Cooling device

Abstract: A cooling device (1) according to the present invention, in which a space (37) is formed by means of flat electrodes (31, 32) and side plates (5, 6) disposed on the ends of the flat electrodes (31, 32), and a flexible film (4) fastened in that space (37) in a shape of the letter S, is fixed to a semiconductor package (2) so that one of the flat electrodes (31, 32) is fitted on the semiconductor package (2). The respective flat electrodes (31, 32) are alternately powered to move the S shaped section of the flexible film (4).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a cooling device, a semiconductor package on which the cooling device is mounted and a computer which includes this semiconductor package, and more particularly to a preferred cooling device which dissipates heat from the semiconductor package or a substrate on which the semiconductor package is mounted. DESCRIPTION OF THE RELATED ART With the recent progress of the integration of electronic circuits, in substrates including semiconductor packages in which semiconductor chips containing electronic circuits are packaged, heat generated per unit area tends to increase. Some packaging substrates such as a printed circuit board, on which a variety of semiconductor packages and electronic parts are mounted, often include semiconductor packages which generate more heat than ordinary semiconductor packages. When these substrates are mounted on electronic devices in high concentration, in order to have the semiconductor packages stably work it is necessary to maintain the temperatures of the semiconductor package or the substrate at predetermined values and thus it is necessary to dissipate heat from the surface of the substrate or an electronic device effectively. As the method for dissipating heat from these semiconductor packages, substrates and electronic devices including the semiconductor packages or substrates, Japanese Patent Unexamined Publication No. 2-130894, for example, has disclosed a method for blowing air to a plurality of substrates by means of a large size rotation fan which produces air flow in a computer. Japanese Patent Unexamined Publication Nos. 2-83958 and 2-196454 have disclosed a cooling method for directly providing a circuit substrate with a miniature size rotation fan. Moreover, Japanese Patent Unexamined Publication No. 62-149158 has disclosed a method for directly providing a circuit substrate with a reciprocating type fan employing a piezo-vibrator instead of the miniature size rotation fan. According to the method for blowing air to a plurality of substrates by means of a large size rotation fan in the prior arts described above, it is difficult to feed air uniformly to all the substrates and if a semiconductor package having a high heat generation density is mixed into the substrate, it is difficult to cool the respective elements uniformly. A further problem is that the fan occupies a large area in the computer. The method in which a miniature size fan or a reciprocating type fan unit employing a piezo-vibrator is directly mounted on a substrate is effective for the substrate on which the semiconductor packages having high heat generation density are mixedly mounted. However, with the miniature size rotation fan, the efficiency of the fan tends to decrease and additionally, since a space for air to be blown to the semiconductor package is needed, a wide space in a direction perpendicular to the substrate is required. With the reciprocating type fan employing the piezo-vibrator, it is necessary to locate the fan away from the surface of the substrate to blow air to the surface of the substrate. On the other hand, if the height of the fan from the substrate is reduced so that the fan edges operate as if they sweep the surface of the substrate, the fan scans only a part of the substrate surface, thereby the cooling efficiency is not satisfactory. In Technische Rundschau, vol. 84, no. 42, 16 October 1992, Bern, pages 72-73 a micro-pump for pumping a flow of gas to be used in the epitaxial layer growing process is disclosed which as a pumping element employs a electrically conductive flexible film which is extending across a space defined between two flat plates which are held in a distance from each other by means of spacer plates upon which the ends of the film are clamped by the flat plates. The conductive film and electrodes disposed on inner surfaces of the flat plates and coated with isolating linings may get potentials applied thereto to provide an electrostatic field across the space in such a manner that the flexible film performs a reciprocating movement across the space between the flat plates thus moving a gas into and from that space so as to pump that gas to a vacuum chamber for performing the epitaxial layer growing process. The micro-pump may serve for the purpose of feeding the