Purpose of Acid in Copper Plating

A. Hi Guarav.
I don't know what "bad structure" means. Can you explain, in different words, precisely what your polishing department means by "bad structure"? If you can't explain precisely what they mean, you have a communication problem in-house which ought to be resolved first. Attempting to resolve a problem without agreeing on exactly what that problem is will prove hopeless :-)

I will guess that they mean the plating is "burned". Such a defect will occur first at high current density edges and corners. Is that what the polishing department reports?

Do you run Hull Cell panels? Is there a noted difference when they are run at 63 gm/l vs. 70 gm/l?

In my opinion until you understand exactly what the problem actually is, and/or have Hull Cell panels to demonstrate that high sulfuric acid is problematical, you should not be making adjustments to your operating range. Some shops operate at 70 gm/l, although it is high, and some operate at half that. Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

"The Properties of Electrodeposited Metals and Alloys"
by William Safranek

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A. Hi Andrew.

The black smut is probably copper rather than copper oxide -- very finely divided particles of copper or other metals tend to appear black because their shape is such that little light tends to be reflected from them.

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 simple salts of copper in solution at high pH.

As a general rule when you electroplate, a small portion of the applied electricity electrolyzes 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.

When it comes down to the very fine points of exactly what intermediate reactions are occurring in the boundary layer of the plating process, it is extremely complicated and much of our knowledge is empirical, so I hesitate to try to answer your question of whether the sulfuric acid "becomes involved with the surface electrochemical reaction". Safranek wrote a whole book on how the properties of the deposited metal vary according to the deposition parameters .

A. 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 copper sulphate will partially get Cu(OH)₂ . Also the sulfuric acid gives you a better 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 add, 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 holes to plate in, contains about 220 g/l sulfuric acid.

What you plated out of your solution is probably copper; to get a better deposition I would suggest you use a 1m solution of copper sulphate ⇦this on eBay or Amazon [affil links] with 60 g/l sulfuric acid and also about 100 mg/l Hydrochloric acid ⇦this on Amazon [affil link] .

A. 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 while copper can deposit at very close to 100% efficiency, it's probably not quite, so there will be some loss to the evolution of hydrogen, although it is good to get it close to zero. 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 and migrationare part and parcel of electroplating, and boundary layer phenomena are poorly understood, and your electrolyte has to be capable of allowing the copper to go ionic and stay ionic. And this may involve acids or complexors, rather than deionized water. Good luck.

A. Thanks, Andrew, I understand.

I think your principal problem then is an anode to cathode spacing that is far too close for that voltage. Can you change this to 3 cm instead of 3 mm? If not, then greatly 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 96,485 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 are not being oxidized into solution, traveling across the solution, convecting themselves through the boundary layer, and being reduced to metallic copper at a rate sufficient to keep up with the electron flow you are imposing -- so electrolytic side reactions are forced to occur by the surfeit of electron flow. Among these is the generation of hydrogen and extremely finely divided copper instead of proper crystal growth, 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.

Ed. note: It's a shame that someone e-mailed you rather than posting -- this isn't intended to be a private hook-up service which leaves the poster satisfied while cutting hundreds or thousands of readers adrift 8 postings into an issue  :-(

Try to imagine a Dear Abby or Ann Landers column where she advises the lovelorn that she'll tell them what to do about their posted quandary in private rather than in the newspaper   :-)

A. Andrew,

"This would be a data base, preferably online, that contained density and viscosity data for electrolytes. Is such a list available anywhere?"

www.chemspider.com might be the kind of thing you are looking for. There are many more chemical databases out there, but chemspider should be a good jumping off point.

Mike

"Principles of Electroplating and Electroforming"
by Blum & Hogaboom

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or eBay or

Amazon

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A. Hi Andrew,
The Principles of Electroplating and Electroforming, Blum & Hogaboom (1929 1932 & 1948) &8680; gives a table showing that copper sulphate and of sulfuric acid give the same effect on specific gravity according to the number of grams per litre in a solution.

The book may be in the university library.

I carry out "gasless" copper deposition on a regular basis.

A. I am not a theorist and really can't help much. The best book on such matters is probably Safranek's "The Properties of Electrodeposited Metals and Alloys" ⇦[this on eBay , Amazon, AbeBooks affil links] , 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 (NASF).

