Abstract:
An electronic apparatus including a motherboard, a plurality of multi-chip modules mounted to the motherboard and cooling members for cooling the multi-chip modules. A refrigeration unit is arranged to deliver cooling water to the cooling members. A connection structure is provided for each multi-chip module for thermally and mechanically releasably coupling each multi-chip module to the refrigeration unit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an electronic apparatus used in a computer system or the like.  
           [0003]    2. Description of the Related Art  
           [0004]    A computer system comprises an arithmetic operation section with integrated arithmetic circuits, a cache section operating at a high speed, for processing instructions and data, so as not to reduce the processing capacity of the arithmetic operation section, a memory section for storing the instructions and data sent to the cache section, and peripheral units for reading programs and data from the memory. Also, a system having a plurality of arithmetic operation sections for concurrent operation comprises a common memory section accessible by a plurality of arithmetic operation sections. Especially, the arithmetic operation section and the cache section operate at a high clock frequency in order to improve the operating speed, and involve considerable data changes for large power consumption and heat generation.  
           [0005]    Means for cooling these circuits include natural air cooling, forced air cooling, liquid cooling, immersion cooling, etc. Natural air cooling is low in cooling capacity and used for small computers which generate a small amount of heat. Forced air cooling and the liquid cooling are used for computers having a high processing capacity. The main application of immersion cooling is to special computers, in test stages, using the Josephson device or the like.  
           [0006]    The mainframe computer, as shown in FIG. 47, has a large arithmetic operation circuit, which is configured of a motherboard  201  having a plurality of LSIs  203  mounted thereon. This motherboard generates so much heat that it is cooled by a cooling plate which, in turn, is cooled by a cooling liquid. The cooling plate  206  includes bellows  207  on the portion thereof in opposed relation to the LSIs  203  which generate heat. These bellows  207  are brought into contact with the LSIs  203  to cool the latter. The refrigerant used for this cooling plate  206  is cooled down to about room temperature by a heat exchanger not shown. The refrigerant absorbs the heat of the LSIs  203  and returns to a heat exchanger not shown. Also, the bellows  207  have a spring property to closely contact the LSIs  203 , so that the LSIs  203  and the bellows  207  can thermally contact each other simply by mounting the cooling plate  206  at a predetermined position on the motherboard  201 .  
           [0007]    This apparatus has LSIs  203  mounted on the two sides of the motherboard  201 , though not shown, in order to improve the package density. In order to cool the LSIs  203  mounted on the two sides, two cooling plates  206  are arranged on both sides of the motherboard  201 . As a result, when the motherboard develops an abnormality or the requirement for replacement occurs due to the version up of hardware, it is necessary to remote the cooling plate  206  from the housing and then remove the motherboard  201 .  
           [0008]    Also, with the improvement in the performance of this apparatus and with the increase in the processing capacity of the LSI  203 , more heat is generated. To increasing the flow rate of the cooling water, however, a lower pressure resistance in the cooling plate, piping, etc. is required. However, the cooling plate or the like is arranged in a limited space and it is impossible to increase the size. If the temperature of the cooling water is decreased below the room temperature, on the other hand, water drips attach to the motherboard, thereby causing a malfunction. The area at which the motherboard  201  and the cooling plate  206  are thermally connected to each other is limited to the contact area between the LSIs  203  and the bellows  207 . To increase this area, a total change of the apparatus structure is required and this is difficult. For improving the cooling performance, therefore, the thermal resistance at the contact surface must be decreased. If the bellows  207  is pressed against the LSIs  203  with a greater force, the LSIs  203  and the motherboard  201  would be damaged. Also, a larger force would be required for mounting the cooling plate  206  for deteriorated maintainability. In view of this, a metal of low melting point is held between the LSIs  203  and the bellows  207  and melted to mount and demount the cooling plate. In this way, the contacting force is increased for a reduced thermal resistance.  
           [0009]    As a result, each time the motherboard is replaced or the cooling plate is mounted or demounted, the bothersome labor of spraying hot air higher in temperature than the melting point of the metal between the motherboard and the cooling plate is required for a very deteriorated maintenance efficiency.  
           [0010]    With the advance in CMOS techniques, the present-day mainframe computer uses the forced air cooling using radiation fins  204  as shown in FIG. 48. A circuit as large as the arithmetic operation circuit mounted on the motherboard  201  in FIG. 47 is formed of one chip by the recently-developed micromachining technology. Thus, the arithmetic operation circuit which has conventionally been configured of a plurality of motherboards is now formed of a single multi-chip module  202 . Further, the reduced size of the circuit has shortened the signal line and the operation with higher clock frequency is made possible. At the same time, the power saving unique to the CMOS technique eliminates the need for liquid cooling. In this way, the operation capacity of each motherboard  201  has been remarkably improved while the conversion of the cooling method from liquid cooling to forced air cooling has improved the maintenance efficiency.  
           [0011]    The change in cooling method from liquid cooling to forced air cooling has eliminated the need of installing the cooling plate and makes it possible to replace the motherboard directly. Also, since there is no need of piping work, the radiation fins can be divided into small units on the motherboard, and the radiation fin can be installed on each multi-chip module constituting a unit of parts on the motherboard. As a result, the multi-chip module can be handled with the radiation fin mounted thereon, and parts can be replaced without removing the cooling plate or the motherboard.  
           [0012]    Even with the recent technological development, however, the reduction in circuit size and the operation at a high clock frequency due to the reduced circuit size have reached a limit. As disclosed in JP-A-1-318295, it is known that a semiconductor device can operate at a higher rate of reaction and a higher clock frequency, when the temperature thereof is lower. It is, therefore, unavoidable to introduce such a technique for meeting the prevailing requirement of the processing performance. The technique of cooling the semiconductor devices, which generally includes the natural air cooling, forced air cooling and the liquid cooling, is the one for preventing the thermal breakdown due to the heat generated in the semiconductor itself. The immersion cooling, on the other hand, which is used for cooling the Josephson device or the like, is realized as a cooling method for maintaining the operating temperature of the device, but the application of this technique to the computers in general is difficult due to the maintenance problem.  
         SUMMARY OF THE INVENTION  
         [0013]    The object of the present invention is to provide a practical cooling structure, taking maintainability into consideration, in which a semiconductor circuit is cooled to its operating temperature adapted for operation at a high clock frequency.  
