Bob,
        Were the BGA's baked by chance?  I have seen BGA's in a tape and
reel get baked at too high of a temperature, partially melting some of the
plastic from the tape onto the balls.  (After realizing their screw-up, the
operators put them in a matrix tray.  Could be hard to find this out!)  In
special circumstances, a nice coating of plastic can prevent the two liquids
(BGA ball and solder paste) from coalescing.  If the two liquids can
intimately touch at some point, surface tension will drive the contamination
to the exterior.  If the plastic coating from a melted tape and reel isn't
too severe, most BGA balls will be able to coalesce.  It will only be on a
few severe cases that you will see the "water-balloon sitting on a
water-balloon" situation.

Thanks

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


> -----Original Message-----
> From: bbarr [SMTP:[log in to unmask]]
> Sent: Wednesday, November 15, 2000 7:45 AM
> To:   [log in to unmask]
> Subject:      [TN] BGA Soldering Problem
>
> I have a PBGA357 that is exhibiting a strange (to me) soldering condition.
> Two to three balls (not necessarily adjacent, but usually along an edge)
> do
> not appear to be wetting to the solder paste on the land. This is what it
> looks like after reflow: The paste on the land has reflowed to a mound,
> the
> ball on the BGA is compressed in shape but not wetted to the mound on the
> land (so the ball must have reached reflow temperature to get compressed.)
> A
> clear line of demarcation between the compressed ball and the mound of
> solder on the land is visible. The component fails in test, but sometimes
> pressing on it can get it to work. In most cases the balls on either side
> of
> the one not wetted look fine. Process is OA flux with convection reflow.
> Profile run several months ago with thermocouples imbedded in one center
> ball and one edge ball showed max. temp. of 217 (center) and 220 (edge)
> degrees C with time above 183 of 56 seconds. Max slope is 2 (center) and
> 2.8
> (edge) degrees C. I am going to go run a verification profile, but in the
> mean time your thoughts are much appreciated.
>
> Bob
>
>
> Robert Barr
> Manufacturing Engineering
> Formation, Inc.
> Voice: 856-234-5020 x3035
> Fax: 856-234-6679
> email: [log in to unmask]
>
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