Patent Publication Number: US-2011075376-A1

Title: Module substrate radiating heat from electronic component by intermediate heat transfer film and a method for manufacturing the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a microelectronic module assembly, and more particularly to a module substrate having an electronic component, such as a heat-generative electronic component, mounted thereon. The present invention also relates to a method for manufacturing such a microelectronic module assembly. 
     2. Description of the Background Art 
     A high frequency module is known which has high frequency components, such as a high frequency power amplifier component, a high frequency filter component and a high frequency branching filter component, integrally mounted by flip-chip bonding on a dielectric substrate, as disclosed in Japanese patent laid-open publication Nos. 2006-203652, 2003-051733 and 2003-304048, for example. In addition, a power amplifier module is known in which a power amplifier component for use in communication in a microwave bandwidth is mounted on a dielectric substrate, as disclosed in Japanese patent laid-open publication No. 2005-191435, for example. 
     Such high frequency modules and power amplifier modules are actively used for mobile communication devices, such as cellular phones. Mobile communication devices are strongly required to be downsized. When downsizing, the devices are also strongly required to efficiently radiate to the exterior heat generated by heat-generative electronic components, such as a high frequency power amplifier component. 
     In the case of forming a module substrate, in order to provide effective electric insulation and anti-moisture or dust sealing against heat-generative electronic components mounted on a dielectric substrate, a measure is taken to cover those components with a resin, such as an epoxy resin or a phenol resin. Since the resin and the dielectric substrate are low in heat conductivity, it can be said that the heat-generative electronic components have the periphery thereof covered with a material having poor heat conductivity. Consequently, the temperature of the heat-generative electronic components is increased in operation, thus deteriorating the operating characteristics thereof or causing malfunction in some cases. 
     Therefore, with conventional module substrates having heat-generative electronic components mounted by flip-chip bonding on a dielectric substrate, a variety of mechanisms are contrived such as to efficiently radiate heat from the lower surface of the heat-generative electronic components to the exterior. Regarding another heat from the opposite surface, i.e. the upper surface, to the lower surface of the heat-generative electronic components of the components, however, effective mechanisms have not been taken for efficiently radiating this heat to the exterior. It is thus limitative to effectively prevent the temperature of the heat-generative electronic components due to heat generated by themselves from increasing. 
     SUMMARY OF THE INVENTION 
     The inventor of the present patent application has dedicated himself to studying mechanisms for efficiently dissipating or radiating heat from the upper surface of a heat-generative electronic component. As a result, a simulation the inventor conducted confirms that the problems are solved by the structure in which a material having high heat conductivity is positioned adjacent to the upper surface of a heat-generative electronic component with an insulating layer intervening, thus effectively preventing the temperature of the heat-generative electronic component from increasing. 
     It is thus an object of the present invention to provide a microelectronic module assembly having a heat-generative electronic component mounted thereon to be capable of efficiently radiating heat generated by the heat-generative electronic component to the exterior. It is also an object of the invention to provide a method for manufacturing such a microelectronic module assembly. 
     In accordance with the present invention, a microelectronic module assembly has a heat-generative electronic component which is flip-chip bonded through a solder bump on a wiring layer formed on a main surface of a first dielectric substrate. In the module assembly, a second dielectric substrate is attached on an upper surface opposite to a lower surface of the heat-generative electronic component facing the first dielectric substrate through an insulating layer. 
     The module assembly having a heat-generative electronic component mounted thereon in accordance with the invention is configured by sandwiching the heat-generative electronic component between the first and second dielectric substrates, and further has the following configuration. 
     An intermediate heat transfer film for transferring heat generated by the heat-generative electronic component to the second dielectric substrate is attached between the insulating layer formed on the upper surface of the heat-generative electronic component and the second dielectric substrate so as to make the intermediate heat transfer film in close contact with a lower surface of the second dielectric substrate adjacent to the heat-generative electronic component. 
     The module assembly in accordance with the invention can be manufactured by a method including the following steps. 
     The method for manufacturing the module assembly in accordance with the invention may include a flip-chip bonding step, an intermediate dielectric substrate attaching step, and a second dielectric substrate attaching step. 
     In the flip-chip bonding step, a first dielectric substrate having a wiring layer formed on its main surface is prepared, and an electrical connection of the heat-generative electronic component to the wiring layer is formed through a solder bump. 
     In the intermediate dielectric substrate attaching step, an intermediate dielectric substrate provided with a hole for surrounding the heat-generative electronic component is prepared, a dielectric adhesive agent is applied to the main surface of the first dielectric substrate and an upper surface of the heat-generative electronic component, and the intermediate dielectric substrate is attached to the main surface of the first dielectric substrate through the dielectric adhesive agent so as to fit the heat-generative electronic component into the hole. The dielectric adhesive agent is solidified to form an insulating layer. 
