Abstract:
To provide a heat dissipation structure for an electronic circuit board, a recessed portion is formed in a surface of a heat dissipation board to extend to at least one side surface thereof. A gel-like resin having a high thermal conductivity is coated over the recessed portion. The electronic circuit board is brought into facial contact with the heat dissipation board with the electrical insulation sheet interposed therebetween so that at least one electronic component is buried in the gel-like resin. The gel-like resin coated in the recessed portion is such an amount that a gap between the electronic component and side surfaces and bottom surface of the recessed portion is filled with the gel-like resin and an excess amount of the gel-like resin is expelled out of the side surface of the heat dissipation board The gel-like resin protruded from the side surfaces of the heat dissipation board is removed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a heat dissipation structure for use in combination with an electronic circuit board. The invention relates further to a method of producing an electronic circuit board assembly including a heat dissipation board. 
     2. Description of the Prior Art 
     Heat dissipation boards have been extensively used for electronic circuit boards on which heat generating electronic components are mounted. Japanese Patent Application Publication No. 10-84063, for example, discloses a heat dissipation structure as shown in FIG.  1 . As shown therein, a heat dissipation board  33  is in an inversed U-shaped configuration in cross-sectional view with heat dissipation fins  33   a  formed on the top surface thereof. An electronic circuit board  32  is fixed to the lower face of the heat dissipation board  33  to form a space  33   b  in which heat generating electronic components  31  are accommodated. Other electronic components  34  are mounted on the reverse side of the electronic circuit board  32 . 
     A molten resin  35  is injected into the space  33   b  from a resin injection port  33   c  formed in the side surface of the heat dissipation board  33  to mold the electronic components  1 . Air discharge port  33   d  is formed in the opposing side surface of the heat dissipation board  33  to ease the injection of the molten resin. With this structure, molding the heat generating electronic components with the resin improves heat dissipation efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new heat dissipation structure that has excellent heat dissipation capability. The present invention also provides a manufacturing method for manufacturing an electronic circuit board assembly having the heat dissipation structure. 
     An electronic circuit board assembly according to the present invention includes an electronic circuit board on which a plurality of electronic components are mounted, an electrical insulation sheet, and a heat dissipation board in facial contact with the electronic circuit board with the electrical insulation sheet interposed therebetween. The heat dissipation board is formed with at least one recessed portion on a top surface facing the electronic circuit board and having side surfaces connecting to the top surface. A heat dissipation material, such as a gel-like resin, is coated over the recessed portion and at least one electronic component from the plurality of electronic components is accommodated in the recessed portion over which the heat dissipation material is coated. An amount of the dissipation material is such that the heat dissipation material fills a gap between the electronic component and the side walls and the bottom wall of the recessed portion. 
     With the electronic circuit board assembly thus constructed, the heat dissipation board dissipates heat through the heat dissipation material and also from a side surface of the electronic component, thereby improving the heat radiability. Moreover, in the case where the electronic component is a semiconductor device, the heat of the high-temperature terminal is radiated through the resin as well as through the wiring pattern on the circuit board and the insulating sheet between the circuit board and the heat dissipation board, whereby the heat in radiated with high efficiency. 
     For the heat dissipation board formed with a plurality of recessed portions on the top surface thereof, at least two recess portions may have different depths. In this case, the depth of the recessed portion is determined depending on a height of the electronic component to be accommodated. 
     A through-hole and/or a notch may be formed in the electronic circuit board to allow the heat dissipation material to escape from the recessed portion when the electrical component is intruded into the recessed portion. With the through-hole or notch formed, air bubbles can be prevented from being generated between the electronic circuit board and the recessed portion, thereby being capable of improving the heat radiability 
     Further, the recessed portion formed in the heat dissipation board may extend to at least one of the side surfaces of the heat dissipation board. In this case, the heat dissipation material is filled over an area covering the one of the side surfaces of the heat dissipation board. By doing so, the electronic component can be mounted in alignment with the side surface of the heat dissipation board, so that a space for accommodating the electronic components is enlarged enough to increase a degree of freedom of mounting the electronic components, thereby attaining a higher density of mounting the electronic component on the printed-circuit board. 
