Brad,
        Bridging in the center grouping of pins on a BGA's is commonly
caused by delamination.  When you remove those BGA's check for a bubble  on
the bottom center of the BGA.  If the BGA is delaminated, it is because of
excess moisture.
        Your reflow profile can cause bridging.  This is because plastic
BGA's do not hold their shape very well during reflow.  Like a bimetallic
strip, the CTE mismatch of the plastic encapsulate and the BT laminate cause
the PBGA to wiggle like a worm.
        I personally feel like profiling is still a black art.  (though
others disagree).  For the nastiest of BGA's (BGA's that are way to big),
what I have seen work is a long soak just below liquidous to stabilize the
BGA's shape, then ease in and out of reflow and hope shorts don't occur.
However, if you don't have too, I'd prefer to ramp straight to reflow.

Thanks

Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]


> -----Original Message-----
> From: Brad L. Matthies [SMTP:[log in to unmask]]
> Sent: Monday, June 26, 2000 7:21 AM
> To:   [log in to unmask]
> Subject:      [TN] Reflow Oven Causing Bridging???
>
> TN,
>
> I have been asked to take a look at our convection reflow ovens to
> verify that they are not causing bridging on an array packages?!?  The
> ovens have recently been relocated and the CFM is slightly less than
> what the previous set-up had, but is still above what the equipment
> manufacturer requires.  The array package has recently begun to show
> bridging between pins (not always the same two pins) on the center
> grouping of balls on only one of the three array packages.  There are
> two other array packages on this same board, and they show no bridging.
> We are using a x-ray to view these three compontents post reflow.  Our
> analysis indicates that the stencil is showing signs of wear and needs
> to be replaced, but I have been asked to look at the reflow profile as
> well.  I have no idea if a reflow oven (convection) can cause the type
> of bridging we are seeing.  If if could, wouldn't the bridging present
> globally or randomly rather than in the same location on the same
> component ID?  I am also wondering if a Wave profile can cause
> re-reflowed joints that would result in bridging on array packages.  Any
> thoughts would be greatly appreciated.
>
> Regards,
> blm
>
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