Copper pyrophosphate plating bath Q&A's

A. Dear Adam

For a critical job it was necessary to use copper pyrophosphate and no supplier offered that in India. So I formulated the bath from off shelf chemicals, and converting ortho phosphate to pyro phosphate. If you want to go to those root levels, I can dig up my notes.

A. 74% is actually pretty good for Pyro.

"Current efficiency" includes two terms. Anode efficiency and Cathode efficiency. Instead of Hydrogen at the cathode you may see Oxygen at the anode.

For good operation you should see neither.

In many solutions (Electroless Copper comes to mind) gases like Hydrogen can bump around in solution for a while before bubbles begin to form.

A. We, too, have recently checked the cathode efficiency of a pyrophosphate copper bath, and found it to be close to 80%. We are also noticing an increase in the specific gravity of the solution, that may indicate that the copper concentration in it is rising due to the difference between the anode (presumably 100%) and cathode efficiencies.

A. Hi Waldermar,

To lower the ortophosphite content is only one way to do it that's is to discarded to reduce its concentration in the solution. If you have a high contamination of zinc that will show by producing brassy looking deposit and should be dummied out at medium current density.

Regards,

A. We don't use the term polyphosphoric. If it means pyrophosphoric, is O.K.

A. Copper pyrophosphate and proprietary baths are probably the only cyanide-free approaches.

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A. Pyrophosphate copper :
copper pyrophosphate.......... 110 gm
potassium pyrophosphate...... 400 gm
citric acid ⇦ on eBay or Amazon [affil links] .................. 10 gm
water........................1 lit
pH 8,5 -temp.50 C, 5 A/dm2

Tartaric copper (19.century):
add copper carbonate ⇦ on eBay or Amazon [affil links] to solution of potassium tartrate( 100 gm/lit.water), until some carbonate remained undissolved, then add potassium carbonate to pH 10

A. A variety of alkaline non-cyanide copper baths are available from suppliers. Each manufacturer has baths with different properties in plating or waste treatment. Pyrophosphate baths are outdated technology unless you are trying for a "home brew".

A. You can use ceramic or enamel coating [as heat treatment maskant].

A. Hi Diana,

pH increase could be due to hydrolysis of water at cathode instead of copper ions reduce to copper metal:

2e-
2H2O ------> H2 (hydrogen gas) + 2OH-

You may verify cathode efficiency by:

1. Observe if significant gassing around cathode;
2. Observe if bath solution copper concentration is climbing up (anode dissolution more than cathode plate out);
3. Perform weight gain test to compare actual copper deposit weight vs. theoretical weight under Faradays Law.

Other than that, drag in of alkaline species may be a reason as well.

Regards,
David

A. Hi,

In the early days of Copper Pyrophosphate plating, we used and recommended "Tetraphosphoric acid". It was made by mixing water and phosphorus pentoxide to give a polyphosphoric containing 4 P atoms - H6 P4 O13. In reality it contained a mixture of polyphosphates with an average P of 4. It worked very well and rapidly became pyrophosphate (P2) in the plating bath.
We soon abandoned it in favour of of the cheaper and readily available sulfuric acid. In moderate quantities sulphate has no adverse effects. Ultimately it begins to lower the maximum current density as it adds to viscosity of the bath.

Your rate of pH rise is too high and the question you should have asked is how to prevent it. pH rises in pyro baths because of excessive anode area. There are several drawbacks, the need to add acid being one of them. Others are:
1) lowering of the optimum P207:Cu ratio
2) If you are using brightener, it will precipitate on the sludge which forms on the anodes at LCD.

Controlling anode area is the is key to running a copper pyro bath. The pH should slowly rise, say 0.2 - 0.3 units per week. There are also disadvantages with falling pH.

My advice is to get the anode area down to meet the above condition. When the anode current density is correct, the anodes should have a clean, pink or slightly dusty appearance. Anode CD too low gives a brown sludge and too high, a bright electropolished appearance.

Well, there's some advice - whether you want it or not!

Harry

? Is the volume of your acid solution staying constant?

A. I run a Copper Pyrophosphate Plating Bath. It's small, about 150 gallons. My additives (with each load) is a brightener and ammonium hydroxide. Both of these items have a higher pH than my bath. So with each load, my bath pH increases. However my DI water is a bit of a lower pH to my bath, and so this tends to even out. Once a week I usually add a solution of 10% Sulfuric Acid to 90% DI Water. This solution is generally (for my bath) made up in a 3L bottle and poured into our sump tank. This generally drops the pH from 8.9 (which is the highest I will allow it to go), to 8.6. I sometimes have to add 2 x 3L bottles, depending on my concentration amounts.

Hope this helps.

A. TO LOWER pH USE POLYPHOSPHORIC ACID (THE TERM POLY PHOSPHORIC ACID REF. TO CONC. GRADES OF PHOSPHORIC ACID ABOVE 95%). TO RAISE pH USE 10% POTASSIUM HYDROXIDE.

A. Hi Ben.

We appended your inquiry to this thread because Harry Parkes' response of November 10, 2011 seems to directly answer your question. Good luck.

Regards,

A. I would suggest taking a look at your part while it is being plated if possible. Find out at what step of the process these dots are occurring and if they are occurring consistently in the same area at the same part of the process.

From what you said, it appears that your part goes in not cleaned fully as the dots appear at the beginning and are covered up by your brightener? (If I understand you correctly).

Items to check would be:

1. Are there dots before the part goes in? Can you check with a scope before putting it in the bath?

2. If they are not there, look at the part as it goes through the process and see at which point the dots start appearing and narrow that problem down

3. Are you using a consistent substrate? If you were not getting dots before, but are getting dots now, did you change the type of substrate you were using?

4. It looks like you could have a chemical imbalance, possibly too much of one thing, too little of another? Can you have it analyzed by a lab?

5. Are you putting in too much additive (PT61H or Ammonia) or too little of it?

That's my two cents on the subject.

A. I don't think that there is a practical method to remove iron from a copper pyrophosphate bath. In many years of involvement with copper pyro I can't recall a problem cause by iron. Except that it is important to allow sufficient potassium pyrophosphate to complex the iron as well as the copper. Think in terms of total metal: pyrophosphate rather than just copper.

May I ask what problems you have?

A. Sounds like the copper is getting passive and you need to activate the surface. You should look at a mild copper etch after the copper and before the next plating solution.

• Topic #353/61 "Non-cyanide copper strike/plating process problems & solutions"