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
||Air Agitated Etching Tank
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.
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:
- 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.
- 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
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.
- 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.
- 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
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.
- 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.
- 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.
- 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
- 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
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Copyright © 1994 - 2014 Think & Tinker, Ltd.