Abstract:
A cooling system for absorbing thermal energy from an integrated circuit device while preventing the formation of condensation thereon. The cooling system comprises a evaporator unit mounted on an integrated circuit device, housed within an airtight enclosure which is evacuated and then pressurized with dry air.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to cooling technology for electronic packaging for computer systems, and, more particularly, relates to means for cooling and preventing the formation of condensation on cooled components within a computer system. 
     The high circuit densities and operating frequencies in modern integrated circuit devices and multi-chip modules of today&#39;s computer systems has resulted in a significant increase in the power dissipated by such chip and module components. No matter how fast one wishes to operate a given electronic circuit chip, there almost always is the potential for running it faster if the chip were to be cooled further and more thermal energy is removed during its operation. This is true of computer processor circuit chips and more particularly of such chips disposed within multi-chip modules that generate significant amounts of heat. Because of the demand to run processor modules at increasingly higher speeds, the clock frequencies at which the devices must operate also increases. Power generation correspondingly rises in proportion to the clock frequency, generating thermal demands in terms of energy which must be removed for faster, safer, and more reliable circuit operation. It is required that cooling arrangements be provided so that the heat generated by the operation of these components be effectively and efficiently removed in order to maintain the temperature of the devices within the limits that will keep the operating parameters of the devices in a predetermined range, and, that will keep the operating parameters of the devices in a predetermined range, and, further, to prevent the damage or destruction of the integrated circuit devices by overheating from the high temperatures generated. 
     Using refrigeration technology, integrated circuit chips and multi-chip modules readily can be cooled to appropriately low temperatures. In addition, however, to the essential of cooling to prevent damage from overheating, it is also recognized that cooling offers marked advantages in circuit speed, system throughput, and component reliability. For example, it is known that a CMOS circuit will operate at higher clock speeds as the circuit temperature is lowered, and, it has been reported that the processor frequencing of a CMOS processor has been improved by nearly threefold by cooling the processor to temperatures around −200° C. 
     Various techniques for the cooling of integrated circuit electronic devices are known and many have been implemented with success. Some practiced techniques involve conventional methods such as by directing ambient air onto the components to be cooled; by sealing the computer cabinet and refrigerating the interior of the cabinet; as well as by immersing components in coolants such as liquid nitrogen. Individual integrated chip or multi-chip module components also have been cooled through specialized devices such as hollow cold plates which are attached to the components to be cooled. Liquid coolants can be circulated through the hollow cold plates to effect cooling of the attached components. 
     However, cooling of the electronic components to a temperature below the ambient environment dew point results in condensation problems in that moisture will condensate on the cooled components and the structures and components to which the cooled components may be attached. This condensation can damage and literally destroy the electronic circuitry associated with the integrated chip or multi-chip module and the circuit board on which it is mounted. 
     Accordingly, a cooling system is needed for an integrated chip or multi-chip module device which can effectively reduce the temperature of the device to below ambient dew point without causing damage to the device through the effects of condensation. 
     SUMMARY OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the present invention. 
     Now, an improved system for cooling integrated circuit devices within a computer system has been developed whereby condensation is prevented from forming on the cooled devices. According to the present invention, a cooling system is provided which comprises a cooling evaporator unit having a thermal interface in thermal communication with an integrated circuit device mounted on a substrate within a computer system. The integrated circuit device typically is a multi-chip module (MCM) mounted on a planar circuit board contained within the cabinet of a computer system. The evaporator, generally comprising a thermal block, typically copper or aluminum, having coolant flow passages through its mass, is connected to a refrigeration unit to circulate coolant through the evaporator to cool the integrated circuit device to a desired temperature. To optimize performance of the chip device, it generally is cooled to a temperature below the dew point of the surrounding atmosphere. Typically, the device is cooled to a temperature ranging from about −10° C. to about +10° C. 
     The refrigeration unit typically includes a compressor, condenser, and expansion device connected in a closed refrigeration loop with the coolant flow passages in the thermal block. The coolant used may be any coolant fluid; R134A and R507, standard refrigerants known and used in the art, are preferred because of their environmentally friendly composition. 