vacuum chamber with well-defined doses of gas but since its flexible film is operated through electrostatic fields there is a tendency of generating electric charges which would adversely affect the provide an effective flow of cooling medium through a very limited space as required for cooling semiconductor chips. SUMMARY OF THE INVENTION It is an object of the present invention to provide a device for effectively cooling semiconductor packages which is very small, has a very low height to enable a very thin design and allows to be operated by a very small amount of energy and in a variable manner. According to the present invention that object is achieved by means of a cooling device as defined in the appended claims 1 and 4; advantageous further developments of the invention are indicated in subclaims. According to the present invention, there is provided a cooling device mounted on a semiconductor package, comprising a flexible film both ends of which are fastened and flexible film driving means for changing shape of the flexible film. Moreover, there is provided a cooling device comprising a pair of flat plates which face each other with a space therebetween, a pair of side plates which are disposed on both ends of the plates so as to define that space with the pair of flat plates and a flexible film which is disposed in the space, both ends of the flexible film being fastened near different side plates on different flat plates so that the shape of said flexible film is changeable in said space. Further, there is provided a cooling device comprising a magnetic flexible film both ends of which are held, two magnetic field generating devices which generate magnetic fields according to electric signals and side plates which are mounted between the two magnetic field generating devices, the shape of the magnetic flexible film being changed by the generated magnetic fields. According to a still further aspect of the present invention, there is provided a semiconductor package including a semiconductor chip containing an electronic circuit, a substrate on which the semiconductor chip is mounted and which has an I/O portion for inputting and outputting electric signals to and from the semiconductor chip and a heat dissipation part which expels heat generated from the semiconductor chip, any one of the cooling devices mentioned above being used for the heat dissipation part. In the cooling device of the present invention both ends of the flexible film are fastened near different side plates on different flat plates so that the shape of the flexible film is changeable in the space which is surrounded by the plates. In this manner, the flexible film is fixed so that it is loose, crossing the gap between the flat plates. Accordingly, upon operation the flexible film comes into contact with one flat plate successively from the portion where the flexible film is fastened to the flat plate. Thus, the portion which crosses the space between the flat plates of the flexible film, is moved along the surface of the flat plate. During the next step of operation, the flexible film in contact with the surface of the one flat plate is separated therefrom and comes into contact with the surface of the other flat plate. At this time, the portion which crosses the space, of the flexible film is moved reversely. By repeating this action, the air heated by a semiconductor package on which the cooling device is mounted is dissipated from the space. The flexible film repeats contact and separation from the flat plates which are heated by the semiconductor package in order to destroy the thermal boundary layer created over the surfaces of the flat plates. This action accelerates heat conduction on the surfaces of the flat plates and expels the heated cooling medium, introducing a new cooling medium from the surrounding area. With air cooling by means of a fan, it is well known that the thermal boundary layer formed over the surface of a heat generating object blocks heat conduction from the surface of the heat generating object. Some countermeasures such as increasing the air flow rate or dispersing the air flow have been taken to decrease the effect of the thickness of the boundary layer. The present invention allows such heat to be expelled very effectively because the thermal boundary layer formed over the film in contact with the electrode plate is removed together with the film. As regards this action, the present invention is considerably different from conventional cooling devices such as a cooling device comprising a reciprocation type fan the end faces of which operate as if they sweep the surface of a substrate by employing a piezo-vibrator. Use of a flexible film allows the flexible film to reciprocate over the entire surface of the flat plates. With the electromagnetic drive type cooling device, of the present invention the magnetic