           [0014]    An electronic apparatus according to this invention comprises a motherboard, multi-chip modules mounted to the motherboard, cooling members for cooling the multi-chip modules, a refrigeration unit for cooling the cooling members to the room temperature or lower, and a connection structure provided for each multi-chip module for thermally and mechanically releasibly coupling each multi-chip module to the refrigeration unit.  
           [0015]    The cooling members can be mounted and dismounted while the multi-chip modules are being mounted to the motherboard. As a result, the motherboard can be easily separated from the cooling members, and thereby a cooling structure is realized which secures a sufficient maintainability. Also, in the case where the number of multi-chip modules is increased or decreased depending on the system configuration, the cooling members can be selectively arranged easily for a highly practicable configuration.  
           [0016]    The following features can be added to the configuration described above.  
           [0017]    The cooling member is fixed to the heat radiating member mounted to the multi-chip module by a fixing member (a screw, for example). Therefore, the electronic element can be cooled positively with the cooling member in close contact with the heat radiating member. By releasing the fixing member (a screw, for example), the cooling member and the multi-chip module can be mechanically separated without considering the troublesome leakage of the cooling water which may otherwise occur in the liquid cooling.  
           [0018]    The cooling members are collectively and floatingly supported by a cooling member holding mechanism. In the case where the cooling member is fixed to the heat radiating member by a fixing member (a screw, for example), therefore, the displacement between the cooling member and the heat radiating member is guaranteed. Also, when a plurality of the cooling members are moved away from and toward the heat radiating member, the job can be accomplished in a single operation of moving the cooling member holding mechanism. The maintenance work, therefore, can be done simply by operating the cooling member holding mechanism, thus realizing a cooling mechanism very high in maintainability for the motherboard and the multi-chip module.  
           [0019]    The cooling member has a refrigerant inlet, a refrigerant outlet, and a refrigerant path extending between the refrigerant inlet and the refrigerant outlet to circulate the refrigerant at lower than the room temperature. The multi-chip module can thus be cooled efficiently.  
           [0020]    The cooling member holding mechanism, together with a plurality of the cooling members, can be moved toward or away from the motherboard. In this case, the cooling member holding mechanism is wholly movable along a slide mechanism. In the maintenance work to be performed on the multi-chip module, therefore, the electronic element mounted on the motherboard is exposed by pulling out the cooling member holding mechanism along the slide mechanism and rotating it like the door.  
           [0021]    The cooling member holding mechanism includes a movable portion formed to move, together with each cooling member, toward or away from the motherboard. When the maintenance work is performed on a single multi-chip module, for example, the particular multi-chip module can be exposed by opening only one movable portion of the cooling member holding mechanism.  
           [0022]    The multi-chip module is mounted on the motherboard by a connector, and the movable portion of the cooling member holding mechanism is configured to operate in coordination with the connector removing means. By doing so, at the time of maintenance work on a single multi-chip module, the particular multi-chip module can be released from the motherboard by opening only one movable portion of the cooling member holding mechanism.  
           [0023]    The multi-chip module is a common board on which a multiplicity of connecting pins for connecting the motherboard and a plurality of semiconductor chips are mounted. A cooling member is mounted on the other side of the board to provide the cooling performance. This multi-chip module is formed for each set of arithmetic operation circuits. In realizing a computer system comprising eight arithmetic operation circuits in parallel, for example, eight multi-chip modules may be mounted on the motherboard.  
           [0024]    The connection structure includes, for example, a coupler arranged at the refrigerant inlet and the refrigerant outlet of the cooling member. In this way, the cooling member can be mounted on the multi-chip module to constitute a unit, thereby facilitating the system configuration. Also, the refrigeration unit and the multi-chip module can be separated from each other by the coupler, so that the thermal and mechanical replacement is facilitated. Further, the use of the coupler with a refrigerant stopper makes the replacement possible conveniently without removing the refrigerant from inside the cooling member. Furthermore, if the cooling member is of the same size as the multi-chip module, and if clogged with a refrigerant, it can be handled with high maintainability by making the unit of such a weight as to be capable of being carried in hand. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The present invention will become more apparent from the following description of the preferred embodiments, with reference to the accompanying drawings, in which:  
         [0026]    [0026]FIG. 1 is a view showing a computer system according to the embodiment of the present invention;  
         [0027]    [0027]FIG. 2 is a perspective view showing an electronic apparatus according to an embodiment of the invention;  
         [0028]    [0028]FIG. 3 is an exploded perspective view of a part of the electronic apparatus shown in FIG. 2;  
         [0029]    [0029]FIG. 4 is an exploded perspective view of the remaining part of the electronic apparatus shown in FIG. 2;  
         [0030]    [0030]FIG. 5 is a view showing the electronic elements mounted to the motherboard;  
         [0031]    [0031]FIG. 6 is a cross-sectional view showing the cooling member holding mechanism holding the cooling members;  
         [0032]    [0032]FIG. 7 is a front view of the cooling member holding mechanism;  
         [0033]    [0033]FIG. 8 is a view showing the moving mechanism for the cooling member holding mechanism;  
         [0034]    [0034]FIG. 9 is a view showing the cooling member holding mechanism in the outwardly slid position;  
         [0035]    [0035]FIG. 10 is a view showing the cooling member holding mechanism in the rotated position after being slid;  
         [0036]    [0036]FIG. 11 is a plan view showing the cooling member holding mechanism of FIG. 10;  
         [0037]    [0037]FIG. 12 Is a view showing a configuration in which the cooling member holding mechanism is partially moved;  
         [0038]    [0038]FIG. 13 is a view showing the movable portion of FIG. 12 in open state;  
         [0039]    [0039]FIG. 14 is a view showing an example in which the box structure is a hermetically closed structure;  
         [0040]    [0040]FIG. 15 is a view showing another example in which the box structure is a hermetically closed structure;  
         [0041]    [0041]FIG. 16 is a view showing still another example in which the box structure is a hermetically closed structure;  
