Cooling device of electronic part having high and low heat generating elements

[0002] 1. Field of the Invention

[0003] The present invention relates to a cooling device of an electronic part arranged in a PC server or a UNIX server, for example. The present invention also relates to an electronic apparatus including such a cooling device.

[0004] 2. Description of the Related Art

[0005] In a PC server and a UNIX server playing the central role in the network that has seen a rapid development in recent years, a plurality of substrate units are mounted, each including one or a plurality of integrated circuit elements or packages, typically represented by a processor generating much heat.

[0006] Performance of the processor has recently been improved and the number of semiconductor circuits mountable in one chip has increased sharply, resulting in a correspondingly increased heat generated by the processor. Once the temperature of the processor exceeds an operation guaranteed temperature (generally, 80° C. to 100° C.), however, the normal function of the processor may be adversely affected.

[0007] In order to solve this problem, Japanese Unexamined Patent Publication (Kokai) No. 6-334374, for example, discloses a cooling structure in which a heat conductive member is mounted on the back of a high heat generating chip such as a processor, and the heat conductive member is inserted in a heat sink base plate having a multiplicity of radiation fins thereby to form a heat transmission path, thereby making it possible to efficiently radiate the heat generated by the high heat generating chip.

[0008] The PC server and the UNIX server described above are often used in a stack of multiple layers in a rack or the like. In order to install a multiplicity of servers in a small space, each server is desirably as thin as possible. Nevertheless, the prior art described above poses the problem that the server cannot be made thin for the reason described below.

[0009] An object of the cooling structure described in Japanese Unexamined Patent Publication (Kokai) No. 6-334374 is to prevent the heat generated by a high heat generating chip from being transferred to the other chips. In order to achieve this object, the heat sink base plate is installed in considerably spaced relation with the chips, and the heat conductive member for coupling the high heat generating chip and the heat sink base plate is bent substantially in the shape of Z as viewed sideways. Due to this structure, a large space is formed between the heat conductive member and a chip arranged downstream of the high output heat generating chip in the cooling air (hereinafter referred to as the low output heat generating chip), with the result that the heat conduction to the low output heat generating chip is suppressed.

[0010] An attempt to reduce the thickness of the server having the conventional structure described above would make it necessary to reduce the intervals between the multilayered substrates which in turn leads to a correspondingly smaller interval between the heat sink base plate and the substrates. This smaller interval reduces the space for preventing the heat conduction in the neighborhood of the low output heat generating chip, thereby making it impossible to achieve the object of the conventional structure described above, i.e. the prevention of heat conduction to the low output heat generation chip. Also, a smaller interval between the heat sink base plate and the substrates further causes the heat conductive member arranged upstream of the low heat generating chip in the cooling air to further block a smooth flow of the cooling air, with the probable result that some areas are exposed to the cooling air more than other areas. Since the processor is expected to be improved in performance more and more in the future, with an increased heating value, it is considered difficult to form a thinner server with the conventional structure described above.

[0011] On the other hand, various components are mounted on the substrate and therefore both the heating value and the size are not uniform. In the area in the neighborhood of the processor constituting a high heat generating element, for example, the heat generation density is locally increased, while the heat generation density is comparatively small in the area where a integrated circuit element or a package constituting a low heat generating element is mounted. An attempt to reduce the temperature of the area having a large heat generation density below the operation guaranteed temperature by increasing the amount of the cooling air, for example, would excessively reduce the temperature of the area having a small heat generation density, with the result that the areas not required to be cooled are undesirably cooled. Especially in the case where the rotational speed of the cooling fan is limited for noise suppression, the uneven distribution of the heat generation density can be a stumbling block to a thickness reduction of the server.

[0012] In view of the aforementioned problem of the prior art, the object of the present invention is to provide a cooling device for an electronic part in which uneven distribution of the heat generation density is eliminated for an improved cooling efficiency of the electronic part.

[0013] In order to achieve this object, according to the present invention, there is provided a cooling device of an electronic part having a substrate, a first heat generating element mounted on the substrate, and a second heat generating element mounted on the substrate and generating less heat than the first heat generating element, the height of the first heat generating element when mounted on the substrate being greater than a height of the second heat generating element when mounted on the substrate, the cooling device comprising a heat conductive member to which heat is transmitted from the first heat generating element and radiation members mounted on said heat conductive member in heat transferring fashion, said radiation members extending toward the second heat generating element, the radiation members being also formed in a space between the combination of the first heat generating element and the heat conductive member and the second heat generating element.

[0014] An electronic apparatus according to the present invention comprises a plurality of package units including at least one substrate unit, which includes the feature.

[0015] In this cooling device, in the case where the elements are arranged in a limited space, the cooling efficiency of the first heat generating element can be improved by reducing the difference of the heat generation density between the neighborhood of the first heat generating element and the neighborhood of the second heat generating element through the heat conductive member and the radiation members.

