Hi Robert,
        You don’t specify which way the boards are warping.  It could be that the
weight of the major components on second reflow is causing the boards to
sag- the cutouts in the panelized pallet would reduce the stiffness of the
panel.  How are the boards conveyed in the oven?  Is there a center
support?
        My guess, however, is that the added thermal mass of the major components
is creating a thermal differential from top to bottom layer through the
board.  The components act as heat sinks, slowing down heating of the major
component side during preheat.  They may also be acting as heat sources
during the cool down phase, keeping the major component side hotter than
the other side.  A large z axis thermal differential is bad news for 2
reasons.  First, since thermal elongation is the product of CTE and
temperature, each layer will have a different elongation, which leads to
applied bending (warping) moments.  Second, as the epoxy goes through its
glass transition temperature, it *rapidly* (with respect to temperature)
loses its stiffness- its ability to resist the applied load.  Thus, even a
balanced layup which experiences similar applied loads on both sides of the
centerline of the board has essentially a weaker material on the hotter
side.  This asymmetry of material strength results in warpage.
        The good news is, you may be able to play with your oven settings
(hopefully you’ve got a 7 stage oven and aren’t tightly constrained with
your belt speed) to counter the thermal mass of the components and get good
uniform through board temperatures during heating and/or cooling.
        <PLUG>You may want to drop the guys at Electronic Packaging Services a
line- (404) 881-1114, or www.warpfinder.com.  They can put your populated
board in a computer controlled oven (matches your oven’s profile) and
measure its warpage in real time. This gives you some quantitative data to
use in setting your process up (i.e. rather than blindly trying a bunch of
different settings in your production environment, you can have these guys
optimize based on good experimental measurements.)</PLUG>
        Hope that helped,
                                Ciao,
                                        Andy

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