Alain is right on with his comments about thermal effects, however
generally for military purposes, you would thermal shock coupons prior to
sectioning.  It's not common to find lifted pads even with that.  Pads lift
primarily due to plating thickness being to thick at the corners.  When the
board swells in the Z axis from heat and then shrinks back during cooling
it will bring the pad with it unless the plating thickness is strong enough
to overcome the bond between the copper foil and the laminate.  Typically
the problem starts to show up when the thickness reaches 0.0022" or greater
of plating thickness.  I have seen problems where the amount of pad
actually bonded to the laminate is very small which also contributes to the
problem, as it's the same area of bond verses thickness of copper problem.
In rare instances, I suppose the ductility of the deposit could have an
effect, but I would assume if it was bad enough to cause this, you would be
seeing other problems as well.

Questions I would think about would be; are all the pads and panel
thickness' the same, or are you looking at a particular design?  Are these
thick panels?  (which would expand more)  Is your plating evenly
distributed or do you have a significant "dog boning" problem?  As Alain
said are you thermally shocking the board more then normal, like maybe at
your hot oil reflow dip tanks, because the preheater isn't hot enough?

Let us know how you make out.

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