     The cooling evaporator is enclosed within a housing. The evaporator block is attached, through a thermal grease layer, to the cover or hat of the integrated circuit devices. The integrated circuit devices are mounted on a printed circuit board substrate via solder interconnections. The cooling evaporator unit and the integrated circuit devices with which it is in thermal communication both are enclosed within the evaporator housing. The housing typically comprises a sealed aluminum chassis to which the thermal block is attached by suitable screws and insulating spacers. The thermal block in turn is attached to the integrated chip device, such as a MCM, by a series of bolts passing through the thermal block into the MCM cover or hat. The housing is designed to seal to the substrate so as to enclose the evaporator unit and the MCM mounted on the substrate. The seal between the housing and the substrate typically is accomplished through a suitable compliant gasket member, preferably a c-shaped gasket. A preferred gasket material is a butyl rubber. The housing is designed to form an airtight enclosure for the evaporator unit and the integrated circuit device when the housing is sealed to the substrate. 
     The housing is fitted with various sealed ports which connect to the evaporator unit. These ports provide flow communication for the coolant in and out of the evaporator from the refrigeration unit, and also include flow communication from a vacuum source to an air cavity around the MCM and evaporator. The vacuum source can be activated to reduce the pressure within the enclosure. This pressure reduction serves to draw off any moisture trapped within the enclosure during attachment of the housing enclosure to the substrate. A pressure detection device also is provided in communication with the enclosure in order to determine that the reduced pressure within the enclosure is maintained after the vacuum source is deactivated. The maintenance of the reduced pressure evidences the accomplishment of an airtight condition within the enclosure. A pressurized dry gas system, typically pressurized dry air, also is in flow communication with the enclosed evaporator, so that, once the airtight condition of the enclosure is verified, a positive pressure of dry gas can be maintained within the enclosure during operation of the cooling system. Preferably, the source of dry gas can be air drawn from the blower system of the computer cabinet, first passed through a desiccant canister to remove moisture. 
     The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 is a perspective view of an embodiment of a cooling evaporator forming one component of the cooling system pursuant to the present invention. 
     FIG. 2 is an exploded view illustrating an embodiment of a cooling evaporator unit and its housing enclosure. 
     FIG. 3 is an exploded view showing an evaporator housing enclosure illustrating chassis structure and accompanying seals, membranes and gaskets. 
     FIG. 4 is an end view illustrating an evaporator housing enclosure and its various external seals and connections. 
     FIG. 5 is a cross sectional view depicting an evaporator unit housed within an enclosure and installed on a substrate in a configuration to cool a multi-chip module. 
     FIG. 6 is a simplified front view showing the arrangement of components in a typical computer server cabinet. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 a cooling evaporator unit  10  is shown comprising thermal block  28 . Thermal block  28  includes dual coolant flow passage circuits, and accordingly shows dual coolant inlet conduits  12  and  14  and dual coolant outlet conduits  16  and  18 . Only one of the coolant circuits is operated at a time; the second circuit is a redundant backup in the event one circuit becomes non-operational. Coolant conduits  12 ,  14 ,  16 ,  18  terminate at face plate  20  through offset insulators  22  and sealable connectors  24  in a diagonally offset configuration. 
     Refrigeration units (not shown) are connected in flow communication through connectors  24  to circulate coolant through the conduits. Insulators  22  help prevent cooling loss by thermal conduction through face plate  20 . Two thermistor tubes  26 , one each located in the top and bottom portions of thermal block  28  receive respective thermistors whose respective cables  92  are sheathed in tubes  26  and terminate at connectors  30  mounted on chassis faceplate  20 . A vacuum tube  32  passes through the thermal block  28  and terminates in a pressure detection test valve  34  mounted on faceplate  20 . 
     FIG. 2 illustrates an evaporator unit  10  and its housing enclosure  36  in an exploded view. The enclosure  36  is a welded aluminum chassis. The evaporator unit  10  is secured within housing enclosure  36  through a series of four bolts (not shown) passing through bored holes at the four corners of the thermal block  28 . The bolts secure the evaporator unit  10  through spacers  38  which serve to help insulate the unit  10  from the walls of enclosure  36  to prevent cooling loss by thermal conduction through the aluminum walls. 
