Homebrew PCBs

Stefan Trethan wrote:
> Hi!
> 
> I'm thinking about my etch tank.
> Even if i make a vertical tank how can i make a airtight lid?
> I want to keep the etchant in and prevent too much corrosion.
> 
> The lid shoud keep all gases in.
> it should have a overpressure valve (because of decomposing H2O2, that is 
> no problem).
> 
> Additionally it should prevent spattering.
> (when the tiny bubbles (from the h2o2) burst there is etchant splattered 
> out of the tank.)
> 
> The lid ideally should be constructed in a way ensuring any condensed (or 
> accumulated from splattering) drops
> building up will run down in the tank again, and not run off to the side 
> when i take the lid off.
> 
> Now to the problem:
> 
> How can i ensure a airtight seal if i make the tank myself.
> even if i use silicone to make something like a seal?
> Would it work to bend the top of the tank outward to make something like a 
> flange?
> and on that a bead of silicone (nice flat) as a seal?
> How to press the lid (with the pcb holder) against it?
> 
> 
> The other idea: (horizontal etch tank)
> Take a big tupperware container.
> (the seals of these containers are tight enough for me i think.)
> fill it a inch or so with etchant.
> now suspend the pcb (somehow) in mid-etchant.
> some holding mechainsm on the lid is needed again.
> 
> 
> The whole jar can be mounted to a heavy wood plate, just to make it harder 
> to knock it over
> while removing the lid.
> 
> 
> While etching the lid may only rest loosely on the cointainer, this would 
> be enough to catch the splattes.
> To store it away just press on the lid.
> 
> Two questions bother me to this idea:
> 
> a)is the lid really tight enough
> b)is it a problem to etch horizontally (for uniformity?)
> in tray etching the edges are faster, but it doesn't cause trouble.
> i nudge the jar sometimes to mix the etchant.
> 
> I don't really like the idea of a bubbler but mechanic agitation is ok.
> (it might be enough to slightly press on the lid of the tupperware 
> container a few times
> to make the etchant mix? assume the pcb is hold to the lid and acts as a 
> stirring paddle)
> 
> 
> Let me know what you think of the idea.
> 
> 
> Stefan



Can I ask why you decided that the lid must be air tight ?

The only possible reason I can see why you may want an air 
tight seal is if your planning on storing concentrated HCl 
and you don't want the any fumes to escape. If you are 
running HCl+CuCl2+H2O2 mix then the acid concentration 
should not be so high that any significant amounts of HCl 
gas escapes.
A 5% HCl solution is normally what they run at. Yes, HCl 
gets consumed during etching, but assuming you have 
relatively large volume of solution to the etching copper 
mass then your HCl additions should not be too frequent.

To make air tight seal then you must have a flanged tank and 
a flat rubber seal between the flange and lid. Then 
depending on the thickness of the plastic used the lid must 
be tightly clamped to the flange at every 20 mm to 60 mm 
with screws. This is how I sealed my spray etcher chamber, 
but as Marvin pointed out this is essential when spraying.

I think a partly sealed lid is all you need, just so that 
evaporation and  any air flow are restricted. There is a 
large range of rubber foam strips available for door seals. 
These will make a good seal for a etchant tank lid just from 
the weight of the lid on the foam.


The biggest problem you may have is spillage from the 
dripping etchant from the lid as its removed. All the 
supermarket plastic containers I've seen are not designed to
avoid dripping. I made my own tanks and did it in such a way 
to avoid this problem. The top of the tank has a flange 
which is right angled. This makes like a step. A cross 
section of the tank looks something like this;



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There is a lid which sits on the step. There is also a 
rubber foam strip between the horizontal part of the flange 
and the flat plastic lid (not shown). The lid normally never 
completely leaves the tank as it can sit on its edge inside 
the flange and rest vertically against something. This way 
the lid is out of the way while the PCB is being immersed in 
the tank.
How you make a tank with a flange like this may be difficult 
without plastic folding and welding. It might be possible 
with acrylic sheet (perspex, plexiglass)) and the solvent 
cement.