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Subject:
From:
Stephen R Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stephen R Gregory <[log in to unmask]>
Date:
Wed, 27 Jul 2005 15:27:46 -0500
Content-Type:
text/plain
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text/plain (215 lines)
                                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 <[log in to unmask]>
                        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 <[log in to unmask]>
                                	
        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


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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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