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Low-cost Bubble Assisted Process Tank

Introduction

Many of the electrochemical processes that have been developed for, or adapted to the fabrication of printed circuit boards, rely on solutions that benefit from rapid, uniform mechanical agitation. Turbulent, chaotic mixing of the process solution is often an effective means of insuring the:
  • isotropic distribution of bath constituents
  • minimization of depletion regions
  • thorough removal of particulate contaminants
Of the many mixing schemes that have been conceived and adopted, the easiest to implement is the use of compressed air bubblers (also known as air spargers) to randomly disturb and mix the bath in question.

Assuming that your bath is of modest volume (10 gallons, 40 liters or less), adequate agitation can be achieved using an inexpensive air compressor or regenerative blower. In some applications, an old vacuum cleaner can be used in exhaust mode if you make absolutely certain that the unit has been totally cleaned out and that absolutely no dirt or debris is present in the air stream. In the case of very small plating baths (less than 1 gallon or 4 liters), an air bubbler from a large aquarium can often be used.

As with most of the equipment mentioned in the Reference Stack, it is probably easier (and cheaper) to make your own bubbler than it is to chase down pre-made components.

Components you will need:

  • NEW #60 (0.040") & 1/8" (0.125") carbide PCB drill bits
  • 5' schedule 80, 1/4" PVC pipe
  • 2 ea. schedule 80, 1/4" PVC pipe caps
  • 2 ea. schedule 80, 1/4" PVC 90o elbows
  • 1 ea. schedule 80, 1/4" PVC tee
  • 1 ea. polyethylene or polypropylene process tank (at least 6" longer and 4" deeper than the largest than the largest PCB you plan to make)

Construction:

  1. Cut two lengths of PVC pipe such that the total length of each section, with a pipe cap on one end and a 90o elbow on the other, is at least 4" longer than the longest PCB your will be processing.
  2. Drill a linear array of through holes into each section.
  3. Assemble the drilled pipe sections and the other components into a "U" shaped manifold with a supply pipe (stand pipe) centrally located in the base of the "U".
  4. Turn each drilled section such that the holes are facing inwards toward the board being plated and angled 45° downward to encourage the formation of large bubbles.
  5. Center the sparger about 1/2" off of the bottom of your process tank. A fair degree of symmetry is required to insure that one side of your board processes like the other. This requirement, as well as the tendency of the sparger to float during operation, suggests that mounting the sparging manifold in a holding frame is usually a good idea. In the absence of a frame, spacers and weights can be used to hold the unit in position.
  6. Hook the stand-pipe to a source of filtered clean, dry compressed air. A compressor capable of generating 3 to 4 CFM (cubic feet per minute) at 1 to 2 psi is usually adequate for most purposes. As mentioned above, for very small tanks (max. 1 gal), a large aquarium "fish bubbler" works very well. For production environments however, a regenerative blower (a.k.a. ring compressor) or a high flow rate air compressor is required.
  7. Do not proceed until you put on your safety glasses, chemical handling gloves and lab smock.
  8. Thoroughly clean the tank and sparger with a mild detergent solution. The detergent used in electric dishwashers is ideal for this purpose if you use about 1 oz. of detergent per gallon of hot tap water (7 grams per liter).
  9. Thoroughly rinse with hot tap water.
  10. Fill the tank with a hot 10% solution (50°C) of sulfuric acid and allow to sit for at least 4 hours. This should leach out any remaining contaminants and will slightly acidify all surfaces in the tank.
  11. Thoroughly rinse with hot deionized (or distilled) water.
  12. Fill the tank with your process solution. Make sure that the liquid level is the same as it will be during actual operation.
  13. Start the compressor and observe the agitation of the solution's surface. Your goal is a gentle, uniform agitation (specifically about 2 CFM of air flow for every square foot of surface area of process solution). If the flow rate is too low, you will need to increase the capacity of your air source. If the agitation is too vigorous, reduce the air flow to the sparger (use an in-line restrictor valve to throttle down a conventional air comressor or a side tapped by-pass valve to bleed off the output of a ring blower). Usually, less is better, as long as you achieve good uniformity.
  14. Once the flow rate and uniformity is set to your satisfaction, your air-agitated process tank is ready to use.


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