flexible film which is supported between two magnetic field generating devices is moved up and down by means of magnetic force. It is possible to provide a thin structure cooling device by narrowing the gap between the magnetic field generating plates. Additionally, because, in the present invention, the film is driven by a magnetic force, the cooling device can be driven by low voltages below 5V and easily integrated with a LSI chip. According to a still another aspect of the present invention, there is provided a computer comprising a substrate on which a plurality of semiconductor packages are mounted, a case which incorporates the substrate, a keyboard which is disposed on a surface of the case to input information, and a display screen, at least one of the semiconductor packages including a heat dissipation part which expels heat to a surface opposite to a surface fitted to the substrate, any one of the aforementioned cooling devices being used for the heat dissipation part. According to the present invention, by disposing a cooling device of the present invention in a semiconductor package, which is contained in a computer including substrates on which semiconductor packages are mounted, and which generates a large amount of heat, it is possible to suppress a rise of the temperature of the semiconductor package and prevent a rise of the temperature of the substrate, both of which allow the electronic circuits to operate properly. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view with a fragmentary cross section of a computer including cooling devices according to an embodiment of the present invention. Fig. 2 is a perspective view of an electromagnetic drive type cooling device according to the basic principle of the present invention. Figs. 3a and 3b are a sectional view of the electromagnetic drive type cooling device according to a second detailed embodiment of the present invention and a plan view of a magnetic field generating unit, respectively. Fig. 4 is a sectional view of an electromagnetic drive type cooling device according to a third embodiment of the present invention. Figs. 5a, 5b and 5c are diagrams showing method for producing a magnetic field generating unit of the electromagnetic drive type cooling device according to a third embodiment of the present invention. Figs. 6a and 6b are a sectional view of an electromagnetic drive type cooling device according to a fourth embodiment of the present invention and a plan view of a magnetic field generating unit, respectively. Fig. 7 is a plan view of a magnetic field generating unit of an electromagnetic drive type cooling device according to a fifth embodiment of the present invention. Figs. 8a and 8b are a sectional view of an electromagnetic drive type cooling device according to a sixth embodiment of the present invention and an explanatory view showing condition of magnetization of a magnetic film used in the cooling device, respectively. Fig. 9 is a sectional view of an electromagnetic drive type cooling device according to a seventh embodiment of the present invention. Fig. 10 is a perspective view of an electromagnetic drive type cooling device according to an eighth embodiment of the present invention. Figs. 11a and 11b are diagrams showing operation principle of the electromagnetic drive type cooling device according to a eighth embodiment. Fig. 12 is a time-chart showing timing for flowing current to respective coils of the electromagnetic drive type cooling device according to the eighth embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS A basic embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a fragmentary perspective view including an exterior and a fragmentary cross section showing an interior structure of a computer. The computer comprises a case 800 which includes a display screen 803 and a key board 805 mounted on a surface of the case, a substrate 8, on which various semiconductor packages and electronic parts are mounted, disposed inside of the case 800, an external connector 806 and a disk drive 804. The case 800 is provided with air ports 801, 802 which are formed on a keyboard side and an external connector side of the case to allow air to circulate in the computer. A plurality of semiconductor packages 2 with high heat generation density and semiconductor packages 20 with low heat generation density are mixedly mounted on the substrate 8 disposed in a flat space inside of the case 800. The substrate 8 is a multi-layer wiring board on which signal lines and power lines are disposed in multi-layers with insulating layers interposed. The semiconductor packages 2, 20 and other electronic parts are electrically connected to each other by means of through holes provided in the substrate and wirings placed on the surface layer. A cooling device 1 is bonded to the semi-conductor package 2 which has high heat