Electromigration mechanisms in Sn-0.7Cu/Cu couples by four dimensional (4D) X-ray microtomography and electron backscatter diffraction (EBSD)

The microstructure evolution of surface-encapsulated Sn-0.7Cu solder volumes of approximately 230 μm diameter is observed from electromigration tests at 150 °C and with current densities between 4.0·104 and 1.6·105 A/cm2. X-ray computed tomography (XCT) is implemented to characterize the volumetric evolution of the microstructural features, and electron backscatter diffraction (EBSD) is used to characterize the grain structures. The growth of Cu–Sn intermetallics is quantified using XCT in order measure the effective diffusivity and effective charge product, D˜Z*, of Cu in the solder. The D˜Z* parameter for Cu has been measured as roughly 8·10−7 atoms/cm2s. The measured D˜Z* parameter increases with greater c-axis grain alignment with the electrical current pathway. Literature reports for Z* of Cu are polled in order to decouple the effective diffusivity from D˜Z*. The D˜ of Cu in the tests is compared with literature reports for diffusivity along the a-axis and c-axis of Sn, and the comparison is interpreted using EBSD orientation image microscopy (OIM) results of the tested specimens.