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August 2000

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 2 Aug 2000 09:53:45 EDT
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Hi Bill,
You are technically right when you say:
>1. Some people try to manipulate Coffin-Manson for time dependency by
>using modified slopes of the graph.
Some people call this a modified Coffin-Manson equation; this is not correct,
because historically the Coffin-Manson equation was not modified, but the
solder joint fatigue results lead to a power law similar to the Coffin-Manson
equation, which is actually a simplification of the more complete but less
well known Morrow equation. I developed this Engelmaier-Wild fatigue
relationship for solder based on Roger Wild's data with the exponent being
able to account for the time- and temperture-dependencies of solder
creep-fatigue.
>2.  Do you mean strain range not stress to equate to the temperature range?
It certainly is more strain range based than stress range based, but for
solder joints with compliant leads, it is necessary to consider the cyclic
visco-plastic strain energy (hysteresis loop).
>3. I think that if there are different rates of heating and cooling (lower
than those
>for shock) then there could be an effect because of the imbalance in time
>dependence effects. This phenomenon certainly occurs in mechanical fatigue.
Different rates of heating and cooling would have a role in shaping the
cyclic hysteresis loop, and thus the cyclically accumulating fatigue damage.
Werner Engelmaier

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