         [0042]    [0042]FIG. 17A is a view showing an example in which a heating element is arranged in the narrow space between the motherboard and the electronic elements;  
         [0043]    [0043]FIG. 17B is a graph showing the humidity change in the box structure from the start of the operation of the drying unit;  
         [0044]    [0044]FIG. 18 is a view showing an example in which blowers move the air in the narrow space between the mother board and the electronic elements;  
         [0045]    [0045]FIG. 19 is a view showing an example in which air introduction means for introducing dry air is arranged in the narrow space between the motherboard and the first electronic elements;  
         [0046]    [0046]FIG. 20 is a plan view showing one plate member constituting the cooling member;  
         [0047]    [0047]FIG. 21 is a cross-sectional view of the cooling member;  
         [0048]    [0048]FIG. 22 is a plan view showing another example of one plate member constituting the cooling member;  
         [0049]    [0049]FIG. 23 is a plan view showing another example of one plate member constituting the cooling member;  
         [0050]    [0050]FIG. 24 is a graph showing the relationship between the load and the thermal resistance with the surface roughness of the cooling member and the radiator as parameters;  
         [0051]    [0051]FIG. 25 is a cross-sectional view showing an example of the method of mounting the heat radiating member and the cooling member;  
         [0052]    [0052]FIG. 26 is a cross-sectional view showing another example of the method of mounting the heat radiating member and the cooling member;  
         [0053]    [0053]FIG. 27 is a plan view showing another example of the method of mounting the heat radiating member and the cooling member;  
         [0054]    [0054]FIG. 28 is a cross-sectional view of the heat radiating member and the cooling member of FIG. 27;  
         [0055]    [0055]FIG. 29 is a view showing an example of cooling the second electronic elements;  
         [0056]    [0056]FIG. 30 is a view showing another example of cooling the second electronic elements;  
         [0057]    [0057]FIG. 31 is a view showing still another example of cooling the second electronic elements;  
         [0058]    [0058]FIG. 32 is a side view from the arrow of FIG. 31;  
         [0059]    [0059]FIG. 33 is a view showing another example of cooling the second electronic elements;  
         [0060]    [0060]FIG. 34 is a view showing still another example of cooling the second electronic elements;  
         [0061]    [0061]FIG. 35 is a view showing a further example of cooling the second electronic elements;  
         [0062]    [0062]FIG. 36 is a view showing a yet further example of cooling the second electronic elements;  
         [0063]    [0063]FIG. 37 is a view showing an example of the structure of the cooling member for the second electronic elements;  
         [0064]    [0064]FIG. 38 is a view showing another example of the structure of the cooling member for the second electronic elements;  
         [0065]    [0065]FIG. 39 is a cross-sectional view showing the refrigerant path in the configuration of FIG. 38;  
         [0066]    [0066]FIG. 40 is a cross-sectional view showing the refrigerant path in the configuration of FIG. 41;  
         [0067]    [0067]FIG. 41 is a view showing another example of the structure of the cooling member for the second electronic elements;  
         [0068]    [0068]FIG. 42 is a view showing still another example of the structure of the cooling member for the second electronic elements;  
         [0069]    [0069]FIG. 43 is a view showing an example of arrangement of the hoses of the refrigerant supply means in the cooling member holding mechanism;  
         [0070]    [0070]FIG. 44 is a side view of the manifold in FIG. 43;  
         [0071]    [0071]FIG. 45 is a view showing another example of arrangement of the hoses of the refrigerant supply means in the cooling member holding mechanism;  
         [0072]    [0072]FIG. 46 is a view showing still another example of arrangement of the hoses of the refrigerant supply means in the cooling member holding mechanism;  
         [0073]    [0073]FIG. 47 is a view showing a conventional liquid cooling structure; and  
         [0074]    [0074]FIG. 48 is a view showing a conventional forced air-cooling structure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0075]    Embodiments of the present invention will be explained below with reference to the drawings.  
         [0076]    [0076]FIG. 1 shows a computer system  10  according to the embodiment of the invention. The computer system  10  includes an electronic apparatus  12  having a plurality of multi-chip modules making up the essential parts of the system such as a CPU, and other peripheral unit  14 . The multi-chip modules of the electronic apparatus  12  making up the essential part of the system require cooling for high-speed operation. Therefore, a refrigeration unit  16  is arranged in a position adjacent to the electronic apparatus  12 .  
         [0077]    The refrigeration unit  16  includes a compressor, a condenser and an evaporator (not shown), to decrease the temperature in the refrigeration unit  16  to the room temperature or lower. The refrigeration unit  16  has arranged therein a tank  18  and a pump  20 . The refrigerant (water) cooled in the refrigeration unit  16  to the room temperature or lower is sent to the electronic apparatus  12  by the pump  20  from the tank  18  as shown by the arrow A. The refrigerant that has left the electronic apparatus  12  is returned to and recooled by the refrigeration unit  16 .  
         [0078]    A drying unit  22  is arranged under the electronic apparatus  12 . The drying unit  22  includes drying or dehumidifying means such as silica gel. The dry air is sent from the drying unit  22  to the electronic apparatus  12  as shown by the dashed arrow B. The drying air that has left the electronic apparatus  12  is returned to and dried again by the drying unit  22 . In this way, by sending the dry air to the electronic apparatus  12 , the strongly-cooled interior of the electronic apparatus  12  is protected from dewing.  
         [0079]    [0079]FIG. 2 is a perspective view showing the electronic apparatus  12  of FIG. 1. FIG. 3 is a view showing a part of the electronic apparatus  12  of FIG. 2, and FIG. 4 is a view showing the remaining part of the electronic apparatus  12 . In FIGS.  2  to  4 , the electronic apparatus  12  includes a casing  24 . The casing  24  is shown with its side wall removed. A box structure  26  is arranged in the casing  24 , and a frame  28  is arranged in the box structure  26 . The box structure  26  has a door  26   a.    
         [0080]    A motherboard  30  is mounted to the frame  28 . A plurality of first electronic elements  32  and a plurality of second electronic element groups  34  are mounted to the motherboard  30 . Further, a cooling member holding mechanism  36  collectively holds plate-like cooling members ( 56  of FIG. 6) for cooling the first electronic elements  32 . The cooling members  56  are arranged in a facing relationship with the multi-chip modules (first electronic elements)  32 . The cooling member holding mechanism  36  has hoses  38  connected to the cooling members  56 . Also, the cooling member holding mechanism  36  holds the cooling members ( 14  in FIG. 29) of the second electronic element groups  34 , described later.  