generation density by high heat conductive adhesive on a side, opposite the substrate 8, of the semiconductor package. Fig. 2 shows an expanded view of the semiconductor package 2 on which the cooling device 1 is mounted. Another and detailed embodiment of the present invention will be described with reference to Figs. 2, 3a, 3b. Fig. 2 is a perspective view showing the construction of the cooling device. The cooling device comprises a flexible magnetic film 70, side plates 103, 104, 102 which hold both ends of the film, upper and lower magnetic field generating units 73, 74 which drive the film held by the side plates 102, 103, 104 and power supply pads 105 which supply electric power to the magnetic field generating units. The cooling device is mounted on a semiconductor package 90, to be cooled, so that the cooling device is integrated with the semiconductor package 90. The power supply pads 105 are connected to power pads 106 which are formed on the semiconductor package 90 through gold wires. The power supply pads 106 are connected to leads 107 which are provided on the semiconductor package 90. Except for the portions of the magnetic film held by the side plates, the magnetic film is held so that the film is partially bent in the shape of the letter S. The portion which is bent in the shape of the letter S is elastically deformed. The magnetic field generating units 73, 74 drive the magnetic film so that the bent portion is moved. When a voltage of 5V is supplied to the upper magnetic field generating unit 73, a top surface of the bent portion of the magnetic film is moved so that the top surface successively comes into contact with a bottom surface of the upper magnetic field generating unit 73. On the other hand, when the voltage to the upper magnetic field generating unit 73 is turned off and a voltage of 5V is supplied to the lower magnetic field generating unit 74, the S-shaped bent portion of the magnetic film is moved in reverse to the direction mentioned above. These actions are repeated in an interval of several hundreds milli second to several tens milli second in order to expel heated air caught between the upper and lower magnetic field generating units. As a result, the semiconductor package which is disposed under the cooling device is cooled. Fig. 3a is a sectional view of the cooling device according to a second embodiment. Fig. 3b shows the magnetic field generating plate which is a major part of the magnetic field generating unit. The cooling device measures 20 mm long, 20 mm wide, 2 mm - 5 mm high. These dimensions are only an example and must be almost the same as the size of the semiconductor package which is required to be cooled. The magnetic film 70 is made of Permalloy foil measuring 20 mm long, 15 mm wide, 5 µm thick. The upper and lower magnetic field generating units comprise insulating substrates 92, 93, coil patterns 95, 96 and magnetic substances 97, 98. The coil patterns 95, 96 of the magnetic field generating units are produced by using semiconductor microprocessing technology. After copper foil of 50 - 100 µm is bonded to the insulating substrates 92, 93 of 500 µm in thickness, several tens turns of coil patterns 95, 96 of 50 - 100 µm in width are formed by photolithography. Then, nickel of 50 - 100 µm in thickness or Permalloy magnetic substance 97, 98 is bonded inside of the coil patterns 95, 96. Finally, to insulate between the coils and between the coil and the magnetic film, resin 110, 111 such as polyimide or silicon resin which has heat resistance of over 200°C is coated on the surface of the magnetic field generating plate, that is, the surfaces of the coil pattern and the magnetic substance. Silicon oxide or silicon nitride which has a higher withstand voltage than these resins may be used for this insulation coating. It is possible to change the thickness of the copper foil, the width of the coil or the number of the turns in response to the magnitude of the magnetic force required to drive the magnetic film. The coil patterns 95, 96 and the magnetic substances 97, 98 may be produced by electrolytic plating as well as by the above mentioned method. Fig. 4 is a sectional view of a third embodiment of the present invention. Figs. 5a, 5b and 5c are process diagrams which show the method of producing a magnetic field generating plate which is a major part of the magnetic field generating unit. The cooling device shown in Fig. 4 is a modification of the magnetic field generating unit of the cooling device shown in Figs. 3a, 3b. The magnetic field generating unit comprises a magnetic substrate 112, 113 having cutout sections on the outside edge, a magnetic substrate 115, 116 both surfaces of which are flat and a coil 117, 118. Figs. 5a, 5b and 5c show the method for producing the cooling device. The flat magnetic substrate 116 is bonded to the magnetic substrate 113 with the