This paper demonstrates a non-destructive technique to evaluate the internal microstructure in the Sn-Ag-Cu (SAC) solder joint through synchrotron X-ray radiation tomography. Synchrotron X-ray tomography is increasingly utilized for characterizing the internal microstructure of materials in 3D images. A 3D model is reconstructed from a set of 2D projection images taken from different angles and angular position during the sample rotation, thus it could provide a more comprehensive description of the microstructure of an alloy compared to 2D images. In this paper, it is successfully observed and evaluated the internal microstructure of a 900 μm solder joint sample. The key principles and methods of synchrotron X-ray tomography are briefly described. Examples of quantitative and qualitative assessments on the grain refinement effect of Mg addition to SAC35 solder joint are also presented in this paper.