METHOD FOR PRODUCING HEAT SINK AND HEAT SINK THUS PRODUCED

Abstract: In a heat sink (1) for semiconductor elements and the like, a pipe (12) made of a heat-conductive material such as copper, aluminum and the like is bent at a middle portion (15) thereof and wound such that a forward flow passage (13) and a return flow passage (14) for a liquid coolant are both formed into a spiral shape with the bent portion held at the center of the spiral.

Description: BACKGROUND OF THE INVENTION This invention relates to a method for producing a cooling member and a cooling member produced by the method. According to the recent progress in the field of semiconductor technique, semiconductor elements of a large capacity operable at 4000V - 3000A or more one triggered directly with light signal have been developed and are used practically. In such a semiconductor element, since several kW of heat is generated in a small area of the electrodes of the semiconductor element, a cooling device of an extremely large capability is required. In order to satisfy such a requirement, heretofore has been proposed a heat sink made of a highly heat-­ conductive metal such as copper or aluminum, through which a circulation passage of a liquid coolant is provided in a zigzag form. In a typical application, a required number of the heat sinks are placed between semiconductor elements such as diodes, thyristors, gate turn off (GTO) thyristors and the like, respectively, and the coolant passages formed in the heat sinks are connected together through a number of coolant supplying pipes made of an electrically insulating material so as to provide a coolant circulating path. By circulating a liquid coolant such as water through the coolant circulating path by means of a coolant circulating pump provided in the path, the heat generated in the semiconductor elements is transferred to the coolant and carried outside of the semiconductor elements. In the aforementioned heat sink, however, the thickness of the heat sink becomes comparatively thick not only because of the heat-conductive metal member through which the zigzag formed circulation passage is provided, but also because of the zigzag-formed configuration, the head loss caused when the liquid coolant flows through the circulation passage in each heat sink becomes high, thus requiring a large capacity of the pump for circulating the liquid coolant therethrough. Furthermore, the temperature of the liquid coolant becomes much different between the inlet portion and outlet portion of the coolant passage in each heat sink, which entails a difficulty of increasing the thermal stress in the semiconductor element and requiring a heat exchange of a large capacity in the coolant circulating path. SUMMARY OF THE INVENTION An object of this invention is to provide a heat sink for cooling stacked means such as semiconductor elements, and a method for producing the heat sink wherein the disadvantages of the conventional technique can be substantially eliminated. Another object of the invention is to provide a heat sink and a method for producing the same, which are simple and economical, and capable of eliminating the possibility of leakage. Still another object of the invention is to provide a heat sink, the thickness of which is substantially reduced while the operational characteristics thereof are improved remarkably. These and other objects of the invention can be achieved by a method for producing a heat sink comprising the steps of preparing a pipe made of a heat conductive material, bending the pipe at a middle portion thereof into a loop form such that two portions of the pipe provided on both sides of the middle portion extend side by side in one direction from the bend middle portion, and winding the thus bent pipe into a spiral shape with the bent middle portion held at the center, and both the side portions of the pipe formed into a forward flow passage and a return flow passage for a liquid coolant circulating through the heat sink thus produced. According to the present invention, in another aspect thereof, there is provided a heat sink for cooling stacked means consisting of a pipe made of a heat conductive material, comprising a forward flow passage for a liquid coolant, a return flow passage for the liquid coolant, and a middle portion provided between the forward flow passage and the return flow passage, the middle portion being bent and wound so that the forward and return flow passages are formed into spirals starting from the middle portion. Preferably, the pipe is constructed to provide a rectangular cross-section, and the forward flow passage and return flow passage are formed by two side portions of the pipe which are wound side by side into the spiral shape. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: FIG. l is a diagram showing an ordinary arrangement of heat sinks for cooling semiconductor elements; FIG. 2 is a modification of the conventional construction of the heat sink; FIG. 3 is a plan view showing a heat sink according to a preferred embodiment of this invention; FIG. 4 is a sectional view taken along the line IV-­ IV in FIG. 3; FIG. 5 is a sectional view similar to FIG. 4, which constitutes a portion of another embodiment of this invention; FIG. 6 is a plan view of a heat sink which constitutes still another embodiment of this invention; FIG. 7 is a sectional view taken along the line VII-­ VII in FIG. 6; FIG. 8 is a plan view showing still another embodiment wherein the pad is otherwise used for electrical connection; FIG. 9 is a sectional view taken along the line IX-­ IX in FIG. 8; FIG. l0 is a diagram showing a semiconductor circuit wherein the heat sink shown in FIG. 8 is advantageously used for its connection; FIG. ll is a diagram showing a semiconductor circuit wherein a heat sink as shown in FIG. l2 is advantageously used for cooling semiconductor elements; and FIG. l2 is a plan view of a duplicated heat sink constituting a further embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Before entering the description of this invention, a conventional construction of a heat sink will be described in more detail with reference to FIGS. l and 2. In its application, a number of heat sinks l of a liquid cooled type are ordinarily piled up in the form of a stack 4, each pad being interposed between two adjacent semiconductor elements 2 of a flat shape, such as diodes, thyristors, GTOs and the like as shown in FIG. l, and compressed together under application of a pressure of sseveral tons. The heat sinks l are interconnected by a number of pipes 3 made of an insulating material. A liquid coolant 5 such as water is supplied through main pipes 6 also made of an insulating material under the action of a pump 8, and the heat in the circulated coolant is exchanged by a heat exchanger 7 provided in the circulation passage of the liquid coolant. Each of the heat sinks l in the above described arrangement comprises a block of a heat-conductive material such as copper or aluminum, with a coolant passage 9 of an undulated configuration form through the block. The liquid coolant 5 flows in and out of the coolant passage 9 of each pad l through inlet and outlet ports l0 provided at the ends of the passage 9. The aforementioned passage 9 may otherwise be formed by use of a tube ll made of copper or aluminum, which is undulated and casted in the block as shown in FIG. 2. Regardless of the way of construction of the heat sink l, the undulated passage 9 provides a long length and increases the heat-exchanging surface area of the heat sink l. However, an increase in the number of bending portions and steepening of the curvature increases the head loss of liquid coolant 5, and increases the required capacity of the circulation pump 8. Furthermore, since the passage 9 is simply undulated without duplication or else, the temperature of the coolant passing through the passage 9 is low at the inlet portion and high at the outlet portion of the passage 9. Such a temperature difference in a single pad creates a thermal stress in the semiconductor element 2 contacting the heat sink l. For eliminating such a disadvantage, the capacity of the heat exchanger 7 must be increased, which in turn increases the size of semiconductor apparatus cooled by the heat sinks. In order to overcome the above described difficulties of the conventional construction, a heat sink l according to this invention comprises solely a cooling pipe l2 made of a heat conductive material such as copper or aluminum without using the block of the conventional construction. The cooling pipe l2 is bent in a middle part thereof into a loop form so that both the side portions of the pipe l2 excepting the middle part are brought together to be extended side by side in one direction from the bent middle part of the pipe l2. The pipe l2 thus bent is then wound into a spiral with the bent middle part held at the center and both the side portions of the pipe l2 held in a closely contacting relation with each other, thereby providing a heat sink as shown in FIG. 3. In this heat sink l, a liquid coolant 5 such as water is passed through a forward flow passage l3 formed by one of the two side portions of the pipe toward a central portion l5 of the spiral, and then through a return flow passage l4 formed by the other of the two side portions of the pipe toward a periphery of the spiral. In this construction, the pipe l2 is preferably formed into a rectangular cross-sectional configuration as shown in FIG. 4, while the number of turns of the spiral wound pipe l2 may be selected as desired according to the required diameter of the heat sink l. Likewise, the positions of coolant introducing and delivering ports l0 formed at the ends of the passages l3 and l4, respectively, may be selected as desired. Although a small void is formed at the center of the spiral for inserting a jig or the like at the time when the pipe l2 is subjected to bending and winding processes, such a void is preferably filled with a heat-­ conductive material similar to that used for producing the pipe l2. Since the pipe l2 is provided with a rectangular cross-section, a final machining