     According to another aspect of the present invention, there is provided a method of manufacturing an electronic circuit board assembly includes forming a recessed portion in the top surface of the heat dissipation board to extend to at least one of the side surfaces, coating the recessed portion with a gel-like resin having heat dissipation capability, bringing the electronic circuit board into facial contact with the heat dissipation board with the electrical insulation sheet interposed therebetween so that at least one electronic component from the plurality of electronic components mounted on the electronic circuit board is buried in the gel-like resin. The gel-like resin coated in the recessed portion is such an amount that a gap between the electronic component and side surfaces and bottom surface of the recessed portion is filled with the gel-like resin and an excess amount of the gel-like resin is expelled out of the one of the side surfaces of the heat dissipation board. Finally, the gel-like resin protruded from the side surface of the heat dissipation board is removed. 
     Thus, the gel-like resin is partially made to swell out of the opening of the recessed portion on the side surface of the heat dissipation board by intruding the electronic component into the resin to be removed. Therefore, the electronic component can be intruded in a state where the recessed portion is sufficiently filled with the resin, and the air bubbles is prevented from being generated, thereby being capable of ensuring the thermal conductivity in the resin. Further, the space for accommodating the electronic component is enlarged enough to increase the degree of freedom of mounting the electronic component, thereby attaining a higher density of mounting the electronic components on the printed-circuit board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a cross-sectional view showing a conventional electronic circuit board assembly; 
     FIG. 2A is a plan view showing an electronic circuit board assembly with a heat dissipation capability according to an embodiment of the present invention; 
     FIG. 2B is a bottom view of the electronic circuit board assembly shown in FIG. 2A; 
     FIG. 2C is a front view of the electronic circuit board assembly shown in FIG. 2A; 
     FIG. 2D is a right side view of the electronic circuit board assembly shown in FIG. 2A; 
     FIG. 3 is a bottom view of a printed-circuit board facing to a heat dissipation board according to the embodiment of the present invention; 
     FIG. 4A is a front view explaining depths of recessed portions in the heat dissipation board according to the embodiment of the present invention; 
     FIG. 4B is a cross-sectional view showing a structure of the heat dissipation board for mounting a MOSFET according to the embodiment of the present invention; 
     FIG. 4C is a cross-sectional view taken along the line E—E in FIG. 3, and shows a structure of the heat dissipation board for mounting a choke coil according to the embodiment of the present invention; 
     FIG. 5A is a cross-sectional view taken along the line F—F in FIG. 3, and shows a structure of the heat dissipation board for mounting the choke coil according to the embodiment of the present invention; and 
     FIGS. 5B and 5C are explanatory views showing steps of assembling the choke coil into the recessed portions of the heat dissipation board. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the invention will be described with reference to the accompanying drawings. FIGS. 2A to  2 D show a whole structure of an electronic circuit board assembly according to the embodiment of the present invention, in which FIG. 2A is a plan view: FIG. 2B is a bottom view; FIG. 2C is a front view; and FIG. 2D is a right side view of FIG.  2 A. 
     In the following description, a switching power source will be taken as a desirable example for applying the heat dissipation structure according to the present invention. The switching power source includes DC—DC converter for converting a DC voltage to another DC voltage having a different level. Because power loss yields as a result of voltage conversion, heat generated due to the power loss needs to be dissipated. In recent years, the switching power sources are made compact in size while providing a higher level of power output, so that an increased amount of heat is generated locally from the switching power source. Therefore, a heat dissipation structure Is particularly needed to the switching power source. On the other hand, the switching power source is made up of a plurality of heat generating components different in shape and material as will be described later. Therefore, the heat dissipation structure has to collectively dissipate heat generated from the various components regardless of the shape and material of the components. 