[0016] Preferably, the heat transferring efficiency can be improved by appropriately making the heat conductive member from a plate formed of a material having an isotropic heat conduction and/or a material having both a high heat conductivity and an anisotropic heat conduction. By the way, the material having the isotropic heat conduction is a metal such as copper and aluminum and the material having the anisotropic heat conduction is carbon graphite or the like, but the invention is not limited to these materials.

[0017] The plate can transmit the heat more efficiently through heat pipes if one or a plurality of heat pipes are built therein.

[0018] Further, if the plate has a first heat pipe and a second heat pipe built therein in such a manner that the first heat pipe extends in a first direction and the second heat pipe extends in a second direction different from the first direction along the plane of the plate with heat pipes of one of the heat pipes bent to avoid interference with heat pipes of the other heat pipe at each intersection thereof, then the heat transmission in two directions of the plate can be improved further and the two heat pipes can be built into the plate in a compact form thereby making it possible to reduce the thickness of the plate, for example.

[0019] Also, if the plate extends in parallel to said substrate and has a notch and/or a hole for avoiding the parts mounted on the board, the plate can be extended more widely regardless of the size of the parts mounted on the substrate.

[0020] Further, if the plate has a plurality of portions having different spaced relations with the substrate, the plate can be bent to avoid tall parts such as a small substrate module arranged in a position perpendicular to the board. By doing so, the plate can be extended more widely regardless of the size of the parts mounted on the substrate.

[0021] Furthermore, if the portions of the plate described above are coupled to each other by heat pipes, different plate portions can be arranged in the neighborhood of tall parts and short parts, respectively, and coupled by heat pipes. In this way, the plate can be arranged in a wider range regardless of the size of the parts mounted on the board. Even in the case where the heat pipes are arranged in a direction crossing the flow of the cooling air, the cooling air flow is only slightly blocked as long as the heat pipes have a circular section.

[0022] In the electronic apparatus, in the case where the elements are arranged within a limited space, the cooling efficiency of the first heat generating element can be improved by reducing the difference in heat generation density between the neighborhood of the first heat generating element and the neighborhood of the second heat generating element through the heat conductive member and the radiation members. Thus, the thickness of each package unit can be reduced, so that a greater number of package units can be stacked without increasing the total height of the electronic apparatus.

[0023] The present invention will become more apparent from the following description of the preferred embodiments, with reference to the accompanying drawings, in which:

[0024] FIG. 1 is a perspective view showing a UNIX server assembly including a cooling structure of an electronic part according to a first embodiment of the present invention, with several portions omitted;

[0025] FIG. 2 is a perspective view showing one of the UNIX server units according to this embodiment, with several portions omitted;

[0026] FIG. 3 is a sectional view of the structure of FIG. 2, taken along the lines III—III in FIG. 2;

[0027] FIG. 4 is a sectional view of the heat conductive plate;

[0028] FIG. 5 is a perspective view showing the heat conductive plate and the radiation fins removed integrally from the substrate;

[0029] FIG. 6 is a graph showing the cooling characteristic of ordinary radiation fins;

[0030] FIGS. 7A and 7B are views showing a second embodiment of the heat conductive plate, in which FIG. 7A is a planview and FIG. 7B is a side view;

[0031] FIG. 8 is a diagram showing a third embodiment of the heat conductive plate;

[0032] FIGS. 9A and 9B are views showing a fourth embodiment of the heat conductive plate, in which FIG. 9A is a planview, and FIG. 9B is a side view;

[0033] FIG. 10 is a perspective view showing a fifth embodiment of the heat conductive plate;

[0034] FIG. 11 is a sectional view of the heat conductive plate shown in FIG. 10;

[0035] FIG. 12 is a perspective view showing a sixth embodiment of the heat conductive plate;

[0036] FIG. 13 is a sectional view showing a UNIX server unit including a seventh embodiment of the heat conductive plate;

[0037] FIG. 14 is a view of the UNIX server unit of FIG. 13 taken along the direction of arrow XIV;

[0038] FIG. 15 is a perspective view showing an eighth embodiment of the heat conductive plate;

[0039] FIG. 16 is a perspective view showing a ninth embodiment of the heat conductive plate;

[0040] FIG. 17 is a partial perspective view showing a modification of the heat conductive plate with a further improved cooling efficiency;

[0041] FIG. 18 is a perspective view showing a tenth embodiment of the heat conductive plate;

[0042] FIG. 19 is a view showing a modification of the UNIX server unit using the heat conductive plate shown in FIG. 15;

[0043] FIG. 20 is a view showing a modification of the UNIX server unit using the heat conductive plate shown in FIG. 16; and

[0044] FIG. 21 is a partial sectional view showing a UNIX server unit including an 11th embodiment of the heat conductive plate.