of, for instance, 0.5 mm depth is sufficient for obtaining smooth surfaces l6 to be brought into contact with the semiconductor elements 2 on both surfaces of the heat sink l. The liquid coolant 5 such as water supplied in the inlet port l0 flows through the forward flow passage l3 toward the central portion l5 of the heat sink l, and then from the central portion l5 through the return flow passage l4 and the outlet port l0 to outside of the pad l. Since the pipe l2 of the pad l is wound into a spiral form, and the electrodes of the semiconductor elements 2 are brought into direct contact with the pipe l2, the cooling effect of the heat sink l can be improved remarkably. Furthermore, since the forward flow passage l3 and the return flow passage l4 are disposed alternately, the temperature of the heat sink is distributed evenly, and the creation of the thermal stress in the semiconductor element can be substantially eliminated. In addition, since the number of the bending portions is reduced, the head loss in the passages of the heat sink l can be reduced, and hence the required capacity of the circulation pump 8 can be reduced. The improved cooling effect of the heat sink also reduces the capacity of the heat exchanger 9 so that the size of the semiconductor apparatus can be minimized. However, the heat sink l according to this invention is ordinarily subjected to a high compressive force when it is stacked together between semiconductor elements, and hence a sufficient amount of wall thickness is required for withstanding the compressive force. For instance, in case of producing a heat sink having a cooling surface l6 of approximately l00 mm diameter, compressive force of about l0 tons is applied to the pad, and hence a wall thickness of 4 ∼ 5 mm is required for the cooling pipe l2 forming the heat sink l. FIG. 4 is a sectional view taken along the line IV-­ IV in FIG. 3. Since the cooling pipe l2 used for providing the heat sink l of this embodiment, which is preferably formed into a rectangular cross-section, has a wall thickness equal in both directions, parallel and perpendicular to the surfaces l6 contacting with semiconductor elements as shown in FIG. 4, the heat conductivity of this embodiment cannot be sufficiently good. According to another embodiment of this invention, the wall-thickness of the cooling pipe l2 used in the pad l is made different between the two directions. That is, the thickness of the wall of the pipe l2 extending in parallel with the contacting surfaces l6 is selected to be less than that extending perpendicular to the contacting surfaces l6 as shown in FIG. 5. As a consequence, the heat conductivity of this embodiment can be substantially improved. Furthermore, since the thickness of the wall of the pipe l2 extending in parallel with contacting surface l6 is reduced, bending and winding of the pipe l2 can be substantially facilitated, and hence the productivity of the heat sink can be improved. Since the cross-sectional area of the internal bore of the pipe l2 can be increased in accordance with the reduced thickness of the wall, the head loss of the heat sink l, and hence the required capacity of the circulation pump 8 (see FIG. l) can be reduced. FIG. 6 and 7 illustrate still another embodiment of this invention, wherein a thermocouple element l8 is provided in a central portion of the heat sink l. According to this embodiment, the thermocouple element l8 is placed at a suitable position in a small void formed at the center of the spiral wound pad l, and lead wires connected to the thermocouple element l8 are extended to be inserted between the portions of the pipe l2, forming the forward and return flow passages l3 and l4, during the pipe winding stage of the heat sink production. The remaining part of the small void is completely filled with a heat conductive material similar to that forming the cooling pipe l2, and then the heat sink l thus produced is slightly machined to provide smooth surfaces l6 contacting with the semiconductor elements. In order to prevent the thermocouple element l8 from being damaged mechanically and thermally, a protection member made of a synthetic resin may be provided around the thermocouple element. In case where a thick insulation is required for the lead wires l7 due to a high voltage of the semiconductor elements, a recess or groove of a suitable size may be provided in the walls of the pipe portions between which the lead wires l7 are inserted. Since the lead wires l7 are extended between the spirally wound portions of the pipe l2, there is no necessity of forming a groove in a contacting surface of the pad l for accommodating the lead wires l7, and the cooling efficiency of the heat sink l can be improved in comparison with a type of heat sink having such a groove in the contacting surface thereof. Since the lead wires l7 are extended between the wound portions of the pipe l2 having a square or rectangular cross-section, there is no possibility of displacing the thermocouple l8 even in a case where the lead wires l7 are pulled during the production of the