You may wish to send the photos to the Technet's treasured and respected
photo archivist, Steve Gregory at [log in to unmask]
for posting on his web site as has become the custom here
Joe
In a message dated 12/7/2008 11:45:24 A.M. Pacific Standard Time,
[log in to unmask] writes:
Just for info.
There is no attachment with pictures, I tried the link in the E-mail and
that wasn't the pictures either.
-------------- Original message --------------
From: David Tremmel <[log in to unmask]>
> Hello Technetters,
>
>
>
> Since I have had some people wanting to see pictures, I have resubmitted
my
> email with some corrections.
>
>
>
> I had someone who got a used Xbox360 that failed and when we disassembled
> it, we found they had put an aftermarket heat sink on two of the largest
> BGAs.
>
>
>
> The original Microsoft heatsinks make contact with the dies (the BGAs are
> flipchip designs) and have four arms that extend past the corners of the
BGA
> and then have posts which go through the board to a latching piece on the
> bottom of the PCB directly underneath the BGA which adds tension to make
> good physical contact with the top of the die and the bottom of the heat
> sink but does not add sufficient pressure to deform the PCB. The
> aftermarket heatsinks have posts with spacers above the PCB and washers
> below the PCB but the spacers above the PCB are not the proper length and
> after time, the PCB warps upwards and the spacing at the corner of the
BGAs
> was severely compressed to the point where I could not put 2 pieces of
paper
> between the corner of the BGA and the PCB. The solder spheres are 25 mil
> and are probably being compressed to less than 10 mil.
>
>
>
> Anywho, this is what I think has the failure mechanism is and I would
> appreciate some feedback from the gurus:
>
>
>
> While the chip operates, it generates heat which expands the device to
some
> degree in the X,Y axis
>
> Part of the job of the solder spheres is to sink the heat to the logic
board
> so the thermal expansion of the BGA does not act like a sheer force at the
> solder ball interfaces. The logic board also expands due to the heat and
> any CTE differences between the BGA and the logic board deform the solder
> spheres to some degree. The BGA substrate size is 35mm x 35mm and the
> solder spheres on the corner of the BGA are subject to the greatest sheer
> forces which, in this case, is detrimental.
>
>
>
> Due to the compression of the solder spheres at the corners because of the
> poor heatsink design, they are unable to deform and any CTE difference
> between the BGA and the logic board are turned into sheer forces and cause
> ball/interface failures.
>
> I also think that the constant pressure of the lead free solder spheres
> would greatly increase the chances of tin whiskers.
>
>
>
> I have a picture of just how bad the solder sphere compression is if
anyone
> is interested.
>
>
>
> Am I way off base?
>
>
>
> Thank you in advance for any correction in my theory. A confirmation would
> be better!!
>
>
>
> Thank you,
>
>
>
> David Tremmel
>
> http://ValuRecovery.com
>
>
>
>
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