We have experienced the same problem as well, but we are short of board
pallets. We cannot afford to use the board pallets to hold the boards
while the boards are cooling. We end up using a cheaper method but it
seems to work.

After soldering, we put the board upside down onto some "standoffs" such
that the board is "flat" (i.e. all edges are straight). We also use some
weights on certain area to make sure the does not bow nor twist in
cooling due to uneven contraction. This method is slightly messy and
requires a bit of experiment about the height of the "standoffs" but it
is cheap.

Joseph Lam

-----Original Message-----
From: jim [mailto:[log in to unmask]]
Sent: Tuesday, 12 January 1999 1:15
To: [log in to unmask]
Subject: Re: [TN] PWB Warping


You may want to consider pallets to hold the board flat while it heats
up
over the way and as it cools down afterwards. The best solution is to
fix it
in the design, but if your already stuck with a design a stiffener or
pallet
will do the trick. Give me a buzz if you need stiffener or pallet info.
Jim 972-494-1911
-----Original Message-----
From: Vascukynas, Brian <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, January 08, 1999 3:09 PM
Subject: [TN] PWB Warping


>Hello,
>
>We have been experiencing warping on certain PWB's from a specific
>supplier.
>I am curious on what the causes could be.
>
>In each instance, the PWB design layout was checked and found to be
>well balanced. Also our SMT Reflow Profiles were looked at for each PWB
>and found
>to be typical. The boards sometimes exhibit a slight warp right out of
>the MFG
>packaging and will warp out of spec on the first run.
>
>1. We looked at dwell times, ramp up rate, actual temp, speed of PWB
>going thru the
>oven and all appears to be typical per each individual case. Is there
>anything else we
>could be checking for?
>
>2. Is moisture absorption a concern - could it cause warping ?
>
>3. Currently we do not bake PWB's prior to SMT Reflow - Is this
>something we should
>consider.
>
>Thanks in advance.
>
>Brian V.
>
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