cutout section on the outside edge, to form a groove on a periphery thereof. In this groove, insulation coated copper wire is wound several hundred turns to form the coil 118 which is molded with a resin. The magnetic substrate with the cutout section on the edge and the flat magnetic substrate are made of Permalloy, nickel, cobalt or iron. The coil is made of copper wire having a diameter of less than 0.2 mm. If the depth of the groove is small, it is possible to use a substrate having a concave groove on its side face. Because the coil patterns of the cooling device shown in Fig. 3a is produced by using photolithography, the thickness of the coil pattern and the number of turns are restricted. The strength of the magnetic field is proportional to the number of turns of coil and a current flowing in the coil. Accordingly, a magnetic field generating plate using a coil produced by photolithography is suitable for a thin cooling device but it has a limitation in the strength of magnetic field which can be generated by it. On the other hand, the assembly-type cooling device which uses copper wire is capable of increasing the number of turns of and the current flowing through the coil as compared with a coil produced by photolithography. Thus a stronger magnetic field can be produced but the thickness of the device exceeds 4 mm. In a magnetic field generated by the coil, magnetic force acting on the magnetic film depends on the gradient of the magnetic field. Because the electromagnetic drive type cooling device shown in Figs. 3a, 3b, 4, 5a, 5b and 5c uses a pair of the upper and lower flat magnetic field generating plates, the gradient of the magnetic field in the magnetic film is low, so that a magnetic force acting on the magnetic film is small. Figs. 6a and 6b show a fourth embodiment of the present invention in which the gradient of the magnetic field in the magnetic film is increased by dividing the upper and lower magnetic field generating plate into a plurality of sections. Fig. 6a is a sectional view of the electromagnetic-drive-type cooling device and Fig. 6b shows a division-type magnetic field generating plate which is a major part of the magnetic field generating unit. The electromagnetic-drive-type cooling device shown in Figs. 6a and 6b includes five magnetic field generating units which are obtained by dividing the magnetic field generating unit of the cooling device shown in Figs. 15a, 15b into five stripes. The magnetic field generating unit comprises substrates 120, 121, magnetic substances 122 - 131 and coils 135 - 144. Supplying power to each magnetic field generating unit is performed by power supply pads 150 - 159. Fig. 6a shows a state in which magnetic fields are generated from the upper and lower magnetic substances 122 and 128 - 131, respectively. This state is obtained by feeding current to the upper coil 135 and the lower coils 141 - 144. In this case, the left portion of the S shaped section is attracted by the upper magnetic substance 122 and the right portion of the S shaped section is attracted by the lower magnetic substances 128 - 131. According to the present embodiment, the divided magnetic field generating unit produces magnetic fields to move the magnetic film. For example, when moving the S shaped section to the right, by feeding current to the upper coils 135 - 139 successively, magnetic fields are produced in the magnetic substances 122 - 126 in order. The currents fed to the lower coils, 140 to 144 in order, are turned off accompanied by the movement of the magnetic film. To move the S shaped section reversely to the left, current is fed in reverse to the order described above. Because the division type magnetic field generating unit includes divided magnetic field generating sections, the gradient of the magnetic field at the magnetic film which is produced by the divided magnetic field generating section is larger than the flat plate type magnetic field generating unit. Thus, use of the division type magnetic field generating unit enables the S shaped section of the magnetic film to be moved rapidly. The division type construction shown in Figs. 6a, 6b is applicable to the third embodiment which uses copper coils. The number of divided magnetic field generating units are determined in accordance with the size of the cooling device and the speed of the movement of the magnetic film. A magnetic substance has a property referred to as form antisotropy. For example, if the magnetic substance is a cylinder, the magnetic substance is readily magnetized along the longitudinal axis of the cylinder. That is, it tends to be magnetized along the length of the substance. The magnetic field generating units of the electromagnetic drive type cooling devices shown in the seventh and the present embodiment employ thin sheets of a magnetic substance, which produce magnetic fields by feeding