Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Patent
1985-04-02
1987-07-28
Griffin, Donald A.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
324158R, G01R 3122
Patent
active
046828575
ABSTRACT:
This improvement is the process of using a few well collimated and even radiative heating lights to heat up the liquid crystal film, which is spread over the surface of the die of an integrated circuit. The radiative heating of the liquid crystal film from the top helps to form an even temperature profile on the liquid temperature film. The rapid time response of the heating filament temperature and the radiative heating process induces a rapid response in the liquid crystal film temperature. By repeatedly turning on and turning off the heating lights at an appropriate power setting, the temperature of the liquid crystal film will repeatedly rise above and drop below the liquid crystal phase transition temperature. During this temperature rise and drop process, the liquid crystal film temperature is brought to infinesimally close below to the liquid crystal phase transisiton temperature, for a limited length of time. We call this temperature control process the infinitesimal temperature control method. During this limited length of time, a small ohmic heating dissipated from the die into the liquid crystal film would induce a localized phase transition in the liquid crystal film. Under a cross polarized light, the nematic liquid phase transition process exhibit a change in the liquid crystal's transparency and colors. The transition process is most easily visible when the die is periodically dissipating heat into the liquid crystal film at a 1.2 Hz and at 50% duty cycle. At this periodic heat dissipating mode, the periodic phase transistion induces a blinking appearance at the region where the phase transition is taking place. I call this blinking the `hot spot induced blinking`. This periodic ohmic heating is accompanied with periodic voltage change in the die. The voltage changes will induce a blinking appearance similar to the hot spot induced blinking. With the use of the infinitesimal temperature control method to vary the ambient temperature of the liquid crystal film the blinking spot size of the hot spot induced blinking increases as the ambient temperature increases. But the blinking spot size of the voltage induced blinking does not respond to temperature changes, as long as the liquid crystal's temperature is not beyond the phase transition temperature. Thus, the varied heating light provide a means to differentiate the two types of blinkings. We have illustrated heating lights as the means of heating up the liquid crystal film. However, if the heating lights were replaced by other heating means, such as a conductive hot plate, or a convective oven, the infinitesimal temperature control method would still work, as long as these alternate heating means were operated at a repeatedly turning on and turning off mode. Therefore, this hot spot detection process will also work well with other heating means.
REFERENCES:
patent: 4242635 (1980-12-01), Burns
John Hiatt, "A Method of Detecting Hot Spots On Semiconductors Using Liquid Crystals," 19th Annual Proceedings of the IEEE Reliability Physics Symposium, 1981, pp. 130-133.
E. M. Fleuren, "A Very Sensitive, Simple Analysis Technique Using Nematic Liquid Crystals," 21st Annual Proceedings of the IEEE Reliability Physics Symposium, 1983, pp. 148-149.
J. L. Fergason, "Liquid Crystals in Nondestructive Testing," Applied Optics, vol. 7, No. 9, Sep. 1968, pp. 1729-1737.
G. D. Dixon, "Cholesteric Liquid Crystal in Nondestructive Testing," Material Evaluation, Jun. 1977, pp. 51-55.
Peng Tan and David Burgess, "Improved Sensitivity for Hot Spot Detection Using Liquid Crystal", 22nd Annual Proceeding of the IEEE Reliability Physics Symposium, 1984, pp. 119-121.
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