     In the second dielectric substrate attaching step, a second dielectric substrate provided with an intermediate heat transfer film is prepared, and the second dielectric substrate is attached so as to position the intermediate heat transfer film above the upper surface of the heat-generative electronic component through the dielectric adhesive agent and so as to bring a portion not provided with the intermediate heat transfer film on a lower surface of the second dielectric substrate into close contact with an upper surface of the intermediate dielectric substrate. 
     In accordance with an aspect of the invention, an alternative configuration of module assembly may be configured by further adding the following mechanism to the above-described module assembly. 
     In the alternative configuration of module assembly, on the top heat radiation film formed on the upper surface opposite to the lower surface of the second dielectric substrate has a structure including a through hole for connecting the intermediate heat transfer film with the top heat radiation film. 
     The alternative configuration of module assembly in accordance with the invention can be manufactured by a method including the following steps. 
     The method for manufacturing the alternative configuration of module assembly in accordance with the invention includes, in addition to the flip-chip bonding step, intermediate dielectric substrate attaching step and second dielectric substrate attaching step described above, a through hole forming step and a top heat radiation film forming step following the second dielectric substrate attaching step. 
     In the through hole forming step, a through hole is formed in the second dielectric substrate. 
     In the top heat radiation film forming step, a top heat radiation film is formed on the upper surface of the second dielectric substrate provided with the through hole opposite to the heat-generative electronic component. 
     According to the prevent invention, in the module assembly, the intermediate heat transfer film is positioned between the insulating layer formed on the upper surface of the heat-generative electronic component and the second dielectric substrate so as to make the intermediate heat transfer film in close contact with a lower surface of the second dielectric substrate. By the intermediate heat transfer film thus dispose, heat emanated from the upper surface of the heat-generative electronic component can be efficiently radiated to the exterior. Therefore, by such a heat radiation mechanism provided on the upper surface of the heat-generative electronic component, the temperature of the operating heat-generative electronic component can more effectively controlled from increasing than a conventional module substrate. 
     Furthermore, according to the prevent invention, in the alternative configuration of module assembly, the structure is provided which interconnects the top heat radiation film formed on the upper surface of the second dielectric substrate and the intermediate heat transfer film with each other by the through hole. By the structure including the through hole, heat reaching the intermediate heat transfer film is efficiently transferred through the through hole to the top heat radiation film formed on the upper surface of the second dielectric substrate. Therefore, the alternative configuration of module assembly in accordance with the invention can more efficiently radiate heat emanated from the upper surface of the heat-generative electronic component to the exterior. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention will become more apparent from consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A ,  1 B and  1 C are schematic cross sectional views cut along a plane perpendicular to a main surface of a conventional dielectric substrate on which a heat-generative electronic component is mounted; 
         FIGS. 2A ,  2 B and  2 C are schematic plan views as viewed in a direction perpendicular to the main surface of the conventional dielectric substrate shown in  FIGS. 1A ,  1 B and  1 C, respectively; 
         FIG. 3A  is a top plan view showing a conventional module substrate to be simulated for use in understanding conditions set for the simulation; 
         FIG. 3B  is a cross sectional view showing the module substrate cut along line IIIB-IIIB in  FIG. 3A ; 
         FIG. 3C  shows the shape and position of the heat-generative electronic component shown in  FIG. 3B ; 
         FIG. 4  shows how the surface temperature of the heat-generative electronic component is distributed, obtained from the simulation made on the conventional module substrate; 
         FIGS. 5A ,  5 B and  5 C are cross sectional views cut along a plane perpendicular to a main surface of a dielectric substrate on which a heat-generative electronic component is mounted in accordance with an illustrative embodiment of the present invention; 
         FIGS. 6A ,  6 B and  6 C are plan views as viewed in a direction perpendicular to the main surface of the dielectric substrate shown in  FIGS. 5A ,  5 B and  5 C, respectively; 
         FIG. 7A  is a top plan view showing a module substrate to be simulated for use in understanding conditions set for the simulation in accordance with the illustrative embodiment; 
         FIG. 7B  is a cross sectional view showing the module substrate cut along line VIIB-VIIB in  FIG. 7A ; 
         FIG. 7C  shows the shape and position of an intermediate heat radiation film; 
         FIG. 8  shows how the surface temperature of the heat-generative electronic component is distributed, obtained from the simulation made on the illustrative embodiment shown in  FIGS. 7A ,  7 B and  7 C; 
