Short version: I found a clear container which seals
properly and seems to work well with
ferric chloride + citric acid.
I just tried a 0.5 litre plastic container from this set:
http://www.maxwellandwilliams.com/mw/maxwell-williams-storage-canisters-jars-snap-and-store-rectangular-container-ls3290.aspI bought what I think is the middle container in this Maxwell and
Williams "Snap and Store" range. Its internal dimensions are about
140mm x 89mm x 40mm. It is polypropylene and is transparent with a
slightly milky appearance. The bottom and sides are without
obstructions, so visibility is good there. The lid has 90 x 47mm
window-like section in the middle, with the rest of the top textured, so
there is no clear visibility outside the window.
What I think sets these containers apart from others is the
exceptionally good seal. It is a soft, smooth, plastic seal which is
part of the lid, and which is pressed onto a fine ridge in the top of
the edge of the main part of the container. There are four very firm
clips to hold the lid in place, and the structure of the lid ensures
that there is pressure on the seal all around, including at the corners.
I placed a small single-sided board into this with a small quantity of
"Edinburgh Etch". This is a mixture of ferric chloride solution and
citric acid crystals, according to:
Etching Copper and Brass: The Edinburgh Etch
A new etching solution for copper and brass
Friedhard Kiekeben
http://www.nontoxicprint.com/etchcopperandbrass.htmKiekeben suggests (with full safety instructions which I won't repeat
here):
"4 liters ferric chloride solution + 1 liter hot water mixed
with 300ml citric acid powder (ratio 3:1)"
I made up 312ml of it by mixing 250ml of commercially available ferric
chloride solution from Altronics here in Australia:
http://www.altronics.com.au/index.asp?area=item&id=H0800plus 62.5ml hot water mixed with 18.75 ml by volume of citric acid
powder (from the supermarket).
According to the above page, Edinburgh etch can be used for a variety of
other metals. It etched our stainless steel kitchen sink when I
accidentally left some drops on the surface.
The addition of citric acid apparently avoids the build-up of sludge at
the copper surface. Kiekeben states that it works best at 18 to 30C and
that it has a very high capacity:
"An Edinburgh Etch solution can remain active for several years. The
citric acid component in the solution allows for literally dozens of
kilograms of metal to be dissolved without causing any
precipitation. The bound, chelated copper compounds in the solution
are considered a much lower hazard to aquatic life than pure copper
ions, but precautions should be taken not to allow a concentrated
solution to go down the drain. Once the Edinburgh Etch solution is
of an oily consistency and of a black olive color, its ability to
etch suddenly drops; and it is time for replacement.
I warmed my bottle of it in a sink of hot water, so the small amount I
poured into the sealable container was mildly warm at the start of the
process. I didn't time it, but it probably took 10 to 15 minutes with
0.5 oz copper.
No gas is produced by this etching process, so it is convenient to do it
in a sealed container. This particular container did not leak any
etchant at all, even when I turned it upside-down and shook it. With a
relatively small quantity of etchant, I was able to rock and gently
shake it, without concern about splashing or corrosive vapours. I would
do this, leave it for a few minutes, inspect the progress and do it again.
The container is clear enough that with good light, the progress of the
etching can be seen clearly. Since the etchant is opaque, I tilted the
container so the etchant drained away from the board, so I could see the
progress.
As the copper is cleared from the board, I could illuminate the
container from behind and see the growing green area of light coming
through the circuit board.
With the container upside down, the whole bottom and the four sides
provide very good visibility.
Perhaps there's a way of clamping one of these, or a larger version, to
something which vibrates or rocks. Perhaps an orbital sander running
from a low voltage would be good. However, for visual inspection it
would be necessary to stop the vibration and to tilt the container so
some parts of the board are out of the etchant.
For my modest needs, I will stick with manual tilting and inspection.
Since Edinburgh Etch apparently has a very high capacity, I think it is
fine to use a small amount of it. I used about 40ml, and tipped it back
into the larger etchant bottle when I had finished. However, it
probably makes more sense to leave the etchant in the container ready
for the next circuit board, since the seal prevents evaporation.
- Robin
http://www.firstpr.com.au/pcb-diy/