Achieve uniform thickness in electroplating

A. Hi Nikhil. We appended your inquiry to a thread which talks about the many different approaches to resolving this problem. But the easiest and most obvious way to improve your situation sounds like removing the anodes at the ends of the tank.

Regards,

A. Hi Rajkamal. Per Faraday's Law, the thickness at any spot is directly proportional to the current flow to that part (this is a slight exaggeration as there are some more obscure relationships between thickness and other partameters, but it's almost surely the problem here). To get even plating thickness to different spots you'll need to insure even current flow to the different spots. One fairly common problem is anode rods being too long, so you get a lot of current flow to the bottom of the part. Good luck and feel free to send sketches.

Regards,

A. Hi Pintu. Your posting is a bit too short to be understood. Are you saying that your present nickel plating thickness is 5-6 microns and you want to reduce the thickness to about 1-2 microns, or are you saying that the thickness varies over the part by 5-6 microns and you would like to reduce the variation to 1-2 microns?

Reducing the thickness to 1-2 microns would be very easy; simply cut the plating time or the plating current proportionally. But there are very few applications where 1-2 microns of nickel plating would be satisfactory. Perhaps for certain diffusion coatings or strikes, I suppose, but not as an actual plating layer. 5 microns is required for even "mild" environmental conditions (indoor applications). What are the parts? What is the application?

Reducing the variation in plating thickness is difficult. We would need a lot of info about the substrate material and geometry, and the target plating thickness. Good luck!

Regards,

A. Hello Jeremy,
I would first check the calibration on your rectifier. Next, contact points between anode and cathode buss bars, and last for any polarization in your anode material supply. Although it is not uncommon to have a fluctuation in thickness from high to low current density areas, or an area where metal must travel further than the shortest path to the work, 50% less seems a bit high for the part in your image. If once you have insured all currents and anodes are preforming correctly you are still left with an an unacceptable difference in thickness, it's time to consider auxiliary anodes. An auxiliary anode is a supplementary anode that alters the current distribution in electroplating to give a more uniform plating thickness. A small piece of anode material placed in close to the area you are having trouble with connected to the anode current supply.
If it is still not as uniform as you would like you could move over to electroless Ni. Electroless Ni solutions are touted for their uniformity in thickness compared to any electroplating solution. Good luck.

A. Hello Jeremy, Chance had some good suggestions and I just wanted to add a few more. I experienced a similar problem a few years ago and was given the task to reduce thickness variation in our Ni plating tank. First I had to bring the anode to cathode ratio down to 2:1. We were at about 5:1. Then I lowered the current density to 20 ASF, we were at 28. Higher current densities will favor the high CD areas on the part. Lower CD's will help you deposit more metal in the LCD areas. You will have to increase dwell time accordingly to meet your thickness specs. Hope this helps.

A. Hi Kaushik. Sorry but I do not understand the main question regarding you getting unwanted plating in holes. Unwanted plating is usually stopped by masking; but since I am misunderstanding you, a picture with labels showing the problem would probably help.

My career was mainly as a plating equipment engineer, designing, starting up, and troubleshooting plating lines. So I know very little about the deeper academic end of electrochemistry, but feel that most questions of anode-to-cathode spacing revolve around the simple practicalities: You don't want the anode and cathode to ever accidentally touch, and you don't want them so close that the plating on the close spots burns; but you want them reasonably close because solution resistance is terribly costly. Greater spacing requires higher voltage rectifiers, consumes more electrical power, requires more cooling, needs bigger tanks with greater capital cost, requires more fume ventilation and more make-up air, etc. This is, to me, the 'desired standard distance'. Again, for a deeper understanding you'll need an answer from someone with different background & experience.

But I do not agree that close spacing "creates roughness". Roughness, as most people use the term, implies the deposition of particulates ... whereas if the anodes are well bagged and you have efficient filtration, roughness should not be a problem. Too much current causes burning, which is rough, and this may be what you are referring to. I think you might look into the 'shields' discussed on this page as a way to minimize that burning, and to discourage plating in the areas where you don't want it. Best of luck.

Regards,

A. Hi,

Send us a photo of the part.

To have good leveling a longer distance between anode and cathode is important; if it too close the current will go a shorter way. If there are holes in the part you should use inductive/auxiliary anodes.