[0045] The present invention will now be explained below with reference to the embodiments and the drawings. FIG. 1 is a perspective view of a UNIX server assembly including a cooling structure of an electronic part according to a first embodiment of the invention, with several portions omitted. A UNIX server assembly 1 constituting an electronic apparatus includes five UNIX servers 10 according to this embodiment. Other units 10′ can also be mounted. In other words, the UNIX server assembly 1 is a combination of a plurality of UNIX server units (package units) 10 each having a processor mounted therein to meet a given specification or provide expandability.

[0046] More specifically, the inner side wall of a box-shaped rack 2 having a front opening 2ais formed with rails (not shown) extending in the depth direction from the opening 2a, and the UNIX server units 10 can be mounted by being inserted into the rack 2 by way of the opening 2aand along the rails. The UNIX server units 10 mounted in the rack 2 are interconnected by wiring to function an integrated UNIX server assembly or several individual units. The UNIX server units 10, which are vertically stacked in the rack according to this embodiment, can be used alternatively by being juxtaposed. horizontally in the rack 2.

[0047] As described above, a plurality of UNIX server units 10 are stacked and make up the UNIX server assembly 1. As far as the height of the rack 2 is constant, more UNIX server units 10 can be accommodated in the rack 2 by reducing the thickness of the UNIX server units 2, thereby making it possible to improve the performance of the UNIX server assembly 1 and the efficiency with which the space is used.

[0048] FIG. 2 is a perspective view showing one of the UNIX server units 10 according to this embodiment, with several portions omitted. FIG. 2 shows the state in which a cover 12 is removed upward. A rectangular base 11 makes up a housing with the cover 12, and a cooling unit 13 including two fans 13aare installed at the left end portion of the base 11. The motor for driving the fans 13ais not shown.

[0049] A substrate 14 is arranged above the base 11 and supported by four supports 11a(of which only two are shown). Various electronic elements are mounted on the substrate 14 to constitute one electronic circuit. More specifically, an integrated circuit element/package (hereinafter referred to as processor) 14awhich generates a comparatively large amount of heat (about 50 to 60 W), and integrated circuit elements/packages 14bwhich generate a comparatively small amount of heat (about 2 to 3 W) are mounted on the substrate 14 either directly by soldering or through a connector. The number of processors is either 1 or plural depending on the specification of the UNIX server assembly 1. FIG. 2 shows the case in which a single processor 14ais mounted. To facilitate understanding, connectors for transferring external signals and power, various capacitors and resistors, mechanical parts such as fittings, and a DC-DC converter for supplying power are neither shown nor described here.

[0050] FIG. 3 is a view of the electronic part of FIG. 2, taken along the lines III—III in FIG. 2. In FIGS. 2 and 3, a heat conductive plate 15 extending substantially over the same range as the substrate 14 is arranged above the integrated circuit elements/packages 14aand 14b. The heat conductive plate 15 making up a heat conductive member is arranged in contact with the upper surface of the processor 14a, which is higher than the other integrated circuit elements/packages 14b, and extends horizontally (the direction of the cooling air flow of the cooling unit 13).

[0051] The heat conductive plate 15 and the processor 14amay be contacted each other directly or, to reduce the thermal resistance, desirably through Radiation Silicone Rubber Sheet Type TC-TX (trade name) or Radiation Oil Compound G750/G751 (trade name) marketed by Shin-etsu Chemical Industry or SC102 Radiation Compound (trade name) marketed by Toray Dow Corning Silicone.

[0052] FIG. 4 is a sectional view of the heat conductive plate 15. As shown in FIG. 4, a thin hollow 15ais formed in the heat conductive plate 15 and fluorocarbon F constituting a solvent of low boiling point is sealed in the hollow 15a. The heat conductive plate 15 may be formed of a metal material having a high heat conductivity and an isotropic heat conduction such as copper or aluminum or a material having a high heat conductivity in a predetermined direction and an anisotropic heat conduction such as carbon graphite. In the case where the heat conductive plate 15 is formed of a material having the anisotropic heat conduction, the cooling efficiency of the processor 14ais desirably improved by arranging the heat conductive plate 15 in such a position that a high heat conductivity is exhibited along the horizontal direction as shown in FIG. 4.

[0053] In FIGS. 2 and 3, a multiplicity of radiation fins 16 and 17 in the form of metal foils (copper, aluminum, etc.) are mounted to extend in the direction perpendicular to the upper surface and the lower surface of the heat conductive plate 15. The radiation fins 16 and 17 making up radiation members may be mounted on the heat conductive plate 15 either directly or, to reduce the thermal resistance, desirably through a material having a superior heat conductivity such as Radiation Silicone Rubber Sheet Type TC-TX (trade name) or Radiation Oil Compound G750/G751 (trade name) marketed by Shin-etsu Chemical Industry or SC102 Radiation Compound (trade name) marketed by Toray Dow Corning Silicone. In the case where the radiation fins 16 and 17 are mounted on the heat conductive plate 15 with adhesive, on the other hand, the bonding is accomplished while maintaining a satisfactory heat conductivity desirably by use of SE4420 Radiation Adhesive (trade name) marketed by Toray Dow Corning Silicone.