         FIGS. 9A ,  9 B and  9 C are cross sectional views cut along a plane perpendicular to a main surface of a dielectric substrate on which a heat-generative electronic component is mounted in accordance with an alternative embodiment of the present invention; 
         FIGS. 10A ,  10 B and  10 C are plan views as viewed in a direction perpendicular to the main surface of the dielectric substrate shown in  FIGS. 9A ,  9 B and  9 C, respectively; 
         FIG. 11A  is a virtual plan view showing a module substrate to be simulated for use in understanding conditions set for the simulation in the alternative embodiment; 
         FIG. 11B  is a cross sectional view showing the module substrate cut along line XIB-XIB in  FIG. 11A ; and 
         FIG. 12  shows a surface temperature distribution of the heat-generative electronic component obtained from the simulation made on the alternative embodiment shown in  FIG. 9A to 10C . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order to have the invention better understood, reference will first be made to  FIGS. 1A to 4  to describe the configuration and thermal characteristics of a typical, conventional module substrate having a heat-generative electronic component mounted thereon, such as a high frequency module or a power amplifier module, to specifically clarify problems to be solved by the present invention. Secondly, illustrative embodiments of the present invention will be described with reference to  FIGS. 5A to 12 , which merely conceptually show the illustrative embodiments to the extent of understanding the basic configuration and characteristics of a module substrate in accordance with the present invention. Preferred embodiments in accordance with the present invention will be described with materials, numerical conditions or the like of components or elements being just exemplified. Therefore, the present invention is not understood as restrictive to the specific details of the illustrative embodiments. In the figures, like components or elements are designated with the same reference numerals and repetitive descriptions thereon may be refrained from. 
     With reference first to  FIGS. 1A to 4 , description will be made on the configuration of a typical, conventional module substrate having a heat-generative electronic component mounted thereon, such as a high frequency module or a power amplifier module, and on the radiation characteristics of heat generated by the heat-generative electronic component. In the context, the term “module substrate” may be understood simply as a substrate per se for use in mounting a microelectronic module as well as more broadly as a module unit having, and hence including, a micro electronic component, such as a heat-generative component, mounted as a modular on a substrate, such as a dielectric substrate. 
       FIGS. 1A to 1C  and  2 A to  2 C are cross sectional and plan views, respectively, useful for understanding the configuration of a conventional module substrate. The module substrate has its structure layered. Thus, in order to expedite understanding a module substrate having a layered structure, the structures of a module substrate are shown in the figures in the time-sequential order of the fabrication at important ones of the initial to final steps of fabricating the module substrate.  FIGS. 1A ,  1 B and  1 C are cross sectional views cut along a plane perpendicular to a main surface of a dielectric substrate on which a heat-generative electronic component is mounted.  FIGS. 2A ,  2 B and  2 C are plan views as viewed in a direction perpendicular to the main surface of that dielectric substrate shown in  FIGS. 1A ,  1 B and  1 C, respectively. 
     As seen from  FIGS. 1A and 2A , in the configuration of a typical conventional module substrate, a heat-generative electronic component  14  is flip-chip bonded through a solder bump  16  on a wiring layer  12  formed on one  34  of primary surfaces of a first dielectric substrate  10 . 
     As seen from  FIG. 1C , the heat-generative electronic component  14  has a generally rectangular vertical cross section, and has its lower, in the figure, surface  32  facing the first dielectric substrate  10  and its upper surface  30  opposite to the lower surface  32 . Above the upper surface  30 , a second dielectric substrate  26  is positioned with an insulating layer  20  intervening. In other words, the configuration of the typical conventional module substrate has the heat-generative electronic component  14  sandwiched between the first and second dielectric substrates  10  and  26  to form a module substrate having a heat-generative electronic component built-in. 
     That conventional module substrate can be manufactured by a process including steps of mounting the electric component  14  by flip-chip bonding and attaching the second dielectric substrate  26 . 
     In the flip-chip bonding step, the first dielectric substrate  10  having the wiring layer  12  formed on the main surface  34  is prepared, and electrical connection of the heat-generative electronic component  14  to the wiring layer  12  is formed through the solder bump  16 . 
     In an intermediate dielectric substrate attaching step, an intermediate dielectric substrate  24  is prepared which has a hole  36 ,  FIG. 1C , cut therein for surrounding the heat-generative electronic component  14 , and is attached by a dielectric adhesive agent to the main surface  34  of the first dielectric substrate  10  and the upper surface  30 , opposite to the first dielectric substrate  10 , of the heat-generative electronic component  14  so as to fit the heat-generative electronic component  14  into the hole  36 . The dielectric adhesive agent is then solidified to complete the insulating layer  20 . 