Regards

A. Hi Basant
Please do us a favor and re-post your actual situation? Who you are, what you do, why this is important to you, what type of plating you do, what unevenness you experience, etc.

Changing from acid to alkaline might help, brighteners and levelers might help, shields might help, thieves might help, racking differently might help, rotation might help, every plating variable from temperature to pH to concentration to current density has some effect on distribution. Thanks for your understanding. Regards,

A. Hi again, Basant. We appended your inquiry to a thread on the subject which you might wish to study. Shields & thieves would not be applicable to barrel plating, but everything else I mentioned would be.

I would suggest that you start tabulating which plating shops are doing a satisfactory job in controlling thickness and steer your work towards them. The idea that hundreds of plating shops don't understand the importance of uniform thickness and the ways to achieve it, but someone will be able to tell you in an internet posting what to tell them to fix it, just isn't realistic. Send your work to the better platers. Good luck.

Regards,

A. Hi Tirath. First impressions:

1. You need a filter and anode bags rather than relying on the particulates to settle.
2. The deposition on the anode rods and tank rim is simply the drying out of splashes & fumes, and is not unusual nor the result of electricity.
3. The black jig needs to be stripped and re-coated with plastisol because the treeing is excessive.
4. The anode rods look too small to carry the current. Do they get quite hot? What is their ID/OD and how much plating current is used per tank or per rod?

Regards,

A. Hi again. What I was getting at was that, from a quick look at a picture, your anode rods look quite small to properly carry the current -- which would result in the poor plating distribution problems you describe. Copper can only conduct 1000 Amps/inch² (1-1/2 Amps/mm²). That is why I was asking whether they were hot (indicating overloading), and how much current they must carry, and their OD and ID dimensions -- in your last picture they look to me to be hollow bars, but I can't see things well enough to figure out what that "W shaped" support or connection is. Maybe it returns power from the outside anodes to the more heavily loaded middle anode?

Accumulated stuff on the anode rods does not affect their ability to carry electricity through them, but it does affect the contact resistance where the anode hooks make contact with the anode rod. All contact point of all electrical circuits must, of course, always be free of any tarnish or trash. Contact surfaces like your anode hooks really ought to be "knife-edge" rather than long flats.

But what is puzzling me about your questions is that this is obviously an old and well-established plating line. Did it never produce satisfactory work in all its history? Or is this a new product line which you are trying to plate? Or have specifications tightened up on the product? Or did it once work fine but now inadequate maintenance and/or loss of experienced operators is now causing problems? In other words, what has changed that makes an old established plating line no longer satisfactory? Step one in the trouble-shooting exercise is usually to ask what has changed :-)

Regards,

A. Hi again Tirath. Doubling the main conductor from the rectifier was a good first step and, although 2 A / sq mm is still more than the 1.5 A / sq mm that we'd like to see, at least the overload is not outrageous :-)

Each load bar carries 700 Amp, so it should ideally be not less than 450 sq mm if bussed from one ends or 225 sq mm if bussed from both ends; it's actually 300 sq mm.

But the anode rods, at 50 ^²⁰⁰ sq mm, are overloaded (as we can see from the melted polypropylene). The center anode in each tank must carry 700 Amps whereas the rod would be appropriate for 75 ^³⁰⁰ Amps if bussed from one end. When tanks are bussed from only one end, the cathodes should be powered from one end of the tank and the anode rods from the other to help distribute the power.

Please retain the services of an electrical engineer or plating equipment engineer to review the whole system and guide in an upgrade. You may have other problems with this line as well, but you'll never get uniform plating thickness when the DC bussing is so terribly overloaded. Good luck.

Regards,

A. Hi Tirath. Myself and other readers are always happy to help with questions, but that help cannot extend to trying to remotely supervise the redesign of a plating line -- that's a job for an on-site paid plating equipment consultant, like one of those whose support is helping make this site possible :-)

A. It is possible to run copper bus bar from your rectifier to the anode rods and cathode saddles at either end of your plating tank or to both ends. If that bus bar connects on both ends of the plating tank, then the anode rods and work bar only need to carry half of the current they have to carry if bussed from only one end.