current to coils disposed about the thin magnetic sheets. Thus, in the second and present embodiments, the direction of magnetization of the thin magnetic sheet is the same as that of the normal line (extending along the thickness of the sheet), and thus it is unlikely to be magnetized. Fig. 7 shows the construction of the cooling device in which the thin magnetic sheets of the magnetic field generating unit are divided into a plurality of small sections so that the thin magnetic sheets are likely to be magnetized along the normal line of the thin sheet. Fig. 7 is a plan view of the magnetic field generating plate showing a condition in which the thin magnetic sheet is divided into a plurality of small sections. A sectional view of the cooling device according to the present embodiment is omitted because this is almost the same as the ninth embodiment described above. As shown in Fig. 7, according to the present embodiment, magnetic substances 165 - 169 are divided into 40 small sections. Thus, the respective magnetic substances are more likely to be magnetized vertically relative to the substrate as compared with the construction shown in the sixth embodiment. As a result, the present embodiment forms a stronger magnetic field than the aforementioned embodiment. Naturally, the number of divisions of the magnetic substances are appropriately changed in accordance with the size of the cooling device and the required strength of the magnetic field. Figs. 8a, 8b show a sixth embodiment of the present invention. Fig. 8a shows a sectional view of the electromagnetic drive type cooling device according to the present embodiment and Fig. 8b shows the directions of the magnetization of the magnetic film. The electromagnetic drive type cooling device of the present embodiment is a partial modification of the electromagnetic drive type cooling device of the ninth embodiment. Differences between the former and latter embodiments are as follows. According to the electromagnetic drive type cooling device of the present embodiment; (1) The magnetic film is made of a magnetic substance (permanent magnet) having a high coercive force. (2) The magnetic field generating unit is mounted on a single side and a magnetic field is produced with AC current. In the second - fifth embodiments described above, a plurality of the upper and lower magnetic field generating units are employed to drive the magnetic film. This is because Permalloy having a small coercive force is used for the magnetic film. If a magnetic substance (permanent magnet) which holds magnetization along the normal line of the film as shown in Figs. 8a, 8b is used, the magnetic film can be moved up and down by only reversing the line of magnetic force going through the film. A reversal of the line of magnetic force can be obtained by applying AC voltage to one magnetic field generating unit. Thus, in the electromagnetic type cooling device of the present embodiment, the magnetic film can be driven by means of a single magnetic field generating unit. Although it is possible to employ either the flat plate type or the division type shown in the seventh - tenth embodiments, for the magnetic field generating unit, the division type produces a larger magnetic force to drive the film. An seventh embodiment of the present invention will be described with reference to Fig. 9. Fig. 9 is a sectional view of the electromagnetic drive type cooling device of the present embodiment. Like the constructions shown in the fourth and fifth embodiments, divided magnetic field generating units are used to drive the magnetic film. In the present embodiment, coils are wound around magnetic substances having U shape cross sections to provide electromagnets 171 - 177 and 181 - 187 and these electromagnets are disposed on the substrate so that concave sides face each other. In the flat type magnetic substance shown in the sixth and seventh embodiments, a magnetic circuit formed by the magnetic film and the flat type magnetic substance is open. Thus, lines of magnetic force are emitted to the space. On the other hand, because a magnetic circuit formed by the magnetic film and the U shape magnetic substance is enclosed (closed circuit) by using the U shape magnetic substance, most of lines of magnetic force are enclosed in the magnetic circuit. As a result, a larger magnetic force than that of the construction of the flat type magnetic substance acts on the magnetic film, thereby making it possible to move the film rapidly. Fig. 9 shows a case when the electromagnets 171, 172 and 181 - 184 are actuated. In this case, the upper electromagnets 171, 172 and the lower electromagnets 181 - 184 form closed circuits together with the magnetic film. The left portion of the S shaped section is attracted by the upper electromagnets 171, 172 and the right portion of the S shaped section is attracted by the lower