Letter 29662

Purpose of Acid in Copper Plating  

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I am creating some numerical models for electrochemistry. I am hoping to use electroplating as a form of global validation. I have run a few preliminary tests trying to plate copper on copper using 0.1M Copper (II) Sulphate solution using distilled water at around 1V Potential difference across a 3mm gap. I have noticed a black deposit on the –ve electrode. I am suspecting this is Copper Oxide formed from the dissolved oxygen in the distilled water. On a number of website I have found that the addition of H2SO4 is used in the electrolyte, although I cannot find why this is the case. Again, I suspect that it is used to remove the dissolved oxygen. Is this the case, and if not, then why is it there? If I were to use a solution of de-oxygenated Copper sulphate, would I get the same results as including the Sulphuric acid? What I am really interested in is weather the sulphuric acid becomes involved with the surface electrochemical reaction?

If anyone has any comments or suggestions I would be very interested in your opinion. Also I would be very interested in any recommended links or resources in this matter. Cheers.

Andrew Garrard
Student - Sheffield, UK

The black smut is probably copper rather than copper oxide--very finely divided particles of copper and other metals appear black. The acid is necessary for two reasons. First, the solution needs to be conductive in order for plating to take place, and acids like sulfuric acid exhibit great conductivity because of the hydrogen ions they loose. Second, you cannot deposit metal out of solution unless it is in solution, and you can't keep a simple salt of copper in solution at high pH.

As a general rule when you electroplate, a portion of the applied electricity electrolizes water into H2^ and 2OH-, the hydrogen leaves the solution as a gas and the OH- continuously raises the pH of the solution, which must be counteracted by additions of acid.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

The sulfuric acid in a copper electrolyte is there for two reasons, one is to keep the copper in solution, you only used a 0,1 m solution, if you use higher concentrations your coppersulfate will partialy get Cu(OH)2 . Also the sulfuric acid gives you abetter conductivity and with this comes a better throwing power. Throwing power is the ability of the electrolyte to get uniform depositions in areas with different current densities. The more sulfuric acid you ad the better is your throwing power, for example a usual copper electrolyte contains about 60 g/l Sulfuric acid, a copper electrolyte for plating printed circuit, which have very small hols to plate in, contains about 220 g/l Sulphuric acid.

What you plated out of your solution is probably copper, to get a better deposition I would suggest you use a 1 m solution with 60 g/ l Sulphuric acid and also about 100mg/l Hydrochloric acid.

Marcus Hahn
- Lucerne, Switzerland

Thank you for your comments. I have continued to attempt to plate copper, but I am having little success. I tried adding the recommended amount of acid to my copper sulphate, but this resulted in a large number of bubbles of H2 being evolved. I would very much like to conduct plating without the evolution of gas, as this will add unnecessary complexity to my model. I know that it is possible to undergo an electrochemical reaction with aqueous solutions without hydrogen evolution. Is there any way of obtaining acceptable results from a simple system of copper anode, copper cathode and copper sulphate solution? If so I would very much like to know it, and if not could anyone recommend an alternative system that I could try?

Andrew Garrard
- Sheffield, UK

Unfortunately I don't understand what it is that you are trying to do when you speak of your model, Andrew. I don't know what exactly you mean by "numerical model" in this particular context, and although I understand the words "global validation", I don't know what you mean with respect to this plating demonstration. And when people don't understand the mission, their suggestions usually result in the wheel-spinning and crosstalk we've seen to date on this topic. Please spend another paragraph explaining exactly what you are trying to do, prove, or demonstrate--and why.

If you are trying to demonstrate Faraday's Law, I agree that hydrogen evolution upsets the demonstration. But copper does not deposit at 100 percent efficency, so there will be some loss to the evolution of hydrogen, although it is good to minimize it. It is possible that copper pyrophosphate may be a better electrolyte for your needs, but I still don't know what those needs are :-)

Electroplating in aqueous solution is not equivalent to a vacuum process where you might try to remove "everything else" so the reaction can proceed unhindered. Rather, ionization is part and parcel of electroplating and your electrolyte has to be capable of allowing the copper to go ionic and stay ionic. And this involves acids or complexors, not deionized water. Good luck.

Ted Mooney finishing.com Brick, New Jersey

Hello,

In an attempt to keep my requirements simple I have clearly made them unintelligible, and for this I apologies, so please permit me to tell you a little about what I am trying to achieve. As part of my studies, I am trying to create a comprehensive model for electrochemistry. A model is a mathematical prediction of various parameters, such as current density, potential distribution etc based on a set of equations and solved using a computer. I believe that I have developed such a model, primarily using the Butler Volmer equation, which I am sure that electrochemists are familiar with. In order to validate that model is performing correctly, that is to say that the output from the computed equations gives an accurate prediction of what happens in real life, I was hoping to use a simple electroplating system. I have set up a small test rig with a Power supply and naively thought that if I immersed two lumps of copper into copper sulphate that I would produce excellent electroplated results. Very much like the opening paragraph in your "FAQ: Electroplating--How It Works" section. I am neither an electrochemist nor an electroplater, so I have very little experience in this area and am deeply grateful for any help.

