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From:
Werner Engelmaier /* <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Fri, 24 Apr 2009 09:50:07 -0400
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Hi John,
You need to consider that solder creep-fatigue is not a stress issue but a strain issue; and that creep is a time-dependent process. Thus, the slower the transition the more complete the creep process the larger the fatigue damage.
Your worst case scenario are cycles of -50 to +50 giving you a maximum 100C delta-T; anything else will give you longer life/higher reliability.
The easiest way to deal with all of this is calculate the the mean fatigue lives for all the cycles involved; that tells you if you have a problem reliability-wise in 50 cycles of any of them---you would realy have to have a very large CTE-mismatch for 50 cycles to be a probem.

Werner

 

-----Original Message-----
From: Nieznanski, John A - SSD <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>; [log in to unmask] <[log in to unmask]>
Sent: Thu, 23 Apr 2009 6:51 pm
Subject: RE: [TN] solder fatigue analysis for slowly changing thermal environment






















Hi Werner,


 


Thanks for responding. Sorry for the confusion. I’ll try to clarify further. [1] and [2] are the limiting cases: i.e., [1] = HOT case
 = +50C Heat Sink, [2] = COLD case = -50C Heat Sink.. They do not occur sequentially as implied by [4]. The daily heat sink temperature drift [3] = 10C is a worst case value.



 


Assume when the part is OFF it is at the heat sink temperature and when ON it is 20C above the heat sink temperature.


 


For the example20given, there are only 50 operational cycles in total. An operational cycle is defined as 30 minutes ON + 30 minutes OFF
 at some steady-state heat sink temperature between [1] = HOT case and [2] = COLD case.


 


The 50 operational cycles do not occur periodically or at predictable temperatures or time intervals, but can be separated in both time
 and temperature. In between operational cycles, the system is OFF but can slowly drift in temperature <=10C per day.  So going from the HOT case heat sink to the COLD case heat sink will take a minimum of 100C delta / 10C per day = 10 days.


 


With this much variability, I want to try to bound the upper/lower fatigue damage calculations and perhaps estimate the expected fatigue
 (if possible). 


 


It seems a reasonable high damage estimate can be calculated assuming 50 consecutive operational cycles in the HOT case: hold heat sink
 steady at +50C, complete 50 operational cycles (each operational cycle = 30 minutes ON + 30 minutes OFF), and ignore the 10C daily temperature drifts. Do you agree? A variant here which will be more damaging yet is to allow more OFF time between operational
 cycles while in the HOT case (perhaps days). Do you agree?


 


It seems a reasonable low damage estimate can be calculated assuming 50 consecutive operational cycles in the COLD case: hold heat sink
 steady at -50C, complete 50 operational cycles (each operational cycle = 30 minutes ON + 30 minutes OFF)
, and ignore the 10C daily temperature drifts. Do you agree? A variant here which will be more damaging yet is to allow more OFF time between operational
 cycles while in the COLD case (perhaps days). Do you agree?


 


I think the other case of interest from a fatigue analysis standpoint is as follows: The system starts out at the upper temperature limit,
 runs ONE operational cycle (30 minutes ON + 30 minutes OFF) while heat sink is HOT=+50C, shuts off, cools down over 10 days to the lower temperature limit, runs ONE operational cycle (30 minutes ON + 30 minutes OFF) while heat sink is COLD=-50C, heats up over
 10 days to the upper temperature limit where the cycle repeats. This would give me a total of 25 cycles while heat sink is HOT and 25 cycles while heat sink is COLD.  


 


So there are quite a few variables here to consider. Any confirmation or repudiation of these ideas or other insights you can offer up
 are greatly appreciated. I recognize that the +50C/-50C range is in the realm of “Large Temperature Excursions” where additional damage mechanisms may apply.



 


Thanks again.


 


John Nieznanski


 


 


 


 











From:
[log in to unmask] [mailto:[log in to unmask]]


Sent: Thursday, April 23, 2009 2:44 PM

To: [log in to unmask];
Nieznanski, John A - SSD

Subject: Re: [TN] solder fatigue analysis for slowly changing thermal environment




 




Hi John,
=0
AForm 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+ = +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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