Tim,
 I was running into almost the same problem(bowing) when trying to
replace PBGA 388.  Bowing of the corners is caused by excess heat  coming
from the corners of the nozzle that you're using.  Increase the preheat
time so that the temp across the component is more stable.  Warpage on
the PCB might be due to localized heating of the bottom heaters depending
on what rework station you're using.  Some will heat up the whole PCB
strenghtening the heat under the component.  Are the vias still tented
after removal of solder on the pads?  If the solder mask is removed,
solder shorts are eminent.


Tom Han
Ardent Systems

On Sat, 31 Oct 1998 07:29:00 -0500 "Stammely, Tim"
<[log in to unmask]> writes:
>Hello all,
>
>We are trying to remove and replace two ORCA 432 pin BGA's on a big
>14-layer
>board. We are getting alot of shorts after the rework(I'm x-ray
>inspecting).
>At first we tried solder pasting the pads with a mini stencil, but the
>amount of bridging we got caused me to try just fluxing the pads and
>letting
>the solder balls collapse, but we still got a lot of shorts. I am also
>noticing a lot of board warpage  during the rework heat cycle and it
>also
>appears that during reflow if I watch the component, the corners of
>the BGA
>collapse very early and the component almost appears to be bowing.
>Most of
>the shorts we are getting are in the corners where this last
>observation
>occurs making me think it is causing the shorts. Any suggestions on
>how to
>eliminate this problem?
>
>Thanks,
>
>Tim Stammely
>Manufacturing/Process Engineer
>MSI
>Manassas, VA
>
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