Visualization of electrical trees by 80 MHz-band pulsed ultrasound

To develop a non-destructive observation system of electrical trees in electric components such as power modules, we used an acoustic microscope with a center frequency of 80 MHz to image electrical trees in a simulated sample. The fine structure of trees approximately 0.5 mm in size could be observed. The spatial resolution for trees via this measurement system was about 40-50 μm. Furthermore, image processing was employed to acquire a clearer image. Assuming a Gaussian blur function, the original image was deconvolved, improving the spatial resolution by optimizing the standard deviate, which was 20 μm in this experimental condition. To confirm the spatial resolution of trees in an optically non-transparent sample, trees in epoxy resin containing alumina as well as an epoxy resin were observed. The half value width of the spatial resolution at tree channels for a non-transparent sample was about 50 μm, which corresponds to that for a transparent sample. Hence, ultrasonic microscope observations are suitable for the non-destructive inspection of electrical trees.