> Hallo Klaus,
> You focus on 5 possible causes for the Xbox 360 failures:
>  (1) the higher thermal stress during Pb-free soldering (possibly at Tpeak 
around 240 C) 
A: that certainly is a concern, specifically because for larger BGAs, a peak 
T of 240 is inadequate for reflow soldering of SAC solders;—however, if this 
were the root cause, failures would happen before shipping or shortly 
thereafter, and the failed SJs would be in the center of the BGA.;

> (2)  the higher thermal stress during specified operation had been the main 
> ones of several failure promoters.
> A: thermal stress is a poor descriptor—a high thermal expansion mismatch 
> between BGA and PCB is the most likely cause—the thermal expansion mismatch is 
> the product of component size, delta-CTE and delta-T; failures would be of SJs 
> near the die or BGA periphery.
> 
> (3) flaws in thermal board design (maybe no thermal steady state and 
> transient simulations)
>  A: see delta-T above.

> (4) flaws in material selection (of the type of Pb-free solder used)
>  A: not much difference in the common Pb-free solder—however, the EU's Pb-ban 
under RoHS is clearly a major contributor to the problem.

> (5) flaws of processing (maybe omission of an underfill under the BGA 
> and/or non-optimal reflow solder profile
> A: see (1) above—the right underfill would help.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com



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