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December 2008

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Subject:
From:
Oscar Fallah <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Oscar Fallah <[log in to unmask]>
Date:
Sat, 6 Dec 2008 11:59:16 -0800
Content-Type:
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Hi David,

I would appreciate it of you could share your pictures with me.

thanks,
Oscar

On Fri, Dec 5, 2008 at 8:38 PM, David Tremmel <[log in to unmask]>wrote:

> Hello Technetters,
>
>
>
> 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 which
> adds tension to make good physical contact with the top of the die and the
> bottom of the heat sink.
>
>
>
> The aftermarket heatsinks do not have adequate spacing from the top of the
> board to the bottom of the arms which extend from the heatsink base and
> after time, the spacing at the corner of the BGAs was severely compressed
> to
> the point where I could not put 2 pieces of paper underneath the corner
> balls of the BGA.  The solder spheres are 25 mil in diameter 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 with the solder spheres on the corner of the BGA are
> subject to the greatest sheer forces.
>
> 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 J
>
>
>
> Thank you,
>
>
>
> David Tremmel
>
>  <http://valurecovery.com/> http://ValuRecovery.com<http://valurecovery.com/>
>
>
>
>
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