         [0081]    Hoses  40  and  42  extending from the refrigeration unit  16  and constituting a part of the refrigerant supply means are mounted to the under side of the box structure  26 . The hoses  40  and  42  are surrounded by insulating members  44 . Hoses  38  are connected to the hoses  40  and  42  through manifolds described later. Further, ducts  46  extending from the drying unit  22  are mounted to the under side of the box structure  26 .  
         [0082]    The first electronic elements  32  comprises a chip module including a plurality of semiconductor chips (CMOS) mounted to a common board. This chip module is called a multi-chip module (MCM) and makes up the essential part of the system. The second electronic element group  34  comprises a plurality of second electronic elements  34   a  (FIG. 29). The second electronic elements  34   a  comprises semiconductor chips for RAM, for example, mounted to a printed board. The first electronic elements  32  and the second electronic element groups  34  are mounted to the same motherboard  30 .  
         [0083]    The casing  24  includes a power unit  48 . When the frame  28  is arranged in the box structure  26  and the box structure  26  is arranged in the casing  24 , the power unit  48  is connected to the motherboard  30  by a cord (not shown), so that power is supplied to the first electronic elements  32  and the second electronic element groups  34  through the motherboard  30 . A hole  50  through which the power cord is passed is shown in the box structure  26  of FIG. 3. The power cord is passed through the hole  50  and connected to the motherboard  30 , after which the hole  50  is hermetically sealed.  
         [0084]    [0084]FIG. 5 shows the first electronic elements  32  and the second electronic element groups  34  mounted to the motherboard  30 . Connectors  52  are mounted to the motherboard  30 , and the first electronic elements  32  are adapted to be plugged in the connectors  52 . Heat radiating plates  54 , called heat sinks, are fixed to the first electronic elements  32 . The plate-like cooling members  56  are brought in contact with the heat radiating plates  54  to thereby cool the first electronic elements  32 . The cooling members  56  are held by the cooling member holding mechanism  36 , and a refrigerant is supplied therethrough. Also, connectors  53  for the second electronic elements  34   a  of the second electronic element groups  34  are mounted to the motherboard  30 .  
         [0085]    [0085]FIG. 6 is a cross-sectional view showing the cooling member holding mechanism  36  for holding the cooling members  56 , and FIG. 7 is a front view of a part of the cooling member holding mechanism  36 . The cooling member holding mechanism  36  is a tabular member having a size approximately identical to that of the motherboard  30 , and has openings  36   a  slightly larger than the cooling members  56  at positions where the cooling members  56  are to be held (the positions opposed to the first electronic elements  32 ). The cooling members  56  are arranged in the openings  36   a  and supported on the side wall of the opening  36   a  by springs  58 . In other words, the cooling members  56  are floatably held by the cooling member holding mechanism  36  and absorb the positional error between the motherboard  30  and the frame  28 .  
         [0086]    The cooling member holding mechanism  36  is formed such that the cooling members  56  are brought into contact with the heat radiating plates  54 . Hoses  38  are connected to the cooling members  56 , and the refrigerant is passed through the refrigerant paths in the cooling members  56 . Preferably, each cooling member  56  is fixed to the heat radiating plate  54  by a screw  60 . Consequently, the cooling member  56  closely contacts the heat radiating plate  54  to thereby effectively cool the first electronic elements  32  through the heat radiating plates  54 .  
         [0087]    [0087]FIG. 8 shows a moving mechanism for the cooling member holding mechanism  36 . FIG. 9 shows the cooling member holding mechanism  36  in outwardly slid position, and FIG. 10 is a view showing the cooling member holding mechanism  36  in a rotated position after sliding. FIG. 11 is top view showing the cooling member holding mechanism  36  of FIG. 10.  
         [0088]    As shown in FIGS. 4, 8,  9  and  10 , the frame  28  arranged in the box structure  26  has a slide guide  62  on both sides thereof, and the cooling member holding mechanism  36  has slide portions  64  slidable along the slide guides  62 . Therefore, the cooling member holding mechanism  36  is movable toward and away from the motherboard  30  together with the cooling plate members  56 .  
         [0089]    The cooling member holding mechanism  36  at a position near the motherboard  30  is shown in FIG. 8, and the cooling member holding mechanism  36  at a position far from the motherboard  30  is shown in FIG. 9. Further, the cooling member holding mechanism  36  is wholly rotatable like a door around a supporting point  66  arranged on the slide portion  64 . As a result, as shown in FIGS. 10 and 11, the cooling members  56  and the cooling member holding mechanism  36 , after being pulled out, rotate collectively like a door around the supporting point  66  of the cooling member holding mechanism  36 . This facilitates the maintenance and replacement of the essential parts of the system as a whole.  
         [0090]    FIGS.  8  to  11  show a configuration in which the whole cooling member holding mechanism  36  moves, while FIGS. 12 and 13 show a configuration in which the cooling member holding mechanism  36  moves partially. As shown in FIG. 7, the cooling member holding mechanism  36  has defined areas  36 X slightly larger than the area of the opening  36   a  in which cooling members  56  are arranged. FIGS. 12 and 13 also show the defined area  36 X of the cooling member holding mechanism  36 .  
         [0091]    The cooling member holding mechanism  36  includes a movable portion  36 Y formed to move toward and away from the motherboard  30  together with each cooling plate member  56 . The movable portion  36 Y is arranged to move about a supporting point  68  arranged in the defined area  36 X of the cooling member holding mechanism  36 , and is movable between the position where the cooling member  54  is in contact with the heat radiating plate  54  as shown in FIG. 12 and the position where the cooling member  56  is apart from the heat radiating plate  54  as shown in FIG. 13. Thus, by opening the movable portion  36 Y, the maintenance and replacement work on the first electronic element  32  which otherwise poses a problem can be accomplished. The movable portion  36 Y is also arranged at the position of the second electronic element group  34 .  
         [0092]    Further, in FIGS. 12 and 13, the connector  52  for mounting the first electronic element  32  to the motherboard  30  is shown. The connector  52  mounted to the motherboard  30  is attached to a cam  70 . The movable portion  36 Y of the cooling member holding mechanism  36  is coupled to the cam  70  by a link  72 , so that when the movable portion  36 Y is moved, the cam  70  rotates to thereby mount or demount the package for the connector  52 . When the movable portion  36 Y of the cooling member holding mechanism  36  opens, therefore, the first electronic elements  32  can be removed from the connector  52 .  
