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topic 0203

Plating by solution voltage rather than current density

A discussion started in 1996.
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Electroplating is universally performed on the basis of an optimum range of current density. Some folks in our research group are proposing that plating be done on the basis of the solution potential relative to the plated part (using a reference electrode). Is there any merit to this idea and if there is why isn't it done in practice? I have some thoughts but I would like to know what you think.

Best regards and thanks for your time,

William L. Bishop
University of Virginia EE Dept.

Two pragmatic factors favor plating by current density:

1). Faraday's Law tells the plater how much metal has been deposited if s/he knows the current and can accurately estimate the efficiency.

2). Brightener consumption is in proportion to current.

You are proposing measuring the voltage of the half-cell from the solution to the part, if I understand correctly? Note that there isn't really an exact half-cell though: I think you'll find that the majority of the voltage drop is across the solution rather than at either electrode, so your numbers will vary greatly based on where you put the reference electrode. This is probably another pragmatic reason people usually measure the voltage from electrode to electrode.

If I misunderstood the question, sorry and please rephrase it.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


In addition to those things mentioned by Ted Mooney I would include the following negative comments:

1. If you plotted current density vs Voltage, I think you would find a steep curve, that is small changes in voltage yield large changes in CD, making for difficult control.

2. Changes in plating cell resistance, such as solution temperature, chemical composition, agitation will also affect voltage needed for a specific CD.

On the positive side (no pun intended) you might dig into information on how oxidation / reduction potential (ORP) is used to control addition of reducing agent (usually formaldehyde) in electroless (autocatalytic) copper plating.

Paul Stransky
- Putnam, Connecticut

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