[0054] The radiation fins 16 mounted on the upper surface of the heat conductive plate 15 extend over substantially the whole of an area defined by the cover 12 indicated by the one-dot chain in FIG. 3 and the heat conductive plate 15, whereby a large radiation area is secured.

[0055] The radiation fins 17 mounted on the lower surface of the heat conductive plate 15, on the other hand, are formed over substantially the whole surface, except for an area (the area 15bin FIG. 5) where the heat conductive plate 15 contacts the processor 14aeither directly or indirectly. The radiation fins 17 are in such a shape as to avoid the remaining integrated circuit elements/packages 14b. The substrate 14, the elements 14aand 14b, the heat conductive plate 15 and the radiation fins 16 and 17 make up a substrate unit. The shape of the radiation fins 17 will be described more specifically.

[0056] FIG. 5 is a perspective view showing the heat conductive plate 15 and the radiation fins 16 and 17, with these members integrally removed from the substrate 14. In FIG. 5, reference is made to the second radiation fin 17 viewed from this side (left side). The radiation fin 17 is formed with rectangular notches 17aat four points. As shown in FIGS. 2 and 3, when the heat conductive plate 15 and the radiation fins 16 and 17 are installed on the substrate 14, a predetermined gap of, say, about 2 to 3 mm is secured in order to keep the radiation fins 17 out of contact with the four integrated circuit elements/packages 14bunder them. The presence of the gap keeps the integrated circuit elements/packages 14bout of contact with the radiation fins 17 on the one hand and prevents the heat transferred by the processor 14afrom being transferred directly to the other integrated circuit elements/packages 14bthrough the heat conductive plate 15 and the radiation fins 17 on the other hand. The other radiation fins 17 basically have a similar shape. The six radiation fins 17 on the rear side shown in FIG. 5, however, are shorter than the radiation fins 17 on the front side in order to secure the area 15bfor contact with the processor 14a.

[0057] The cooling efficiency of the UNIX server unit 10 according to this embodiment is considered next. FIG. 6 is a graph showing the cooling characteristic (thermal resistance Rout of the radiation fins) of ordinary radiation fins for the air flow velocity of 1 m/sec. As an example, assume that a processor package 14aof 30 mm square in size has an internal thermal resistance Rin of 0.2 deg/W and a generating heat value Pi of 50 W and the temperature Ta of the cooling air supplied from the cooling unit 13 is 30° C. Also assume that the junction temperature of the processor chip 14ais required to be controlled to not higher than 80° C. at which the operation and the reliability are guaranteed.

[0058] Assuming that this processor package 14aincludes radiation fins of 35 mm³and that the air is supplied at the rate of 1 m/sec, ignoring all the detailed conditions. The junction temperature Tj1 of the processor chip can be determined as

[0059] Tj1=P×(Rin+Rout)+Ta  (1)

[0060] From FIG. 6, it is seen that Rout is about 2.3 deg/W, and therefore the calculation in the foregoing equation shows that Tj1 is 155° C. Thus cooling is impossible.

[0061] In contrast, according to this embodiment comprising the heat conductive plate 15 in which the heat is transferred along the horizontal direction and released by way of the radiation fins 16 and 17 arranged over the maximum range possible in the space defined between the substrate 14 and the cover 12, the apparent size of the radiation fins of the processor package 14acan be increased to, say, 50 mm×35 mm×100 mm by utilizing the dead space in the housing. In such a case, Rout assumes the value of 0.8 deg/W from FIG. 6 and so Tj1 is 80° C., thereby making it possible to secure a sufficient cooling performance.

[0062] As described above, according to this embodiment, the heat transferred from the processor 14agenerates convection in the fluorocarbon F in the hollow 15aof the heat conductive plate 15. Therefore, by utilizing what is called the heat siphon effect, the heat can be efficiently transferred to all the radiation fins 16 and 17 mounted on the upper and lower surfaces of the heat conductive plate 15. Also, the extensive arrangement of the radiation fins 16 and 17 in the dead space in the housing of the UNIX server unit 10 makes possible efficient heat exchange with the cooling air from the cooling unit 13, whereby the junction temperature of the processor 14aconstituting a high heat generating element is reduced and thus the reliability of the UNIX server unit 10 can be maintained.

