect the grain structure, leveling and physical properties of the deposit. Strict control of the additive chem- istry requires instrumentation such as cyclic voltammetric stripping (CVS) and perhaps ion chromatography. Additional agent imbalances will reduce the ductility of the copper depos- it, thin plating at the knee of the via and poor leveling. Influence of Mechanical Aspects of the Process In addition to the influence of the organic additives and the inorganic chemistry, there are several other factors that influence plating distribution and throwing power. One major influence is solution movement or agitation. Agitation influences mass transport. As one will recognize, the organic addition agents af- fect plating distribution and throwing power, among other attributes. And mass transport of the plating solution is key to "pushing" these additives from the bulk plating solution to the cathode (circuit board to be plated) surface. One can look at agitation as solution mixing and ag- itation at the interface of the cathode and solu- tion boundary layer. Solution agitation of the copper plating electrolyte maybe accomplished with air agitation, eductors, solution impinge- ment or cathode bar movement. The main pur- poses of agitation have been stated many times and include: • Elimination of solution stagnation and dispersal of reaction products • Increase of deposition rates by mass transfer enhancement • Dissipation of heat at electrode/solution interfaces [1] Air agitation suffers from three main dis- advantages: It has a chemical oxidative action towards solution constituents; it is electrically resistive when present as a cloud or foam of bubbles; and, the general plating rate enhance- ment is modest despite several possible param- eters for adjustment. The least appreciated char- acteristic is the resistivity, which can lead to an increase of electroplating power of 25–30% and is therefore a significant electrical cost fac- tor. It also generates environmental pollution through dispersion of air bubbles. In addition, these tiny bubbles can lodge into the through- holes and blind vias, leading to a reduction in plating thickness or voids [1] . This micro-bubble void inducing condition is well-documented in the industry and can be seen in Figure 3. Conversely, solution movement supplied by eductor agitation is much more effective than air. When agitation is supplied via eductors, the solution movement is significantly more uniform across the panel surface. One can de- scribe this as 'laminar flow.' With air agitation, the solution is moving in a turbulent fashion and is often quite non-uniform. This turbulent flow creates areas within the plating cell con- sisting of dead spots, which appear as if there is no or very minimal agitation in that portion of the cell. This is a recipe for disaster. When this condition occurs, plating uniformity is not ideal and the cosmetics of the deposit as well as the physical properties suffer. Eductors, on the other hand, can be imple- mented at a very low cost with respect to retro- Figure 3: air bubble entrapment in via leading to plating void.