     On the main surface  34  of the first dielectric substrate  10 , the wiring layer  12  is partially formed. Therefore, in such a region of the main surface  34  of the first dielectric substrate  10  where the wiring layer  12  is formed, the intermediate dielectric substrate  24  is attached to the wiring layer  12  formed on the main surface  34  through the dielectric adhesive agent, rather than the main surface  34  of the first dielectric substrate  10 . 
     In the second dielectric substrate attaching step, the second dielectric substrate  26  is arranged in close contact onto an upper surface  38  of the intermediate dielectric substrate  24 , which surface will face the second dielectric substrate  26 , thereby forming the second dielectric substrate  26  over the entire structure. 
     In the step shown in  FIG. 1B , the dielectric adhesive agent is applied for forming the insulating layer  20  so as to cover the main surface  34  of the first dielectric substrate  10 , the wiring layer  12  formed on the main surface  34  and the upper surface  30  of the heat-generative electronic component  14 . After having applied, but before solidifying, the dielectric adhesive agent, the intermediate dielectric substrate  24  having the hole  36  formed for surrounding the heat-generative electronic component  14  is placed so that the heat-generative electronic component  14  is fitted into the hole  36  and a lower surface  42  of the intermediate dielectric substrate  24  facing the first dielectric substrate  10  is parallel to the main surface  34  of the first dielectric substrate  10 . 
     Subsequently, the lower surface  44  of the second dielectric substrate  26  adjacent to the heat-generative electronic component  14  is adhered in close contact to the upper surface  38  of the intermediate dielectric substrate  24 . Thus, the intermediate dielectric substrate  24  is attached to the first dielectric substrate  10  and in turn the second dielectric substrate  26  is positioned, thereby forming the upper surface  30  of the heat-generative electronic component  14  and the lower surface  44  of the second dielectric substrate  26  in parallel with each other. 
     The adhesive agent, not shown in the figures, for adhering the lower surface  44  of the second dielectric substrate  26  onto the upper surface  38  of the intermediate dielectric substrate  24  may be the same as or different from that for forming the insulating layer  20 . 
     As shown in  FIG. 2A , when the flip-chip bonding step of forming the conventional module substrate is completed, the heat-generative electronic component  14  can directly be viewed. The heat-generative electronic component  14  is electrically connected to the wiring layer  12  formed on the main surface  34  of the first dielectric substrate  10  through the solder bump  16  at a total of ten points, thereby accomplishing the flip-chip bonding. As shown in  FIG. 2B , when the step of applying the dielectric adhesive agent for forming the insulating layer  20  so as to cover the wiring layer  12  and the upper surface  30  of the heat-generative electronic component  14  is finished, the wiring layer  12  and the heat-generative electronic component  14  are covered with the dielectric adhesive agent. As shown in  FIG. 2C , after having attached the second dielectric substrate  26 , a top heat radiation film  28  is formed on the upper surface  40 ,  FIG. 1C , of the second dielectric substrate  26 , the upper surface  40  being opposite to one surface  44  of the second dielectric substrate  26  adjacent to the heat-generative electronic component  14 . 
     In contrast to the top heat radiation film  28 , a bottom heat radiation film  18  is formed on the other main surface, i.e. bottom surface,  46  opposite to the main surface  34  of the first dielectric substrate  10  adjacent to the heat-generative electronic component  14 . Therefore, in the configuration of the conventional module substrate, heat generated by the heat-generative electronic component  14  is eventually radiated through the top and bottom heat radiation films  28  and  18  to the exterior. 
     Note that the figures do not depict a downward heat radiation mechanism for radiating heat generated by the heat-generative electronic component  14  through the bottom heat radiation film  18  to the exterior, which will be described below. 
     Usually, a conventional type of module substrate similar to the above has a space between the lower surface  32  of the heat-generative electronic component  14  facing the first dielectric substrate  10  and the main surface  34  of the first dielectric substrate  10  filled with a well-known underfill material  22 . The underfill material  22  is electrically insulative and not high in heat conductivity. 
     In the downward heat radiation mechanism, some measures, such as a via hole for heat radiation, not shown, are often formed to render a heat-generating point of the heat-generative electronic component  14  communicate with the bottom heat radiation film  18 . The heat radiation via hole is formed so as to penetrate the first dielectric substrate  10 , and composed of a material having high heat conductivity, such as copper. 
     To a dielectric material for forming the first dielectric substrate  10  and the second dielectric substrate  26 , an insulative resin material, such as a Teflon (trademark) or glass epoxy, is applicable. Generally, such a dielectric substrate has its surface, on which the wiring layer  12  composed of an electrically conductive material, such as copper, is deposited to form a circuit pattern. To the dielectric adhesive agent for forming the insulating layer  20 , an epoxy resin adhesive may be applied. To the underfill material  22 , a thermoset epoxy resin may be applied. 
