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Copper Concentration Analysis

The technique of analyzing the concentration of dissolved copper in an acid copper plating bath is identical to that for analyzing the copper in "peroxy-sulfuric" type etchants. The only real difference is the range of allowable concentrations and the methods used to reduce the copper level in an overdosed bath. In the following, the basic technique is presented, followed by specific discussions of plating and etching baths.

If you have not done so, please read the DOs and DON'Ts section now.

Equipment Required:
  • Titration flask (125 ml)
  • 10 ml Automatic pipettor
  • Digital titrator
Reagents Required:
  • Deionized water
  • pH 9.5 Buffer
  • PAN indicator
  • .800M EDTA tetrasodium salt (titrant)


Acid Copper
Plating Analysis





Peroxy-sulfuric
Etchant Analysis

Procedure:

  1. Measure 50 ml of heated deionized water into the titration flask. (about 40 degrees C will yield optimum results)
  2. Using a pipette, add .50 ml of sample etchant (5 charges from automatic pipettor).
  3. Add the pH 9.5 buffer one drop at a time until the solution turns a light blue or violet color (two or three drops max.). This point is rather subtle so watch for it closely.
  4. Add 2 drops of PAN indicator to yield a bright violet solution. Swirl flask to mix thoroughly.
  5. Using the digital titrator, titrate with the .800M EDTA tetrasodium salt to a green (or yellow-green) end point. Record the number of drops of titrant from the digital indicator.
  6. The concentration of dissolved copper present (as cupric ion in grams per liter (g/L)) can be directly calculated using (digital titrator reading for copper analysis is denoted NumCop):
Copper concentration (g/L) = 0.127 x NumCop

Acid Copper Plating Baths

The acceptable range of NumCop is:
Cu Optimum Range : 17 to 23 (grams/Liter)
(or 8 to 12 oz./gal. [60 to 90 g/L] of copper sulfate)
NumCop: 136 to 181

In normal operation (after the bath stabilizes through regular use), the copper concentration will never be outside the acceptable range.

Low copper level

Too low a level is usually an indication that:
  1. the surface area of the anode material is insufficient to source the amount of copper being removed from the bath during plating, or
  2. the bath has become contaminated and the anodes are not sourcing adequate copper.
In the first case, you should check with the vendor who supplied your plating bath to find out the optimum ratio between the anode and cathode (workpiece) surface areas. Unfortunately, determing the actual area of the anodes in your bath is not as straightforward as one might like. Surface roughness typically contributes the majority of the total area and is almost impossible to reliably estimate. Luckily, this is not much of a problem since, if you have determined that the low level of copper is not due to organic contamination, all you have to do is add more material (phosphorized copper) to your anode arrays. If the copper level continues to decline, add a bit more material until stability is achieved (or at least approximated) Using anode baskets as opposed to anode bars makes it much easier to tweak the anode area to the optimum value.

Contamination is usually evidenced by a thick, muddy gray coating on the anodes. This coating effectively insulates the copper surface from the electrolyte and significantly reduces the dissolution of copper into the plating bath. Ultimately, the bath becomes depleted of copper ions, the conductivity goes down, and plating ceases. If organic contamination is found to be the culprit, it is necessary to carbon treat the bath before proceeding.

High copper level

Of greater concern is the situation where the level of dissolved copper has gotten too high and begins to interfere with the proper operation of the bath. The usual causes of a bath becoming "overdosed" on copper are:
  1. a significant imbalance exists in the organic components of the electrolyte, and/or;
  2. too much copper anode surface area for the workpiece being plated.
Assuming that you do not to own a liquid chromatagraph or spectraphotometer, or a CVS (cyclic voltametric stripper), the most practical way to determine if the various organic components of the electrolyte are out of balance is through a technique known as "dilution Hull-cell analysis".

If the dilution Hull cell indicates that the organic additives are within operating parameters, it is likely that there is simply too much anode surface area contributing copper to the electrolyte. If the copper level was slowly drifting up, remove about 10 to 15% of the anode material and dummy plate for a couple of hours at the maximum current available from your power supply. Analyze the copper ion content. If the copper level has risen or stayed the same, remove 10% of the remaining anode material. Continue plating, testing,and removing anode material until the level of dissolved copper begins to fall.

Continue dummy plating until the level of dissolved copper in the bath is within operating limits. Return 10% of the anode material (the pieces you removed just before the copper level began to fall) and dummy plate at full power for 1 hour. If the copper concentration continues to fall, return a bit more of the anode stock to the bath. Continue in this fashion until stability is achieved. Carefully record the total amount of anode material remaining. This should represent the ideal charge of anode material for your setup, assuming that you are always plating the same size board. You might also want to calculate the effective area of the anode material by dividing the apparent geometric area by the area of the panel you were dummy plating. This ratio will come in handy if you routinely plate boards of varying sizes since it relates the "ideal charge" of anode stock to the unit area of the workpiece.

Peroxy-sulfuric Etchants

The presence of too much dissolved copper in the etching bath will lead to excessive etch times, accelerated peroxide consumption and a reduction of the edge quality of an etched circuit. This analysis will need to be performed after every 30 sq.ft. of copper clad is etched and should be done on a Friday or at any time that the etcher can be idle for one or two days. A marked increase in the consumption of hydrogen peroxide is a fairly reliable indicator that there is too much dissolved copper in the etchant. Careful attention to your log book will alert you when the need for this test is becoming critical.

The acceptable range of NumCop is:
Optimum Range:
(grams/Liter)
37 to 56
NumCop: 295 to 441

A reading in excess of 450 drops indicates that the bath has become supersaturated with copper. To prevent accelerated H2O2 consumption, the excess dissolved copper should be precipitated out of the bath.


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