Hull Cell Analysis of Acid Copper Plating Baths

The Hull Cell

The brightener/leveler/carrier formulation used in most acid copper plating systems is basically a two part organic additive system that functions to smooth out and brighten the copper plated onto the surface and in the holes of a PCB. At the current time there is no simple test to directly analyze the brightener or carrier level in the bath or to determine if it is in balance with the other functional constituents. There is, however, a rather simple technique to infer when the brightener level needs attention and to determine just how much material to add. This technique uses a miniature plating cell commonly known as the Hull Cell. In the procedure described below, the cell is used to test plate a series of sample boards to determine when the bath needs adjustment, and, when used with the colorimetric analysis techniques, to determine how much of each component to add.

Principle of Operation

The Hull Cell is intended to act as a quick check on the health of the electroplating bath. Using the cell in conjunction with chemical analysis it is possible to qualitatively and quantitatively analyze all of the major constituents of the bath. Further, by making all additions to the cell before adding anything to the main bath, you can avoid making mistakes that might result in irreparable damage to the bulk of your electrolyte.

When filled to the line marked on the side of the cell, a "standard" Hull Cell (i.e. the one that I have in my lab) contains 267 ml (or 267 cc if you prefer) of electrolyte. Any additions made to the cell during the following tests must be multiplied by:

Scaling factor A = [Volume of main bath (ml) / 267 ml] x the volume of the addition

to yield the volume of the particular additive that must be added to the main plating bath.

Equipment Required:
267 Hull Cell Copper test plate Corrugated copper anode 2 Amp DC current source Low pressure air compressor (fish tank bubbler)
Reagents Required:
5 cc syringe of 35% sulfuric acid (battery acid) 5 cc syringe of CopperGleam CLX Maintenance (combined brightener and carrier) 5 cc syringe of 35% hydrochloric acid

Procedure:

1. If this test is being conducted prior to using the bath for the day, dummy plate a test board (i.e. plate a piece of scrap copper for about 45 minutes). It is important that the bath be adequately stirred up and filtered and that the copper level adjusted by dummy plating before using the Hull Cell.
2. Fill the cell to the line marked on the side.
3. Peel the thin plastic cover sheet off of the copper test plate and face the mirror bright surface toward the anode. This VERY smooth surface will act as the target for copper plating and will vividly show any defects in the plating process.

4. Connect the corrugated anode to the positive (+) terminal of the power supply and the test plate to the negative (-) terminal. Use flexible vinyl tubing to connect the output of the air compressor to the clear acrylic standpipe located at the back of the Hull Cell.
5. Turn on the air compressor and the power supply.
6. Adjust the air compressor until a vigorous, uniform curtain of bubbles forms in front of the test plate.
7. If the power supply can source more than 2 Amps, adjust it's output until it is sourcing 2 Amps.
8. Plate the test panel for 10 minutes.
9. Remove the panel from the bath, thoroughly rinse under cool tap water, and carefully dry with a non-abrasive tissue.
10. Examine the brass test plate while holding it against the 2 AMPS scale on the plastic rule included with the Hull cell 
11. Examine the CORRUGATED ANODE.

    If the chloride ion level of the bath is within the proper range, the anode will form a thin coating that will vary in color from very dark brown to pitch black.

    If the chloride level is too high, a loose, gray film will form that will effectively stop any plating action. If it too low, the anode coating will be light to dark brown.

    DO NOT ADD HCl TO THE BATH WITHOUT PERFORMING A CHLORIDE LEVEL ANALYSIS. IT IS VERY DIFFICULT TO READJUST THE BATH AFTER TOO MUCH CHLORIDE HAS BEEN INTRODUCED!!!

12. Examine the test panel.
13. A properly adjusted bath will yield a test plate that looks something like:
14. If the test plate shows burning that extends to current densities lower than 45 amps per square foot (ASF) or if the region below 40 ASF is anything but mirror bright, the plating bath needs attention. To save time (and brass plates), check the copper and sulfuric acid levels with the a titration analysis kit and adjust if necessary.
15. Once you have ascertained that the copper content, sulfuric acid, and chloride ion (black anode deposit) are at acceptable levels and that the copper to acid ratio is to within 10% of optimum, run another test plate. If the board shows dullness or rough plating in the low current density area, the bath is low on brightener.
16. Using the syringe filled with brightener, add PCM+ to the sample bath. If the test plate run above was slightly dull in the low current area, add 0.4 milliliters (cc) of brightener to the sample bath and run another test plate.

    If the first plate was very dull or rough add 0.7 milliliters (cc).

17. Continue adding brightener 0.4 cc at a time until you achieve an acceptable test plate. Record the total amount of brightener you added to achieve acceptable results.
18. Multiply this number by A to determine the amount that should be added to the plate through module.
19. Repeat steps 1 through 8 until acceptable results are achieved.

    Do not use a drawn sample for more than 3 Hull cell tests. If more than 3 are needed, pour the used sample back into the plating tank (REMOVE CORRUGATED ANODE FIRST!!!) and draw a new sample.

20. Once the plating bath is properly adjusted you may proceed with your board plating.
21. If the plate refuses to brighten with the addition of more brightener, the bath has either become polluted with excessive organic plating by-products or the individual components of the additive system are out of balance. Performing a dilution Hull Cell test is about the only way, short of cyclic voltametric stripping (CVS, think $12,000 or more), to accurately determine the root cause of the problem. If the bath is found to be contaminated, it should be carbon treated to remove the bulk of the dissolved organics.

Proactive Brightener (HTL) Maintenance

The carrier / brightener is constantly being consumed by the plating operation at the rate of approximately 0.50 milliliter (cc) per Amp hour. Before plating each board, determine the amount of CLX Maintenance to add. This will help insure that the bath is always operating at an optimum level and should reduce the need for large periodic carrier / brightener adjustments.

To determine the amount of CLX Maintenance to add:

1. Calculate the total area to be plated. Be sure to include both sides of the board as well as robber bars and any other areas of exposed copper. AREA
2. Calculate the total plating time. An acid copper plating bath based on the Lea Ronal CLX additive system deposits 0.0011" (1.10 mils, 28 microns, 0.81 oz) of high ductility copper in 1 hour at 20 ASF(Amps per Square Foot). TIME
3. In a well maintained bath, Lea Ronal Copper Gleam PCM and PCM+ are consumed at a rate of 0.59 ml of CLX Maintenance per Amp-hour of plating. To calculate the total probable consumption use:

CLX Maintenance to add (ml) = AREA x TIME x 20 ASF x 0.59