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Understanding and Prediction of Bi Electromigration
This paper will review an effort to describe the expected microstructures in use case settings as well as a model to predict effective Bi boundary layer conditions created during current stress. The estimates will be compared with actual accelerated samples and the impacts of the current stress plus thermal aging on solder joint mechanical performance.
Technical Paper
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Authored By:
Yunfei Wang, Ph.D., Brian Franco, Ph.D., Kevin Byrd, Jacob Schichtel, Ph.D.
Intel Foundry
OR, USA
Summary
The movement of bismuth atoms under the influence of electric current flow, referred to as electromigration, has been extensively studied in relation to tin-bismuth based low temperature solder joints; however, literature work most commonly uses idealized test vehicles under highly accelerated conditions of electric current and/or ambient temperature. While useful for a fundamental understanding and for comparing the electromigration performance of various solder material compositions, the resulting microstructures and failure mechanisms may not be representative of solder joints in applied applications. This paper will review an effort to describe the expected microstructures in use case settings as well as a model to predict effective Bi boundary layer conditions created during current stress. The estimates will be compared with actual accelerated samples and the impacts of the current stress plus thermal aging on solder joint mechanical performance will be documented.
Conclusions
Understanding and predicting Bi electromigration behavior in solder joints is becoming increasingly important as the use of SnBi LTS pastes are extended into higher current/higher temperature product segments. The techniques presented in this paper offer a useful tool to understand the Bi concentration changes that develop at the IMC interface with stressing under current and temperature.
By developing an understanding of the expected solder joint microstructure at solder joint end of life, post-accelerated stress results can be more accurately assessed to ensure the represent real world conditions. As well, these results demonstrate that while current stress and temperature stressing can reduce the mechanical capability of solder joints, the impact is not necessarily catastrophic under some product use conditions.
Initially Published in the SMTA Proceedings
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