Hull Cell Analysis of Tin/Lead Plating Baths

The Hull Cell

The brightener/leveler formulation developed by Lea Ronal is a two part part additive system that functions to smooth out and distribute the solder plated onto the surface and in the holes of a PCB. At the current time there is no simple test to directly analyze the additive or carrier levels in the bath or to determine if they are in balance with the other functional constituents. There is, however, a rather simple technique to infer when the additive 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.  Using 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 above analytical techniques, to determine how much of to add.  PC SolderOn Carrier is normally maintained through routine replenishment of PC SolderOn Additive which contains small quantities of Carrier.

Principle of Operation

The Hull Cell is intended to act as a quick check on the health of the tin/lead plating bath. Using the cell in conjunction with the chemical analyses it is possible to qualitatively and quantitatively analyze all of the major constituents of the bath.

When filled to the line marked on the side of the cell, the volume of the test sample is 267 ml. If we denote:

• V = volume of main plating tank (liters)
• H = amount of addition agent added to Hull Cell to produce acceptable test plate (milliliters)
• C = amount of addition agent needed by main tank (milliliters)

the multiplicative factor that relates what you add to the Cell and what you will need to add to your plating bath is given by:

Equipment Required:

• No. 267 Hull Cell
• Copper test plate
• Corrugated solder anode

Reagents Required:

• 5 cc syringe of SolderOn additive

Procedure:

1. Read the manual that came with the Hull Cell carefully before testing your bath! Or you can not read the book, flounder around for half of the day, burn up $25 worth of test plates, and still not know what is going on. READ IT!
2. 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 tin/lead level be adjusted by dummy plating before using the Hull Cell.
3. Fill the cell to the line marked on the side.
4. 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 solder plating and will vividly show any defects in the plating process.
5. Connect the corrugated anode to the positive (+) terminal of the power supply and the test plate to the negative (-) terminal.
6. If the power supply can source more than 2 Amps, adjust it's output until it is sourcing 2 Amps.
7. Plate the test panel for 10 minutes.
8. Remove the panel from the bath, thoroughly rinse under cool tap water, and carefully dry with a non-abrasive tissue.
9. Examine the test plate carefully. A properly adjusted bath will yield a test plate that is a uniform gray-white matte across the entire panel.
10. If the plate shows a dark band in the mid to high current density areas or bright banding in the mid regions, the additive level in your bath is probably too low, or the bath has become contaminated with organic leachings from dry film plating resist. (This assumes that you have conducted all of the previous chemical analysis tests and have adjusted the bath to be at the optimum point for each parameter)
11. Using the syringe filled with additive, add 10 ml to the sample bath and run another test plate. This will be equivalent to adding C mL to the main bath and should be enough to compensate for any additive consumption from regular use (assuming, of course, that you have been somewhat diligent in maintaining your chemistry).  Examine the second plate carefully.
12. If the high current “burn” and bright band have disappeared and the test panel acceptable, your bath is probably not contaminated and it is safe to add the equivalent amount of additive to the main bath.  

    It is safe to dump the contents of a Hull Cell back into the main bath but MAKE SURE TO REMOVE THE CORRUGATED ANODE FROM THE CELL PRIOR TO DECANTING THE LIQUID.  IT IS A CLASS “A” PAIN TO FISH AN ANODE OUT OF THE BOTTOM OF THE PLATING TANK.

13. If the test panel indicates that the plating solution could use a bit more tuning, start with a new sample, add 10 ml additive (to bring the additive up to the level used in step 11 and add an additional 2 ml to the cell.  Plate a new panel and examine it very carefully to see if its appearance has improved.  Continue in this fashion until no further improvement is observed.  Total up the additions to the final test cell, multiply by C, and add the resulting quantity of PC SolderOn Additive to the main bath.

    If the addition of additive in step 11 did nothing to improve the panel, the bath is probably contaminated and should be carbon treated prior to further use.

14. Once the plating bath is properly adjusted you may proceed.  Remember to diligently record any and all additions to your plating bath so that you can build a “consumption database” to assist you in cost of goods determination when estimating the operating costs of your shop.
15. Once the plating bath is properly adjusted you may proceed with your board plating.

Proactive Additive Maintenance

The additive is constantly being consumed by the plating operation at the rate of approximately 0.35 to 0.50 ML (cc) per Amp hour. Before plating each board, use this relationship to determine the amount of additive to add prior to using the bath. This will help insure that the bath is always operating at an optimum level and should reduce the need for large periodic additive adjustments.  A small amount of carrier is present in the additive so the proper carrier level in the bath is accomplished by additive maintenance.

Example

If you are plating a board with 100 sq. in. of copper exposed (both sides including robber bars), and you are plating up 260 micro-inches at 20 Amps per Square Foot (ASF).  Total plating time will be 6.5 minutes and the plating current will be 14 Amps (20 ASF x 100 sq.in. / 144 sq.in per sq.ft).  This plating job will require about 1.5 Amp-hours of current and consume about .65 ML of additive.

HANDLE WITH CARE

Always wear protective clothing, acid resistant gloves, and a  full coverage face shield
when handling this or any other  corrosive material.

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