Peter Clifton wrote:
We run a robot design project,
.
.
.
the desire is (from the project's leader) that the students use gschem
or similar to draw their schematics. We aren't yet at the stage where
these students build custom PCBs, however various ideas for rapid
prototyping (miniature milling setups) have been discussed as a future
possibility.
I saw at one Univ. site, (maybe MIT) a lab procedure about using their owned
spray etching station. I have tested a very environmentally friendly etchant
using HCl and H2O2 and pure copper metal to start off a solution and it works
well enough without spray, and precipitates copper hydroxide when pH neutralized
leaving slightly salty water that can go down the drain. The copper hydroxide
can be sent to the landfill or sold scrap even. This etchant recipe, originally
published by Leo Van Loon, is easy to see through, easy to replenish by color
change, and low risk of eating holes in clothes, and makes no stains on hands or
clothes. Baking soda in water is all you need to neutralize/rinse just etched
boards, and lye, (NaOH), and pH paper or meter is all you need to neutralize
excess etchant. Replenishing is by adding HCl 35% and H202 35% -- it increases
the volume of etchant, so you drain out some before replenishing the etchant.
The chemicals are available and cheap -- $6/gal for acid, $13/liter for H2O2
35%, and those sizes are the right proportions to buy in to make the recipe.
Here's a board etched with it:
http://shop.cottagematic.com/elab/etched-board-epson-photo-paper.jpg
This photo was out of focus, but it's easy to see in a bubble tank when laminate
substrate is showing -- copper can still be seen on the bottom edge.
http://shop.cottagematic.com/elab/etch-done.jpg
With spray, it would be a more even etch over panels of boards, and easier to
see the progress of the etch for first time success. When you make some
standardizing assumptions like your board sizes are 90% 2x4 cm, 5% 3x6 cm, and
none are longer than 6 inches; 95% of boards are single sided copper plus wire
jumpers, surface mount only; the etch station is small and easy.
The main thing to buy is an acid proof pump for the etch spray. If you have a
fume hood to put it in, any old clear plastic or glass can be used for the low
requirements of the tank size and strength, and assembled with silicone RTV like
a fish tank. If you didn't have a fume hood, some kind of box with a slight
vacuum fan to pull air through some baking soda would neutralize any HCl mist or
vapor as it is done and purify the air inside the etch tank after the spray pump
stops. the final thing required is a fish tank heater. That attaches to the
lid so you can make a glass tube to lid wall seal with silicone RTV (again).
I have all the parts and will be trying it out soon and report more on this list
about it, and would negotiate to make you a system if there is no one who can
budget time on it at your place -- It would be about $400 for a new pump, $100
for a used one and about 6 hours or less work to hire to get one assembled.
John Griessen
_______________________________________________
geda-user mailing list
geda-user@moria.seul.org
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user