抄録
An experimental visual inspection technique using the slit light for the detection of cracks in/on alumina ceramics circuit boards has been presented. Since the alumina ceramics has a well light-diffusible property over all of visible wavelength range, it is difficult to detect cracks in the material by an ordinary inspection with the human eye or optical microscope observation with coaxial illumination. In the present technique, the surface of the specimen placed on a X-Y stage is illuminated from a proper direction by the slit light, which is adjusted to a small width with several condensing lenses, and the line image of the surface is sensed by a CCD line sensor camera through an object lens and inputted into the image device as intensity signal. The X-Y stage, which is driven by pulse motors with high accuracy, is used for the mechanical scanning for obtaining the two dimensional image of the surface.
As for the results, under specific observing conditions, including the camera position relative to the slit light, the incident angle to the surface, and the width of the slit light, some characteristic intensity changes were found not only around the crack, but also on the edge of the board and of the installation holes for IC parts. The optimum condition for the detection of the crack was experimentally examined. In order to interpret these features, a simple model is proposed, and it is clarified that the light uniformly diffused into the specimen gives rise to the characteristic pattern due to the reflection at the crack surface. Finally, utilizing these characteristic patterns, one image processing algorithm, which detects only cracks in the ceramics board and has a possibility to process in high speed, is presented.