heat sink l, and the reliability in the operation of the thermocouple can be improved without increasing the production cost. Furthermore, when heat sinks are stacked together with the semiconductor elements and the like for cooling the elements and when terminals T are provided for connecting the anode side and cathode side of each semiconductor element to a series connection of a resistor and a capacitor as shown in FIG. l0, in case of the conventional construction of the heat sink as shown in FIG. 3, for instance, a machine screw has been driven into the block of copper or aluminum through which the cooling pipe has been provided. Such a procedure of providing terminals T cannot be applied to the present invention where the heat sink l is made solely of a cooling pipe l2. FIGS. 8 and 9 illustrate still another embodiment of this invention wherein such terminals T are provided without requiring the machine screw and the like. In the illustrated embodiment, a bare copper or aluminum wire l9 is inserted between the wound portions of the cooling pipe l2, and terminal 20 is secured to an outer end of the bare wire l9. The terminals 20 of the heat sinks inserted between the semiconductor elements can be used for connecting the anode side and cathode side of each element to the series connected resistor and capacitor as shown in FIG. l0. FIG. l2 illustrates a further embodiment of the invention wherein two heat sinks l, each constructed as in the case of the above described embodiment, are connected with each other. In a case where the combined circuit of the semiconductor elements 2 is used for a switching purpose, the semiconductor elements 2 are frequently connected in an anti-parallel manner as shown in FIG. ll. In such a case, the embodiment shown in FIG. l2 may be used advantageously in consideration of the electric potential of each element, and since the number of connecting points is reduced, the possibility of leakage of the liquid coolant can be thereby reduced. According to this invention, there is provided a heat sink for semiconductor elements and the like, which can reduce the thermal stress created in the element and the head loss caused in the pad. Furthermore, the thickness of the heat sink is reduced in comparison with the conventional heat sink because the block of heat conducting material through which a cooling passage is formed is not utilized, while the number of the spiral wound turns of the cooling pipe can be varied in accordance with the required diameter of the semiconductor element, and a heat sink of a high cooling effect, high reliability and highly economical in production can be thereby produced.

Claims: 1. A method for producing a heat sink comprising the steps of preparing a pipe made of a heat conductive material, bending said pipe at a middle portion thereof into a loop form in such a manner that two portions of the pipe provided on both sides of said middle portion extend side by side in one direction from said bent middle portion, and winding the entirety of thus bent pipe into a spiral shape with the bent middle portion held at the center, said both side portions extending side by side outwardly to be formed into a forward flow passage and a return flow passage of a liquid coolant circulating therethrough. 2. A heat sink for cooling stacked means consisting of a pipe made of a heat conductive material comprising a forward flow passage for a liquid coolant, a return flow passage for a liquid coolant, and a middle portion formed at substantially a central portion of said forward flow passage and said return flow passage, said middle portion being bent and wound so that said forward and return flow passages are formed into spirals starting from said middle portion. 3. The heat sink according to claim 2 wherein said pipe is formed into a rectangular cross-section. 4. The heat sink according to claim 3 wherein said pipe having a rectangular cross-section is formed such that the thickness of walls of said pipe extending perpendicular to said pad is made larger than that of walls extending in parallel with the heat sink. 5. The heat sink according to claim 2 wherein said stacked means are semiconductor elements. 6. The heat sink according to claim 2 wherein said middle portion of said pipe is bent into a loop having a predetermined radius of curvature. 7. The heat sink according to claim 2 wherein a thermocouple element is provided in a void portion formed at the center of said heat sink, while lead wires connected to said element are extended to be inserted between said two side portions of said pipe. 8. The heat sink according to claim 2 wherein a bare metal wire is inserted between said two side portions of said pipe wound side by side for providing a terminal of said stacked means. 9. The heat sink assembly according to claim 2 wherein a pair of heat sinks are combined to be arranged side by side in a plane and connected together in series.

Inventors: IKEGAME, HIROO, ITO, TAKURO, MATSUMOTO, TOSHIAKI, NONAKA, SHIGEO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01L23
IPC codes: H01L23