         [0093]    According to this invention, the refrigerant temperature is not higher than room temperature the refrigerant is supplied by refrigerant supply means to the cooling members  56  through the hoses  38 ,  40  and  42 . Therefore, the first electronic elements  32  can be effectively cooled and operate at a high speed. By supplying the refrigerant having a temperature not higher than room temperature to the cooling members  56 , the system performance is improved at a low cost. Preferably, the refrigerant supply means supplies water having a temperature not lower than 3° C. but not higher than 10° C. to the cooling members  56 . More preferably, the refrigerant supply means supplies water containing an antifreeze liquid at a temperature not higher than 3° C. to the cooling member  56 .  
         [0094]    Further, as shown in FIGS.  14  to  16 , the box structure  26  is formed as a substantially hermetically sealed container. Preferably, the box structure  26  is hermetically sealed with the water penetration rate not more than 1 gram per day (1 g/day) in an environment of 30° C., 70%RH and not higher than 26° C. in wet-bulb temperature.  
         [0095]    The motherboard  30 , the electronic elements  32  and  34   a , the heat radiating plates  54 , the cooling members  56  and the cooling member holding mechanism  36  are housed in the box structure  26 . In FIGS.  14  to  16 , only the motherboard  30  is shown as representative. By making the box structure  26  hermetically sealed and circulating dry air, the dew point of the neighborhood of the electronic elements  32  and  34   a  in the box structure  26  is kept at a temperature not higher than the refrigerant supply temperature so that the electronic elements  32  and  34   a  are not affected by dewing.  
         [0096]    Further, the drying unit  22  removes moisture in the box structure  26  by supplying the dry air into the box structure  26  through the duct  46 . As shown in FIG. 15, the box structure  26  preferably includes a partitioning wall  74  for separating the air flow supplied from the drying unit  22  from the air flow returning to the drying unit  22 . Also, at least one of the duct  46  and a fan  76  is provided in the box structure  26  to ensure smooth circulation of the dry air supplied from the drying unit  22  in the box structure  26 . In this way, moisture is removed from the interior of the box structure  26  to prevent the electronic elements  32  and  34   a  from being affected by moisture.  
         [0097]    Furthermore, the wall of the box structure  26  has a portion with a thermal insulating layer and a portion without an insulating layer. The wall with an insulating layer can maintain the interior of the box structure  26  at a low temperature. In the case where the atmospheric temperature outside of the box structure  26  is higher than the internal air temperature of the box structure  26 , however, the outer surface of the box structure may be dewed. In the case where the atmospheric temperature outside of the box structure  26  is lower than the internal temperature of the box structure  26 , on the other hand, the interior of the box structure  26  may be overheated. In order to prevent these inconveniences, the wall of the box structure  26  is sometimes preferably partially provided with an insulating layer.  
         [0098]    In FIG. 14, the insulating layer  78  is provided only at the under side of the box structure  26 . This corresponds to the fact that the portion at the under side of the box structure  26  is liable to decrease in temperature. In FIG. 15, the insulating layer  78  is formed only at the downstream portion of the dry air circulating from the drying unit  22 . This corresponds to the fact that the portion at the downstream portion of the drying air in circulation is liable to decrease in temperature. Also, by positively agitating the internal air, the internal environment of the box structure can be made uniform and the humidity in the narrow gap which tends to remain humid at the time of start of the drying unit can be sharply reduced within a short time. In FIG. 16, the radiation insulating layer  78  is formed only on the inner surface of the box structure opposed to the surface of the electronic elements  32  and  34   a  and both sides of the motherboard  30 . This corresponds to the fact that the temperature of the inner surface of the box structure opposed to the cold surface is liable to decrease due to heat exchange by the radiator.  
         [0099]    [0099]FIG. 17A shows an example in which a heating element  80  is inserted in a narrow space between the motherboard  30  and the first electronic element  32 . The heating element  80  is energized by control means  82  through a lead wire  84 .  
         [0100]    [0100]FIG. 17B shows a humidity change in the box structure  26  from the start of the operation of the drying unit  22 . The curve P indicates the humidity change in the narrow space between the motherboard  30  and the first electronic element  32  (the position P in FIG. 17A, for example), and the curve Q indicates the humidity change in the broad space around the first electronic element  32  (the position Q in FIG. 17A, for example). As the drying unit  22  starts operation, the large air flow reduces the humidity with comparative rapidity in the broad space around the first electronic element  32 . In the curve P, however, the air flow is so small in the narrow space between the motherboard  30  and the first electronic element  32  that the humidity decreases more slowly. Thus, the first electronic element  32 , when cooled, may receive dew on the surface thereof.  
         [0101]    In view of this, the heating element  80  of FIG. 17A is supplied with power within an initial predetermined time of operation of the drying unit  22 , whereby the moisture existing in the narrow space between the motherboard  30  and the first electronic elements  32  is evaporated while at the same time generating an ascending air current, so that the humidity of the narrow space between the motherboard  30  and the first electronic elements  32  is rendered to approach the humidity of the broad space around the first electronic elements  32  as the curve P′ of FIG. 17B. In this case, however, the provision of the heating element  80  is not always required but the first electronic element  32  may be used as the heating element  80 .  
         [0102]    [0102]FIGS. 18 and 19 show examples in which air introduction means  86  is provided for introducing dry air into the narrow space between the motherboard  30  and the first electronic elements  32 . In FIG. 18, the air introduction means  86  is a fan which is opposed to the first electronic elements  32  on the motherboard  30 , and blows the air at the broad space around the first electronic elements  32  into the narrow space between the motherboard  30  and the first electronic elements  32 .  
         [0103]    In FIG. 19, the air introduction means  86  is formed as a small tube, and the dry air is introduced as an air flow by the drying unit (device)  22 . In these cases, too, within an initial predetermined time of operation of the drying unit  22 , the humidity in the narrow space between the motherboard  30  and the first electronic elements  32  is rendered to approach the humidity in the broad space around the first electronic elements  32  as early as possible.  