[0063] In the prior art, priority is given to the prevention of heat transfer to the integrated circuit elements/packages of a low heat value, and therefore the problem is that the temperature distribution in the housing of the UNIX server unit 10 is not uniform. According to this embodiment, in contrast, the heat is radiated also in the neighborhood of the integrate circuit elements/packages 14bhaving a low heat value to assure a substantially uniform temperature distribution in the housing, and therefore a processor 14aof a high heat value can be cooled efficiently even in a smaller housing. This embodiment thus is suitable for use with a PC server assembly and a UNIX server assembly of such a type that the UNIX servers 10 are stacked in the rack 2 as shown in FIG. 1. Also, this embodiment can meet the requirement of the UNIX server assembly or the PC server assembly having mounted thereon a processor which is considered to assume a high heating value in the future.

[0064] Also, according to this embodiment, the heat conductive plate 15 extends from the processor 14ain horizontal direction over the whole housing. Therefore, the blocking of the cooling air flowing in the horizontal direction from the cooling unit 13 by the heat conductive plate 15 is minimized, so that the cooling efficiency of the processor 14acan be further improved. Also, a thermal coupling as thin as not more than 1 mm can be realized on the processor 14aon the substrate 14 extending in parallel to the substrate 14 through a high heat conductive filler such as a thermal compound or a high heat conductive adhesive, with the result that a high cooling performance is expected. The advantage of the embodiment is that a comparatively short thermal coupling, even though not more than 1 mm thick, can also be formed in view of the opposed relation with the substrate 14c.

[0065] FIGS. 7A and 7B show a heat conductive plate according to a second embodiment, in which FIG. 7A is a top plan view and FIG. 7B is a front view of the heat conductive plate. In FIG. 7A, the radiation fins are not shown and the cooling air is assumed to flow in the direction of arrow. The heat conductive plate 115 shown in FIG. 7 is formed of a material similar to that of the heat conductive plate shown in FIG. 4, but not formed with an internal hollow. A plurality of heat pipes 115aare embedded in the heat conductive plate 115.

[0066] More specifically, eight heat pipes 115awith sealed ends are embedded in the heat conductive plate 115 and extend in the direction of the cooling air flow. The hollow heat pipes 115ahave sealed therein a metal fiber, water or alcohol, for example. However, the structure of the heat pipe is well known and therefore will not be described in detail.

[0067] According to this embodiment, the convection, evaporation or condensation of water or alcohol is caused in the heat pipes 115aembedded in the heat conductive plate 115, and therefore the heat can be efficiently transferred to the whole of the radiation fins (not shown) mounted on the upper and lower surfaces of the heat conductive plate 115 utilizing what is called the heat pipe effect.

[0068] FIG. 8 shows a heat conductive plate according to a third embodiment and is a view similar to FIG. 7A. The embodiment shown in FIG. 8 is different from the second embodiment shown in FIG. 7 in that a single heat pipe 115bis embedded in the heat conductive plate 115. More specifically, the heat pipe 115bextends in the direction of the cooling air flow as indicated by arrow and is folded back in a reverse direction at each point leaving the heat conductive plate 115 and arranged in zigzag form as viewed from above.

[0069] According to this embodiment, the single heat pipe 115bis arranged two-dimensionally on the heat conductive plate 115 and therefore the heat transferred from the processor (not shown) can be two-dimensionally transferred through the heat pipe 115b. Thus, the heat can be efficiently transferred to all the radiation fins (not shown) mounted on the upper and lower surfaces of the heat conductive plate 115.

[0070] FIG. 9A shows a heat conductive plate according to a fourth embodiment, and is a top plan view similar to FIG. 7A. FIG. 9B is a side view similar to FIG. 7B. In FIGS. 9A and 9B, the present embodiment is different from the second embodiment shown in FIGS. 7A and 7B in that a plurality of heat pipes 115cand 115dembedded in the heat conductive plate 115 are arranged at right angles to each other. More specifically, the six heat pipes 115care arranged in parallel to each other in the direction of the cooling air flow indicated by arrow, and the four heat pipes 115d, as clear from FIG. 9B, are arranged above and at right angles to the heat pipes 115c. The spaced relation between the heat pipes 115cand 115dis desirably adjusted to such an extent that the processor 14aextends over a plurality of the heat pipes 115cand 115d. By the way, the configuration of the heat pipes 115cand 115dthemselves is similar to that of the second embodiment.

[0071] According to this embodiment, the heat pipes 115cand 115dare arranged two-dimensionally on the heat conductive plate 115, and the heat transferred from the processor (not shown) can be rapidly transferred, two-dimensionally, through the heat pipes 115cand 115dwhich are shorter than the heat pipes 115bof the third embodiment. Therefore, the heat can be efficiently transferred to all the radiation fins (not shown) mounted on the upper and lower surfaces of the heat conductive plate 115.

