Interesting. Yes, I would like to see a pic of this...even several....
Probably couldn't get hold of enough proper heatsinks or the price was too
irrestible....


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Tremmel
Sent: Friday, December 05, 2008 9:38 PM
To: [log in to unmask]
Subject: [TN] Xbox360 BGAs

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

 


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