         [0104]    [0104]FIG. 20 is a plan view showing a plate member  56   a  of the cooling member  56 , and FIG. 21 shows a cross-sectional view of the cooling member  56 . The cooling member  56  consists of two plate members  56   a  and  56   b  laid one on the other and coupled to each other by brazing. The cooling member  56  has a refrigerant path  88 , a refrigerant inlet  90  and a refrigerant outlet  92 . The refrigerant path  88  is formed of a plurality of parallel passages  88   a  grooved on a surface of the plate member  56   a.    
         [0105]    In FIG. 20, the refrigerant path  88  is formed of four parallel passages  88   a . The refrigerant path  88  of the refrigerant member  56   a  turns two returns on the surface of refrigerant member  56   a , and the parallel passages  88   a  merge at each bend. Thus, the refrigerant can be distributed widely over the surface of the cooling plate  56 , resulting in advantages in increasing the effective cooling area and improving the cooling efficiency.  
         [0106]    [0106]FIG. 22 is a plan view showing another example of the plate member  56   a  constituting the cooling member  56 . In this example, the refrigerant path  88  includes two parallel passages  88   a , which turns five returns over the surface of the plate member  56   a.    
         [0107]    [0107]FIG. 23 is a plan view showing another example of the plate member  56   a  constituting the cooling member  56 . In this example, a refrigerant path  88  consists of three parallel passages  88   a , and the three parallel passages  88   a  make three turns over the surface of the plate member  56   a.    
         [0108]    A pressure loss of refrigerant flow through the refrigerant path  88  of the cooling member  56  is less than 700 Pa per cm 2  of the heat transfer area of the cooling member  56 , and a cooling performance is less than 7623° C./W per cm of the heat transfer area of the cooling member  56 .  
         [0109]    [0109]FIG. 24 is a diagram showing a relationship of the thermal contact resistance to the mechanical load, with surface roughness of the cooling member  56  and the heat transfer plate  54  varying as parameters. As seen in FIG. 24, the smoother the surface roughness, the smaller the thermal contact resistance and the higher the heat transfer from the heat transfer plate  54  to the cooling member  56 . In FIG. 24, the surface contacting roughness is 0.025 μm, 0.05 μm, 0.1 m, 0.2 μm and 0.8 μm respectively. If the surface roughness Ra of the cooling member  56  and the heat transfer plate  54  are not more than 0.2 μm in terms of the average roughness along the center line, as shown in the figure, the thermal contact resistance then can be kept in within a satisfactory level without imposing a large mechanical load.  
         [0110]    [0110]FIG. 25 is a cross-sectional view showing an example of mounting the heat transfer plate  54  and the cooling member  56 . Spacers  94  are arranged between the motherboard  30  and the heat transfer plate  54  in such a manner as to maintain a gap between the first electronic element  32  and the heat transfer plate  54 . Spacer mounting means  96  are mounted on the motherboard  30 . The spacer mounting means  96  can adjust the position of the spacers  94  as to maintain a gap between the first electronic element  32  and the heat transfer plate  54 . The heat transfer plate  54  is fixed by the heat transfer plate mounting means  98  at a position predetermined by the spacers  94 . The cooling member  56  is attached directly to the heat transfer plate  54  as described above. In this way, the cooling member  56  collectively held by the cooling member holding mechanism  36  is positively placed in contact with the heat transfer plate  54 .  
         [0111]    A high heat conductive material  100  is filled between the first electronic element  32  and the heat transfer plate  54 . The high heat conductive material  100  is a compound for filling the gap between the heat transfer surface of the first electronic element  32  and the heat receiving surface of the heat transfer plate  54 .  
         [0112]    [0112]FIG. 26 is a cross-sectional view showing another example of mounting the heat transfer plate  54  and the cooling member  56 . This example induces pressure means  102  for bringing the heat transfer plate  54  and the cooling member  56  into pressure contacting with each other. As a result, the cooling member  56  is positively brought into contact with the heat transfer plate  54 , and the first electronic element  32  can be effectively cooled. In this embodiment, the pressure means  102  is an expandable container. By injecting a fluid into this container, the container expands and presses the cooling member  56  against the heat transfer plate  54 . The pressure means  102  is inserted between the cooling member  56  and a support wall  104 .  
         [0113]    [0113]FIGS. 27 and 28 are cross-sectional views showing another example of mounting the heat transfer plate  54  and the cooling member  56 . In this example, the spacers  94  are movable in the direction perpendicular to the heat transfer plate  54 . Specifically, the spacers  94  are formed as spacer pins inserted into the holes of the heat transfer plate  54 , and the tip of the spacer  94  is adapted to be in contact with a seat  96  mounted on the motherboard  30 . The tip of each spacer  94  in contact with the seat  96  on the motherboard  30  sets the spacer  94  in position relative to the heat transfer plate  54  in such a manner as to maintain a gap between the first electronic element  32  and the heat transfer plate  54 . After being set in position in this way, the spacer  94  is fixed on the heat transfer plate  54  by an adhesive  106 .  
         [0114]    For a gap to be maintained between the first electronic element  32  and the heat transfer plate  54 , a sheet of predetermined thickness is placed on the heat transfer surface of the first electronic element  32 , and with the heat transfer plate  54  placed thereon, the spacers  94  are set in position. After bonding the spacers  94 , the heat transfer plate  54  is removed together with the spacers  94 , and a compound is coated on the heat transfer surface of the first electronic element  32 . Then, the heat radiating plate  54  is mounted together with the spacers  94  and fixed by screws  108 . The cooling member  56  is fixed on the heat transfer plate  54  by screws  60 .  
         [0115]    FIGS.  29  to  36  are views for explaining the cooling of the second electronic elements  34   a . The first electronic element  32  making up a CPU is cooled by bringing the cooling member  56  into direct contact with the heat radiating plate  54 . The second electronic elements  34   a  for a RAM, in contrast, is cooled by air, without bringing the cooling member  114  into contact with the second electronic elements  34   a.    
         [0116]    [0116]FIG. 29 is a view showing an example of cooling the second electronic elements  34   a . As described above, the second electronic element group  34  includes a plurality of second electronic elements  34   a  arranged in lines, and each second electronic element  34   a  comprises RAM semiconductor chips  110  mounted to a printed board  112 . The printed board  112  is arranged in the direction perpendicular to the motherboard  30 , and an end of the printed board  112  is mounted to the motherboard.  
