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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