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March 2009

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From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Mon, 30 Mar 2009 16:09:54 -0400
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Your last q: No! 
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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 16:05:20 2009
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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