     As is best shown in FIG. 2C, the electronic circuit assembly includes a heat dissipation board  1 , a first circuit board  2 , an insulation sheet  3 , and a second circuit board  4 . As described above, in this embodiment, the circuit boards  2  and  4  are mounted with electronic components that configure the switching power source. Mounted on the first circuit board  2  are power electronic components requiring heat dissipation, including power semi-conductor devices, such as a MOSFET (metallic oxide semi-conductor field effect transistor) and magnetic components, such as a transformer  12  and choke coils  10 ,  11  as shown in FIG.  2 C. The second circuit board  4  is mounted chiefly with electronic components used for controlling purpose. As shown in FIG. 2C, the electronic components  9  are mounted on both top and bottom surfaces of the second circuit board  4 . Heat dissipation is not essential for the components mounted on the second circuit board  4 . The insulating sheet  3  is interposed between the first circuit board  2  and the heat dissipation board  1 . The heat dissipation board  1  is formed with recessed portions  1   a ,  1   b , and  1   c  in a surface facing the first circuit board  2  as will be described in detail hereinafter. 
     To assemble the heat dissipation board  1 , the insulation sheet  3 , the first and second circuit boards  2  and  4 , four fixing rods  5  and a pair of plastic fixing members  6  are used. The heat dissipation board  1 , the insulation sheet  3 , the first and second circuit boards  2  and  4  are stacked one on the other in the stated order, and the fixing members  6  are placed over the first circuit board  2  at two opposing short sides thereof while bridging the second circuit board  4  between the pair of fixing members  6 . Each fixing member  6  is formed with two holes  6   a  at both end portions thereof. The heat dissipation board  1 , the insulation sheet  3 , the first and second circuit boards  2  and  4  are also formed with holes corresponding to the holes  6   a  of the fixing members  6 . The fixing rods  5  are force-fitted into the holes  6   a  while aligning the corresponding holes of the components to be assembled, whereby the insulating sheet  3  and the circuit boards  2  and  4  are sandwiched between the heat dissipation board  1  and the fixing members  6 . 
     The circuit board assembly further includes first terminals  7  and second terminals  8 . The first terminals  7  protrude from the fixing member  4 , one from the right-side fixing member  4  and three from the left-side fixing member  4 . The first terminals  7  penetrate into holes formed in the fixing members  6  to reach a land of the first circuit board  2 . The end of each terminal  7  is fixed into a hole formed in the land of the first circuit board  4 . The second terminals  8  also protrude from the fixing members  4 , two from each fixing member. The second terminals  8  protrude to be the same height as the first terminals  7 . The second terminals  8  penetrate into holes formed in the fixing members  6  to reach a land of the second circuit board  4 . The end of each second terminal  8  is fixed into a hole formed in the land of the second circuit board  4 . The second terminals  8  are each provided with two flange-like portions in a spaced-apart relation in the lenghtwise direction. The second circuit board  4  is supported by the upper flange-like portions of the second terminals  8 . The lower flange-like portions of the second terminals  8  are in abutment with the surface of the first circuit board  2 . The first and second circuit boards  2  and  4  are, therefore, spaced apart from each other by a predetermined distance corresponding to the distance between the upper and lower flange-like portions of the second terminals  8 . 
     The lower portions of the fixing rods  5  are meshed to allow attachment of a case, a chassis, a frame or the like thereto. The meshed portions  5   a  of the fixing rods  5  can be accessed from the lower surface of the heat dissipation board  1 . 
     FIG. 3 is a bottom view showing a layout of the electronic components protruding from the lower surface of the first circuit board  2 , and notches (recessed portions) for the layout, where reference symbols  1   a ,  1   b ,  1   e  denote the portions corresponding to the recessed portions of the heat dissipation board  1 . Reference symbols  2   a  and  2   b  denote a hole and a notch into which a central leg and side legs of the choke coil  10  are inserted, respectively. Further, reference numerals  13 ,  14 , and  15  denote MOSFETs, and  16 , an IC for driving these transistors. These electronic components are accommodated in the recessed portion  1   a . The recessed portion  1   a  in which the above-noted electronic components are accommodated is filled with a resin for improving heat dissipation. 
