steve,
did you keep the distance between the PWA the same (thermal mass)? Is your
oven is six zone? I believe the reflow oven is convection only...if it is
IR, you may have non-uniform hot spot (particularly the corners will reflow
early than the center if it is IR)... jk
At 09:58 AM 7/24/00 +0200, you wrote:
>Hi Steve,
>
>I can't explain why your bridges are all in the corners, but I do have an
idea why some folks are more succesfull than others: are the pads soldermask
defined or not ? With soldemask defined pads you'll get a larger stand-off
height but less "squishing" then with non soldermask defined pads (see
Motorola's application note AN1231). Or maybe these boards have some kind of
undefined mixture of SMD and NSMD? Other factors of influence are size of
the pads and the soldermask quality (how good does it prevent solder from
flowing over the edge of the pad). With an unlucky combination of these and
your metal-plate BGA you have the ingredients for misery.
>
>Kind regards,
>
>Daan Terstegge
>Unclassified mail
>Personal Website: http://surf.to/smtinfo
>
>>>> "Stephen R. Gregory" <[log in to unmask]> 07/22 2:42 am >>>
>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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