aaaahhhh, aluminum silicon carbide. what a nightmare. printed wiring
boards were structurally bonded to it in an effort to restrain the cte
of the pwbs to a point where leadless ceramic chip carriers
(ahhhhhh...another nightmare) could be soldered to them. brittle and
expensive (they could only be 'machined' using edm processes. the raw
blocks were created by dipping SiC 'felt' into molten aluminum. the
fear of it all is permanently etched in my synapses. please, please
don't ever mention this again on this listserv.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leo Higgins
Sent: Wednesday, July 27, 2005 4:38 PM
To: [log in to unmask]
Subject: Re: [TN] Strange Looking BGA Heatsink Gap...
Hi Steve,
Cannot give quantitative answers to the squishy balls/stand-off
reduction question. I have never seen this analyzed but it does seen
correct to assume that the BLR would be reduced to some degree. Altera
released a notice a while back that they were changing from the use of
AlSiC heat spreader lids to Cu, which is more common, and much less
expensive, but which also adds mass to the package.
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
[log in to unmask]
www.asat.com
The information contained in this electronic message is
CUSTOMER/SUPPLIER PRIVILEGED AND CONFIDENTIAL INFORMATION intended only
for the use of the individual or entity named above. If the reader of
this message is not the intended recipient, you are hereby notified that
any dissemination, distribution and copying of this communication is
strictly prohibited. If you have received this communication in error,
please immediately notify the sender by electronic mail. Thank you.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Stephen Gregory
Sent: Wednesday, July 27, 2005 3:28 PM
To: [log in to unmask]
Subject: Re: [TN] Strange Looking BGA Heatsink Gap...
Hi Leo!
Thanks for the information! I think I'm starting to feel a little better
about things now.
Your information is pretty darned accurate!
Since my last email, I discovered that there were two different Altera
devices on these assemblies. We built two each of 4-different part
number assemblies (these are prototypes).
We had two different matrix trays of what appeared to be the same part,
and were told to use one part on one of the pair of assemblies, and the
remaining one on the other.
I didn't really investigate what the differences were back when we built
them, because they at first glance, looked the same...I thought maybe a
different die or something like that.
But one is a commercial grade Stratix (EP1S30F780C5) that was made in
Korea, and the other is a industrial grade (EP1S30F780I6) that was made
in Taiwan. I gathered that information from the part numbers...
The pictures that you saw on my web page is the commercial grade device.
I took a picture of the industrial grade device and posted it on my
page. Go directly to:
http://www.stevezeva.homestead.com/files/AlteraNoGap.jpg
You can really see the difference. This seems to be the one with the
2-piece lid\stiffener assembly that you described. So I'm starting to
breathe a little easier now...
I've tried to get a hold of someone at Altera all afternoon, because I
just want to make sure that the commercial device didn't popcorn as some
have suggested. The devices were sealed with dessicant and a HIC card
(which was all blue), and weren't opened until they went down on the
board
Another thing I noticed between the two devices and the way they went
together on the board, is that the industrial device has more of what I
call; "Squishy Ball Syndrome". Meaning that the weight of the heatsink
and the rest of the metal in the device "Squishes" the balls and reduces
the stand-off of the part to less than the commercial part does.
My question is; at what point does that reduced stand-off become a
reliability issue? At what part weight is it determined that high-temp
balls should be used to maintain good stand-off?
Kind regards,
-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
|---------+---------------------------->
| | Leo Higgins |
| | <Leo_Higgins@ASAT|
| | .COM> |
| | Sent by: TechNet |
| | <[log in to unmask]>|
| | |
| | |
| | 07/27/2005 01:11 |
| | PM |
| | Please respond to|
| | TechNet E-Mail |
| | Forum |
| | <[log in to unmask]>|
| | ; Please respond |
| | to Leo Higgins |
| | <Leo_Higgins@ASAT|
| | .COM> |
| | |
|---------+---------------------------->
>-----------------------------------------------------------------------
>----
-----------------------------------|
|
|
| To: [log in to unmask]@SMTP@Exchange
|
| cc: (bcc: Stephen R Gregory/LABARGE)
|
| Subject: Re: [TN] Strange Looking BGA Heatsink Gap...
|
>-----------------------------------------------------------------------
>----
-----------------------------------|
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
[log in to unmask]
www.asat.com
The information contained in this electronic message is
CUSTOMER/SUPPLIER PRIVILEGED AND CONFIDENTIAL INFORMATION intended only
for the use of the individual or entity named above. If the reader of
this message is not the intended recipient, you are hereby notified that
any dissemination, distribution and copying of this communication is
strictly prohibited. If you have received this communication in error,
please immediately notify the sender by electronic mail. Thank you.
-----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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