Werner, without knowing the thernal mass, no would be a safe answer :-). I saw thermal shock parts (small package in large container), with short dwell without defrose at hot. It "pass" without failure for 500 :-(. I'll leave it to you to address the answer.
--------------------------
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----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Mar 30 17:03:19 2009
Subject: Re: [TN] solder fatigue predictions for highly asymmetrical duty cycles
Hi Joyce & John,
Let's not be too hasty with a categorical NO--the real answer is: [drum roll] 'It depends.'
First, what does 'valid' mean. It needs to be understood that any reliability prediction is just an estimate of the outcome. This varies by the accuracy of quantifying the cyclic load conditions [geometry, physical properties, temperatures, time, etc.] which in most cases are approximations and median values.
Second, what is the purpose of the exercise. Are you comparing the projected reliability relative to some other set of inputs? That can be done validly with some insights of what is going on and what is important.
Third, the dwell at the higher temperature is more important because of the much higher creep rates and therefore a more complete creep process. Thus, in an asymmetric cycle, the inputed dwell should not be just an average of the high and low-T dwells, but needs to be judged according to the T's involved. There have even been arguments advanced by some that because of the slow creep rates at the lower T, it can be ignored altogether. [Not that I agree with this argument]
Fourth, your 6/54 minute argument would give you somewhat of an overprediction of the actual cycles, because the hysteresis loop will be larger because of the 54-minute dwell at T(low)--the actualt's involved would determine the level of inaccuracy.
Fifth, it is these kind of judegements by whichwe consultants earn our keep.
Werner
-----Original Message-----
From: Nieznanski, John A - SSD <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, 30 Mar 2009 4:05 pm
Subject: [TN] solder fatigue predictions for highly asymmetrical duty cycles
Hello Tech-Net Gurus,
The solder fatigue parameters called out in IPC-D-279, IPC-SM-785 and IPC-9701
all assume symmetrical (i.e. 50%) duty cycles and temperature swings. This
raises the question as to how far these calculations can be pushed to predict
solder fatigue in low duty cycle, variable duty cycle or high duty cycle
applications.
In particular, Tsj ( = mean cyclic solder joint temperature), and tD ( =
half-cycle dwell time in minutes) are both used and are directly affected by
duty cycle. It is simple enough to recalculate these values based on
asymmetrical duty cycles, but how do we know that the predicted solder fatigue
results are still valid? If we know, do we know what assumptions are required to
make the results valid? This seems like a fairly common situation. Has anyone
correlated measured results to predictions in this regard?
Specifically, there are numerous caveats in the specs including for example,
high-frequency (e.g, vibration) conditions where tD < 1 second, but none of
these caveats apply here.
For example, with a power on dwell time of 6 minutes (system reaches thermal
equilibrium in < 1 minute) and a power off dwell time of 54 minutes (10% duty
cycle), I assume I can simply use tD = 6 minute dwells and Tsj = average solder
joint temperature over 60 minute cycle. Do you agree with this approach?
Another potential case of interest is if the duty cycle is reduced to something
less than 1 minute (but more than 1 second; not a vibration situation), such
that the stabilized operating temperature is not reached. Can any valid fatigue
calculations be performed in these cases if I can determine a valid Tsj for a 1
minute tD?
Thanks in advance for your insights and assistance.
Best regards,
John N.
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