John

I don't know about these specs. But the problem seems to be mechanical and
material related.

- Solder fails, if a certain ammount of  deformation is accumulated. How
much deformation a solder joint can bear depends to a good deal on the
deformation mechanism activated ( Grain boundary sliding or dislocation
climb ). The mechanism activated is determined by how fast solder is
deformed.

- Leaded components do relief stress applied to a solder joint due to
thermal cycling because the leads will bend and therefore accommodate the
differences in the thermal expansion coeffitions. To bend a lead , a
certain force is necessary and so, a certain stress is induced into the
solder. This stress is the same, no matter how fast you apply the
deformation or at what temperature you do so. On the other hand the
mechanical behaviour of tin lead depends very much on the deformation speed
( creep behaviour ) and on the temperature. Low stress causes low
deformation speeds. However, at temperatures above say 20°C a continuous
deformation occurs no matter how small your load is until all deformation
is gone into the solder joint ( try that with a chewing gum, solder behaves
the same way ). But as mentioned above, with very small loads as those
occurring if a IC lead is bend, the deformation might be so slow that only
part of it is transferred into the joint during a thermal cycle in service.
Thus due to the bending of the leads less strain is induced into a solder
joint. This would agree with IPC SM-780.

- However, because above 20°C solder is so readily deforming, solder in the
bend will deform too. But how much it does stiffen the lead is extreamly
difficult to say. I think finite element analysis could be a way to
estimate the stiffening. This might be said in J-STD-001. Anyway, there it
is stated that you can do anything you want as long as you don't do any
bad.

- Another point is the solder that should't touch the package. If the
solder touches the plastic the local heat shock might cause cracking in the
components body, rather by stiffening the leads.

Well I could fill pages about these aspects. Hope the above helps you.

Guenter Grossmann
Reliability Laboratory
Federal Institute of Technology
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