[0072] FIG. 10 is a perspective view showing a heat conductive plate according to a fifth embodiment. This embodiment is different from the fourth embodiment shown in FIGS. 9A and 9B in that a plurality of heat pipes 115eand 115fembedded in two portions of a heat conductive plate 215 are arranged at right angles to each other in substantially the same plane. More specifically, longitudinal and transverse. grooves 215care formed in the upper surface of the lower half portion 215aof the heat conductive plate 215, and the heat pipes 115eand 115fare arranged in the grooves. The upper half portion of the heat conductive plate 215 is not shown.

[0073] The five heat pipes 115earranged in parallel to the flow of the cooling air indicated by an arrow are linear while the four heat pipes 115farranged to cross the heat pipes 115eabove are formed with bends 115habove the crossing points, respectively, thereby to prevent the interference with the heat pipes 115e.

[0074] FIG. 11 is a partial sectional view of the heat conductive plate shown in FIG. 10. An upper half portion 215bcombined with a lower half portion 215ato make up the heat conductive plate 215 is formed with a cut-out hole 215das a relief to avoid the interference with the bend 115hof the heat pipe 115f. By the way, the cut-out hole 215dmay alternatively be a recess.

[0075] According to this embodiment, the heat pipes 115eand 115fare arranged two-dimensionally on the heat conductive plate 115, so that the heat transferred from the processor (not shown) can be rapidly transferred two-dimensionally. Thus, the heat can be efficiently transferred over the whole of the radiation fins (not shown) mounted on the upper and lower surfaces of the heat conductive plate 115. Further, unlike in the fourth embodiment, the heat pipes 115eand 115fare arranged substantially in the same plane and therefore the thickness of the heat conductive pipe 215 can be reduced to provide a thinner UNIX server unit.

[0076] Small board modules called daughter boards packaged with RAM or the like may be mounted on the substrate as need arises. In many such cases, the small board modules are mounted in a position perpendicular to the substrate, and this poses the problem of interference with the heat conductive plate. According to the embodiment described below, the interference with the small board modules can be avoided.

[0077] FIG. 12 is a perspective view showing a heat conductive plate according to a sixth embodiment. This embodiment is different from the embodiments shown in FIGS. 4 to 7B in that the heat conductive plate 315 is partially notched. More specifically, in FIG. 12, four small board modules 14care arranged at a corner of the substrate 14 in a position perpendicular to the substrate 14. The area of the heat conductive plate 315 having the small board modules 14cis formed with a rectangular notch 315a. The other portions of the heat conductive plate 315 are configured in a similar manner to the heat conductive plates described above. In FIG. 12, the radiation fins not shown are mounted on the upper and lower surfaces of the heat conductive plate 315.

[0078] According to this embodiment, the heat conductive plate 315 is formed with the rectangular notch 315ain the area thereof having the small board modules 14c, and therefore the interference between the heat conductive plate 315 and the small board modules 14cis prevented. Even with the UNIX server unit having the small board modules 14c, therefore, the interior of the UNIX server unit can be efficiently cooled. A cut-out hole instead of a notch may be formed in the heat conductive plate for avoiding interference with tall electronic parts such as the small board modules.

[0079] As a result of cutting off a part of the heat conductive plate as a notch for avoiding the interference with tall electronic parts as in the aforementioned embodiment, the radiation fins are absent in the notched portion, and the correspondingly reduced radiation area may reduce the cooling efficiency. The embodiment described below can obviate or alleviate this problem.

[0080] FIG. 13 is a sectional view showing a UNIX server unit including a heat conductive plate according to a seventh embodiment. This embodiment is different from the embodiments shown in FIGS. 4 to 7B in that the heat conductive plate 415 is configured of two stages. More specifically, the heat conductive plate 415 includes a horizontally-extending first plate portion 415ain direct or indirect contact with the upper surface of the processor 14aconstituting a high heat generating element, a second plate portion 415bextending horizontally above the small board module 14ctaller than the processor 14aand a vertical plate portion 415ccoupling the ends of the first plate portion 415aand the second plate portion 415bto each other.

[0081] FIG. 14 is a view showing the UNIX server unit of FIG. 13 as viewed from the direction of arrow XIV. As shown in FIGS. 13 and 14, radiation fins 416aare mounted on the upper surface of the first plate portion 415aand radiation fins 417aare mounted on the lower surface of the first plate portion 415ain a similar manner to the embodiments described above. On the other hand, radiation fins 416bare mounted on the upper surface of the second plate portion 415b, and radiation fins 417bare mounted on the lower surface thereof.

[0082] The second plate portion 415bis shifted upward with respect to the first plate portion 415ain order to avoid the tall small board module 14c. As a result, the distance to the lower surface of the cover 12 indicated by a one-dot chain line is reduced. The radiation fins 416bmounted on the upper surface of the second plate portion 415b, therefore, are shorter than the radiation fins 416amounted on the first plate portion 415a.

