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Local fatigue in lead-free SnAg3.8Cu0.7 solder
|Pascal P. Jud 1, Guenter Grossmann 1, Urs Sennhauser 1, Peter J. Uggowitzer 2
1. Empa, Materials Technology and Research (Empa), Überlandstrasse 129, Dübendorf 8600, Switzerland
In this study unified shear samples with a thin (100 μm) solder gap and an area of 10x10 mm2 were investigated. The test setup was especially designed for pure shear stress. To understand the deformation behaviour the specimens were marked with various surface patterns. The patterns were etched and Pt deposited by Focused Ion Beam (FIB). It was found that the deformation of the solder is inhomogeneous in unidirectional shear tests and as well in cyclic testing. The deformation is concentrated in zones mostly parallel to the shear direction. The width of these areas found was between 5 to 20 μm by a length of 20 to several hundreds microns. The FIB etched patterns in the submicron range allowed investigating the deformation in detail with dislocation climbing, grain boundary sliding and recrystallization. With cross-sections done by FIB in the before mentioned zones it could be shown that there were no cracks generated during the primary and secondary creep.
Unidirectional shear creep tests showed an intense microstructure dependence of the shear creep rate. Two different microstructures were investigated. Water quenched samples, which showed a distinct finer microstructure with β-Sn phases of about one fourth of the ones of the slowly solidified samples. The water quenched samples exhibited a larger temperature sensitivity of the stationary creep rate and showed higher strain rates, especially at elevated temperature, compared to the slowly solidified samples.
In isothermal cyclic testing it could be shown by the FIB etched markers that there were as well zones with concentrated deformations established. There are two different mechanisms at work in the degradation of the sample. The first was found to form deformation zones and the second was the fatigue to rupture in these zones. The microstructure sensitivity is observed as well in the cycling tests. It was found that the quenched samples withstand less cyclic deformation.
Presentation: oral at E-MRS Fall Meeting 2005, Symposium H, by Pascal P. Jud
See On-line Journal of E-MRS Fall Meeting 2005
Submitted: 2005-05-30 10:46 Revised: 2009-06-07 00:44