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A Model for Life of SnAgCu Solder Joints
A Model for Life of SnAgCu Solder Joints
Almost all reliability assessment is focused on the anticipated life in service. The question, how large a cycling temperature range is required for a model to apply.
Analysis Lab

Authored By:
P. Borgesen, L. Yang, A. Qasaimeh, L. Yin
Department of Systems Science & Industrial Engineering
Department of Mechanical Engineering
Binghamton University
Binghamton, NY, USA

M. Anselm
Universal Instruments Corporation
Conklin, NY, USA
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Summary
We have shown the life of a SnAgCu solder joint in a typical BGA or CSP assembly in thermal cycling to scale with the time to completion of a network of high angle grain boundaries across the high strain region of the joint. This provides for a credible materials science based model. Indepth studies did however show this to require significant temperature variations. Isothermal cycling may also lead to recrystallization, albeit at a much lower level depending on alloy, processes, and cycling parameters, but a quantitative model would need to be completely different.

The question therefore arises as to how large a cycling temperature range is required for our model to apply. We present results indicating that repeated cycling between 20 degrees C and 60 degrees C should be sufficient, i.e. the model should allow for extrapolation of accelerated test results to realistic service conditions.

Many practical applications involve a combination of thermal excursions and mechanical cycling, and there is little doubt that thermal cycling induced recrystallization will tend to lead to much faster crack growth through the solder in subsequent vibration, etc. We discuss how this greatly complicates the definition of a conservative but still practical accelerated test protocol.
Conclusions
The rate of failure of SAC305 solder joints in BGA, CSP, TSOP, flip chip, or LGA assemblies in thermal cycling is controlled by the recrystallization induced formation of a network of grain boundaries across the high strain region. This mechanism does require numerous repeated alternations between pile up of dislocations at low temperature and the coalescence and rotation of dislocation cell structures at high temperature. However, a maximum dwell temperature as low as 60 degrees C and a minimum dwell temperature as high as 25 degrees C appear to be sufficient for this to be the dominant mechanism, i.e. conventional accelerated thermal cycling should be relevant for thermal mismatch induced failure under even relatively benign service conditions.

Many realistic service conditions involve a combination of thermal excursions and repeated mechanical loading, e.g. vibration. Depending on the details this may lead to failure much faster than predicted based on Miner's rule of linear damage accumulation. Research to establish an accelerated test approach aimed at conservative assessments for those cases where solder fatigue is the dominant failure mode is proposed.
Initially Published in the SMTA Proceedings
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