John,
I think I have a good process now. I made 4 1" square samples to verify
repeatability. The root cause of my problem was inadequate exposure time.
What a difference that made! Warming up the ferric chloride did help shave a
few minutes off the etching time but it was still less than 10 minutes. I
was getting etching times over 30 minutes when my exposure time was way off.
I'll put the rest of my comments in line.
Thanks!
Rick
___________________________________________
-----Original Message-----
From:
Homebrew_PCBs@yahoogroups.com [mailto:
Homebrew_PCBs@yahoogroups.com]
On Behalf Of John Anhalt
Sent: Friday, November 23, 2012 6:12 AM
To:
Homebrew_PCBs@yahoogroups.comSubject: Re: [Homebrew_PCBs] first step into convection etching
@Rick Sparber
Your last picture is most revealing.
Apparently, you are faced with a difficult etching process as described in
the Electronics_101 thread you sent.
___________________________________________
>>>> what do you see that you call difficult?
___________________________________________
I would recommend getting a good etching process first, then tackle that
harder problem. I suspect that is what you are doing.
When I etch a PCB, I do not seek convection etching as a goal; although, I
am sure there is some of that going on.
1) Why do you want convection and laminar flow? Considering the chemistry,
even without considering the interaction of oxygen and oxidized forms of
copper, I would think that turbulent flow would be better. With laminar
flow, there will always be that surface layer of etchant in contact with the
copper that is not well mixed.
___________________________________________
>>> someone recommended it so I have it a try. One of my 4 samples used it
but I found the sloshing method was just as good and didn't require floats
on the board.
___________________________________________
2) Your experiment shows "edge effect," i.e., the edges etch faster than
the center. I am sure there are several reasons for that to occur, but
certainly having laminar flow across the board will make it worse. I do my
boards either copper on top or inverted with continuos magnetic stirring and
heated to 60 to 70°C. Copper down goes faster. I made some glass trivets
from 3 mm glass stirring rods to rest my boards on.
___________________________________________
>>> So the glass rods don't block etching?
___________________________________________
Depending on the severity of any edge effect, I remove the board and use a
sponge with etchant to speed up the central etching. With 1/2 oz and 1 oz
copper, that is generally not necessary to do.
3) You don't say what your resist is. If it is a photoresist,
underdevelopment and/or underexposure (for positive resist)can lead to
difficulty etching.
___________________________________________
>>> you nailed it. I am using pre-treated photo resist board and did have
the exposure time way off. My process now has the UV bulb 2.5" from the
board and I expose for 8 minutes. Then after about 15 seconds in the
developer, I see clean copper. Etching takes less than 10 minutes.
___________________________________________
With my photoresist (Injectorall), it is easy to tell when development is
complete. When properly developed, a water rinse will show wetting of all
exposed copper.
___________________________________________
>>> great test!
___________________________________________
If in doubt, a quick dip in ferric chloride will show if there are any
areas that are to be etched that still have remaining resist on them. If
so, it can be rinsed and put back in the developer for more time. That
final picture, however, shows far more scratches and defects than I have
ever seen with photoresist.
___________________________________________
>>> I had developed that board the previous day and had it kicking around
the bench. The resist was damaged but since it was a test board, no loss.
___________________________________________
That is not meant as criticism, but only as an observation that something
is wrong. Now, if you are using the toner-transfer method, then such
defects may be more common. I don't use toner transfer and can't comment on
that further on that.
___________________________________________
>>> your guidance is most welcome!
___________________________________________
My recommendations for ferric chloride would be to:
1) Increase your etching temperature to 60°C
___________________________________________
>>> The warning on the bottle says to not get above 140F. I'm leery of going
that hot.
___________________________________________
2) Improve stirring and/or agitation. Remember, of the etchants available
to hobbyists, ferric chloride provides the least undercutting and best
edges. You may get better results, if you dilute concentrated ferric
chloride a little bit -- say 10 to 20% with water (i.e., 1 part water to 4
to 9 parts concentrated ferric chloride). The Edinburgh etch
(
http://www.lawrence.co.uk/acatalog/etch.pdf), which adds citric acid to the
ferric chloride has been reported to give sharper etching. I have tried it,
but found the difference for what I do was negligible. It might be worth a
try for you, though.
___________________________________________
>>> I found that rocking the translucent plastic container with a strong
light under it works well to monitor progress. Now that I have the exposure
right, I'm happy with the etch time.
___________________________________________
Regards, John
----- Original Message -----
From: Rick Sparber
To: Homebrew_PCBs@yahoogroups.com
Sent: Friday, November 23, 2012 7:15 AM
Subject: [Homebrew_PCBs] first step into convection etching
Although I have done analog circuit design for 50 years, I'm a relative
newbie when it comes to circuit board etching. Furthermore, my last
chemistry class was in 1967. Here is my first step into convection
etching.
http://rick.sparber.org/electronics/ce.pdf
How do I improve this process? I understand that bubbling air into the
acid
will speed it up but won't that spoil the laminar flow?
Thanks in advance,
Rick
[Non-text portions of this message have been removed]
[Non-text portions of this message have been removed]
------------------------------------
Be sure to visit the group home and check for new Links, Files, and Photos:
http://groups.yahoo.com/group/Homebrew_PCBsYahoo! Groups Links