         [0117]    Like the cooling member  56 , the cooling members  114  supplied with a refrigerant through hoses  38  inserted between the second electronic elements  34   a . Specifically, the cooling members  114  are arranged in the vicinity of the second electronic elements  34   a  and cooled by cooling air. The second electronic elements  34   a  are cooled in this way, and the air thus heated has the effect of reducing the humidity in the box structure  26 .  
         [0118]    [0118]FIG. 30 is a view showing still another example of the cooling of the second electronic elements  34   a . In this example, the cooling member  114  is arranged substantially outside of the lines (second electronic element group  34 ) of the second electronic elements  34   a , heat conductors  116  are connected to the cooling member  114  and inserted between the second electronic elements  34   a . In this way, the second electronic elements  34   a  are cooled by the cooling member  114  and the heat conductors  116  through an air layer.  
         [0119]    [0119]FIGS. 31 and 32 are views showing still another example of cooling the second electronic elements  34   a . In this example, the cooling members  114  are arranged substantially outside of the lines of the second electronic elements  34   a  (second electronic element group  34 ), the heat conductors  116  are connected to the cooling members  114  and inserted between the second electronic elements  34   a . In FIG. 30, the cooling member  114  is arranged in parallel to the motherboard  30 , whereas in FIGS. 31 and 32, the cooling members  114  are arranged in the direction perpendicular to the motherboard  30 , and the two cooling members  114  are located on the two sides of the second electronic element group  34 . With this configuration, not only the second electronic elements  34   a  can be cooled as described above, but also the second electronic elements  34   a  can be inserted or removed with the cooling members  114  and the heat conductors  116  kept intact.  
         [0120]    [0120]FIG. 33 is a view showing a further example of the cooling of the second electronic elements  34   a . The cooling members  114  and the heat conductors  116  are arranged in a manner similar to FIGS. 31 and 32. In this example, however, the cooling member  114  has rails  118  for inserting the second electronic elements  34   a  into a connector arranged on the motherboard  30 . Specifically, the cooling member  114  and the second electronic elements  34   a  are movable relatively to each other in the direction perpendicular to the page (the horizontal direction in the page of FIG. 31). When the cooling member or the heat conductors  116  are inserted or unloaded, or when the second electronic elements  34   a  are inserted or unloaded, the inclination of the second electronic elements  34   a  is suppressed thereby to prevent the cooling member  114  and the heat conductor  116  from contacting the second electronic elements  34   a.    
         [0121]    [0121]FIG. 34 is a view showing a yet further example of cooling the second electronic element  34   a . In this example, the cooling member  114  and the heat conductors  116  are arranged in a manner similar to FIGS. 31 and 32, and a heat pipe is inserted in the heat conductors  116 . As a result, the heat transfer capacity from the second electronic element  34   a  to the cooling member  114  is enhanced.  
         [0122]    [0122]FIG. 35 is a view showing still another example of cooling the second electronic element  34   a . The cooling members  114  and the heat conductor  116  are arranged in a manner similar to FIGS. 31 and 32. In this example, pins  120  are arranged in parallel to the second electronic element  34   a  and the cooling members  114  on the motherboard  30 , and holes  122  through which the pins  120  are applied are formed in the cooling members  114  or a member integrated therewith. When the cooling member holding mechanism  36  is moved and the heat conductor  116  is inserted between the second electronic elements  34   a , therefore, the pins  120  act as a guide and set the second electronic elements  34   a  and the cooling members  114  in position thereby to prevent the second electronic elements  34   a  and the cooling members  114  from contacting each other.  
         [0123]    [0123]FIG. 36 is a view showing still another example of cooling the second electronic element  34   a . The cooling member  114  and the heat conductor  116  are arranged in a similar manner to FIGS. 31 and 32. In this example, a latch mechanism  124  of a connector  53  for the second electronic element  34   a  is operable by a tool  126 , so that the second electronic element  34   a  can be inserted or unloaded with the cooling member  114  and the heat conductor  116  kept intact.  
         [0124]    [0124]FIG. 37 is a view showing an example structure of the cooling member  114  for the second electronic element  34   a . Like the cooling member  56  shown in FIGS. 20 and 21, the cooling member  114  includes two plate members  114   a ,  114   b  overlaid and coupled by brazing or the like. The cooling member  114  has a refrigerant path  128 , a refrigerant inlet  130  and a refrigerant outlet  132 . The refrigerant path  128  is formed as a groove in the surface of a plate member  114   a . The refrigerant inlet  130  and the refrigerant outlet  132  have nipples  134 ,  136  mounted thereon.  
         [0125]    Further, a heat conductor  116  made of a metal plate is brazed at a position designated by  138  on the outer surface of a plate member  114   b.    
         [0126]    [0126]FIG. 38 is a view showing still another example of the structure of the cooling member  114  for the second electronic elements  34   a . In this example, the cooling member  114  includes a first plate  140   a , a second plate  140   b  and a spacer  142  arranged between the first and second plates. The first plate  140   a , the spacer  142  and the second plate  140   b  are coupled hermetically to each other through seal rings  144 .  
         [0127]    The spacer  142  has annular grooves  146  for arranging the seal rings  144  and a through hole  148  for forming a refrigerant path. The first plate  140   a  and the second plate  140   b  have through holes  150   a ,  150   b  forming a part of the refrigerant path. The through hole  148  of the spacer  142  is formed inside the annular grooves  146 . The through holes  150   a ,  150   b  of the first and second plates  140   a ,  140   b  are arranged vertically staggered, and are formed in such a manner as to be located in the annular grooves  146  when the first plate  140   a , the spacer  142  and the second plate  140   b  are coupled to each other.  
         [0128]    Further, as shown in FIGS. 38 and 39, the first plate  140   a , the spacer  142  and the second plate  140   b  are combined with a spacer  142  and a third plate. The third plate has fundamentally the same structure as the first plate  140   a  or the second plate  140   b . As a result, a hermetic multilayer assembly is formed of the second plate  140   b , the spacer  142 , the first plate  140   a , the spacer  142  and the second plate  140   b . The number of layers in this multilayer assembly can be changed.  
