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United States Patent 
Taraci et al.
ill ii:: mi 11 Hi 11:111:1 ill Ii: Iiiiii
 Patent Number:  Date of Patent:
4,745.354 5/1988 Fraser 324/158 F
4.884.169 11/1989 Cutchaw 361/385
5,001.548 3/1991 Iversen 357/82
Shock, R. A. W., "Nucleate Boiling . . . ", Int. J. Heat Mass Transfer; vol. 20; pp. 701-709; Dec. 1977. Yokouchi et al.; "Immersion Cooling . . . ", IEEE Trans, on Components Hybrids and Manufacturing Technology; vol. CHMT-12; No. 4; Dec. 1987; pp. 643-646.
Primary Examiner—Ernest F. Karlsen
Attorney, Agent, or Firm—Cahill, Sutton & Thomas
A method and apparatus for maintaining an electrically operating device at a desired temperature includes immersing the device in a bath of an inert liquid having a boiling point less than the desired device temperature. The device generates heat during electrical operation which is transferred to the bath by nucleate boiling of the liquid. The device temperature is monitored until it stabilizes at a temperature between the boiling point of the liquid and the desired device temperature. An inert liquid having a boiling point greater than the desired device temperature is slowly added to the bath to modify the rate of nucleate boiling of the lower-boiling liquid, while the device temperature is simultaneously monitored, until the case temperature reaches the desired device temperature.
8 Claims, 2 Drawing Sheets
U.S. Patent june 2,1992 sheet 2 of 2 5,119,021
METHOD AND APPARATUS FOR MAINTAINING ELECTRICALLY OPERATING DEV ICE TEMPERATURES
This application is a continuation in part of application Ser. No. 07/379,083, filed Jul. 13. 1989, now U.S. Pat. No. 5,004,973, entitled "METHOD AND APPARATUS FOR MAINTAINING ELECTRICALLY OPERATING DEVICE TEMPERATURES". On even date herewith, a continuation-in-part application having the same parent application is also being filed.
FIELD OF THE INVENTION
The invention relates generally to a method and apparatus for maintaining an immersed electrically operating device at a desired temperature by controlling the rate of nucleate boiling of the immersion liquid under elevated thermal conditions.
DESCRIPTION OF THE PRIOR ART
Many electrical devices which produce heat during operation have a high percentage failure rate during the first year of service. Such devices include capacitors, 25 resistors, and semiconductor devices including diodes, transistors and integrated circuits. Circuits are of particular concern because advances in semiconductor processing and circuit design have led to increased component density on the circuit, with a consequent increase 30 in heat generated per unit area of semiconductor chip surface.
The majority of the electrical devices that will fail during the first year of operation can, however, be eliminated from commercially available products by 35 subjecting the devices to a "burn-in" test. During a burn-in test the devices are subjected to extreme thermal or electrical operating conditions for a short period of time, typically, one to eight weeks, thereby simulating one year of operation under normal conditions. 40
Originally, burn-in tests were conducted in air or nitrogen. Recently, however, as disclosed in U.S. Pat. No. 4.745,354 to Fraser, burn-ins have been conducted with the devices immersed in a non-reactive electrically insulating liquid. The liquid serves to increase the ambi- 45 ent operating temperature of the devices and to dissipate excess heat produced by the operating devices. The liquid used in the Fraser process is unusually expensive and has a significant evaporation rate at the ele- J0 vated temperatures associated with burn-in tests. Therefore, to prevent extensive losses from evaporation, the liquid is carefully maintained below its boiling point by pumping it over mechanical cooling coils to remove heat generated by the operating devices. 55
Computer manufacturers also use immersion cooling to dissipate heat from semiconductor devices in operating computers. In such cooling systems, the devices are immersed in a dielectric, low boiling point liquid. During operation the heat generated by the devices causes ^ vapor bubbles of the liquid coolant to form in nucleation sites on the surface of the chip. A portion of the heat generated is absorbed as the latent heat of vaporization of the liquid, the remainder being absorbed by the convection of the liquid at the chip surface. The 65 process is called "nucleate boiling".
The following U. S. Patents are representative of nucleate boiling heat transfer methods and apparatus.
In each of the above cited patents, modules having heat generating components such as semiconductor devices are located within a low boiling point dielectric liquid. A vapor space is located above the liquid level. The electronic components heat the liquid causing nucleate boiling at the surface of the electronic components. The R. C. Chu, et al., U.S. Pat. No. 4,050,507 describes electronic chips having nucleate boiling sites located on at least the back surface of the chip and mounted so that the back surface is exposed and is oriented vertically. The Frieser, et al, U.S. Pat. No. 4,312,012 describes enhancing the nucleate boiling characteristics of silicon devices by forming lattice defects on the backside surface of the device by sandblasting and etching the damaged surface. The R. C. Chu, et al., U.S. Pat. No. 4,709,754 discloses a fin structure with an improved nucleate boiling surface. In general, nucleate boiling cooling technology has been directed at improving the nucleation sites at which boiling commences, rather than maintaining the temperature of the immersion bath or device case.
One problem with nucleate boiling cooling is that the liquids appropriate for immersion cooling exhibit an unexplained hysterysis characteristic, requiring a higher temperature to initiate nucleate boiling than is required to sustain boiling. The superheat required to initiate nucleate boiling may exceed the desired operating temperature of the heat-generating device, potentially damaging it, even though the eventual boiling temperature stabilizes at an acceptable level. The hysterysis effect can be best illustrated by referring to FIG. 1. FIG. 1 is a graph of temperature versus time for the nucleate boiling of immersion fluids. Curve 20 represents the temperature overshoot necessary to initiate nucleate boiling in the immersion fluids used in prior art nucleate boiling cooling systems.
Another problem associated with nucleate boiling cooling is that as heat is dissipated from the device, more and more bubbles are formed, eventually creating a film. The onset of film boiling marks the upper limit of nucleate boiling because the film blocks the liquid from reaching the chip surface, thereby significantly restricting heat transfer from the devices and potentially causing destructive overheating, or thermal runaway, of the devices.
The following publications discuss nucleate boiling and immersion cooling in general. R. A. W. Shock , "Nucleate Boiling , in Binary Mixtures", International Journal of Heat Mass Transfer, Vol. 20, 1977, pp 701-09. Yokouchi et al., "Immersion Cooling for High-Density Packaging", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-12, No. 4, December 1987, pp 643-46.
Problems associated with film boiling can be ameliorated by attaching heat sinks to the heat dissipating surface to increase the heat transfer area. U.S. Pat. No. 4,203,129 to Oktay, et al. discloses a heat sink containing tunnels so that the attached devices are cooled by nucleate boiling bubbles formed within the tunnels.