So my requirements are only for a simple system of electroplating that I can use to validate my model. My model makes the assumption that the system remains in single phase, that is to say that there is no mixture of liquid and gas, it is all liquid. Incorporating the equations for multiphase flow would add significantly more complexity than I can deal with at the moment. Within the chemical process industry there are numerous documented electrochemical reactions that do not result in hydrogen evolution, so I know that some must exist. The addition of acid produces significant amounts of gassing at relatively low voltage. In a paper by Uziel Landau (Case Western Reserve University) about through hole plating of PCB’s he says that acid is not necessary to copper plate. In addition I would like a system where the electrolyte is not chemically affected so that I can recycle it and the chemical properties are still known. I am at university, so I can get hold of almost all chemicals, but for convience, I would prefer ones that were less toxic or corrisive.

Thank you kindly for your time.

Andrew Garrard
- Sheffield, UK

Thanks, Andrew, I understand.

I think your principal problem then is an anode to cathode spacing that is far too close. Can you change this to 3 cm instead of 3 mm? If not, then reduce the voltage, although that may not work as well. Here's the deal--

The first principal of electrochemistry as far as electroplaters are concerned is Faraday's Law. This states that 96,500 coulombs (amp-seconds) will deposit a gram molecular weight of metal if operated at 100 percent efficiency. This is simply a conversion factor which accounts for how we count electrons vs. how we count atoms, and you'll understand it with just a few minutes of thought and realize that it is incontrovertible. That is, if the plating process operates at 100 percent efficiency (100 percent of the electrons go toward reducing metal ions in solution rather than into losses such as liberation of hydrogen) then a fixed amount of copper is deposited.

But think of this. Suppose the solution is conductive and a lot electricity is flowing, and there is no or almost no copper in solution, what happens to Faraday's Law then? The electricity is flowing but a proportional amount of copper cannot possibly deposit because it isn't even there. What has happened is that the efficiency has dropped to near zero, with most of the electricity liberating hydrogen.

The cause of your smut is probably efficiency lower than 100 percent (although tramp metal contaminants can also cause smut). Copper ions simply cannot be oxidized into solution, travel across the solution, convect themselves through the boundary layer, and be reduced to metallic copper at a rate sufficient to keep up with the electron flow you are imposing--so undesirable electrolytic side reactions are forced to occur by the surfeit of electron flow. Among these is the generation of hydrogen which finely divides the copper, making it into a smut. Increase the anode to cathode spacing by a factor of ten and the solution resistance will increase by a factor of ten, so the current flow will be cut by a factor of ten, and the electroplating process may be able to keep up with the electron flow. You can't rush the plating.

Ted Mooney finishing.com Brick, New Jersey

Thank you for all your help in this matter. I was emailed by someone viewing this post who provided some very interesting documentation in the matter. I have a follow up question regarding a different matter, if anyone could be so kind as to help me with it.

I have seen a number of posts asking about the density of copper sulphate, however, none of them point to the answer that I would like to see. This would be a data base, preferably online, that contained density and viscosity data for electrolytes. Is such a list available anywhere? Obviously I have used an internet search engine to find one, but with no luck. Thank you in advance for any help on this matter.

Andrew Garrard
- Sheffield, UK

Hi

I have just read with interest the chat between Ted Mooney and Andrew Garrard (Sheffield Uni) wrt the effect sulphuric acids has on the electrodeposition of copper onto a copper electrode.

I am growing thin (~100nm thick) copper films on a (gallium arsenide) semiconductor surface by electrolysis, using ~0.3M copper sulphate solution. I have found that upon adding sulphuric acid the films have been much more uniform (as you were suggesting in your previous discussion), and that on a microscopic level it also contributes to film smoothness.

My aim is to grow a uniform, but rough film. I have found that the addition of a chloride to the acidic solution, or the use of HCl, results in a macroscopically uniform film, which is microscopically rough, and the roughness, i.e. the structures that form are of interest to me.

Do you have any idea as to why the addition of chloride to the solution has this profound consequence? I have read many a journal which say 'it has been shown that cl- ions have this effect on the surface' but cannot find any description of the mechanism, or of why it has this effect.

Can you offer any insight?

Many thanks,

Chris Hill
- Bristol, England

I am not a theorist and really can't help much. The best book on such matters is probably Safranek's "Properties of Electroplated Metals and Alloys", but you might do a search for "Rolf Weil" who conducted a number of research projects on similar issues for the American Electroplaters and Surface Finishers Society.

Ted Mooney
finishing.com - Brick, NJ

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