     Reference symbols  2   c  and  2   d  denote holes into which cores of the choke coil  11  are inserted. Reference numerals  17 ,  18 , and  19  denote MOSFETs, and these electronic components are accommodated in the recessed portion  1   b . The MOSFETs are buried by the resin for improving heat dissipation. Reference symbols  2   e  and  2   f  denote through-holes into which a central leg and side legs of the transformer  12  are inserted, respectively. The transformer  12  is accommodated in the recessed portion  1   c  and is also buried by the resins. 
     FIG. 4A explains depths of the recessed portions  1   a  to  1   c . The depths of these recessed portions  1   a  to  1   c  are set in accordance with protruding amounts of the MOSFETs  13  to  15  and the IC  16 , the choke coil  11 , and the transformer  12 , respectively. To be more specific, when the first circuit board  2  is superposed on the heat dissipation board  1  with the insulating sheet  3  interposed therebetween, depths h1 to h3 of the recessed portions  1   a  to  1   c  are set so as to reserve substantially the same gap between the lower surfaces of those electronic components and bottom surfaces of the recessed portions  1   a  to  1   c . Further, the recessed portion  1   b  is configured such that a series of depths of the portions in this recessed portion  1   b , which correspond to the MOSFETS  13  to  15 , the IC  16 , and the choke coil  11 , are made different from each other because of these electronic components having different protruding amounts. 
     FIG. 4B is a sectional view showing a state where the MOSFET 17  is fitted into the recessed portion  1   b . The recessed portion  1   b  is filled with a gel-like resin  20  composed, for example, of silicon compound (silicon gel) exhibiting a high heat conductivity, and the MOSFET  17  is intruded into the recessed portion  1   b  filled with the resin  20 . Thus, the MOSFET  17  is set in a state where a gap between the MOSFET  17  and an internal surface of the recessed portion  1   b  is filed with the resin  20 . The other electronic components are similarly treated. In this case, the insulating sheet  3 , which is a sheet formed of a silicon-group resin having a high thermal conductivity or a double-sided adhesive sheet, is interposed between the heat dissipation board  1  and the first circuit board  2 , excluding the recessed portions  1   a  to  1   c.    
     Thus, according to the structure in which the resin  20  fills the recessed portion  1   b , the heat dissipation board  1  dissipates heat through the resin from a side surface  17   b  of the MOSFET  17  as indicated by an arrow c as well as from a bottom surface  17   a  of the MOSFET  17  as indicated by an arrow a, thereby improving heat dissipation capability. Further, the resin fills the gap between the side surface  17   b  of the MOSFET  17  and the internal surface of the recessed portion, and hence a distance W between these surfaces can be made proximate, whereby the high heat radiability from the side surface can be ensured. 
     Moreover, as in this example, in the case where the MOSFET  17  is a semiconductor device, the beat of a high-temperature terminal  21  is radiated through the resin between the MOSFET  17  and the internal surface of the recessed portion as indicated by an arrow d as well as through a wiring pattern  22  of the first circuit board  2  and through the insulating sheet  3  interposed between the first circuit board  2  and the heat dissipation board  1  as indicated by an arrow b. Thus, heat is dissipated with high efficiency. 
