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52 SMT Magazine • August 2014 INtRODuCtION tO INLINE aXI tEChNOLOgy continues aRTIClE Laminography One of the first practical 3D X-ray technolo- gies, laminography has been developed to pro- vide layer-based imaging of three-dimensional bodies. The objective is achieved through syn- chronized rotating movement of the X-ray source and detector, which ensures that objects on the chosen focal plane are projected onto the same spot on the detector, while all out-of-focus objects get blurred out. Each full scan generates a single focused layer slice, requiring additional scans to acquire slices at other height. Since ob- jects which are out of the focal plane are blurred out, they contribute to a relatively low-contrast image with comparatively sharp edges. All out-of-focus objects are automatically blurred by the physical rotation of the X-ray beam and the detector when each slice image is captured, which makes laminography com- putationally less demanding than other 3D AXI technologies. Computed Tomography Computed tomography (CT) is a solution designed for accurate imaging of complex 3-di- mensional internal structures using volumetric reconstruction from a large number of source images taken from a very wide range of view- ing angles. The main disadvantages of CT are very slow imaging rate (dozens of minutes) and a high radiation dose, but it can offer the high- est available detail resolution. These limitations make CT ideal for detailed inspection of com- plex individual components (e.g., µBGAs, PoPs) on prototype boards, QA and sampling during new product introduction (NPI), but unsuitable during volume production when inspection time is critical. Digital Tomosynthesis A more practical approach to computed to- mography, digital tomosynthesis, uses a limited number of source radiographs from a limited range of angles to reconstruct virtual cross-sec- tion images at arbitrary height. This approach requires significantly shorter imaging time, lim- iting the radiation dose and approaching pro- duction line speeds. A significant advantage of tomosynthesis over laminography is the ability to reconstruct virtual cross-sections of the inspected subject at any arbitrary height using only the images captured in the initial scan, avoiding addi- tional scanning and radiation exposure of the subject. Since objects at the same height will project to known locations on the camera array, we can use a simple shift-and-add operation to align the captured set of images with the in-focus parts overlapping to blur out all other objects. Figure 6: CT reconstruction of a bga package using micro CT reconstruction from a very large number of images. Figure 7: Computed tomography uses circular motion around the subject to gather shadow images from a very wide range of angles.