Authored By:
David B. Witkin
The Aerospace Corporation
El Segundo, California
Summary
SAC-Bi and Sn-Ag-Bi alloys have demonstrated superior performance in thermal cycling reliability tests of printed circuit boards, such as the National Center for Manufacturing Sciences programs in the 1990's and the JCAA-JGPP program of the early 2000's.
They have not been widely used in electronics manufacturing despite these promising results due to their Bi content, which has raised concerns for the potential of forming the low-melting point Sn-Pb-Bi eutectic phase (T 96°C) in mixed SnPb-Pb-free soldering. The recently concluded (December 2011) NASA-DoD program Phase II follow-on to JCAAJGPP revived the possible use of Bi-containing alloys with the recommendation that lower reflow temperatures for ternary Sn-Ag-Bi and quaternary SAC-Bi could reduce potential for pad cratering.
At the same time, an explanation for the observed performance in thermal cycling has not been provided, and basic aspects of the metallurgy of these alloys have not been explored to the same extent as more common SAC alloys. In this study, the relationship between microstructure, aging and mechanical behavior was studied for Sn-3.4Ag-4.8Bi and Sn-3.4Ag-1.0Cu-3.3Bi and compared to SAC305.
The alloys were prepared in bulk form by rapidly quenching from 260°C resulting in an as-solidified microstructure similar to that observed in solder joints. As-solidified properties were compared to those for samples aged two weeks at 150°C. Tensile testing, constant-stress creep tests, and low-frequency dynamic mechanical analysis up to 50 Hz were performed at various temperatures for both microstructural conditions. Aging led to significant microstructural changes in all the alloys, but while aging was accompanied by changes in the tensile and power-law creep properties of SAC305, the corresponding differences in the as-solidified and aged Bi-containing alloys were either smaller or absent.
For example, aging SAC305 led to an increase in the creep stress exponent and a nearly 50% reduction in the activation energy, while for SAC-Bi the reduction in activation energy was similar but the stress exponent was reduced, and in Sn-3.4Ag-4.8Bi neither activation nor stress exponent were changed by aging. These differences do not explain the performance of the solder joints in reliability testing but suggest that thermal fatigue reliability of solder alloys may be enhanced by addition of Bi.
Conclusions
The objective of this research was to characterize the mechanical metallurgy of alternative solder alloys for high-reliability applications. Key differences in use of high-reliability electronic systems compared to mass-produced consumer electronic devices include harsh deployment environments and length of service life. Electronic systems installed in a satellite, for example, may be built years before the spacecraft is launched and then must survive many years in orbit with no opportunity for repair or replacement.
The system life cycle, sustainability requirements and usage environments for space vehicles differ from airborne systems and there is no reason to expect that the optimal solder alloy for space is the same for aircraft. Characterizing behavior and performance of solder joints over long time scales may not be a high priority for consumer electronics manufacturers, although the documented changes in SAC alloy microstructures due to isothermal aging and their impact on reliability have been documented [24, 25].
The selection of Bi-containing alloys was based on their performance in early circuit-board reliability testing programs. The mechanical behavior of these alloys shows that the addition of Bi to SnAg or SAC leads to a different response to isothermal aging than is seen in SAC305. These results do not necessarily explain the performance of Bi-containing alloys in reliability testing, but they do show the extent to which the mechanical behavior of different alloys can vary.
The results of this research demonstrates the need for microstructure-based material properties for modeling of solder joint behavior in long lifetime high-reliability systems but they also suggest that the additions of small amounts of Bi could decrease changes in solder alloy properties over time and result in more readily predictable solder joint deformation.
Initially Published in the IPC Proceedings
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