Bill,
plenty of people have answered your question on warpage and I agree with
the responces. The cooling rate is critical when the board passes through
the glass transition phase. This cooling rate could be 30 degrees / minute
if you could ensure that every point on each board were cooling at that
exact rate, but that would be near impossible. The truth is, cooling slowly
at the 7 degrees/min, as previously mentioned, will give you a better
chance of minimising temperature variation across the board. I suggest you
thermocouple a dummy panel, in several positions, and assess the
temparature variation. Back to Tg, if part of the board is above Tg and
part below, you will get the bi-metal strip effect (warpage) which will be
set in the board.
Hope this helps
Best regards to all and merry christmas
Michael
On Wed, 19 Dec 2001 08:26:21 -0500, Kasprzak, Bill (sys) USX
<[log in to unmask]> wrote:
>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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