Authored By:
Yifan Wu, Hannah Fowler, Nathaniel Weddington, Sean Yenyu Lai, Sukshitha Achar Puttur Lakshminarayana, Ganesh Subbarayan-Shastri, John Blendell
Purdue University
IN, USA
Summary
Low temperature solders based on Sn-Bi are used as substitutes for Sn-Ag-Cu (SAC) alloys, reducing reflow temperature and, hence, warpage-induced defects. Understanding the effects of microalloying elements on solder mechanical, microstructural, and thermodynamic properties is an essential part of alloy design.
This study focuses on the changes in melting and solidification behavior of near-eutectic Sn-Bi alloys with Sb (0.5-2wt%) and Ag (0-1wt%) additions, and microstructure evolution and intermetallic compound (IMC) growth during isothermal aging at 85°C. Our DSC studies show that, while Sb increases the alloy melting temperature, it also reduces undercooling. This reduction in undercooling is largest for 2wt%Sb alloys in which large SnSb IMCs serve as nucleating sites for Sn dendrites producing a 4˚C undercooling. We discuss these results in light of the observed changes in mechanical properties with isothermal aging of Sb and Ag-containing alloys in thermal cycling performance of the Sn-Bi alloy solder joints.
Conclusions
The microalloying effect of Sb and Ag on the thermodynamic, microstructural, and mechanical properties of near eutectic Sn-Bi alloys was examined. The addition of Sb is shown to increase both the melting and solidification temperature of the alloy, though at different rates and for different reasons. The Sb concentration impacts the microstructure of the Sn-Bi alloy in terms of both IMC formation and Sn dendrite morphology. With 0.5 wt% Sb addition, no SnSb forms and the as-reflowed microstructure shows fine and numerous Sn dendrites. Fine SnSb IMC particles form within the primary (Sn) phase in the 1 wt% Sb samples, while relatively large (>20μm) SnSb particles can be seen in the 2 wt% Sb samples. As the Sb concentration increases and the size of the primary SnSb particles increase, the Sn dendrites forming during solidification are fewer but coarser. Based on observations made on the microstructures, we propose that the reduced degree of undercooling at higher concentrations of Sb is a result of large, primary SnSb particles serving as nucleation sites for Sn. The addition of Ag has a less pronounced effect on the thermodynamic behavior of the alloy.
In terms of the improvement on the mechanical properties of the alloys, the addition of Ag shows a more pronounced effect, which is likely due to the formation of Ag3Sn IMC particles. Of the Sb-containing alloys tested, the 42Sn-Bi-1Sb solder had a higher saturation stress compared to the 42Sn-Bi-0.5Sb solder. This can be explained by the lack of precipitation strengthening in the 0.5 wt% Sb case. Aging increased the saturation stress for all the alloys, a surprising result, given that SAC alloys and SnPb typically age-soften. The ductility of these alloys cannot be quantified due to the limits of the mechanical testing set-up where the maximum strain achieved is 0.16, but the results show promising behavior with respect to the effects of Sb on microstructural stability and the effects of aging on stress-strain behavior in LTS alloys as a class.
Initially Published in the SMTA Proceedings
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