Terry please see below,

----- Original Message -----
From: "Terry Harris" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Tuesday, October 08, 2002 6:43 PM
Subject: Re: [PEDA] OT - Complex boards and time to Layout?


> On Tue, 8 Oct 2002 14:54:55 -0700, JaMi wrote:
>
> >> Nothing you can do will eliminate the problem, just hope to reduce the
> >> effects. A lot of copper in one area of the PCB will take longer to
etch
> >> resulting in over etching (undercutting) in areas with less copper. The
> >> board house can't afford to over etch because some tracks will undercut
> >> unacceptably (or even completely and fall off the board), under etching
> >> risks leaving shorts in the dense areas. You are giving them process
> >> control and yield problems seems reasonable they want you to pay extra.
>
> >I can accept this as applies to a board with different weights of copper
on
> >each side, but not for the same thickness of copper . . .


 * * * * * * * * * * > > > > >   (specifically not to the
                                                  --------------------------
--

 * * * * * * * * * * > > > > >   extent that you appear
                                                  --------------------------
--

 * * * * * * * * * * > > > > >   to be stating here),
                                                  --------------------------
--

> >          . . .  where the only issue is "area",
> >and there is proper replensihment and removal of etchant.
>
> The trouble is your idea of 'proper' isn't possible. If each molecule of
> etchant came into contact with its own molecule sized area of the board
and
> was magically whisked away then copper density would make no difference
but
> that doesn't happen.
>

I am not saying there is no difference, I am saying that it is not a "major
problem".

> It hits the board surface then moves around a bit. It reacts with the
first
> copper atom it meets and for the rest of its journey across the board
> surface it is just getting in the way of the remaining active etchant
> molecules.
>

granted, granted, granted, granted, but this is nowhere near the the
monumental problem that you guys are making it out to be.

> The more copper area on the board surface the higher the proportion of
> spent etchant in contact with the board. You can not avoid this effect
just
> try to reduce it by reducing the time each etchant molecule stays on the
> board surface (by spraying or agitation or bubbling or whatever).
>

granted, but this is NOT THE SAME as saying that the acid on this side of
the board is going to get weak, in the sense that we are talking about. The
specific statement I am replying to implies (unquestionably to me) that
there is NOT adaquate circulation  / agitation / replenishment of the
etchant, period.

I am not talking about molecular levels, I am talking about the difference
between dropping the board in a tank and letting it sit, which is directly
implied in the original post, and giving the board the proper agitation /
circulation of the etchant, and removal of copper in solution from the area
that it needs to etch properly.

I cannot see how proper circulation could possibly ever let the "acid on one
side get weak", and I cannot accept that your description above qualifies as
letting "the acid on one side get weak".

How many times do I have to say it, the problem you are discribing is minor
by comparison, to the problem as it appears to be described in the original
description in the post that I responded to.

Are you guys trying to say that the original post does not imply that there
is improper agitation / circulation / solution removal? None of you seem to
be saying that, but you are all telling me I am out to lunch because I am
saying that.

What you are talking about is a minor problem compared to what I am talking
about, and if the quote of what the board house said is anywhere near
accurate, I still say look for another board house.

Please remember that I agree that there are far more important reasons to
balance the copper (and the layer stackup) in a board than this, but I still
maintain that this is a minor problem in comparison to those issues, and the
issue of simply dropping a board in a tank of etchant and letting it sit so
that the "acid on one side gets weak".

-

I have just logged onto Yahoo! Groups and I find that for some reason I have
not received all of the posts for this thread.

I have received all of Brads, and all of yours Terry, but I am missing
number 12425 from Ian (which I just forwarded and will now go read),
although I got a later one from Ian, and the same is true with Thomas
(missed his earlier post but got his later post), and I have not received
the post from Max. This is truely bizzare! They must be stuck in
SBCGlobal.net internet hell somewhere.

-

What I am saying, and what I have always said, is that the statement in the
original post implies no agitation / circulation / replenishment, and that
IS a MAJOR problem.

Do any of you guys actually dissagree with that?

JaMi

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