Wayne,
I have to agree whole heartedly. We use with both cupric chloride and ammoniacal etchants and never encounter "acid traps" or the likes. I suppose if I were able to pull a panel off the conveyor of the etcher prior to rinsing and leaned it against something vertically (not that I would) that any traces or features that could trap the chemical would be a trap.;)
Dan S
>>> [log in to unmask] 11/21/06 12:34PM >>>
Several weeks ago I challenged TechNetters to get a photo of "acid trap"
phenomenon. No one volunteered. Not a big sample set, but I believe
that this is one of those things which was a problem at the dawn of pcb
manufacture, but is inapplicable to today's processes.
The closest respondents could come up with was to point out that
alkaline ammoniacal etch solution has a tendency to eat more copper once
it has some copper in solution. But, obviously, this process will
starve itself out too: There must be a peak copper concentration in the
bath for highest activity, otherwise the reaction would continue until
the solution was 100% copper decomposition products.
I can provide photos of the inverse phenomenon, which most pcb
manufacturers are aware of: Pads spaced way out in the middle of
dielectric tend to become over-etched, whereas tight lines/spaces tend
to be under-etched. No big surprise there, since its harder to get
aggressive fluid activity in tight areas. For cupric chloride etchant,
These lone pads really take a beating because once the dielectric around
them clears, the hungry acid droplets will splash sideways along the
dielectric until they bump into something.
As to boards continuing to etch long after they've come out of the
etching machine, I think that's completely ridiculous.
Wayne Thayer
>>> [log in to unmask] 11/21/2006 12:35:17 pm >>>
Deac,
I agree, acid never stops, but if properly cleaned it is either
diluted
or neutralized to a point of no longer being able to effect the metal
geometry.
Again I emphasize the importance of good practice by the fabricator,
if
properly cleaned an acid trap has no acid left in it.
Good design practice should be to avoid the creation of possible acid
traps, but sometimes due to other circumstances (RF structures being
one) they are unavoidable.
If a design has acid traps then the alternatives are fixing the design
(if time allows) or using good fabricators whose processes will assure
removal of all acids from the PCB prior to completion.
Just my beliefs,
FNK
Frank N Kimmey CID+
Principal PCB Designer
PCB Programs
Powerwave Technologies Inc
Office 916-941-3159
Fax 916-941-3195
Mobile 916-670-0645
-----Original Message-----
From: Deac Descoteaux [mailto:[log in to unmask]]
Sent: Tuesday, November 21, 2006 9:22 AM
To: 'TechNet E-Mail Forum'; Frank Kimmey
Subject: RE: [TN] Acid Traps
Frank,
Acid is like rust.....it never sleeps! The acid will continue, albeit,
at a
slow rate, to eat away at the etch. It will take time, but it will
happen.
The biggest problem is that it is one of the most difficult failures
to
find
on a board.
Deac
Deac Descoteaux
PalPilot, East Coast Corporation, (PEC)
Director of New Business Development
71 Spitbrook Road, Suite #307
Nashua, NH 03060
office # (603) 888-3400, ext. #10
Toll Free # (877) PALPILOT, (725-7456)
Fax # (603) 888-3500
Cell Phone# (603) 557-7568
email address: [log in to unmask]
www.palpilot.com
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Frank Kimmey
Sent: Tuesday, November 21, 2006 12:10 PM
To: [log in to unmask]
Subject: Re: [TN] Acid Traps
John,
In todays world, if a PCB is properly cleaned after etch, deposition
and
prior to mask, then acid traps should not be a concern.
We do a lot of weird angle stuff in the RF world and I have never seen
a
failure due to acid trap type issues.
Bottom line is if your fabricator has good process then it should not
be
an issue, but to keep risk to a minimum good design practice should be
followed when possible.
Hope it helps,
FNK
Frank N Kimmey CID+
Principal PCB Designer
PCB Programs
Powerwave Technologies Inc
Office 916-941-3159
Fax 916-941-3195
Mobile 916-670-0645
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Foster
Sent: Tuesday, November 21, 2006 8:53 AM
To: [log in to unmask]
Subject: [TN] Acid Traps
I would like to ask the board fabricators if acid traps are a real
issue.
I normally try to avoid any feature that creates a structure that is
less
then 90 degrees.
However I am helping out another group by reviewing their boards and
there
are literally
hundreds of acid traps on the board. This board is class 3 and has to
be
high reliability.
I have always been taught that they can be a real problem. But have
never
quite understood
why.
Any input on this will really be appreciated.
Thanks
John Foster
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