In-situ study of electromigration-induced grain rotation in Pb-free solder joint by synchrotron microdiffraction

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In-situ study of electromigration-induced grain rotation in Pb-free solder joint by synchrotron microdiffraction Kai Chen1,2, N. Tamura1, and K.N. Tu2 1 Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720 2 Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095 ABSTRACT The rotation of Sn grains in Pb-free flip chip solder joints hasn’t been reported in literature so far although it has been observed in Sn strips. In this letter, we report the detailed study of the grain orientation evolution induced by electromigration by synchrotron based white beam X-ray microdiffraction. It is found that the grains in solder joint rotate more slowly than in Sn strip even under higher current density. On the other hand, based on our estimation, the reorientation of the grains in solder joints also results in the reduction of electric resistivity, similar to the case of Sn strip. We will also discuss the reason why the electric resistance decreases much more in strips than in the Sn-based solders, and the different driving force for the grain growth in solder joint and in thin film interconnect lines. INTRODUCTION Electromigration (EM) has been known to induce grain rotation in Al (Tm = 933 K) [1-3], Cu (Tm = 1356 K) [4], and white tin (β-Sn) (Tm = 505 K) [5-7] lines at enhanced temperature. However, since the melting points of these three metals are different, so are the diffusion mechanisms involved [8]. During Electromigration stressing in Al, the grains rotate more at the cathode end than at the anode end while there is no trend in grain rotation between the cathode and the anode in Cu lines [1-4]. The average rotation rate in these metal lines is slower than in Sn lines. The grains in Al and Cu lines rotate as a result of dislocation redistribution, while the grains don’t grow significantly in this process. In Sn lines, grains increase in size as the rotation occurs even if the sample has been annealed at high temperature for long periods before the electromigration experiment. Since Sn has a body-centered tetragonal structure, the electric conductivity is anisotropic. It has been measured that the electric conductivity increases by a few percent in Sn lines due to the electromigration-induced grain rotation [9] and the a-direction of the crystal is realigned along the electron flow direction. However, there is no literature report about the conductivity increase coupled with Sn grain rotation induced by electromigration in Pb-free solder joints even though most of the industrial Pb-free solder joints contain over 90% of Sn. Here we report a detailed study of the orientation evolution of Sn crystals in Pb-free solder joints. The high orientation resolution (0.01 degree) and high spatial resolution (1 µm) provided by synchrotron radiation based X-ray polychromatic diffraction makes such study possible. The resistivity evolution is also estimated in this letter based on the precisely measured orientation of each individual grain and reasonable assum

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