Silver plating thickness distribution problems

A. 1.Plate copper as a barrier layer (undercoat)for silver plating on brass.

2. If the electrical contact is a fixed contact, 3 micron Silver is sufficient.

3. If it is a make & break contact, it depends on the current flowing and the freq of make & break. In general practice, for low current it is 10 microns & for high current it would be 25 microns or so.

Q. Do you have any references for these values? I mean any standard that explains the minimum thickness of silver coat based on current value?

A. Hi, Sandesh. According to Faraday's Law, the thickness of the silver will be proportional to the primary current distribution. And it is possible to use conforming anodes to quite precisely control the primary current distribution. So, yes -- while this would take some development effort on the anode design and fixturing, and the results might not be exactly what you wish the first time -- it sounds doable.

Regards,

A. If you can't work it out with the anode, introduce a cathode stealer/thief made up of polypropylene which will prevent excess deposition in the area

A. Hi, Hari. Your thickness distribution doesn't seem really bad, but perhaps it can be improved. The plating thickness follows the primary current distribution according to Faraday's Law. If you can get the primary current distribution more even, via improved racking or the use of non-conductive plastic shields, you can improve the thickness consistency. Good luck.

Regards,

A. Try fixing say 2" x 2" SS sheet at the bottom of the jig which will get deposition and at the same time your bottom components will have 6 microns.You can strip silver by flaking from SS sheets with out loosing costly silver.
Regards

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A. Hi Ankush. Page 812 was the correct link for the edition of the Metal Finishing Guidebook that was on the web earlier; it's page 866 in the 2012-2013 Edition, and it tells us that it takes 6.2 amp-hours to deposit a silver coating of 0.001" on a square foot of surface (at 100% efficiency, and silver plating is close to 100% efficient). Aside from converting units, the only missing factor is how fast you can plate (what current density you can use) and this number is empirical rather than theory-based. 5-10 amps per square foot might be a good estimate for rack plating.

You apparently have looked at our Intro to Faraday's Law but perhaps you haven't really had a chance to work hard to fully understand it yet :-)

But please give us all the details of your own situation. If cast your questions in the abstract all we can do is refer you to books instead of helping solve your actual problem because we can't possibly explain the full process and philosophy of designing silver plating installations in the general case in a brief forum response; the answers will depend on your plating volume; whether you will be rack, barrel, or reel-to-reel plating, etc. Please tell us what kind of parts you want to silver plate, how many, how thick, and why -- all the data you have -- then we can almost surely help with how long the plating time will be, how big the plating tank should be, and what you first steps should be. Good luck.

Regards,

A. Hi Ankush
Tables and calculations will allow you to estimate the weight of material deposited and therefore the average thickness.
However in a practical plating application the metal will rarely deposit equally all over the part. This can be quite useful in, for example, achieving a thick deposit where it is needed while saving metal in less critical areas.
It is usual to estimate the thickness then plate some parts and measure the actual thickness at the critical point and use this to adjust the time accurately.

A. Hi Shreyas. The generalities of this issue have been discussed dozens, maybe hundreds of times over the years on this site. The truism is that the plating thickness is directly proportional to the current flowing to that area per Faraday's Law; and the current essentially follows the path of least resistance, following Ohm's Law.

If you get that thinner coated side closer to the anodes and the thicker coated side further away, the situation will reverse. But true, consistent uniformity, is difficult -- and may even be impractical in some real-world situations. The results you are achieving probably aren't especially bad :-)

Luck & Regards,

A. Hello Sheyras,
Just wanted to add a few more possibilities for your problem. Besides cathode to anode distance you can also check your anode bar cable connections. They should be tight and clean. You will also want to check your anode surface area to ensure that they are equal on both sides. If the anodes are bagged, make sure they are clean and drain well. If you have cathode rocker agitation, measure the distance of the part when it is closest to the anode on both sides. The measurement should be of an equal distance.
I worked for a shop where we had to have a separate rectifier for each side of the part but the surface area on each side of the item to be plated was different. Good luck with your troubleshooting.