Steve,
I hate to be the one to break the bad news, but it looks like the part
popcorned during reflow. The lid peeled up from the corners because the
encapsulant sandwiched in between the lid and the FR-4 bulged with the
corners pushing downward. That is why the encapsulant is also separating
away from the FR-4 interposer or substrate further back from the corners
(you can see the gap between the encapsulant and the FR-4 getting larger
on the right side of the picture.. It is hard to see this for sure, but
from what I can see in the picture thats what it looks like to me.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stephen Gregory
Sent: Wednesday, July 27, 2005 11:35 AM
To: [log in to unmask]
Subject: [TN] Strange Looking BGA Heatsink Gap...

Hi Everyone!

We just finished building 8-assemblies that used a 780-ball Altera
Stratix BGA on them. These BGA's have a pretty decent sized copper
heatsink in the top.

Everything went pretty good. But then I noticed something with the BGA
heatsink after we got them all built. In the corners, there seems to be
some sort of gap beneath the heatsink and the top of the part that I
didn't notice before. Go to: http://www.stevezeva.homestead.com  then
click on Picture Page 2, then look for "Heatsink Gap".

All the BGA's show this in all 4-corners. I don't have any more of these
BGA's around that haven't been through reflow, so I can't do a before
and after reflow comparison. The Altera package drawing doesn't show
this gap either.

I'm thinking (hoping) that the parts are made like this, to somehow keep
the whole part from warping during reflow because of a CTE mismatch
between the substrate and the heatsink...it just looks strange though.

Anybody else out there familiar with these devices that can tell me this
is normal?
(Please tell me this is normal!!)


Kind regards,

-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
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