     Next, with reference to  FIGS. 3A to 3C  and  4 , described will be a result of simulative estimation on the heat radiation characteristics of the conventional module substrate having the heat-generative electronic component.  FIGS. 3A ,  3 B and  3 C will be referred to for describing conditions set for the simulation.  FIG. 4  shows a surface temperature distribution obtained from the simulation on the heat-generative electronic component. 
       FIG. 3A  is a top plan view showing a conventional module substrate to be simulated. In this simulation, the module substrate used has a square shape, in a plan view, having its side length equal to 8000 μm, or 8 mm. 
       FIG. 3B  is a cross sectional view showing the conventional module substrate cut along line IIIB-IIIB shown in the top plan view of  FIG. 3A . 
       FIG. 3C  is a plan view virtually showing the plane of the conventional module substrate at the level indicated by an arrow B in  FIG. 3B , to illustrate the shape and position of the heat-generative electronic component  14 ,  FIG. 3B , which may also correspond to the heat-generative electronic component  14  shown in  FIGS. 1A and 2A . The heat-generative electronic component  14  is a rectangular shape, in a plan view, having a length of 1330 μm and a width of 2550 μm, and is positioned in the center of the module substrate. 
     As shown in  FIG. 3B , the lower and top heat radiation films  18  and  28  have a thickness of 18 μm. The heat-generative electronic component  14  and the intermediate dielectric substrate  24  have a thickness of 132 μm, i.e. in height. The underfill material  22  has a thickness of 50 μm and the first dielectric substrate  10  has a thickness of 327 μm, also in height. The second dielectric substrate  26  has predominantly a thickness of 327 μm and partly 195 μm corresponding to 327 μm-132 μm. 
     The simulation was performed by using the heat-generative electronic component  14  made of base material having its heat conductivity of 68 W/(m·K), and using the first, intermediate and second dielectric substrates  10 ,  24  and  26  made of dielectric material having its heat conductivity of 0.2 W/(m·K). In the simulation, the heat conductivity of the underfill material  22  was 0.4 W/(m·K), and the heat conductivity of the bottom and top heat radiation films  18  and  28 , and the wiring layer  12  was 390 W/(m·K). 
     In the simulation, the heat-generative electronic component  14  was assumed as an MMIC (microwave monolithic integrated circuit) having three-stage structure. In other words, the MMIC having three-stage structure includes first, second and third stage circuits  14 - 1 ,  14 - 2  and  14 - 3 . The first, second and third stage circuits  14 - 1 ,  14 - 2  and  14 - 3  are fed with the electric powers of 3 V-30 mA, 3 V-60 mA and 3.2 V-120 mA, respectively. 
     With reference to  FIG. 4 , described will be a surface temperature distribution obtained from the simulation on the heat-generative electronic component. The outmost rectangle shown in  FIG. 4  represents the outline of the heat-generative electronic component  14 , and has a length of 1330 μm and a width of 2550 μm, see  FIG. 3C  also. In order to facilitate understanding the size of the heat-generative electronic component  14 ,  FIG. 4  also shows a scale indicating the length of 1 mm, or 1000 μm. The temperature distribution shown in  FIG. 4  is depicted as measured on the top surface of the top heat radiation film  28 . 
     As shown in  FIG. 4 , in the three-stage structured MMIC, the ambient temperatures of, i.e. temperatures in the areas surrounding, the first and second stage circuits  14 - 1  and  14 - 2  are 42° C. The ambient temperature of the third stage circuit  14 - 3  is almost 60° C., and the highest temperature of the third stage circuit  14 - 3  is 64.76° C. 
     As described above, the conventional module substrate shown in  FIGS. 1A to 4  is configured with the heat-generative electronic component  14  surrounded by a material having low heat conductivity. Due to such a configuration, it is difficult to radiate the heat generated by the heat-generative electronic component  14  through the lower and top heat radiation films  18  and  28 . As described above, the highest ambient temperature of the third stage circuit  14 - 3  in the three-stage structured MMIC serving as the heat-generative electronic component  14  is 64.76° C., which is significantly high. 
     By contrast, in accordance with preferred embodiments of the present invention, a module substrate is particularly characterized by including a mechanism for promoting the radiation of heat through the top heat radiation film  28 . Therefore, heat radiated through the bottom heat radiation film  18  to the exterior will not be taken into account. 
     The module substrates in accordance with preferred embodiments may be similar to the conventional module substrate described above so far as the structure below the heat-generative electronic component  14  to the bottom heat radiation film  18  is concerned. Therefore, discussion on the temperature characteristics of module substrates in accordance with preferred embodiments will be directed to the surface temperature of the top heat radiation film  28  to thereby clarify differences over the conventional module substrate. 
