Electroplating of copper, Part 4: anode-cathode placement and secondary current distribution.
Anode surface area versus cathode surface area is another important consideration. If the anodes are much larger in area than the cathode, increased current flow around the edges of the panels (cathode) will be plated to a much greater thickness than other areas, as depicted in FIGURE 2. Pattern plating of PCBs represents a similar situation.
To minimize the primary current distribution, it is recommended that anode to cathode surface area ratios should not exceed 2:1 and anode length should be three to six inches shorter than the cathode. Rack design and cathode spacing on the rack will also influence the primary current distribution.
Fortunately, secondary current comes into play through a factor known as polarization. Polarization refers to the additional potential required above the equilibrium potential to drive the deposition of the metal to be plated. When a plating cell contains copper anodes and a copper cathode (circuit board) in a plating electrolyte, an equilibrium potential exists between the anode and cathode. Basically, the potential is determined by Ohm's Law--which is the solution resistance between the anodes and cathodes. Additional resistances arrive through the voltage required to corrode the anode and the resistance required to reduce the metal ions to metal at the cathode.
[FIGURE 1 OMITTED]
In order to improve throwing power and plating uniformity, one can increase the conductivity of the plating electrolyte and the polarization. Polarization and conductivity are both dependent on solution operating temperature, solution agitation and cathode current density.
The three types of polarization encountered in electroplating are gas polarization, concentration polarization and chemical polarization. Gas polarization results directly from oxygen and hydrogen being evolved during the plating process. By increasing solution agitation (solution movement), and more importantly solution movement uniformity, gas polarization is reduced. Chemical polarization results from a thick film forming on the anodes, which results in poor anode corrosion. Typically, one sees a thick sludge-like film form on the anode. A number of factors contribute to this issue: poor quality anodes, anode bags that are plugged with sludge that has fallen from the anodes and requires cleaning and operating parameters of the plating process that can lead to polarization of the anodes. These include too low an operating temperature, imbalance of key addition agents or a combination of too high a concentration of the metal salt in solution in combination with the acid level. Finally, concentration polarization results as the metal ions are depleted from solution close to the surface of the cathode. The concentration polarization can be altered by solution agitation.
[FIGURE 2 OMITTED]
Secondary current distribution effects are complicated. Changes in one variable may have a number of effects. One must control the anode polarization to insure that the anodes are uniformly polarized and a planar equi-potential surface is radiated from the anode. Non-uniform polarization will cause the anodes to sludge causing thickness variations from the top of the plated panel to the bottom. Solution agitation at the anodes will minimize both concentration and gas polarization. There are no guarantees that uniform anode polarization will ensure uniform plating distribution. The main consideration is plating distribution across the surface of a printed wiring board panel, and from the surface of the panel through the holes, it will vary due to resistances. By mitigating the effects of these resistances, plating distribution is improved.
MICHAEL CARANO is vice president for OM Group, Inc. and can be reached at email@example.com.
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|Title Annotation:||POSITIVE PLATING|
|Publication:||Printed Circuit Design & Fab|
|Date:||Nov 1, 2008|
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