electromagnets 181 - 184. Meanwhile, the actions of the lower electromagnet and the upper electromagnet opposite to each other is reverse. For example, when the upper electromagnet 171 is turned on, the opposite lower electromagnet 187 is turned off. The magnetic film 70 is moved to the right or the left by controlling electric signals to be supplied to the upper electromagnets 171 - 177 and the lower electromagnets 181 - 187. The magnetic film is moved so as to close a magnetic circuit produced by each electromagnet. An eighth embodiment of the present invention will be described with reference to Figs. 10 - 12. Fig. 10 is a perspective view of an electromagnetic drive type cooling device of the present embodiment. Figs. 11a and 11b are views showing the principle of operation. Fig. 12 is a time chart showing timings for applying current to each coil. As in the cooling device shown in the seventh embodiment described above, the magnetic film is moved so as to close a magnetic circuit formed by the electromagnets. In the present embodiment, the cross section of the magnetic substance for producing a magnetic field is protruded and a plurality of electrically independent coils are wound around the protruded sections. Whether a coil is formed on each protruded section as shown in Fig. 10 or on every other protruded section as shown in Fig. 11a is determined in accordance with the required speed of the magnetic film. The magnetic film is moved step by step by applying current to the respective upper and lower independent coils successively. Fig. 12 shows a time chart of current to be fed to the upper and lower coils. If currents are fed to the coils U1, U2, U3, U4, L1, L2, L3, L4 in order of the eight upper and lower coils (U1 - U4, L1 - L4), the magnetic film is moved so as to close a magnetic circuit formed by the respective protruded section of an iron core and the magnetic film. When currents are fed to the upper and lower coils U1 - U4, the magnetic film is moved from the left to the right. Thus, if current continues to be fed to the respective coils in order of U1 - U2 - U3 - U4 - L4 - L3 - L2 - L1, the magnetic film reciprocates the right and the left. By changing the frequency of the clock signal shown in the time chart, it is possible to change the time period for feeding current to each coil. Figs. 11a and 11b show the condition of the magnetic film when current is fed to the upper and lower coils. As shown in the time chart shown in Fig. 12 and the condition of the magnetic film shown in Fig. 11a and 11b, currents which are fed to the respective coils are in reverse relationship between the upper and lower coils. That is, when current is flowing to the upper coil, no current is flowing in the lower coil opposite to that coil. Conversely, when current is flowing to the lower coil, no current flows to the upper coil. Thus, when an actuator is driven, current always flows to four coils constituting half of the eight coils. Because the coils are formed on the protruded section in the present embodiment, the eighth embodiment can be manufactured easily in comparison with the seventh embodiment and has an advantage making it possible to produce a thin device. In the electromagnetic drive type cooling devices shown in Figs. 2 - 11, a film supported between two magnetic field generating unit plates is moved up and down by a magnetic force. Thus, it is possible to provide a thin structure cooling device having a thickness of 10 mm by narrowing the space between the magnetic field generating plates. Additionally, because, in the cooling device of the present invention, the film is driven by a magnetic force, the cooling device can be driven by low voltages below 5V and integrated with the semiconductor package. If the cooling device described in the second to eighth embodiments is mounted on a semiconductor package and employed in the computer shown in Fig. 1, it is possible to effectively dissipate heat from the semiconductor package and the substrate on which the semiconductor package is mounted. As is particularly described hereinbefore, according to the present invention, it is possible to form a cooling device having a simple structure comprising a pair of flat plates oppositely disposed with each other with a space therebetween and a film element disposed therebetween. Thus, the cooling device performs cooling with a thin structure in comparison with a rotational fan type cooling device. As a result, it is possible to mount electronic circuit substrates in a high density. Further, the cooling device of the present invention has a remarkable effect to destroy a thermal boundary layer and has a high cooling performance. Further, as the cooling device of the present invention has no sliding portions such as bearings, there is no deterioration of life on account of wear. Further, energy consumption is very small.