     Now, with reference to  FIGS. 5A to 5C  and  6 A to  6 C, described will be a microelectronic module assembly, implemented as a module substrate having a heat-generative electronic component, in accordance with a preferred embodiment of the present invention, specifically in terms of its configuration.  FIGS. 5A ,  5 B and  5 C are cross sectional views cut along a plane perpendicular to the main surface of a dielectric substrate having a heat-generative electronic component  14  mounted thereon.  FIGS. 6A ,  6 B and  6 C are plan views as viewed in a direction perpendicular to the main surface of the dielectric substrate having shown in  FIGS. 5A ,  5 B and  5 C, respectively. 
     The module substrate in accordance with the preferred embodiment is specifically different over the conventional module substrate described above in that the embodiment includes an intermediate heat transfer film  50 ,  FIG. 5C , for transferring heat generated by the heat-generative electronic component  14  to the second dielectric substrate  26 . More specifically, as clearly understood from  FIG. 5C , the intermediate heat transfer film  50  is disposed between the insulating layer  20  formed on the upper surface  30  of the heat-generative electronic component  14  and the second dielectric substrate  26  in close contact with the lower surface  44  of the second dielectric substrate  26 . 
     The module substrate of the preferred embodiment thus differs from the conventional module substrate particularly in the intermediate heat transfer film  50  additionally provided. The remaining part of the structure may be the same as the conventional module substrate described above. Therefore, repetitive descriptions on like components and elements will be avoided. 
     The module substrate in accordance with the preferred embodiment may be fabricated by a process including the following steps. Specifically, the module substrate in accordance with the embodiment may be fabricated in the above-described process for manufacturing the conventional module substrate with its step of attaching the second dielectric substrate modified to read below. 
     With the preferred embodiment, the second dielectric substrate attaching step may be applied in such a fashion that the second dielectric substrate  26  provided with the intermediate heat transfer film  50  is prepared and is then positioned on the upper surface  30  of the heat-generative electronic component  14  through a dielectric adhesive agent, which will be solidified to serve as the insulating layer  20 , with its portion  44   a  not provided with the intermediate heat transfer film  50  on the lower surface  44  being in close contact with the upper surface  38  of the intermediate dielectric substrate  24 , thereby attaching the second dielectric substrate  26 . 
     The module substrate of the embodiment may be fabricated in the same process, except for the second dielectric substrate attaching step described above, as the conventional module substrate. Therefore, repetitive descriptions on the remaining part of the process will be refrained from. 
     Next, with reference to  FIGS. 7A to 7C  and  8 , described will be a result of a simulative estimation on the heat radiation characteristics of the module substrate having the heat-generative electronic component in accordance with the preferred embodiment.  FIGS. 7A ,  7 B and  7 C are for use in understanding conditions set for the simulation.  FIG. 8  shows a surface temperature distribution obtained from the simulation on the heat-generative electronic component  14  of the embodiment. 
       FIG. 7A  is a top plan view showing the module substrate to be simulated in accordance with the preferred embodiment. In this simulation also, the module substrate has a square shape, in a plan view, having its side length of 8000 square μm, or 8 square mm. 
       FIG. 7B  is a cross sectional view showing the module substrate of the embodiment cut along line VIIB-VIIB shown in the top plan view of  FIG. 7A . 
       FIG. 7C  is a top plan view virtually showing the plane of the module substrate of the embodiment at the level indicated by an arrow C in  FIG. 7B , to illustrate the shape and position of the intermediate heat transfer film  50 . The intermediate heat transfer film  50  is a rectangular shape, in a plan view, having a width of 2550 μm and a length of 8000 μm substantially equal to the length of the side of the module substrate  14 . 
     The shape of the intermediate heat transfer film  50  is not necessarily congruous with the upper surface  30  of the heat-generative electronic component  14 . It may be sufficient that at least a part of the intermediate heat transfer film lies immediately on the upper surface  30  of the heat-generative electronic component  14 , for example, and therefore the intermediate heat transfer film  50  may extend substantially broader than the upper surface  30  of the heat-generative electronic component  14 . Rather, so far as the electric operating characteristics of the heat-generative electronic component  14  is not affected, it is effective to form the intermediate heat transfer film  50  as broad in area as possible from the viewpoint of improving heat radiation. 
     As shown in  FIG. 7B , the lower and top heat radiation films  18  and  28  have a thickness of 18 μm. The heat-generative electronic component  14  and the intermediate dielectric substrate  24  have a thickness of 132 μm, and the underfill material  22  has a thickness of 50 μm. The first dielectric substrate  10  has a thickness of 327 μm, the second dielectric substrate  26  has a thickness of 127 μm and the intermediate heat transfer film  50  has a thickness of 18 μm. 
