Some time ago I was on a project in this area. It was long enough ago I can
only remember the conclusions, not any supporting data or references. It was
a production orientated project rather than a rigorous investigation, for
what it is worth:

We concluded that solder joint strength was not effected by void size until
the void was greater than 50% of the joint area. Similarly
electrical/thermal conductivity.

Our other concerns were reliability and process control.
Here we concluded that voiding over 10%  was a concern due to differential
stresses. (Not just in solder but more in die)
WE also took presence or absence of voiding from part to part to indicate
variations in surfaces, work or process parameters and worked to eliminate
them. On a production basis we set a limit of no more than 5% of total area,
with no one void to contribute more than 50% of the total voided area.

The project was die attach so geometries and so on are quite different, but
it could be that this sort of pragmatic approach might be useful in
determining process and inspection parameters to ensure consistent product
and yield.


Mike Fenner
Indium Corporation of Europe
T: + 44 1908 580 400
F: + 44 1908 580 411
M: + 44 7810 526 317


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Guenter Grossmann
Sent: Wednesday, June 06, 2001 11:35 AM
To: [log in to unmask]
Subject: Re: [TN] Antw: [TN] voids in BGA


Pete

Solder is not elastic. It is important to see the difference between solder
and elastic materials in the deformation behaviour and the degradation
behaviour. Solder creeps, same as a wet chewing gum if a constant load is
applied.  Above 0 deg.C the creep rate ( which depends on the stress
applied ) is so fast that in electronic applications no elastic deformation
will occur. All stress is relieved within seconds. In my opinion it is not
accurate to speak of a longer lifetime due to voids in the joint because of
a higher elasticity of the solder joint. Also the comparison with hollow
trees is only valid for elastic materials.
The degradation of solder is due to slow cycle fatigue caused by the strain
in the creep deformation not due to cyclic elastic deformation. It is strain
that degrades the solder not stress.
This is the reason why voids in solder joints don't have such a significant
influence on the reliability. A reduction of the load bearing area does not
affect the strain induced during a thermal cycle. The influence one could
imagine is that if a crack starts growing in a solder joint and it hits a
void one could assume that the void could be looked at as a sudden advance
of the crack. This would for sure be true if a void would be so big that it
goes over the entire with of a joint an be on the surface of the solder.
However, I must admit, I haven't seen hard facts so far that describe the
influence of voids on the reliability of solder joints ( or maybe I forgot
them ).

Best regards

Guenter

Guenter Grossmann

Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :      xx41 1823 4054
mail:     [log in to unmask]

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