     In FIG. 3, an exploded view of housing enclosure  36  is illustrated, including various seals, membranes, and gaskets which serve to create an airtight condition when the enclosure is installed and in use. Stainless steel upper membrane  40 , lower membrane  42 , and lower pocket membrane  44  fit into the peripheral recesses of enclosure  36  and help to preserve the airtightness of the walls. When fitted into place, membranes  40  and  42  form pockets into which two small bags of desiccant (not shown) are positioned to help quickly dry out the inside of the enclosure once the evaporator block is mounted onto the MCM hat. Face plate  20  has a horizontal hole pattern  46  to receive horizontal insulators which secure the sealable connectors from the terminations of the coolant conduits of the evaporator. Insulator  48  fits into face plate hole  50  and serves to receive and secure a pressure detection test valve which is connected by a vacuum tube to the thermal block of the cooling evaporator. Faceplate gasket insulator  45  seals faceplate  20  to enclosure  36 . Gasket  52  typically is a C-ring or similarly shaped compliant seal member (preferably formed from a butyl rubber) and, when the evaporator within enclosure  36  is in operational position over an MCM on a circuit board, forms an airtight, but breakable, seal between the enclosure  36 , and the circuit board substrate. 
     FIG. 4 shows an end view of the housing enclosure  36 , as viewed from the faceplate  20  end of enclosure  36 . FIG. 4 simply depicts the seven screw plug O-ring seals  54  which screw into seven holes in the chassis of the aluminum enclosure  36  through which mounting screws (not shown) pass through to holes in the thermal block of the evaporator to attach the thermal block to the top or hat of the MCM. Seals  56 ,  58 , and  60  on face plate  20  serve to seal the terminations of the refrigeration coolant conduits, the thermistor cables, and vacuum tube, respectively. 
     FIG. 5 is a cross sectional view of a cooling evaporator unit  10  installed in an operational configuration, as the housing enclosure  36  is slid into place, guided into place by inclined ramps  62  mounted to a support bracket  64 . The thermal interface  66  of the thermal block  20  is in thermal communication with the multi-chip module  68  (MCM). A thin layer of oil  70  increases thermal communication between the two surfaces. As the MCM  30  generates thermal energy, it is conducted away through the evaporator  10  which is controlled by a cooling system controller (not shown). 
     In preferred embodiments of the present invention, it is clearly desirable that the thermal block  20  be made as flat as possible to conform to the exterior packaging of electronic module  68 . However, in those circumstances in which module  68  comprises a curved or even a stepped design, it is nonetheless possible to provide an appropriate thermal block by correspondingly machining or molding the thermal interface  66  of the thermal block  20 . However, in general, a good flat thermal mating surface is preferable. 
     The evaporator block  28  is preferably well insulated from the surrounding air to help prevent cooling loss. The housing enclosure  36  is filled with a urethane foam  72  which insulates the thermal block  28  and the refrigerant coolant conduits  74  from thermal losses due to air which is relatively warmer than the thermal block  28 . A desiccant bag  76  located below the thermal interface  66  helps to absorb any condensation which runs off the thermal block  28  or the MCM  68 . 
     The MCM  68  is mounted to a planar board  78  which is strengthened by rear stiffener  80  and front stiffener  87 . The stiffeners  80  and  82  are circumferentially sealed onto the planar board  78  by respective rear and front O-rings  84  and  86 . A gasket  52  circumferentially seals the front stiffener  82  and the housing enclosure  36  to create an airtight seal at the perimeter mating surface. 
     Once the housing enclosure  36  is in place against the MCM  68 , screws (not shown) are inserted into mounting holes  88 . The screw threads are received by matching threaded holes (not shown) in the MCM  68  and the evaporator is further mated to the MCM  68  by tightening of the screws. The holes in enclosure  36  through which these screws pass are sealed with screw plug O-ring seals  54  (shown in FIG.  4 ). 
     The two thermistor holes  90 , one each located in the top and bottom portion of the thermal block  28  receive respective thermistors  26  whose respective cables  92  terminate at their respective connectors  24  mounted on the enclosure faceplate  20 . 
     Vacuum tube  32  (shown in FIG. 1) is connected to thermal block  28  and is in flow communication with air cavity  94  around the periphery of the thermal interface  66  between thermal block  20  and MCM  68  and around and below MCM  68 . After the evaporator unit  10  is attached to MCM  68  and sealed to circuit board  28  through gasket  52 , a vacuum source (not shown) is attached to vacuum tube  32  through pressure detection test valve  34  mounted on face plate  20  (see FIG.  1 ). The vacuum source is activated to reduce the pressure to a desired level within the enclosed air space formed by stainless steel membranes ( 40 ,  42 , and  44 ), evaporator block  28 , circuit board  78 , and gasket  52 . When the vacuum source is deactivated, pressure detection test valve  34  serves to indicate whether the reduced pressure within this enclosed air space is maintained, so as to verify the airtight integrity of the various seals, membranes, gaskets, and plugs. 