     The simulation was performed by using the heat-generative electronic component  14  made of base material having its heat conductivity of 68 W/(m·K), and using the first, intermediate and second dielectric substrates  10 ,  24  and  26  made of dielectric material having its heat conductivity of 0.2 W/(m·K). In the simulation, the heat conductivity of the underfill material  22  was 0.4 W/(m·K), and the heat conductivity of the lower and top heat radiation films  18  and  28 , and the wiring layer  12  was 390 W/(m·K). 
     In the simulation, the heat-generative electronic component  14  of the preferred embodiment was assumed as an MMIC having three-stage structure, similarly to the simulation described above in respect of the conventional module substrate. 
     With reference to  FIG. 8 , described will be the surface temperature distribution obtained from the simulation on the heat-generative electronic component  14 . The outmost rectangle shown in  FIG. 8  represents the shape of the heat-generative electronic component  14  shown in  FIG. 7C , and has its width of 2550 μm.  FIG. 8  show the temperature distribution observed over the upper surface of the top heat radiation film  28  corresponding to the electronic component  14 . 
     As seen from  FIG. 8 , in the three-stage structured MMIC of the preferred embodiment, the ambient temperatures of the first and second stage circuits  14 - 1  and  14 - 2  are 25° C. The ambient temperature of the third stage circuit  14 - 3  is almost 40° C., and the highest temperature of the third stage circuit  14 - 3  is 40.076° C. Thus, the highest temperature of the third stage circuit  14 - 3  of the preferred embodiment is lower than the highest temperature of 64.67° C. in the above-described conventional module substrate by no less than 24.594° C. Therefore, according to the module substrate in accordance with the preferred embodiment, it was confirmed that the temperature of the heat-generative electronic component in operation could more effectively be suppressed from increasing that the conventional module substrate. 
     Now, with reference to  FIGS. 9A to 9C  and  10 A to  10 C, described will be a module substrate having a heat-generative electronic component in accordance with an alternative embodiment of the present invention.  FIGS. 9A ,  9 B and  9 C are cross sectional views cut along a plane perpendicular to the main surface of a dielectric substrate having a heat-generative electronic component  14  mounted thereon.  FIGS. 10A ,  10 B and  10 C are plan views as viewed in a direction perpendicular to the main surface of the dielectric substrate shown in  FIGS. 9A ,  9 B and  9 C, respectively. 
     The module substrate in accordance with the alternative embodiment may be the same as the preferred embodiment show in and described with reference to  FIGS. 5A to 6C  except for the top heat radiation film  28  having through holes  52  cut therethrough to connect its lower surface  44  to its upper surface  40 , opposite to the former, to form a structure for conducting the heat from the intermediate heat transfer film  50  to the top heat radiation film  28 . The through holes  52  may be filled with filler material thermally conductive. 
     More specifically, the module substrate of the alternative embodiment is differed from the module substrate shown in and described with reference to  FIGS. 5A to 6C  in the through holes  52  newly provided to connect the intermediate heat transfer film  50  and the top heat radiation film  28  to each other and in the size of the intermediate heat transfer film  50 . In other part configuration, the alternative embodiment may be the same as the module substrate shown in  FIGS. 5A to 6C . Therefore, repetitive descriptions on like components and elements will be avoided. 
     The module substrate in accordance with the alternative embodiment may be fabricated by a process for manufacturing the module substrate including the following steps. 
     The process for manufacturing the module substrate of the alternative embodiment includes, in addition to the flip-chip bonding, intermediate dielectric substrate attaching and second dielectric substrate attaching steps described above, a through hole forming step and a top heat radiation film forming step following the second dielectric substrate attaching step. 
     In the through hole forming step, the through holes  52  are formed in the second dielectric substrate  26 . In the top heat radiation film forming step, the top heat radiation film  28  is formed on the upper surface  40 , opposite to the heat-generative electronic component  14 , of the second dielectric substrate  26  thus provided with the through holes  52 . The through hole forming step may be performed by a conventional technique disclosed by, for example, Japanese patent laid-open publication Nos. 2001-332650 and 2008-251935. Further details thereon will therefore not be described. 
     Next, with reference to  FIGS. 11A ,  11 B and  12 , described will be a result of simulative estimation of heat radiation characteristics of the module substrate having the heat-generative electronic component in accordance with the alternative embodiment.  FIGS. 11A and 11B  are virtual plan and cross sectional views, respectively, useful for understanding conditions set for the simulation.  FIG. 12  shows a surface temperature distribution obtained from the simulation on the heat-generative electronic component of the alternative embodiment. 
