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Altering the mechanical properties of Sn films by alloying with Bi: mimicking the effect of Pb to suppress whiskers



Nitin Jadhav, Maureen E. Williams, Fei Pei, Gery R. Stafford, Eric Chason


Stress is believed to be the main driving force for whisker formation in Sn coatings on Cu. This suggests that whiskering can be suppressed by promoting stress relaxation in the Sn layer, as is the case with Sn-Pb alloys. However, the need to remove Pb from electronics manufacturing requires finding an alternative alloying element. We used pulsed plating of Sn-Bi alloys to create an equiax microstructure similar to the kind found in Sn-Pb alloys. The mechanical properties of the Sn, Sn-Pb and Sn-Bi alloy thin films were measured using wafer curvature during thermal cycles to monitor the relaxation of stress due to thermal expansion mismatch. Our results show that Sn-Bi films have significantly enhanced stress relaxation relative to pure Sn films. Comparison between equiaxed and columnar microstructures show that both microstructure and alloy composition play a role in enhancing stress relaxation.
Journal of Electronic Materials


SnBi, pulse plating, equiax grain structure, packaging, tin, whisker, thermal expansion induced strain, Pb-free solder, mechanical properties of Sn and Bi alloy Sn thin films, thermal stress


Jadhav, N. , Williams, M. , Pei, F. , Stafford, G. and Chason, E. (2007), Altering the mechanical properties of Sn films by alloying with Bi: mimicking the effect of Pb to suppress whiskers, Journal of Electronic Materials, [online], (Accessed July 25, 2024)


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Created December 28, 2007, Updated October 12, 2021