GAYATRI CUDDALOREPATTA,1 MAUREEN WILLIAMS,2 and ABHIJIT DASGUPTA1
1.CALCE Electronic Products and Systems Center, Mechanical Engineering Department, University of Maryland, College Park, MD 20742, USA.
2.Material Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
The viscoplastic behaviour of as-fabricated, undamaged, microscale Sn-3.0 Ag-0.5Cu (SAC305) Pb-free solder is investigated and compared with that of eutectic Sn-37Pb solder and near-eutectic Sn-3.8Ag-0.7Cu (SAC387) solder from prior studies. Creep measurements of microscale SAC305 solder shear specimens show significant piece-to-piece variability under identical loading. Orientation imaging microscopy reveals that these specimens contain only a few, highly anisotropic Sn grains across the entire joint. For the studied loads, the coarse-grained Sn microstructure has a more significant impact on the scatter in primary creep compared to that in the secondary creep. The observed lack of statistical homogeneity (microstructure) and joint-dependent mechanical behaviour of microscale SAC305 joints are consistent with those observed for functional microelectronics interconnects. Compared with SAC305 joints, microscale Sn-37Pb shear specimens exhibit more homogenous behaviour and microstructure with a large number of small Sn (and Pb) grains. Creep damage in the Pb-free joint is predominantly concentrated at highly misoriented Sn grain boundaries. The coarse-grained Sn microstructure recrystallizes into new grains with high misorientation angles under creep loading. In spite of the observed joint-dependent behaviour, as-fabricated SAC305 is significantly more creep resistant than Sn-37Pb solder and slightly less creep resistant than near eutectic SAC387 solder. Average model constants for primary and secondary creep of SAC305 are presented. Since the viscoplastic measurements are averaged over a wide range of grain configurations, the creep model constants represent the effective continuum behaviour in an average sense. The average secondary creep behaviour suggests that the dominant creep mechanism is dislocation climb assisted by dislocation pipe diffusion.
Key words: Lead-free solder, grains, creep, viscoplastic, primary creep, secondary creep, recrystallization
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