       FIG. 11A  virtually shows in a plan view the module substrate  14  to be simulated in accordance with the alternative embodiment with the top heat radiation film  28  removed simply for illustration, or cut at an arrow D in  FIG. 11B . In this simulation also, the module substrate  14  has a square shape, in a plan view, having its side length of 8000 μm, or 8 mm. As shown in the figure, the through holes  52  are positioned within a region corresponding to the upper surface of the heat-generative electronic component  14 , the upper surface having its length of 1330 μm and its width of 2550 μm. 
       FIG. 11B  is a cross sectional view showing the module substrate cut along line XIB-XIB in the virtual plan view shown in  FIG. 11A . From the figure, it can be seen how the top heat radiation film  28  is connected with the intermediate heat transfer film  50  by the through holes  52 . 
     As depicted in  FIG. 11B , the bottom heat radiation film  18  and the top heat radiation film  28  have a thickness of 18 μm. The heat-generative electronic component  14  and the intermediate dielectric substrate  24  have a thickness of 132 μm and the underfill material  22  has a thickness of 50 μm. The first dielectric substrate  10  has a thickness of 327 μm, the second dielectric substrate  26  has a thickness of 127 μm and the intermediate heat transfer film  50  has a thickness of 18 μm. 
     The simulation was performed by using the heat-generative electronic component  14  made of base material having its heat conductivity of 68 W/(m·K), and using the first, intermediate and second dielectric substrates  10 ,  24  and  26  made of dielectric material having its heat conductivity of 0.2 W/(m·K). In the simulation, the heat conductivity of the underfill material  22  was 0.4 WI (m·K), and the heat conductivity of the lower and top heat radiation films  18  and  28 , and the wiring layer  12  was 390 W/(m·K). 
     In the simulation also, the heat-generative electronic component  14  of the alternative embodiment was assumed as an MMIC having three-stage structure, similarly to the simulation described above in respect of the conventional module substrate. 
     In the simulation conducted as shown in  FIGS. 11A and 11B , conditions set therefor may be the same as the simulation described with reference to  FIGS. 7A and 7B  except for the conditions described above as well as the through holes  52  provided and the area of the intermediate heat transfer film  50 . 
     With reference to  FIG. 12 , description will be made on the surface temperature distribution of the heat-generative electronic component  14  obtained from the simulation. The outmost rectangle shown in  FIG. 12  represents the shape of the heat-generative electronic component  14 , and has its length of 1330 μm and its width of 2550 μm.  FIG. 12  shows the temperature distribution of the upper surface of the top heat radiation film  28 . 
     As understood from  FIG. 12 , in the three-stage structured MMIC of the alternative embodiment, the ambient temperatures of the first and second stage circuits  14 - 1  and  14 - 2  are 4° C. The ambient temperature of the third stage circuit  14 - 3  is approximately 19° C., and the highest temperature of the third stage circuit  14 - 3  is 19.553° C. Thus, the highest temperature of the third stage circuit  14 - 3  of the alternative embodiment is lower than the highest temperature of 40.076° C. in the module substrate of the illustrative embodiment shown in and described earlier with reference to  FIGS. 5A to 6C  by no less than 20.523° C. Therefore, with the module substrate in accordance with the alternative embodiment, it was confirmed that the temperature of the heat-generative electronic component in operation could further effectively be controlled than the module substrate of the embodiment described earlier. 
     As discussed earlier, the illustrative embodiment shown in and described with reference to  FIGS. 5A to 6C  is more effectively applicable to an application where the intermediate heat transfer film  50  is allowed less limitative in designing to its dimension so as to be attached to the heat-generative electronic component  14  having its upper surface  30  different in shape therefrom. Therefore, the intermediate heat transfer film  50 , when designed larger in dimension than the surface of the electronic component  14 , can improve the heat radiation efficiency without requiring a manufacturing step for forming through holes, thereby reducing the cost in manufacturing. 
     The module substrate in accordance with the alternative embodiment is more effective in a case where it is difficult to increase, or forced to render comparable, the area of the intermediate heat transfer film  50 , for example, where the electric operating characteristics of the heat-generative electronic component  14  is sensitively affected by the intermediate heat transfer film  50 . Therefore, even when it is difficult to increase the dimension of the intermediate heat transfer film  50 , the through holes  52  can radiate the heat sufficiently efficiently. 
     The entire disclosure of Japanese patent application No. 2009-223783 filed on Sep. 29, 2009, including the specification, claims, accompanying drawings and abstract of the disclosure, is incorporated herein by reference in its entirety. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.