| Summary: | 博士 === 中原大學 === 機械工程研究所 === 106 === In this study, a high-resolution surface topography measurement system was developed using two line scan cameras for the full-field optical inspection of surface geometry and defects. The two cameras capture and record multiple line images of a test object simultaneously based on stereoscopic vision technique. Reconstruction algorithms were developed to reconstruct visual depth or 3D information of test objects. System performance was evaluated using different test specimens. Compared to area cameras of the same pixel resolution, the use of line scan cameras in the inspection system offers the advantages of processing efficiency, high sensitivity, and better resolution.
In the measurement, one camera vertically scans the object while the second camera captures the image of the same object from an angle. A datum line along the intersection of the two scan directions is assigned forming a datum surface with the scanning axis. Based on this surface, a 2D coordinate of an arbitrary point (x, y) from the image captured by the vertical camera is expressed in terms of the number scan lines and pitch between each scan line-image. Critical matching of the same physical points in the two images determines the 3D shape and height reconstruction of the object. To obtain the correspondence pairs from each line scan, a novel digital image correlation algorithm was developed. Calibration was established by mapping the horizontal coordinates of the object to the image coordinate in the cameras. The position difference in terms of pixel for each inspection point was identified by the algorithm. The height h(x, y) of the specimen was calculated from the difference in the number of scan lines between the two cameras. The horizontal resolution was set by the pixel spacing of the line scan cameras and the height resolution by the pitch of the line scan images. The height differences of three adjacent ladder-type gauge blocks were measured to establish the accuracy of the inspection system. The ball height and co-planarity of a ball grid array (BGA) were measured from 3D surface reconstruction, which identified overlooked defects from 2D inspection data. A Chinese ten-dollar coin specimen was reconstructed to reveal the date inscription and embossed profile on the coin. The 3D surface profiles of the test specimens were resolved with very good accuracy.
Aiming towards in-line measurement, the inspection system was automated for measurement of bond-line thickness (BLT) and die tilt in die attach chip. The same imaging and measurement principles were used. Initially, four points on the flip-chip substrate were marked in advance by the chip maker for reference. The positions of the four corners of the chip were determined from feature extraction on the specified region of interest using Hough transform, edge detection, image binarization, and image thinning from which four straight lines were obtained to delineate the edge of the chip surface. The 3D coordinates were determined from the intersection of two linear graphs on the chip and the center of the markings. The BLT and die tilt of the chip on a 800 mm thick substrate of cross section 15 x 15 mm2 were measured. The corresponding height and tilt were approximately 76.4 μm and 0.006, respectively. These measurements were verified experimentally, which shows that the inspection system is capable of providing sub-micrometer accuracy with high repeatability. Thus the automated measurement of the BLT on product line could be expected with the proposed technique.
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