Steve,
I agree with Gabriela, the source of bridging in the corners is the
warpage of the BGA at reflow temperature. All PBGA's will do this to some
extent because of the CTE mismatches within the BGA. Better yet, the PBGA
will go from a bowl shape to an upside down bowl shape as the various
components in the BGA reach their Tg and their respective CTE's change. It
wouldn't surprise me if the heat slug just amplifies this effect.
Thanks
Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]
> -----Original Message-----
> From: Stephen R. Gregory [SMTP:[log in to unmask]]
> Sent: Friday, July 21, 2000 6:43 PM
> To: [log in to unmask]
> Subject: [TN] BGA bridging...
>
> This is a subject that has come up before...it has to do with the TI BGA's
> with the metal heat sink incorporated into the top of the device. I have
> x-ray but it doesn't have enough power to penetrate the heatsink. We've
> recently aquired a Ersascope, I've been using that to inspect first
> article
> placement to make sure that we're getting it on the pads...it's been fine.
> But we still have had a problem with bridging.
>
> Out of 50 or so boards, there's been maybe 5-6 boards that have bridged.
> The
> bridges have all been in the corners. The balls all seem "squished"
> because
> of the weight of the heatsink. I've been looking closely at everything,
> making sure the print is good, placement is accurate, and that my reflow
> profile is good too, but I still am seeing this bridging problem...
>
> The last time I brought this subject up, I did get conflicting
> perspectives
> on this problem with the T.I. parts with the heatsinks in them. One
> experience shared was that they were having problems processing this part
> without bridging, and another perspective was from someone that said they
> built many, many boards with this part and didn't have a problem at all...
>
> I'm really trying hard to make sure that everything is as it's supposed to
> be, print definition and registration, placement accuracy, and reflow
> profile, but I'm still experiencing this bridging problem, 5 or 6 boards
> out
> of 50 isn't good at all...now that I have a Ersascope (because I can't
> x-ray)
> it's painfully easy to see the bridges...and like I said, they're mostly
> on
> the corners..
>
> Any ideas?
>
> -Steve Gregory-
>
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