         [0129]    An end plate  152  is arranged at an end of this multilayer assembly. The end plate  152  includes a groove  152   a  forming a part of the refrigerant path, an annular groove  152   b  for the seal ring  144  and an inlet/outlet. A nipple  152   c  is mounted on the inlet/outlet. An end plate (not shown) similar to the end plate  152  is arranged at the other end of this multilayer assembly. The multilayer assembly including the first plate  140   a , the spacer  142  and the second plate  140   b  (and the third plate  140   c ) is integrated by a bolt  154 . The bolt  154  is inserted in an insertion hole  154   a  and screwed to a nut (not shown).  
         [0130]    The refrigerant that has entered by way of the nipple  152   b  of the end plate  152 , therefore, can flow in a zigzag path along the refrigerant path formed of the groove  152   a , the through holes  150   a ,  150   b  and the through hole  148 .  
         [0131]    Further, as clear from FIG. 38, the first and second plates  140   a ,  140   b  are larger than the spacer  142 , and the portion of the first and second plates  140   a ,  140   b  that expands outside beyond the spacer  142  (outside beyond the multilayer assembly) acts as a heat conductor  116 . Specifically, the portion of the first and second plates  140   a ,  140   b  and the spacer  142  constituting the multilayer assembly acts as the cooling member  114  of FIGS.  29  to  36 , for example, and the portion of the first and second plates  140   a ,  140   b  expanding outside act as the heat conductor  116  of FIGS.  30  to  36 , for example.  
         [0132]    [0132]FIGS. 40 and 41 show still another example of the structure of the cooling member  114  for the second electronic elements  34   a . The fundamental configuration of this embodiment is similar to that of the preceding embodiments. In the embodiment of FIGS. 38 and 39, the bolt  154  is arranged outside of the seal rings  144 . In this embodiment, however, the bolt  154  is arranged inside of the seal rings  144 . The through holes  150   a ,  150   b  can double as a part of the insertion holes  154   a.    
         [0133]    [0133]FIG. 42 is a view showing still another example of the structure of the cooling member  114  for the second electronic elements  34   a . In this embodiment, the feature of the embodiment of FIG. 37 is combined with the feature of the embodiment of FIG. 38. The cooling member  114  is formed of two plate members  114   a ,  114   b  laid one on the other through the seal ring  144 , and the two plate members  114   a ,  114   b  are integrated by bolts  154 . The nipples  134  are also mounted by the bolts  154 . The heat conductor  116  is fixed on the plate member  114   a  by the brazing  138 .  
         [0134]    In FIGS.  37  to  42 , the configuration (FIG. 37) in which the plate members  114   a ,  114   b  making up the cooling member  114  are coupled by brazing is compact and can be fabricated easily. At the time of brazing, however, the plates  114   a ,  114   b  are heated, and therefore may be softened and deformed. Also, once brazed, the plates  114   a ,  114   b  are difficult to repair when the refrigerant path is clogged. The configuration in which the plates  140   a  and  140   b  are laid one on the other by way of the seal rings  144  and coupled to each other by the bolts  154  or the like, in contrast, can solve the problem of the brazing.  
         [0135]    [0135]FIG. 43 is a view showing an example of arrangement of the hoses  38  of the refrigerant supply means in the cooling member holding mechanism  36 . A manifold  160 L for supplying the refrigerant and a manifold  162 L for discharging the refrigerant are mounted horizontally at the lower end of the cooling member holding mechanism  36 . Further, a manifold  160 U for supplying the refrigerant and a manifold  162 L for discharging the refrigerant are mounted horizontally at the upper end of the cooling member holding mechanism  36 . The hoses  40  and  42  extending from the refrigeration unit  16  shown in FIG. 2 are connected to the manifolds  160 L,  162 L,  160 U and  162 U.  
         [0136]    [0136]FIG. 43 shows the cooling members  56  and  114  arranged in four lines to facilitate the understanding. For the cooling member  114  in the uppermost stage, the hoses  38  are connected in parallel to each other in the direction substantially perpendicular to the upper-stage manifolds  160 U and  162 U. For the cooling member  56  in the second stage, on the other hand, the hoses  38  are connected in the direction substantially perpendicular to the upper-stage manifolds  160 U and  162 U on the one hand and extend horizontally in the area of the cooling member  56  on the other, with the hoses  38  of adjacent cooling members  56  being serially connected.  
         [0137]    For the third-stage cooling member  56 , the hoses  38  are connected in the direction substantially perpendicular to the low-stage manifolds  160 L and  162 L and extend horizontally in the area of the cooling member  56 , with the hoses  38  of adjacent cooling members  56  being connected serially to each other. For the lowest-stage cooling member  114 , the hoses  38  are connected in the direction substantially perpendicular to the low-stage manifolds  160 L and  162 L, with the hoses  38  of adjacent cooling members  114  being connected in parallel to each other.  
         [0138]    Therefore, the hoses  38  of given cooling members  56  and  114  are prevented from crossing other cooling members  56  and  114  in the direction perpendicular thereto. Therefore, the hoses  38  form no hindrance when the cooling members  56  and  114  perform the open/close operation around the horizontal axis as shown in FIGS. 12 and 13. Thus, the maintenance and replacement work of the electronic elements  32  and  34   a  are facilitated.  
         [0139]    Further, as shown in FIG. 44, the upper-stage manifolds  160 U and  162 U are arranged in a staggered fashion, and so are the low-stage manifolds  160 L and  162 L. This piping arrangement effectively simplifies the piping work of the hoses  38  using the same manifolds  160 U,  162 U,  160 L and  162 L and can realize the above-mentioned feature.  
         [0140]    [0140]FIG. 45 is a view showing still another example of arrangement of the hoses  38  of the refrigerant supply means in the cooling member holding mechanism  36 . in this example, the hoses  38  are connected to the manifolds  160 L and  162 L by way of couplers  164 . The manifolds and the hoses can be alternatively configured as shown in FIG. 43.  
         [0141]    [0141]FIG. 46 is a view showing still another example of arrangement of the hoses  38  of the refrigerant supply means in the cooling member holding mechanism  36 . In this example, the hoses  38  are connected to the manifold  160  by couplers  164 . Further, a stopper is arranged in each coupler  164 . Thus, the hoses  38  and the cooling members can be separated from each other without draining the refrigerant.  
         [0142]    As described above, according to this invention, a computer with a multi-chip module mounted to the motherboard is cooled using a cooling member of a temperature lower than the room temperature. Thus, the operation at high clock frequency is made possible, and the maintenance work, including the replacement of the multi-chip module, is facilitated.