I think it is not the same, -40 to 85C, it is not the Delta T that kills
the parts but rather the -40C
Solder act differently at -40 compared at 0C.  We are getting here the
fatigue at extreme temperatures

So it should not be the same (-40 to 85 and 0 to 125).
So Yuan, the parts interconnect  could have been brittle at -40C. Trace
lay-out, board thickness, CTE's
Soldermask or Non-SMD could be a factor to that problem.


regards

Jonathan Noquil
WW Pkg Research Center



                                                                                                                                       
                      Volkmar Huss                                                                                                     
                      <volkmar.huss@DRA        To:       [log in to unmask]                                                               
                      EGER.COM>                cc:                                                                                     
                      Sent by: TechNet         Subject:  Re: [TN] Thermal cycling test failure modes                                   
                      <[log in to unmask]>                                                                                                
                                                                                                                                       
                                                                                                                                       
                      11/25/2002 11:05                                                                                                 
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                      Volkmar Huss                                                                                                     
                                                                                                                                       
                                                                                                                                       




Hi Yuan,

based on what I have learned about thermal cycling tests, it is not the
low temperature that ' kills the part' but the higher delta T of 165K
compared with 125K, an increase of 32%.  To prove this, you can run a
-40C to +85C profile which should have the same result as the 0C to 125C
profile.

             Volkmar

Yuan Li wrote:

>
>
> We have seen some interesting phenomenaduring TCT. We compared 0-125C
> to -40- 125C. For the same package, we had no failures after 2000
> cycles with the 1st profile, but had 1st failure @ 500 cycles with the
> 2nd profile. It appears that the -40 C excursion is what kills the
> part.The most common failure we saw was substrate trace crack. The
> substrate was 2-2-2 build-up BT substrate. I know at -40C Young's
> modulus is higher. Is it a major factor? Any other factors? Why -40C
> would make sucha big difference?
>
> Recall there were some discussions on this topic, however, cannot find
> them. Any papers or on line info available?
>
> Thanks,
>
> Yuan



  Volkmar Huß, C.I.D.

  Engineering Electronic Circuit Boards
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