A method of image preprocessing based on nonlinear diffusion and information extraction

Thermally induced stresses play a very important role in controlling the structural reliability of microchip packages. In order to evaluate the magnitude of the warpage of measured objects caused by such stresses, the shadow moiré technique was suggested, where how the moiré fringes are extracted is the key feature. In this work, an improved filtering algorithm based on a nonlinear diffusion equation is developed and a medial axis transformation and pruning algorithm are applied to extract the skeleton of the moiré fringe. In order to remove the forficate fringe and isolated noise fringe, a main fringe extraction algorithm is then used, and this is discussed in detail. According to the moiré fringe information extracted from the shadow moiré pattern, the three-dimensional image which indicates warpage of the measured object’s surface is reconstructed. This demonstrates that the measuring accuracy can be improved up to 2.0 μm by using a grating of 100 lines per inch.