Hans

The item of the mechanical properties of tin / lead solder is quite
difficult to explain since this material shows extensive creep behaviour.
Creep is a behaviour which looks strange to someone who never looked at it
before:
In creep deformation there is no direct relationship between stress and
strain. If a load is applied to a material in creep a continuous
deformation sets in. This means, with every stress you can achieve every
deformation. It is the deformation-rate which determines the stress
necessary to achieve this rate. I always illustrate this behaviour with a
well chown chewing gum. If one hangs a weight to a peace of chewing gum,
the gum gets longer and longer until the weight is removed. Applying a
higher weight results in a faster deformation.
The relationship of strain-rate and stress are temperature dependent.
Therefore a law prescribing the deformation behaviour has to be extended by
a arrhenius therm ( a cold chewing gum deforms slower than a warm one ).
Tin lead solder deforms with two deformation mechanisms. Grain boundary
sliding ( GBS ) and dislocation climb ( DC ). Which deformation mechanism
is activated depends on the stress applied and therefore on the deformation
rate. This behaviour can be prescribed with the following law:

g'=(t^n)*exp(-Q1/R*T)+(t^m)*exp(Q2/R*T)

where
g' is the strain rate
t the stress applied
n and m are exponents depending on the deformation mechanism activated
Q1 and Q2 are the activation energies for the mechanisms
T is the temperature ( in Kelvin ) where the deformation takes place

This is just the deformation part. Nothing is said about the degradation of
the solder so far. However, it is quite important to activate the same
portion of GBS and DC as in reality if one performs reliability tests,
since each deformation mechanism has its own influence on the degradation.
To quantify the degradation we usually work with a simple Coffin- Mansion
evaluation, since there are a lot of uncontrollable influences coming from
the variations of the production and we believe that these uncertainties
foul up every precise calculation of the lifetime of a solder joint.
Therefore Coffin Mansion is in our opinion accurate enough ( and easy to
handle ).

Best regards

Guenter Grossmann




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