Hi Steve,
I will send you the Stratix Pkgg info in a subsequent email. This
device is a flip chip BGA and is either the EP2S90 or the EP2S130 device,
since these are the only 2 Stratix devices with this pin count. FC-BGAs are
commonly assembled with a heat spreader attached to the back of the die,
leaving a gap to the surface of the build-up substrate that is the thickness
of the FC assembly and the thermal adhesive. This may allow the surface
mounting of capacitors under the overhanging "lid". I cannot make out what
is between the copper lid and the substrate surface in the photo. It looks
like it may be two separate elements, with one on the left and one on the
right in the photo. The one on the right may be a FC-capacitor, but I
cannot tell if the edge view shows the small solder joints that you would
expect.
Another common FC-BGA construction is a 2-piece lid-stiffener assembly.
When the substrate gets over approx 25-27mm on either or both edges, it is
common to adhesively attach a copper stiffener ring around the perimeter of
the substrate after FC assembly/underfill, and any possible capacitor
surface mount assembly. This reduces package warpage and improves FC
assembly reliability. In a subsequent step a thermal interface material is
applied to the die back and an adhesive is applied to the top of the
stiffener ring. A flat heat spreader lid is then mounted against the die
back and the top of the stiffener, and the materials are cured. Sometimes
due to substrate size limitations it is possible that the stiffener ring
will not have constant width around all edges so as to allow capacitor
surface mount.
So, if your package is made with the top-hat lid (no stiffener ring),
and if the two elements seen between the lid and the substrate are caps, you
should be OK. But, if the assembly uses the 2 piece lid (stiffener and heat
spreader), then the assembly is in trouble. I am afraid that that is what
you have, and the stiffener and lid assembly was poor.
It is possible that the stiffener is bound to the substrate and there
is an intentional gap between the stiffener and the lid, but I have not seen
this before, and this would significantly reduce the thermal performance and
stiffening characteristic of the package. And even if this is the case, the
'stiffener' does not appear well attached to the substrate.
Good luck.
Best regards,
Leo
Director of Applications Engineering
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
ph 512-383-4593
fx 512-383-1590
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www.asat.com
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-----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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