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 example given, 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,
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+ = +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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