[0083] In the case where there is a gap between the lower surface of the second plate portion 415band the small board modules 14c, the radiation fins can be mounted in such a gap, but such a gap may not always exist. On the other hand, the small board modules 14c, though tall, are often comparatively thin. As shown in FIG. 14, therefore, a wide radiation area can be secured by arranging the radiation fins 417bby and between the small board modules 14c.

[0084] By configuring the heat conductive plate 415 in two stages as described above, a problem is posed that the vertical plate portion 415cis located perpendicular to the cooling air flow from the cooling unit 13 and blocks the air flow thereof. In the embodiment described below, such a problem can also be obviated or alleviated.

[0085] FIG. 15 is a perspective view showing a heat conductive plate according to an eighth embodiment. The embodiment shown in FIG. 15, like the embodiments shown in FIGS. 13 and 14, has a double-stage structure including a first plate portion 515aand a second plate portion 515b. This embodiment is different, however, in that six openings 515dare formed in the vertical plate portion 515cof the heat conductive plate 515.

[0086] In view of the fact that the openings 515dare formed in the vertical plate portion 515c, the cooling air from the cooling unit (not shown) flowing in the direction of arrow in FIG. 15 is passed through the openings 515dand flows downward on the second plate 515bfor a further improved efficiency of cooling the processor.

[0087] With the configuration of the FIGS. 13 and 14, the heat conductive plate 415 is bent at the vertical plate portion 415c. Since an internal continuous hollow can be formed as shown in FIG. 4, however, the heat transfer ability to the second plate portion 415bcan be secured. In the case where the openings 515dare formed in the second plate portion 515cas shown in the configuration of FIG. 15, however, it becomes difficult to form a hollow in the heat conductive plate 515. According to the embodiment described below, in contrast, the aforementioned problem can be obviated or alleviated.

[0088] FIG. 16 is a perspective view showing a heat conductive plate according to a ninth embodiment. The embodiment shown in FIG. 16, like the embodiment shown in FIG. 15, has a double-stage structure, including a first plate portion 615aand a second plate portion 615b, but is not provided with the vertical plate portion. Thus, the first plate portion 615aand the second plate portion 615bare separated from each other.

[0089] The six heat pipes 615cembedded in the first plate portion 615a, after being bent in the form of crank, extend to the second plate portion 615bwhere they are embedded. Specifically, the heat pipes 615care exposed to the atmosphere between the first plate portion 615aand the second plate portion 615b. The internal structure of the heat pipe 615cis similar to that in the embodiment shown in FIGS. 7A and 7B and therefore will not be explained further.

[0090] As described above, according to this embodiment, the heat transferred to the first plate portion 615afrom the processor not shown can be efficiently transferred to the second plate portion 615bthrough the heat pipes 615cfor an improved cooling efficiency. Also, the heat pipes 615care exposed to the atmosphere between the first plate portion 615aand the second plate portion 615b, and therefore this embodiment has the advantage that the exposed portions are easily cooled by the cooling air.

[0091] FIG. 17 is a partial perspective view showing a heat conductive plate improved in cooling efficiency according to a modification. In the embodiment shown in FIG. 17 which has six heat pipes 615clike the embodiment shown in FIG. 16, three radiation fins 615dare mounted in contact with the exposed portion between the first plate portion 615aand the second plate portion 61b. The radiation fins 615dextend in horizontal direction and therefore the flow of the cooling air along the direction of arrow is not blocked.

[0092] As described above, according to this modification, the heat pipes 615care exposed to the atmosphere between the first plate portion 615aand the second plate portion 615b, and the radiation fins 615dare mounted on the exposed portion of the heat pipes 615c. Thus, heat exchange is efficiently performed with the cooling air through the radiation fins 615d, thereby further improving the cooling efficiency of the processor not shown.

[0093] The electronic elements may be arranged differently on the substrate according to the specification of the UNIX server unit. In such a case, if the position of the radiation fins mounted on the heat conductive plate remains the same, the interference with the electronic elements may occur. Fabrication of the heat conductive plates of different shapes for different specifications of the UNIX server unit, however, would increase the cost. The embodiment described below obviates or alleviates this problem.

[0094] FIG. 18 is a perspective view showing a heat conductive plate according to a tenth embodiment. In FIG. 18, a heat conductive plate 715 before mounting radiation fins 716 thereon is shown. Threaded holes 715aarranged in a regular grid are formed on the upper and lower surfaces of the heat conductive plate 715. In FIG. 18, the heat conductive plate 715 is arranged upside down, i.e. the surface thereof opposed to the board (not shown) up, as compared with the embodiments described above.

[0095] The fin unit 716 has six radiation fins 716amounted on a single base 716b. This fin unit 716 is adapted to be mounted at an arbitrary position on the heat conductive plate 715 where the electronic parts (including the processor) of the board not shown can be avoided, by inserting screws not shown in arbitrary threaded holes 715a. The fin units can be mounted in this way on the two surfaces of the heat conductive plate 715.

