Hi John,
Form your email it is not clear [at least to me] what is happening.
How many cycles [1.]? How many cycles[2.]? How does your assembly go from [1.] to [2.]?


 Cycle [1.] is much more damaging than cycle [2.] because of the higher mean cyclic temperature.
The cycle described in [4.] is even more damaging because of the much larger delta-T.
Fatigue analysis is based on the assumption [and there is very strong evidence that it is correct—Palmgren-Miner's Rule] that fatigue damage occurring at a given cycles is additive to any damage occurring at other cycles.
So, in order to analyze the fatigue life of anything including solder joints, you first must determine the cyclic loading history to be expected, calculate the fraction of life consumed for each cycle type, and add it up.

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



 

-----Original Message-----
From: Nieznanski, John A - SSD <[log in to unmask]>
To: [log in to unmask]
Sent: Wed, 22 Apr 2009 2:11 pm
Subject: [TN] solder fatigue analysis for slowly changing thermal environment









Hello TechNet Gurus,

Using the classical IPC-D-279 methods, how would you determine SMT solder joint 
fatigue for the following situation?

 1.  SMT part turns on at upper system temperature limit (Thi= +50C), runs for 
30 minutes at Thi+ 3D +70C, shuts off, part stabilizes at Thi= +50C for 30 
minutes.
 2.  SMT part turns on at lower system temperature limit (Tlo= -50C), runs for 
30 minutes at Tlo+ = -30C, shuts off, part stabilizes at Tlo= -50C for 30 
minutes.
 3.  And now the twist, the system temperature can slowly vary between these 
limits as much as 10C in a 24-hour period.
 4.  How much fatigue develops after 50 thermal cycles between T-hi and T-lo?

Can I claim that the upper limit on fatigue can be calculated simply by running 
50 operational cycles from the upper temperature limit (Thi= +50C to Thi+ = 
+70C)?

Can I claim that the lower limit on fatigue can be calculated simply by running 
50 operational cycles from the lower temperature limit (Tlo= -50C to Tlo+ = 
-30C)?

Can I claim that the actual fatigue is somewhere between these two limiting 
cases (worst case, best case)?
John




 


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