Dan

It is clear that the actual stresses after soldering a far lower that what
you expect when you take only the difference in elongation during
solidification. Solder creeps above 20°C very fast. This means that the
solder joint will not build up stress during cool down but a continuous
relaxation takes place. The higher the stress and the temperature the
faster the relaxation. Thus HOOKE's law is not applicable for tin lead
solder. The elastic deformation is negligible compared to the creep
deformation occurring. However, forced cooling after soldering may result
in a temperature gradient that leads to a considerable stress that may
damage a weak component ( Again, Captain Hook has not his fingers in this
game. Its just that a higher temperature ramp causes a higher strain rate
and thus in creep a higher stress ). Leaded components do not see the
problem a severe as those without leads since the leads are elastic enough
to deform thus storing the deformation of the CTE mismatch as elastic
deformation ( in the lead not in the solder ). But the stress needed for
the deformation is low especially in copper leads. Thus no damage occurs
due to the global CTE mismatch of component and PCB. On the other
hand.........well there is a lot to say but it might be too much for
TECHNET, as you said this might go very much in depth. Gotta shut up now.


Best regards

Guenter

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