B. I ducked that question because as long as you have grossly inadequate bussing and anode rods totally wrecking the current distribution, trying to account for more localized unevenness in current distribution is a fool's errand. But your plastisol coated jigs need to be adequate size for the current they are carrying (they may be already); and the coating must be good as I already noted ... if the current is able to grow "trees" as on your black jig, it's stealing the current in a way which must be stopped rather than mathematically analyzed. After all that is done, you can look at the relationship between anodes and workpieces to see if some workpieces are too close or too far from the anodic current.

(anode bag pic courtesy of The Bangalore Paper & Pulp Mills)

C. Sorry, I don't know what you have sent a picture of, but anode bags are made of polypropylene cloth and look rather like a sock you put over the anodes and tie in place.

Regards,

A. Hi Tirath. I'm not going to engineer a plating line for you in this forum; you need to retain a plating equipment consultant for the redesign :-)

But I think you're dreaming in hoping to copper plate at 150 ASF in a plating line of this sort. Please focus on the things you need to do to get this line running right rather than what you would need to do to get it plating at over twice the speed at which you're plating wrong :-(

Luck & Regards

A. Hi again Tirath. Apologies, my calculation of anode rod area was wrong. I made the error of dividing by 4 twice instead of once in the formula OD²/4 - π ID²/4 which I didn't catch right away because I unfortunately don't visualize in metric.

When I started designing plating equipment 50 years ago as a fresh engineer, my employer and their competitors had already been using "1000 Amps/sq. in" for decades. Thousands upon thousands of plating installations have used it, and it results in no power distribution problems but no significant wastage (the bussing will get slightly warm but not hot). And it tracks electrical codes pretty well too.

As for what it means, it means the cross sectional area of the copper carrying the current should not be less than that. Any bus bar carrying 2800 Amps should be 2.8 sq in. in cross section or greater; any bussing carrying 1400 Amps should be 1.4 sq. in. or greater; any bussing carrying 700 Amps should be 0.7 sq. in or greater; any bussing carrying 350 Amps should be 0.35 sq. in. or greater.

Your work bars are carrying 700 Amps each if I understood correctly. Your outside anode rods (the ones next to the tank walls are carrying 350 Amps each, and your center anode rods are carrying 700 Amps each.

Bussing from one end is fine as long as the copper is big enough, and the cathode saddles are kept clean, and the anodes are bussed from the opposite end of the tank than the cathode saddles are bussed from. Bussing anodes from one end and cathodes from the other is simple and cost effective, and means that the voltage from anode to cathode remains exactly the same along the length of the bars instead of being slightly higher at one end due to the resistance loss in the copper.

In terms of practical solutions rather than perfection, I would probably move the anode connections to the far end of the tanks, and either replace the center anode with a solid 25 mm dia. bar or squeeze in a second 20 mm OD x 12 mm ID anode rod.

Best of luck.

Regards,

A. Hi Tirath
Before you get involved in technical solutions to your distribution problem there is a much more pressing matter.
From your photographs your connections to both anode and cathode are in terrible condition. It seems unlikely to me that the current is going where you may think it is and certainly not evenly distributed.
All bars and every contact point should be perfectly clean and show shiny copper. They look as if they have not been cleaned for years ! Operators must be trained to clean contacts as standard practice.
I can see that you have air agitation but it is so strong that it is adding to your problem, I would turn it down considerably.

Anode bags will not help distribution but will avoid much of the sludge. A good filter pump should also be a standard item.

A. Hi Abhishek. The abstract question of plating thickness uniformity has been addressed exhaustively on these 2 long pages over a period of 24 years now, so I don't think there's much point in simply starting over and repeating all of those abstract ideas again, from addition agents to temperature and current density control, to shields, to shaped or auxiliary anodes, to spinning the part during processing, etc.

Rather, I think we need to delve into the very exacting situation confronting you ...
Are you plating one single 6" x 6" piece at a time, or are you talking about a rack full of such parts? Can you provide a photo of what you are doing, and tell us the temperature, solution concentration and current density? Can you give us some hints about the thickness problems you are actually experiencing ... is the plating thicker on the top, on the bottom, on all 4 edges, in the middle, or totally random? Are you saying that your current thickness variation is 15%, or are you saying that is your target? If that's your target, what are you currently getting? Thanks!

Luck & Regards,


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