Stefan Trethan wrote:
> On 5/1/08, warrenbrayshaw <warrenbrayshaw@...> wrote:
>> I have been pondering the false walls idea for a while. I would
>> experiment with driving the bubbles up the tank walls to cause the
>> return flow of 'Unbubbled' etchant to flow down the surface of the
>> PCB. -This should take away the etching issues caused by the
>> incorrect bubble size or rate of bubbling.
>>
>> The spacing between the false walls and the PCB should be kept small
>> to ensure there is a decending flow at the surface of the PCB. As the
>> etching by-product is heavier, having a decending flow makes sense.
>>
>> The bubble bar/hose should be a little way up in the bubble channel.
>> I suspect the greater the column height of the bubbles the greater
>> the etchant flow rate will be. Bubbling may need to be much stronger
>> with this arrangement to induce the desired flow
> The false walls mustn't be too close to the PCB either or the flow
> will be impeded. I don't think "bubble drive" will create much
> pressure.
>
> I was thinking to have the false walls guide the bubbles towards the
> PCB (narrowing at the top), but that would still leave the issue with
> distributing bubbles evenly over the length of the PCB. This idea
> would mostly eliminate that problem.
>
> ST
>
The false walls would need to extend the entire length of the tank so
there is no mixing between the rising and falling fluid flow. I'd also
think they bubble tube ∗must∗ also extend the entire length of the tank
so flow rate is equal and consistent across it's entire length.
I don't think it would be difficult to test. I thought about testing
various tank configurations using water and confetti to see the flow
patterns. Weather the bubbles should be on the outside or the inside of
the two false walls could probably only be determined by etching real
PCB material. The PCB to false wall spacing also matters. Too far gap
between PCB and the false wall will reduce flow rate. Too close a gap
will also reduce flow rate.