Claims: A cooling device, comprising a pair of flat plates (73, 74; 110, 111; 115, 116) comprising driving means and facing each other to define a space therebetween, a pair of side plates (102, 103, 104) disposed between and on opposite ends of said flat plates to define said space together with them, a flexible film (70) disposed in said space between said flat plates and said side plates, having one end attached to one flat plate near one side plate and the other end attached to the other flat plate near the other side plate and being disposed in said space in such a way that a first portion of it is in contact with the surface of one flat plate, a second portion is in contact with the surface of the other flat plate and a third portion between said first and second portions crossing said space between said flat plates is movable over the surface of said flat plates in a reciprocating motion between the fixed ends of said flexible film by means of said driving means and power supply means (106) for supplying power to said driving means, characterised in that said flexible film (70) is made of a magnetic material, that each of said flat plates (73, 74; 110, 111; 115, 116) comprises a driving means in the form of a magnetic field generating unit (95, 96; 117, 118; 122-131; 164-169; 171-177; 181-187; U1-U4, L1-L4) generating magnetic fields according to signals for driving said flexible magnetic film (70) into a reciprocating motion in order to expel heated air caught between said flat plates and that one of said flat plates (73, 74; 110, 111; 115, 116) is in thermal connection with a heat generating member (90) to be cooled. A cooling device according to claim 1, characterised in that said flexible magnetic film (70) is made of Permalloy or another material having a low coercive force. A cooling device according to claim 1 or 2, characterised in that said magnetic field generating units (95, 96; 117, 118; 122-131; 165-169; 171-177, 181-187; U1-U4, L1-L4) of said flat plates (73, 74; 110, 111; 115, 116) are alternately supplied with electrical power. A cooling device, comprising a pair of flat plates (120, 121) comprising driving means and facing each other to define a space therebetween, a pair of side plates (103, 104) disposed between and on opposite ends of said flat plates to define said space together with them, a flexible film (70) disposed in said space between said flat plates and said side plates, having one end attached to one flat plate near one side plate and the other end attached to the other flat plate near the other side plate (6) and being disposed in said space in such a way that a first portion of it is in contact with the surface of one flat plate, a second portion is in contact with the surface of the other flat plate and a third portion between said first and second portions crossing said space between said flat plates is movable over the surface of said flat plates in a reciprocating motion between the fixed ends of said flexible film by means of said driving means and power supply means for supplying power to said driving means, characterised in that said flexible film (70) is made of a magnetic material having a high coercive force, that one of said flat plates (120, 121) comprises a driving means in the form of a magnetic field generating unit (127-131) generating magnetic fields according to signals for driving said flexible magnetic film (70) into a reciprocating motion in order to expel heated air caught between said flat plates and that one of said flat plates (120, 121) is in thermal connection with a heat generating member to be cooled. A cooling device according to one pf the claims 1 to 4, characterised in that said magnetic field generating unit or units (122-131; 165-169; 171-177, 181-187; U1-U4, L1-L4) of flat plates (110, 111) are each divided into a plurality of independent sub-units. A semiconductor package including a substrate (8) having at least one semiconductor chip (2, 20) containing an electronic circuit mounted thereon and having an I/0 portion (806) for inputting and outputting electric signals to and from said semiconductor chip and a heat dissipation part (1) disposed on said substrate (8) to expel heat from said semiconductor chip (2) and being a cooling device according to one of the claims 1 to 5. A computer comprising a substrate (8) having a plurality of semiconductor packages mounted thereon and being incorporated in a case (800), a keyboard (805) disposed on a surface of said case (800) to input information and a display screen (803), wherein at least one of said semiconductor packages has a heat dissipation part (1) disposed on a surface opposite to a surface fitting to said substrate (8) and being a cooling device according to one of the claims 1 to 5.

Inventors: ASHIWAKE NORIYUKI, HARADA TAKESHI, HATADA TOSHIO, HONDA YUKIO, OHASHI SHIGEO, SATO KAZUO, SHIKIDA MITSUHIRO, TANAKA SHINJI, ASHIWAKE, NORIYUKI, HARADA, TAKESHI, HATADA, TOSHIO, HONDA, YUKIO, OHASHI, SHIGEO, SATO, KAZUO, SHIKIDA, MITSUHIRO, TANAKA, SHINJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: H01L23
IPC codes: H02K57, F04B45, H01L23, F04B43, H02N1