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/> http://ValuRecovery.com

 


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