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36 The PCB Magazine • January 2017 acid copper solutions for many years, as circuit board designs have evolved into higher density patterns and smaller diameter vias, air agita- tion has reached its limitations. Air suffers from three main disadvantages: It has a chemical oxi- dative action towards solution constituents; it is electrically resistive when present as a cloud or foam of bubbles; and the general plating rate enhancement is modest despite several possible parameters for adjustment. The least appreciat- ed characteristic is the resistivity which can lead to an increase in electroplating voltage power of 25−30% and is therefore a significant electri- cal cost factor. It also generates environmental pollution through its 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. One ideal solution (pun intended) to the need for solution agitation is to employ educ- tors and eliminate the use of air agitation com- pletely. Eductor agitation is based on the Venturi principle, whereby one volume is pumped and up to four volumes are drawn in by the pressure drop, making it a highly efficient jetting sys- tem. When the system is fully submerged, no air is entrained. Such eductors are marketed by several manufacturers. Eductor agitation over- comes several of the disadvantages associated with air agitation. Air bubbles and misting are eliminated. In addition, eductor agitation pro- vides a more uniform mixing of the plating so- lution. This minimizes potential dead spots in the cell where the air agitation is lacking. It is well known that educators provide more uni- form agitation, better known as laminar flow. In contrast, air agitation provides a turbulent flow, and may only promote mixing of the solution. For quality plating results, it is preferable to have interface agitation. That is, one interface agita- tion is directed more at the cathode diffusion layer. This helps to reduce the diffusion layer thickness thus permitting the efficient delivery of additives and ions to the cathode surface[1]. Figure 1 shows a schematic of a plating cell outfitted with eductors. This shows the educ- tors pointing towards the surface to be plated. However, there are additional options available with respect to educator designs. In a second case, the eductors are pointed up from the bot- tom of the plating cell (Figure 2). Both designs have extensive research and field experience that shows that with either de- sign, surface distribution of electrolytic copper plating is greatly improved. ACID COPPER PLATING—UNDERSTANDING WHAT'S OFTEN TAKEN FOR GRANTED Figure 1: Schematic of plating cell outfitted with eductors. (Source: Spray Systems Company's "A Guide to Optimizing in-Tank Agitation") Figure 2: Actual photo of plating cell outfitted with bottom-up eductors. (Source: RBP Chemical Technology)