Paul

Yes we did some work on the item. Let me outline the results as an extract from several IEEE publications:
- Degradation is strain driven (Coffin-Manson, with an exponent of approx.2 for GBS)
- Solder joints degrade due to slow cycle fatigue (creep)
- Creep takes place with 2 main deformation mechanisms (Grain boundary sliding GBS / Dislocation climb DC). GBS=low strain rate, high temperature; DC= high strain rate, low temperature
- Deformation mechanism influences degradation
- Strain rate is stress dependent
- Strain = deformation divided through solder thickness 
- Experiments show that a joint as soldered consists of large tin domains with finely dispersed lead. A joint with plastic deformation shows refinement of the tin phase and growth of the lead phase
- Recristallisation enables GBS ==> if a recristallised zone is formed all deformation takes place in this zone. ==> Transformation from global strain to local strain (higher)
- Shear tests show that slip bands of recristallised solder approx. 15um on each side of the solder layer form during plastic deformation. ==> further recristallisation in the middle of the solder joint is slowed down or stopped

> Decreasing the space between lead and pad causes higher strain thus faster degradation. Because of Coffin Manson exponent 2: 1/2 of thickness = 1/4 of Number of cycles to failure
> Decreasing the space between lead and pad leads to increasing strain rates with given deformation rates (temperature ramps in a test)
> Increasing the space between lead and pads runs into kind of a saturation because of the formation of slip bands where local strain in the bands becomes dominant to the global strain over the whole thickness of the solder

> This only applies to the area between lead and pad. The role of the fillet is another story.

Sorry for the short form. My wive called me, it's time to go home.


Best regards

Guenter

EMPA
Swiss Federal Laboratories  for Materials Testing and Research
Centre for Reliability
Guenter Grossmann,  Senior Engineer

8600 Duebendorf
Switzerland

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

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