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January 1999

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 21 Jan 1999 10:24:05 EST
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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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