Think & Tinker, Ltd.
P.O. Box 1606, Palmer Lake, CO 80133
Tel: (719) 488-9640, Fax: (866) 453-8473
Sales: Sales@ThinkTink.com, Support: Support@ThinkTink.com
Think
&
Tinker
Ltd.





SkypeMe at
"thinkntink"
Air Agitated Etching Tank

Introduction

An air agitated etcher can be used to enhance the action of most popular etchants. With careful attention to the maintenance of the various bath components, significant gains in etch rate and resolution can be realized when compared to tray or immersion etching systems. Often, however, the evolution of corrosive aerosols requires that the tank be located adjacent to an efficient fume collector/scrubber. In the case of a peroxy-sulfuric etchant ,precautions must be taken to insure that any such aerosol is trapped in the etching tank (using a sealed cover) or quickly collected by an external scrubber.

Basic construction

Bubble agitated etching tanks are built using air "sparging" as their primary means of solution agitation because of the ease of implementation and reliability (see Low-cost Bubble Assisted Process Tank). The system should be based on a polyethylene, polypropylene, or PVC tank that is at least 4" wide, 4" longer than the longest PCB you anticipate making, and 2" deeper than the width of your widest board. Tank materials should be rated for continuous service at 140°F (60°C) or greater. Once you have selected your tank and installed the sparger, finish the etcher as follows:
  1. If you will be using an in-tank heater to maintain the temperature of the bath, mount it at least 2" above the sparger to insure adequate mixing of the heated etchant during operation.
  2. Mount the heater off center so that the board can hang down past it. This will keep you from wasting much of the depth of your tank, allowing the system to accommodate larger circuits. If possible, make sure that the heater element is at least 1.5" from your board and a minimum of 1.5" from the side of the tank. This separation is adequate to prevent overheating of both the board and closest tank wall.

    It is better for the heater to be closer to the board being etched than closer than 1.5" to the side of the tank, where overheating can cause the plastic to soften and possibly rupture.

  3. If your tank heater does not have an internal temperature sensor (or thermostat), mount your external sensor directly above the center of the heated length of the element. Get as close as possible to the heater skin to minimize the hysteresis between the actual bath temperature and the temperature controller.
  4. Mount a stainless steel float switch directly over the heater, positioned such that the heater is covered by at least 1" of etchant before the level sensor in the switch activates. Wire the float switch in series with the heater and temperature controller.

    This is not an option! Plastic tanks containing hot, highly corrosive solutions should never be operated without some form of fail-safe level sensor controlling the operation of pumps and/or heaters. Wiring some kind of audible alarm into the circuit is also a VERY good idea.

  5. For tanks that with capacities over 5 gallons, mount the tank in a frame (or into a tabletop) so that the top rim bears the full weight of the solution filled assembly. In case of very large tanks (20+ gallons), side braces should be used to keep the tanks from bulging excessively.
  6. The tank must be equipped with a bottom drain that allows the tank to be completely emptied. This outlet can be connected to the suction side of a recirculation pump through a "T" coupling if continuous solution filtration is implemented.
  7. Etchers that are susceptible to thermal runaway (e.g. immersion etchers based on FT-2 peroxy-sulfuric etchant) should be equipped with cooling coils and/or a way to quickly dump the etchant into a heat-proof secondary container

Tank Heaters

  • Format - Everyone seems to have their own idea as to what configuration makes the best tank heater. "Over the side" immersion heaters are very popular because they do not require that the tank be penetrated below the surface of the process solution. They are available in both all metal and quartz element designs. A disadvantage is that the side hanger makes it difficult to seal the top of the tank with an easy to remove cover. Also available are cartridge style immersion heaters supplied with threaded couplings. These heaters insert into the process solution through special threaded fittings that are either welded or clamped to one of the tank sides. Very compact in design, cartridge heaters are available with internal thermocouples and thermostats for direct monitoring of the bath temperature.

    When designing a cartridge heater for use in a plastic tank, be sure to specify a non-heated length of at least 2" adjacent to the nut that threads into the tank fitting. This will push the heated surface out into the process solution and away from the tank wall.

  • Material selection - Regardless of the style chosen, consideration must be given to the properties of the solution being heated. If it is very corrosive, quartz, 316 stainless steel, or even titanium construction might be required. In the case of the process solutions encountered in the Green CirKit process, etchant, developing solution, and tin/lead (solder) plating solution heaters should be made from 316 stainless steel. Electroless tin baths that contain urea should be heated with titanium heaters. Stainless steel heaters cannot be used on any etcher that uses ferric chloride!
  • Power density - Related to the selection of sheath material is the specification of maximum power density. Measured in Watts per square inch (W/sq.in.), it is calculated by dividing the total power output of the heater by the area of the heated section of the heater. Green CirKit process solution heaters should be designed with power densities of 30 to 40 W/sq.in..
  • Electrical leads - If available, specify Teflon insulation on all electrical leads (power and thermocouple) emerging from the heater. Although not strictly necessary, teflon offers added protection in the event of catastrophic failure of tanks or plumbing hardware.


Established 1990

On the web since 1994

Payment Processing
Sales: 1-(719) 488-9640    Tech Support: 1-(719) 488-9640    Fax: 1-(866) 453-8473
Copyright © 1994 - 2014 Think & Tinker, Ltd. Updated 2/13/2014 8:36:56 AM