Greetings,
I would like to ask for some insight with respect to an assembly process.
I am trying to get a handle on why a 4 layer board would warp @ assembly.
The panel is really an palletized array of two boards, seperated by a 60
mil gap and attached @ five perforated ponits that join the panels
together. The board is manufactured using Allied's FR406. Construction is
as follows;

6" x 9" board  9" x 12.1" array
.5 oz foil
Prepreg 2@7628 & 1@1080
Core .021 1/1  one side is a ground while the other side is circuit related
@ approx. 5% coverage
Prepreg 2@7628 & 1@1080
.5 oz foil

The panels are pattern plated with 1-1.2 mil of copper in the holes
LPI SM Taiyo PSR 4000
Entek 106
 The panels show no sign of warpage at our package stage/shipping
 The panels are seeing two conveyorized reflows in a HotAir Unit? I am not
sure if I have properly identified the assembly device. It does use hot air
and not a solder wave.
The first pass thru the reflow involves some resistors, capacitors. There
are no high profile components. This is layer 4. There is no sign of
warpage after this step.
The second pass with major components added resulted in 30% of the arrays
warpping

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