ICSR (Soldering and Reliability) Conference Proceedings

Microstructure and Whisker Growth of SAC Solder Alloys with Rare Earth Additions in Different Environments

Authors: Doug Perovic, Leonid Snugovsky, John Rutter, Polina Snugovsky, Zohreh Bagheri, and Stephan Meschter
Company: U of T, Celestica Inc., and BAE Systems
Date Published: 5/4/2011   Conference: ICSR (Soldering and Reliability)

Abstract: The ability to withstand drop/shock loading is a major issue of Sn-Ag-Cu (SAC) solders joints in electronic products. It was shown recently that small additions of rare earth elements (REE) such as Ce, La, and Y to Sn-Ag-Cu alloys significantly increase their ductility relatively to conventional SAC alloys. Unfortunately, it was found that SAC+REE solder joints are prone to whisker formation. In this study the microstructural evolution during solidification in different conditions, including directional solidification and variations in cooling rates, is explored. The rare earths are nearly insoluble in the solid state in Sn-based Pb-free solders. As a result, their distribution coefficient with respect to crystallization is very small, resulting in strong segregation during solidification and a long freezing range for a small additions. Phases with different morphologies form in SAC105 solder with Ce, La, and Y additions reflowed on Cu foil depending on solidification conditions will be described in this paper.

The whisker growth phenomenon is studied on solder joints including samples previously cross-sectioned. The samples were exposed to different environments including: open air at ambient temperature, nitrogen atmosphere at ambient temperature, and high temperature/high humidity and checked periodically for whisker evolution. It was found that whisker formation depends on microstructure and may grow in all conditions including nitrogen atmospheres. The whisker length, density, relationship with microstructure and mitigation measures are discussed in this paper. Key words: Tin Whiskers, Lead Free, Rare Earth Elements

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