Homebrew PCBs

fenrir_co wrote:

> I've used vertical bubble etchers, both homemade, and commercial (but 
> cheaper models), and both of them caused terribly uneven etching. I 
> suspect that 1) the strength of the air pump and 2) the size and 
> placement of the bubbles have a lot to do with this. So if you have a 
> vertical bubble etcher that works perfectly, post the name and model 
> number of it. I don't remember where I got the commercial one that I'd 
> bought, I think it was Electronics Express but I could be wrong. I 
> threw it away about two years ago. It etched much, much faster where 
> the bubbles hit the metal, and one edge would etch faster than the 
> other. So I don't think this is entirely a myth, I think it has to do 
> with the quality of the bubble etcher you buy.

If you want the ultimate in etching, you need to collimate that 
process. Bubble or spray etching is a far less collimated process than 
exposing a mask to an uncollimated source.

If you float the board, copper side down on the surface of ferric 
chloride etchant, the etching process proceeds almost straight up. 
The loaded etchant is more dense than fresh, so a convection cell 
forms that provides fresh etchant to the center of the slot (or hole) 
between two edges of resist, and the loaded etchant slides down along 
both sides, preventing undercutting, till the copper is etches all the 
way through.  I have used this technique to etch holes with a diameter 
equal to the copper thickness, and slots narrower than the copper 
thickness.  The sides of the remaining copper traces are smooth and 
almost perpendicular to the board surface.

The skill that must be mastered to make this work is to be able to 
slide the board in, one edge first, so that no bubbles are trapped 
under the board.  I usually slide the board in, to get it wet, raise 
it up, to see if all the exposed copper has been wet, and slide it 
back in.  I usually just use a plate of etchant, selected so that the 
board is supported by the 4 corners just into the surface of the 
puddle.  After that, resist (as much as possible) the urge to peak or 
otherwise agitate the etchant, because that will ramp up undercutting 
until the convection cells reform.  This works especially well with 
single sided boards, where you can see through the board material when 
the etching is about finished.

Spray, bubble or other agitated methods are faster than convection 
etching, but, once you compare the precision and accuracy of the 
results, you may decide that the extra time (to make and drink a cup 
of tea), is well worth it, unless you are on a production schedule.  I 
also like the absence of mist that corrodes everything metallic, nearby.