Homebrew PCBs

This is part of a thread that started on Electronics_101.

Rick

-----Original Message-----
From: Electronics_101@yahoogroups.com
[mailto:Electronics_101@yahoogroups.com] On Behalf Of John Popelish
Sent: Thursday, November 22, 2012 9:40 AM
To: Electronics_101@yahoogroups.com
Subject: Re: [Electronics_101] board etching question

On 11/22/2012 10:01 AM, Rick Sparber wrote:
> John,
>
> This technique certainly does sound promising.
>
> What size board did you last etch? Can you give me a rough guess as to 
> area of copper removed and thickness of the copper? How much acid was 
> in the tub? I'm trying to figure out the ratio of acid to copper 
> removed.

I first came up with convection etching to make a prototype magnetic winding
circuit on flexible base for a magnetic print head that couldn't be made.  I
think it had 3 oz copper with traces and spaces narrower than the thickness
of the copper, where a couple thousand traces had to intersect an 11 inch
long magnetic comb structure.

They were trying to etch the copper from both sides with a commercial spray
etcher (after cutting a slot in the base with plasma etching).  But the
spray broke the overhanging resist off and etched the copper into an inverse
hour glass cross section (isotropic etching), so that when the insulating
coating was applied and the magnetic comb pushed through, the sharp corners
at the midpoint of the cross section cut the insulation.

I set up convection etching in a 9 by 12 inch Pyrex baking dish with about
1/2 inch of ferric chloride etchant (slow convection, because of its high
viscosity, but wonderfully consistent because of the strictly laminar flow)
and in about an hour, had a nearly perfect etch, from one side, with
rectangular cross section traces.  I taped the flex circuit to a piece of
plexiglass to make it float and stay flat.

After that, I etched all my boards at home, upside down. 
Even boards (1/2 or 1 oz copper) with lots of etched area etched pretty
well, with fresh etchant only 1/4 inch deep, usually in a round bowl that
held the board at the surface by its corners.  Sometimes I used more
etchant, because the only bowl that fit held the board up higher, off the
bottom.

-- 
Regards,

John Popelish


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