Bill,
Probably the biggest culprit of warp boards is the cool down cycle during
the press
operation. If the cooling rate temperature is greater than 7 degrees F per
minute this
could induce warp immediately. You stated that  you can bend the bare boards
flat.
Do yourself a favor and check a board that you have bent flat after several
hours
and see if the board retains the warp, if so the board has "memory" and is a
sure
sign of a press load that was cooled to quickly. One other point is the
grain direction
of the material could be different from layer to layer which could cause
this situation
also.

----- Original Message -----
From: Kasprzak, Bill (sys) USX <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, December 19, 2001 8:26 AM
Subject: [TN] Warpage in FR-4


> Holiday Greetings to one and all,
>
> I have an 8 layer FR-4 board that was supposedly built IAW IPC-6011, Class
3
> and IPC-6012, Class 3. The board itself appears to be like any other
board,
> .062" thick, no unusual ground plane layers etc. After wave soldering this
> assembly severely warped. Looking at the bare boards, they are also
warped,
> but can be shaped into a straight position by bending a little. (I
shouldn't
> have to do this.)
>
> Not being up to date on board fabrication techniques, was there anything
in
> the fab process that could lead to a warped board? We're talking a 3/16"
> warp over an 8" length.. Or was there something at my assembly end that is
> introducing the warp. The boards were run in a titanium picture frame
> pallet.
>
> Your thoughts and ideas are greatly appreciated.
>
> Bill Kasprzak
> Moog Inc., Electronic Assembly Engineering
>
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