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50 PCB007 MAGAZINE I MARCH 2022 Understanding Plating Cell Resistances While most engineers work diligently to control the chemical aspects of the electro- plating process to ensure optimum throw- ing power (surface-to-hole ratio), they oen overlook other critical parameters of the pro- cess. ese include resistance through the cabling leading from the power supply to the plating cell, plating rack current-carrying capacity, and additional resistances within the cell. As it stands, electroplating is governed by Ohm's Law: e greater the resistances within the plating cell, the more difficult it will be to achieve uniform plating thickness distribu- tion. I like this analogy: • Current flow (analogy) is likened to a flow of water through a hose • Flow of water: GPM = pressure divided by resistance • In other words, the longer the hose or the smaller the diameter of the hose opening, the less water you will get e same then can be said about current flow to the plating cell. Current is dependent on resistance and voltage. If resistances increase, the flow of current to the plating cell is reduced. us, based on this analogy, I prefer to design the cell with minimal distance of the cables from the power to supply to the plating cell. In addition, the current carrying capacity of the cables is critical to achieve uniform cur- rent flow and to minimize current loss. If the cabling from the power supply (rectifier) to the cell feels "hot" to the touch, this is an indi- cation that there is a loss of current reaching the cathode (circuit board). Figure 1 depicts the connections and solution resistances in the electroplating cell. Understanding where these resistances are and finding opportunities to mitigate them will help improve plating distribution and throwing power especially in high aspect ratio through-holes. e key takeaway from Figure 1 is that in addition to these various resistances in the cell, there is a voltage drop (IR drop) down the through-hole. e greater the resistance through the via, the less uniform the plating in the through-hole will be. Copper plating Figure 1: Schematic of an electroplating cell.