     FIG. 4C is a cross-sectional view cut along the line E—E in FIG. 3, showing a structure for mounting the choke coil  10 . According to the present invention, an insulator board or a metallic board coated with an insulator, in which no conductor is provided, may be used for each of the circuit boards  2  and  4 . However, this embodiment exemplifies a case where the first circuit board  2  employs a composite board composed of a resin or a mixture of the resin and ceramic powders, which is comprised of a multi-layered board provided inside with conductors based on a laminate structure; and a coil conductor  23  configuring the choke coil  10  is provided based on the laminate structure within the circuit board  2 . Reference numerals  24  and  25  denote cores configuring the choke coil  10 . The core  24  is classified as an R-type core (that is a modified form of an E-type core, where a peripheral side leg  25   b  is formed in an arcuate shape). A central leg  24   a  of the core  24  penetrates the through-hole  2   a , side legs  24   b  thereof penetrate the notches  2   b , and the core  24  is thus bonded to the upper plate-like core  25 . 
     As illustrated in FIG. 4C, in the case where the core  24  of the choke coil  10  is intruded into the recessed portion  1   a , an extra resin  20   a  as a part of the resin  20  is filled in a gap formed due to the notch  2   b , thereby preventing air bubbles from being generated and ensuring the thermal conductivity or the heat dissipation in the recessed portion  1   a . Further, variation in the protruding amounts of the electronic components toward the heat dissipation board and variation in the dimensions of the recessed portions can be eliminated by providing an escaping area for the extra resin described above, with the result that a high dimensional accuracy in manufacturing and a high accuracy of the coating amount are not required of the device, which facilitates manufacturing processes. 
     It should be noted that, when the notch  2   b  is formed as a through-hole in this case, this can be ensured as the escaping area for the extra resin  20   a . Moreover, the notch or the through-hole may be provided as one that does not receive the insertion of the core etc., i.e., as dedicated one for the escaping area for the resin. 
     FIG. 5A is a cross-sectional view of a structure for mounting the choke coil  10  as viewed in a direction of a line F—F in FIG.  3 . As illustrated in FIG. 5A, the recessed portion  1   a  is formed extending to the side surface from the surface of the heat dissipation board  1 , and the choke coil  10  is mounted such that the side surface of the core  24  extends to the side surface of the heat dissipation board  1 . 
     FIGS. 4B and 4C are views showing steps of assembling the choke coil  10  of the circuit board  2  into the heat dissipation board  1 . As shown in FIG. 5B, the recessed portion  1   a  is coated with the gel-like resin  20  exhibiting a satisfactory thermal conductivity. In this case, the amount of the resin  20  is set to such an amount that the resin  20  slightly overflows from the recessed portion  1   a  when the choke coil  10  is intruded into the recessed portion  1   a.    
     Thereafter, the choke coil  10  mounted on the circuit board  2  is intruded into the resin  20  in the recessed portion  1   a  and is integrally superposed on the heat dissipation board  1  through the insulating sheet  3  exhibiting the high thermal conductivity, portions of which correspond to the recessed portions  1   a  to  1   c  are removed. Therefore, a gap between the periphery of the choke coil  10  and the internal surface of the recessed portion  1   a  is filled with the resin  20 . 
     Through such a step, as shown in FIG. 5C, a partial resin  20   b  of the resin  20  is swollen out of the side surface of the heat dissipation board  1  and thereafter the extra resin  20   b  swelling out of the side surface thereof is removed by wiping. Note that an unillustrated tape is pasted to the portion thus exposed from the side surface. 
     Through such a step, the electronic components, such as the choke coil  10 , can be intruded in the state where the resin  20  is sufficiently coated over the recessed portion  1   a , thereby making it possible to prevent the air bubbles from being generated and to ensure the thermal conductivity in the resin. Further, a space for accommodating the electronic components, such as the choke coil  10 , is enlarged enough to increase a degree of freedom of mounting the electronic components, thereby attaining a higher density of mounting the electronic components on the circuit board  2 . 
     Furthermore, the electronic components, such as the choke coil  10 , are mounted so that the side surfaces thereof extend to the side surface of the heat dissipation board  1 , whereby the high-density mounting is attained. At the same time, cooling air blows directly against the electronic components when effecting forcible air cooling, whereby a heat radiating effect rises.