Three-dimensional imaging system using a varifocal mirror at video frame rate

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Two types of imaging systems based on perfect projection using a varifocal mirror were devised to achieve high-quality three-dimensional imaging and precise measurement of shape. The first was a constant-magnification imaging system based on orthographic projection and the second was a wide-angle imaging system based on perspective projection. The first system using a piezoelectric-type varifocal mirror operating at an almost video-frame-rate of 20 Hz was used for an experimental setup for measuring an array of solder bumps on an LSI package board 10-mm square in size, on the edges of which 208 bumps with a diameter of 300 μm were arrayed at a pitch of 500 μm. Shape of an upper part of a solder bump could be acquired precisely. Height errors were less than 12 μm, so that precision was sufficient for automated inspection of solder bumps. Some horizontal sections of solder bumps have been estimated to have sufficient information. To obtain sectional data at a specified height, we set a pair of focal points, H1 and H2, with a specified interval along the height axis. By comparing the two images for focal points H1 and H2 with every pixel and selecting pixels with the same focus measure values, we can detect points on the contour of the section located at the mid-point between focal points H1 and H2. We assumed the solder bump was a sphere with a constant radius. By using the horizontal section at a specified height to fit a sphere into the bump, the height of the bump top was efficiently evaluated. Sphere fitting to estimate the bump height from the section could be successfully applied to measuring the coplanarity of a ball bump array. The time to measure an LSI package board was estimated to be 7.2s, using a personal computer.