     The vacuum source utilized to reduce the pressure within evaporator enclosure  36  through pressure detection test valve  34  and vacuum tube  32  preferably is a vacuum source external to the server cabinet  100 . The vacuum source (not shown) used typically is a vacuum device such as a portable hand held pump; the vacuum source is deactivated following pressure reduction in the enclosure  36  after installation of the evaporator unit on the MCM mounted on the circuit board. 
     Once the airtight condition of the mounted evaporator unit  10  is verified by pressure test valve  34 , dry gas, typically dry air, is introduced into the air space formed by the circuit board and the evaporator housing enclosure  36  through dry gas port  96  so as to create a positive pressure of dry gas within the enclosure at all times. Accordingly, not only is the enclosure evacuated to remove trapped moisture during assembly, but also the installed enclosure airspace is filled with dry air under positive pressure, so that if there is a breakdown in any of the seals over time, no ambient moist air will be able to enter the enclosure. The rate at which this dry air is introduced is constrained by using an appropriate length capillary tube between the dry air source and the air space within the evaporator to board assembly. This ensures long life of the dry air supply. 
     FIG. 6 is a simplified front view showing the arrangement of components in a typical server cabinet in which the present invention is included in the system. 
     The server system consists of cabinet  100  having air inlet opening  102  for the passage of air into the system for circulation there through. The flow of air is induced by means of one or more blowers  104 . 
     Fans or other air-moving devices could also be employed for this same purpose. However, blowers are preferred because of their efficiency. In the apparatus shown in FIG. 6 air moves from inlet  102  to be circulated through the cabinet  100  to an exhaust outlet (not shown). The flow of air is employed for the particular purpose of cooling certain of the electronic components disposed on printed circuit cards or so-called “books”  106 . Air flows down between these books or cards through blower(s)  104 . 
     The description provided thus far is therefore seen to disclose a preferred method for cooling certain of the electronic components, namely, those components located on cards  106  which are of sufficiently low power that air cooling is an appropriate modality. However, an important feature of the present invention is directed to the specific means and systems employed for cooling an electronic module (MCM). In preferred embodiments of the present invention the MCM includes circuits for data processor components associated with a mainframe or server computer system. In particular, the system shown in FIG. 6 illustrates the placement of cooling evaporator  10  which forms a significant aspect of the present invention. Evaporator  10  is connected by means of flexible refrigerant lines (not shown for clarity) to a refrigeration system present in the upper portion of cabinet  100  above shelf  108 . The refrigeration system  110  for cooling evaporator  10  and, thus, the MCM directly behind evaporator  10  and in thermal contact therewith, includes items such as motors, compressors and condensers together with an expansion device. Thus, the system shown in FIG. 6 represents an apparatus in which both air cooling and direct refrigerant cooling is employed. It is particularly useful for a proper understanding to note that the MCM is not mounted in a sideways fashion as are cards  106  primarily because of the fact that the MCM typically possesses a very large number of pins for achieving connection and communication with other circuits in the computer and/or with other computer systems. Thus, the I/O (input/output) pin requirements for the MCM dictate that it preferably be mounted in the fashion shown. In FIG. 6, this MCM electronic module is shown disposed beneath evaporator  10  which is visible in FIG.  6 . 
     The dry air used to pressurize the evaporator enclosure  10  through dry air port  96  preferably is air provided from air blowers  104  after passing through desiccant canister  114  to remove moisture from the air source. 
     As shown in the figure area  112  may include the same components as shown in the top half of the cabinet. Additionally, area  112  may also include power supply components (a power cage, e.g.) Along with its own air-moving device. However, the components disposed in are  112  are not otherwise directly relevant to the present invention. 
     While the invention has been described with reference to specific embodiments thereof, it is intended that all matter contained in the above description is shown in the accompanying drawings be interpreted as illustrative and not limiting in nature. Various modifications of the disclosed embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art upon reference to this description, or may be made without departing from the spirit and scope of the invention as defined in the amended claims.