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Why Filter Your Plating Solution?

August - 2008
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By: Jack H. Berg
President of Serfilco

In an article I wrote awhile back for the P.F. Directory based on ‘tell it like it is' opinions, I feel that I may have missed a most important fact in regards to organic removal and filtration. Other authors suggest 1½ oz. of filter aid plus up to 3 oz. of powdered carbon on their disc filters for continuous organic removal and filtration. That's up to 3 - 4.5 oz total solids to remove what solids or organics are present in the plating tank, or 300 to 450 oz. per 100 sq. ft. in a surface filter. That is over 18# to 27#, which goes to waste treatment every time the filter is serviced, sometimes once or twice a week, sometimes less if cleaning is less frequent, plus solution loss.

The fact, which may be overlooked, is that filters remove insoluble solids - carbon absorbs organics which are in solution. The powdered carbon, which is pre-coated onto a filter surface, is not a filter aid. The diatomaceous earth is the filter aid and any powder, such as carbon, is a solid which is to a filter, a solid which lowers solids holding capacity; hence frequent servicing. Not just because the flow rate has been lowered, but because the carbon, acting like a filter, plugs up and fails to adsorb the organic impurities.

During many of my talks around the world, I've opened with a remark indicating that you no longer have to filter your plating solution. The reaction I see is "Why does this speaker tell us we no longer have to filter our plating solutions?" It is simple - No dirt (solids) in, no dirt out.
Another time, a special part to be plated had to be done in a solids free solution. So an instrument which monitors solids was purchased to signal when the part could enter the tank. Of course, the instrument didn't assure the plater that during the time of deposition the solution stayed free of solids from the air (or anodes, or migrated filter media). I feel that the money spent on the instrument should have been used on a filter and particle retention and flow rates to assure the solution was free of solids.

Many years ago, I authored an article which Products Finishing editor emeritus, Ezra Blount, typed as I dictated, entitled The Case for Coarse Filtration at High Turnover Rates (PF 1964). It was based on the Hydraulic Handbook, which states that at 14 turnovers per hour, all of the solution would have passed over the filter at least once. It didn't recognize dead areas in a tank of plating solution.

It also depended on no additional solids from entering the plating tank while deposition was taking place, (which condition would occur when plating sulfate nickel for record masters in what looked like a simple waffle iron.) This is an example of being able to prevent solids from entering the chamber if filtration was done at 100% of flow rate, and the filter media was chosen to be non-migrating and at a level of particle retention to match the desired result.

This brings us to explain the difference between nominal filter media, which stops certain size solids according to its density which could offer 85 - 95% assurance that no particles larger than the rated media would retain, and absolute rating indicating a 100% of certain size particles were stopped. The use of any filter media does not assure that the presence of different size particles aren't in the plating solution, but when media and flow rate (turnover) are put together, than a predictable degree of clarity can be accomplished.

The plating of memory drums for electronic applications sometimes employs 40 - 50 turnovers per hour at .1 micron absolute particle retention from non-migrating membrane media. Compare the above to 1-2 turnovers per hour at even 5 micron retention from a media which migrates, and you see why your plating may sometimes have rough areas of deposit.

Frankly, I have always favored some type of cartridge which, after a first flush, is no longer allowing fibers to migrate. I also favor cartridges because they are available for use on slimy alkaline solutions with coarse windings. (Platers of alkaline zinc used them because their cleaning was inadequate and the coarse filter held more solids - sometimes 6-8 wks. between services.) Other solutions which crystallize, such as acid zinc or nickel, would usually use 5 to 15 micron, but even certain nickel applications may find that a 50 micron at high turnover rates will provide good results.

Now, what about ‘no filter on the plating tank' - why not? Because the filter media needs neutralizing of the plating solution, all steps should be taken to minimize the amount of media and solids to be disposed of. Previous articles have emphasized the choice to "Work Backwards." This reduces the solids, which may reach the plating solution. Therefore, the final rinse before plating is a good opportunity to stop particulates, which from surface tension have traveled from the cleaner to the etch and finally arriving at the plating station.

Attention to prevention could eliminate 80-90% of such solids, and when a separate chamber containing granular carbon is used to recirculate on the etchant and pre-rinse, another problem is avoided. This leaves the organic breakdown products as the only other need for carbon on the plating solution. This in turn can be reduced by switching from air for agitation to pumps with eductors to create the necessary movement of the ions for faster plating without causing breakdown of the brightener chemical.

One recent report indicated that a plater could plate up to 33% faster at higher amps without burning. It made it possible for his company to bring in more parts from his customer without the need to increase his need for another automatic machine. A further benefit of controlling the amount of solids in the plating rank and the level of organic impurities is that captured plating solutions dripping from parts in a 'dry sump' may be added back to the plating tank without the fear that you are building up the level of organic impurities.

With the use of filters, coalescers, and carbon on solutions from the soak to the plating tank, the plater may expect to have eliminated up to 90% of solids from entering the plating solution. This could make it possible for his plating filter to last ten times longer between servicing. Did we completely eliminate the filter on the plating solution? No, but with the steps of prevention in place, we have greatly reduced the need.

You now have a system of filtration and separate purification in place which will pay dividends from less solution contamination, less solids to hazardous waste, less labor for servicing and, as we've said before, "unattended filtration" and purification, from day-to-day and month-to-month. Talk to your chemical supplier; he knows the value of getting rid of solid or organic contamination, as it applies to service requirements. Talk to your customer as well; he knows the value of more uniform deposits with improved ductility. And finally, talk to your production staff; they know the value of less rejects.

Article reprinted, courtesy of:

Jack H. Berg
2900 MacArthur Dr.
Northbrook, IL 60062
Ph: 847-509-2900
Fax: 847-559-1995