An increasing number of analysis techniques requires access to the backside silicon of a functional device. For backside sample preparation of packaged devices, CNC milling tools can perform both package opening and circuit preparation. They offer good versatility in terms of type and size of packages – from ceramic to exotic plastic molding. They are suited for precise silicon thinning as well as polishing. Finally, the automation and software control of the process offer good reproducibility of chip opening and preparation. For some applications, the silicon substrate needs to be thinned as closely as possible to the circuitry with a uniform thickness (less than 100 microns). Bent silicon surfaces are challenging for backside sample preparation. This is the case of C4 packages or large plastic TSOP packages. Conventional approaches would cut off the top of the bent surface. From small flat surface to large bent silicon dies, we will detail our technique for thinning silicon to a uniform thickness with extreme precision. Finally, we will characterize the final surface roughness which plays an important role in backside techniques.