I would like to support Brian's statement about Center Board Support.  When I
was affiliated with a reflow manufacturer we also did similar studies and
achieved similar results.  Most likley the warpage is kicking in when the
substrate goes above the glass transition temperature (which is somewhere in
the vicinity of 150 deg. C or so if I recall correctly).  A good Center Board
Support will definitely help alleviate the problem.
The key word here is good.  Having been involved in design and having since
benchmark tested a number of CBS design, like ovens, they are not all created
equal.  Things to look for are how much deviation from flatness can the system
prevent, whether or not it has an adverse effect on the thermal
characteristics and ability of the oven (it shouldn't) how parallel with the
edge conveyor rails it can remain, how much "freeboard" (path) area on the
bottomside of the board does it require and can it move out of the way when
not in use.  Most CBS' I've tested are ineffective but a few work very well
and a very few meet the above requirements.  Caveat emptor!

Phil Zarrow
ITM, Inc.
Durham, NH
www.ITM-SMT.com

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