gEDA-user: experiences using geda with students

[John Griessen <john_g@xxxxxxxxxx>]

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


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