23 June 1995 Rapid evaluation of hermetic seals using digital shearography
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Packages such as microelectronic packages require hermetic seals, as the reliability of such devices is extremely important. One possible failure is due to leakage resulting from imperfect hermetical seals in microchips and microelectronic packages. Imperfect sealing may allow moisture and other impurities to migrate into the packages and cause the devices to malfunction. Current methods of testing hermetics are very slow and cumbersome and thus they cannot be used for mass inspection in production. This paper presents a novel technique based on digital shearography, which allows hermetic seals to be rapidly tested. This method employs a technique of digital shearography for measuring time-dependent deformation. In this paper, a figure is shown of the technique whereby the object to be studied is illuminated with a point source of coherent light and it is imaged by an image-shearing video camera which produces a pair of laterally sheared images on the image sensor of the camera. In other words, two object points are brought to meet in the image plane; they interfere producing a speckle pattern. When the object is deformed, a phase change is induced in the speckle pattern which measures the relative displacement between the interfering points. In the study of a time-dependent deformation of a test object, the speckle patterns received by the image- shearing video camera are continuously digitized at a predetermined rate depending on the deformation rate via a frame grabber into a micro-computer such as a PC, and stored in the memory of the computer. After recording, the displacement derivative versus time for any point of interest can be extracted by plotting the phase change of the speckle pattern at the point versus time from the computer memory. The total phase change can be obtained by integrating the phase curve. The evaluation of hermetic seals in microelectronic packages is based on measuring the time-deformation of the package's lid. The package to be tested is placed in an enclosure inside which the pressure can be varied. A change of pressure in the chamber is applied and held constant; the package surface will be deformed. If the package is perfectly sealed, the deformation will remain constant. Whereas, if the package leaks, the deformation of package surface will gradually recover cuasing a time-dependent deformation. Hence by measuring the deformation of the package's surface as a function of time with digital shearography, leaky packages can be revealed. Figures in this paper show a typical test result of a perfectly sealed package, and a typical result for a slow leaker, and a fast leaker. In the data, each cycle in the curve represents a deformation of $w/x of about 100 X 10-6. This process of detecting leaks in microelectronic packages is extremely fast, typically a couple of seconds. The conventional methods would probably require a couple of hours.
© (1995) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Y.Y. Hung, Dahuan Shi, "Rapid evaluation of hermetic seals using digital shearography", Proc. SPIE 2545, Interferometry VII: Applications, (23 June 1995); doi: 10.1117/12.212667; https://doi.org/10.1117/12.212667

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