Thermally induced void growth in Cu filled through-silicon vias (TSV) has been studied for reliability purposes with x-ray microscopy and finite element model (FEM). A laboratory-based x-ray microscopy combined with computational tomography imaging is demonstrated to have advantages over other methods of inspecting TSVs. We show that the 8-keV x-rays used by Nano X-ray Computed Tomography (NanoXCT™) are capable of imaging voids inside filled vias before and after annealing without cross-sectioning the TSV. A series of TSV arrays filled conformally and from the bottom up were inspected by the x-ray microscope before and after annealing. Pre-existing voids in the seamline were observed in conformally filled TSVs before annealing, while bottom up-filled TSVs do not have a seamline or voids. The same TSV samples were repeatedly annealed at 200, 225, 250 and 300°C. X-ray micrographs after annealing reveal TSVs with pre-existing voids are prone to void growth. In addition, x-ray measurements show the total volume of void growth increased with annealing temperature. A steady-state FEM was developed to understand the void growth phenomenon. FEM suggests high concentrations of vacancies occurring at the pre-existing void area cause void growth.