[0096] According to this embodiment, assume that the relative positions of the electronic elements are changed in accordance with the specification of the UNIX server unit, etc. By changing the mounting position of the fin unit 716 appropriately, elements can be shared and the cost can be reduced. With the UNIX server unit mass-produced with the electronic elements arranged at predetermined positions, therefore, the number of parts and the cost can be reduced. Also the labor is eliminated for assembling the radiation fins in the heat conductive plate. The heat conductive plate is of one type, and by combining several types of fins, a cooling structure is realized which has the versatility of meeting the requirements of the heating value of the elements and the shape of the parts mounted.

[0097] As a modification of this embodiment, as shown in FIG. 18, protrusions 716dare formed at the lower parts of the end portions of the radiation fins 716cof different lengths, and according to the relative positions of the electronic parts, the radiation fin 716 of an appropriate length is selected. By inserting the protrusions 716dinto the threaded holes 715aappropriately, the radiation fin 716 can be mounted on the heat conductive plate 715. In the case where a multiplicity of types of UNIX servers are available, a radiation fin of an arbitrary length can be arranged in accordance with the shape of the parts mounted. The fins can also be replaced easily for an improved versatility.

[0098] FIG. 19 shows a UNIX server unit using the heat conductive plate of FIG. 15 according to a modification. In FIG. 19, the second plate portion 515bof the.heat conductive plate 515 makes up a part of the housing. More specifically, the upper part of the cover 512 is cut out, and the second plate portion 515bis accommodated snugly in the cut-out portion to make a surface flush with the upper surface of the cover 512. The cooling air from the cooling unit 13 is passed through the opening 515dof the heat conductive plate 515 as indicated by arrow and flows from left to right. The remaining configuration is similar to that of FIGS. 12 to 14 and therefore will not be described in detail.

[0099] According to the modification shown in FIG. 19, as compared with the embodiments shown in FIGS. 13 to 15, the area of the radiation fins 517 mounted on the second plate portion 515bis maintained while at the same time improving the cooling efficiency of the processor 14aby exposing the upper surface of the second plate portion 515bout of the housing. Also, since the second plate portion 515bmakes up a part of the housing, the UNIX server unit can be further reduced in thickness.

[0100] FIG. 20 shows a modification of the UNIX server unit using the heat conductive plate of FIG. 16. Also in FIG. 19, the second plate portion 615bof the heat conductive plate 615 makes up a part of the housing. More specifically, the upper portion of the cover 612 is cut out, and this cut-out portion accommodates the second plate portion 615bsnugly thereby to form a surface flush with the upper surface of the cover 612. According to this modification, a small-sized cooling unit 813 is arranged on the first plate portion 615a, and the cooling air from the cooling unit 813 flows leftward through the space between the heat pipes 615cexposed between the first plate portion 615aand the second plate portion 615b. The remaining configuration is similar to that of FIGS. 13 to 15 and will not be described in detail.

[0101] According to the modification shown in FIG. 20, as compared with the embodiments of FIGS. 13 to 15, the area of the radiation fins 517 mounted on the second plate portion 615bis maintained while at the same time exposing the upper surface of the second plate portion 615boutside of the housing, thereby making it possible to improve the cooling efficiency of the processor 14afurther. The improved cooling efficiency can reduce the size of the cooling unit 813 as in this modification for a lower cost. Also, it is expected that the space can be saved by mounting the cooling unit 813 on the heat conductive plate 615 in the housing.

[0102] FIG. 21 is a partial sectional view showing a UNIX server unit including a heat conductive plate according to an 11th embodiment. An end portion 715aof the heat conductive plate 715 near to the cooling unit 13 is wedge-shaped and can smooth the flow of the cooling air along the direction of arrow. The end portion 715amay be in the shape of streamline instead of wedge.

[0103] The present invention has been described above with reference to embodiments thereof and is not limited to these embodiments but is modifiable in various ways within the scope of the technical concept of the invention. The radiation fins mounted on the heat conductive plate, for example, can be porous to secure a wider radiation area or a multiplicity of pins can provided in place of the radiation fins.

[0104] As described in greater detail, in the cooling device of an electronic part according to the present invention, in the case where the elements are arranged in a limited space, the cooling efficiency of the first heat generating element can be improved by reducing the difference in the heat generation density between the neighborhood of the first heat generating element and the neighborhood of the second heat generating element through said heat conductive member and said radiation members.

[0105] In the electronic apparatus according to the present invention, in the case where the elements are arranged within a limited space, the cooling efficiency of the first heat generating element can be further improved by reducing the difference in heat generation density between the neighborhood of the first heat generating element and the neighborhood of the second heat generating element through the heat conductive member and the radiation members. Thus, the thickness of each package unit can be further reduced, so that a multiplicity of package units can be stacked without increasing the total height of the electronic apparatus.