All,
Thanks for all your responses. Here is the detail you asked for:
I believe that the vendor dewarps all of our boards (we have about 150
different boards).
The problem is with both bow and twist.
Boards are flat at the time of receipt.
Typically it is a multilayer problem.
The oven is Conceptronics HVA 70.
The five zones are: 85, 111, 137, 180, 220 degrees C. Boards are both
panelized and non panelized.
Boards giving us problems are four layer.
This problems affects our wave solder and trim processing. It also is a
question at assembly as the boards are held flat; we're not sure if
flattening them out will damage the board in any way. We run our boards
on the flat belt conveyor of the reflow oven.
I appreciate all of your input.
Thanks,
Ken
______________________________ Reply Separator ____________________________
_____
Subject: Re: [TN] Bowed Boards
Author: [log in to unmask] at VEEDER-ROOT
Date: 1/21/99 10:24 AM
In a message dated 1/20/99 5:12:47 PM Pacific Standard Time,
[log in to unmask] writes:
<< We have been seeing boards come off our reflow oven with bow and
twist
that exceeds IPC specs. Our vendor insists that we are the only
customer that complains about this. We had our reflow oven
calibrated
and it looked fine. The only thing about the profile that may
be
suspect is the cooling rate may be a little too fast. According
to the
vendor, they put all of their boards through a de-warping process.
The vendor has tweaked their de-warping process to cool slower
in an
effort to make the problem go away.
My question is: This is our first bout with warping, what
should we be
looking at for a cause? Is it normal for a pc board vendor to
have to
de-warp boards?
Regards,
Ken Nevius
Veeder-Root
Altoona, Pennsylvania >>
Good Morning Ken!
When you say the boards are exceeding IPC specs, is it that
statement in
the -610B that says:
" Bow and twist after soldering shall not exceed 1.5% for
through-hole and
.75% for surface mount."? Is it just exceeding those specs, or is it
truely
creating a problem in the rest of the assembly process, or in the
form, fit,
and function at the end use of the product?
The reason I ask is that there has been times in the past where
the
attention was focused purely on numbers in a spec, rather than
looking at
things from a practical point of view. We all would like to have
boards that
are flat as a pool table, but because of different things, many times
that
doesn't happen.
I guess it could be caused by something your fab vendor is
doing, but in
my experience, most of the time it has something to do within the
layout of
the whole assembly. Either the copper isn't balanced throughout the
fab, or
with the way the component mass is distributed throughout the board. The
above comments go without saying that the warping isn't
being induced
by doing something wrong in reflow such as running a heavy, dense,
board on
the edge rails without some sort of center support.
But getting back to the warp itself, is it really causing a
problem
somewhere? Just as the statement says at the end of the paragraph
about bow
and twist in the -610B: "Before dispositioning completed assemblies
with bow
and twist as scrap, it is recommended to have in mind how the PWA's
in
question are mounted on their final destination run evaluations. Keep
in mind
form, fit, and function without jeopardizing reliability. Hope this helped
a bit...
-Steve Gregory-
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