A. Hi, Becky.

The anode is the positively charged electrode. I note this because the opposite definition finds use in electronics, and things will get confusing if we are thinking of opposite poles. Nothing should deposit on it; rather, it should be dissolving. In that case, the black smut is usually the impurities in the copper, that did not dissolve along with it. (The cathode is the negatively charged workpiece upon which electrodeposition occurs).

Regards,

A. Hi Ray.

1. Sorry, I don't know the pH of typical copper sulphate plating baths, but it's probably somewhere around 2.0; my texts all specify the sulfuric acid ratio rather than pH.

2. Your theory that the pH will rise if O2 gas evolves and H2 gas does not, is absolutely correct--

Excessive hydrogen evolution, though, causes the pH to rise and excessive oxygen evolution causes it to drop (as you report). My question would be why is your anode gassing and your cathode not? Usually, anodes are larger than cathodes, so dissolution is favored over plating. Usually concentrations are fairly low so dissolution is favored over plating. Are you perhaps using a very small anode or an anode which can't dissolve, rather than a copper anode? Any chance your anode is coated or lacquered so it is not dissolving? It is not uncommon in plating for the anodes to polarize (stop dissolving).

Regards,

A. Hi. Platinum (or platinum clad titanium) anodes are commonly used as DSAs (dimensionally stable anodes). The platinum will not substantially dissolve, I guess it's too noble.

Regards,

A. Mr. ray, in a CuS04 electrolyte, acidity shall be increasing due to Cu ions deposition at the cathode and the S04 radical goes to Cu anode takes Cu ions to become CuS04 and again deposits Cu on to cathode. In this process Cu metallic ions are continuously removed leaving behind more SO4 radicals resulting in increase of acidic pH3 to O.1 if you do not replenish with CuS04. The best method is to replenish with a concentrated CuS04 by DRIP method to maintain equilibrium constant. Secondly add MnO2 in ppm level to avoid polarisation, i.e., bubbles sitting on cathode interrupting the deposition of copper.

A. Hello David, the role of H2SO4 in a acid copper bath has multiple advantages. It is a vital part of the electrolyte where throwing power is essential. It also keeps the copper sulphate soluble in solution. It also keeps the pH of the bath stable. If too much H2SO4 is in a bath it speeds up the deposition rate and plays havoc in metal distribution across the part. Think of voltage as resistance. Cathode to anode spacing, condition of buss bars, contacts and rack or barrel conditions. A low H2SO4 level in the bath increases voltage. Maybe these answers to your questions can be applied to your process. Best of luck.

A. Hi Kushal. I'm not sure which electrochemical law you are trying to use to calibrate your equipment, and in what way you are trying to calibrate it ... but copper will spontaneously plate onto steel, corrupting the usual Faraday's Law relationship. You will likely get an efficiency in excess of 100%.

Regards,

A. For a thin adherent copper layer deposit on steel I suggest using 1 gram of copper sulphate, 5 grams of sodium chloride, 10 grams of tartaric acid ⇦this on eBay or Amazon [affil links] dissolved in a small quantity of DI water.

A. Hi Mal. Is the roughness primarily on the top of horizontal surfaces, i.e., "shelf roughness"? If so, I would suspect torn anode bags and/or a poorly functioning filter. If the roughness is everywhere I would probably suspect loss of required addition agents. Try a Hull Cell test to see if the problem is definitely a solution imbalance.

We may be having a language difficulty, but there is no such thing as "throwing powder", so that is not the cause of the roughness. "Throwing power" refers to the capacity, ability, or power of the plating solution to achieve plating in low current density areas. A plating solution with high throwing power will tend towards offering an even plating thickness whereas a plating solution with poor throwing power will exhibit far greater plating thickness in the high current density areas like edges and corners. Good luck.

Regards,

A. Hi Mal
A common cause of roughness is solid particles in solution.
A simple test is to take a sample in a glass container and shine a narrow beam of light through it. Light scattered by particles is usually obvious. This is the Tyndall effect.
As Ted says, a punctured anode bag is a common cause but it could also be a badly fitting filter element or simply general shop debris that is not filtered properly.