An automatic through-hole inspection system by analyzing laser diffraction pattern

This paper describes a system for inspecting the inner surface of through-holes in a circuit board simultaneously and automatically. The system analyzes a diffraction pattern which appears in the shadow region around the through-holes by projecting a laser illumination on a circuit board. The diffraction pattern in the shadow region is generally observed with circular shape similar to the edge of the through-hole. However, radial fringes appear in the diffraction pattern of a through-hole, if the through-hole has defects on the inner surface itself. Our proposed system inspects a through-hole by processing an image captured the diffraction patter in the shadow region around the through-hole by a camera and finding fringes in the diffraction pattern. The system also inspects all through-holes in a whole circuit board by changing the capture area in the circuit board using a movable clamp automatically. In the experiment to evaluate our system, the system was able to detect only defective through-holes in a captured image including 20 through-holes whose diameter is 0.8 mm in a sample circuit board whose size is 90×68 mm. The time to inspect all 20 through-holes in the image was 0.35 second. And the time to inspect the whole sample circuit board was 77 seconds.