摘要
A small Pb-free solder joint exhibits an extremely strong anisotropy due to the body- centered tetragonal （BCT） lattice structure of β-Sn. Grain orientations can signif- icantly influence the failure mode of Pb-free solder joints under thermomechanical fatigue （TMF） due to the coefficient of thermal expansion （CTE） mismatch of β-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The mi- crostructure and damage evolution in cross-sections of solder joints under thermome- chanical shock tests were Characterized using optical microscopy with cross-polarized light and scanning electron microscopy （SEM）, and orientations of Sn grains were determined by orientation imaging microscopy （OIM）. During TMF, obvious recrys- tallization regions were observed with different thermomechanical responses depend- ing on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to significant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading to an accelerated damage after recrystallization .
A small Pb-free solder joint exhibits an extremely strong anisotropy due to the body- centered tetragonal （BCT） lattice structure of β-Sn. Grain orientations can signif- icantly influence the failure mode of Pb-free solder joints under thermomechanical fatigue （TMF） due to the coefficient of thermal expansion （CTE） mismatch of β-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The mi- crostructure and damage evolution in cross-sections of solder joints under thermome- chanical shock tests were Characterized using optical microscopy with cross-polarized light and scanning electron microscopy （SEM）, and orientations of Sn grains were determined by orientation imaging microscopy （OIM）. During TMF, obvious recrys- tallization regions were observed with different thermomechanical responses depend- ing on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to significant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading to an accelerated damage after recrystallization .
基金
supported by the National Natural Science Foundation of China(No.50905042)
the State Key Lab of Advanced Welding&Joining,Harbin Institute of Technology(AWPT-M12-02)
