Void Identification
Volumetric non-destructive imaging resolves internal structural features of dense electronic assemblies through differential radiation attenuation. X-ray microtomography executes this task by rotating a mounted specimen between a focused point source and a flat panel detector, collecting hundreds of individual projection radiographs across three hundred sixty degrees. A specialized reconstruction algorithm subsequently processes these angular views into a three-dimensional Cartesian array of voxel density values.
This digital volume exposes hidden defects such as micro-voids inside ball grid array solder joints and interfacial cracking beneath packaged semiconductor dies without requiring destructive cross-sectioning.
Density Mapping
Contrast generation relies entirely upon the linear attenuation coefficient of each constituent material within the scanned device. Metallic intermetallic compounds, copper traces, and silicon dies attenuate high-energy photons heavily, yielding bright voxel intensities in the reconstructed dataset. Organic printed circuit board substrates and epoxy molding resins absorb far fewer photons, appearing as dark background regions.
Quantitative greyscale calibration maps these variations directly to material density, allowing technicians to compute total void percentage within thermal pad solder joints directly from the voxel histogram.
Resolution Limits
Geometric magnification governs the smallest detectable defect dimension through the ratio of source-to-detector distance divided by source-to-object distance. Finite focal spot dimensions introduce penumbral blurring that ultimately restricts spatial resolution to the sub-micron scale for specialized high-magnification configurations. Sample thickness and maximum operating tube voltage impose practical boundaries during dense multi-layer board inspection because excessive photon scattering degrades edge sharpness and obscures fine micro-cracks.
Higher tube power penetrates thick copper planes successfully, but the resulting photon scatter reduces signal-to-noise ratios and increases scan duration significantly.
