Nick,
Are all the solder bridges under the middle of the BGAs?  If this is the
case alarm bells should be going off!

When BGAs popcorn they do this on the bottom, not on the top, so it is hard
to spot or visualize without a cross-section or x-ray picture.  This is one
case where electrical test might not be better than these other methods.
What class of BGAs are they?  How were they shipped (container-wise?)  Were
they opened immediately upon receipt?  How are they stored?  How long
between opening and using them?  What was the wait between SMT and wave
soldering?

Another possibility, a little more "far out" - I would presume from what you
said that the solder bumps are eutectic.  Is there any chance that this is a
REALLY heavy BGA, say with a metal insert heat dissipation slug?  This could
smush the paste down and about.

Is there solder mask between the pads on the boards?

Are the BGAs clean on the bottom, no spider trails of solder between solder
bumps?

How smooth running is your oven conveyor?

Just a few things to think about.

Good luck!

Bev Christian
Nortel


> ----------
> From:         Nick Nicolaides[SMTP:[log in to unmask]]
> Sent:         Tuesday, August 25, 1998 5:55 PM
> To:   [log in to unmask]
> Subject:      [TN] Solder Bridges under BGAs
>
> Hi!
> Our contract manufacturer employs HASL for the PCBs'. Previous to this the
> CM was using OSP (Entek). He stated he has better quality using HASL PCBs.
> However we are now experiencing some solder bridging under the BGA's. He
> is
> using HASL,  WS 609 solder paste (6 mils), and what the solder bumps are
> on
> the BGAs. This appears to be acceptable but why the solder bridging under
> the BGA. What are your thoughts!
>
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