In situ synchrotron study of electromigration induced grain rotations in Sn solder joints

Hao Shen,Kai Chen,Yao Li,Nobumichi Tamura,Wenxin Zhu

doi:10.1038/srep24418

Hao Shen, Kai Chen + Show 3 more

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https://doi.org/10.1038/srep24418

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Abstract

Here we report an in situ study of the early stage of microstructure evolution induced by electromigration in a Pb-free β-Sn based solder joint by synchrotron polychromatic X-ray microdiffraction. With this technique, crystal orientation evolution is monitored at intragranular levels with high spatial and angular resolution. During the entire experiment, no crystal growth is detected, and rigid grain rotation is observed only in the two grains within the current crowding region, where high density and divergence of electric current occur. Theoretical calculation indicates that the trend of electrical resistance drop still holds under the present conditions in the grain with high electrical resistivity, while the other grain with low resistivity reorients to align its a-axis more parallel with the ones of its neighboring grains. A detailed study of dislocation densities and subgrain boundaries suggests that grain rotation in β-Sn, unlike grain rotation in high melting temperature metals which undergo displacive deformation, is accomplished via diffusional process mainly, due to the high homologous temperature.

Highlights

We report an in situ study of the early stage of microstructure evolution induced by electromigration in a Pb-free β-Sn based solder joint by synchrotron polychromatic X-ray microdiffraction
In face-centered cubic (FCC) metals Al and Cu, rotation of several degrees is realized via the generation of geometrically necessary dislocations (GNDs) and geometrically necessary boundaries (GNBs) under the stress gradient created by EM. β -Sn has a body-centered tetragonal (BCT) crystal structure and its electrical conductivity[10] and self-diffusivity[11] are anisotropic with greater values along a- and b-axes than along c-axis
The cross-section of a Pb-free Sn-based solder joint was raster scanned with μXRD before and during the EM test, and 11 scans were made in total within a 43 h period

Summary

Introduction

We report an in situ study of the early stage of microstructure evolution induced by electromigration in a Pb-free β-Sn based solder joint by synchrotron polychromatic X-ray microdiffraction. With this technique, crystal orientation evolution is monitored at intragranular levels with high spatial and angular resolution. We study the early stage of the evolution of crystal orientation in β -Sn grains induced by high density of electric current in a Pb-free solder joint using synchrotron polychromatic X-ray microdiffraction (μXRD). The grain rotation is achieved via diffusional deformation induced by the unbalanced atomic diffusion in the current crowding region

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