Application of scanning acoustic microscopy for thickness measurement and examination of topology of fracture surfaces

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Supervisor:
Dr. Gordon Péter Róbert
Department of Electronics Technology

The acoustic microscope allows a non-destructive inspection of the sample, helps detecting internal errors, such as delaminations, cracks, voids. My task was to get to know the theoretical foundations of acoustic microscopy and the operation of the acoustic microscope. I learned to manage the department's acoustic microscope, experienced what settings create a good quality image of the sample. I illustrated the effect of parameters in quality of pictures taken with the most common method, the C-mode scanning.

Time measurement based thickness measurement can be performed with the acoustic microscope. In my dissertation I searched the minimum limits of measurement because of the pulse length of the ultrasound, on the samples chosen by me. It turned out from my measurements, that the acoustic microscope can only be used for measuring thickness with limited conditions, in a sample four or five times thicker than the wavelength of sound in the material, the measurement accuracy is satisfactory. The relative error increases with the decrease of the sample’s thickness. Based on my measurements on steel samples I suggested a formula helping frequency selection for thickness measurements.

I investigated cracks in the die attach layer of power electronic components. I processed the grayscale image of the solder layer made by acoustic microscope in Matlab to get three-dimensional image of the surface of the crack. The results were compared with optical microscope measurements performed on cross-sections. The cross-sectional optical microscopy measurements confirmed my results with acoustic microscope. Acoustic microscope images of the die attach layers processed with Matlab combined with the cross-sectional measurements resulted in method, which gives a relatively accurate position of the crack located in the solder layer.

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