Stephen,
You should make a trip to one of your friends with a stronger X-ray .
But, just a thought- warping is one of the causes of this "squishing", and I have seen
this on inner pads after popcorning. It may happen on the outer pads due to downward
warping because of the thermal mismatch. The stronger X-ray will show in this case
smaller ball diameters on the inner balls.
Bye,
Gaby

"Stephen R. Gregory" wrote:

> 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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