BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a printed circuit board unit including a printed wiring board and an electronic component package mounted on the printed wiring board. The electronic component package includes a heat spreading member covering over a predetermined surface of an electronic component such as a LSI (Large-Scale Integrated circuit) chip. The heat spreading member extends larger than the predetermined surface of the electronic component.  
         [0003]     2. Description of the Prior Art  
         [0004]     A LSI package includes a LSI chip mounted on a small-sized printed wiring board. A heat spreading member or heat spreader is received on the upper surface of the LSI chip. A solidified material, such as a solidified silver paste, a solidified diamond paste, or the like, having heat conductivity is interposed between the LSI chip and the heat spreader. The solidified material serves to improve an efficient transfer of heat from the LSI chip to the heat spreader. The heat is allowed to spread over a larger or wider area of the heat spreader.  
         [0005]     The small-sized printed wiring board may utilize a substrate made of resin in the LSI package, for example. Such a substrate, however, tends to suffer from a larger expansion as compared with the LSI chip in response to a rise in temperature of the LSI chip, for example, because of the linear expansivity larger than that of the LSI chip. This results in a slight camber of the substrate. The camber of the substrate causes separation between the heat spreader and the solidified material. An efficient transfer of heat cannot thus be accomplished between the LSI chip and the heat spreader.  
       SUMMARY OF THE INVENTION  
       [0006]     It is accordingly an object of the present invention to provide an electronic component package as well as an electronic component package assembly both capable of reliably transferring heat from an electronic component to a heat spreading member. It is also an object of the present invention to provide a printed circuit board unit greatly contributing to improvement of heat radiation from the electronic component package such as a LSI package.  
         [0007]     According to a first aspect of the present invention, there is provided a printed circuit board unit comprising: a printed wiring board; an electronic component package mounted on the surface of the printed wiring board, said electronic component package allowing a heat spreading member to cover over an electronic component, said heat spreading member extending larger than a predetermined surface of the electronic component; a contact piece contacting with the surface of the heat spreading member over a contact area smaller than the predetermined surface of the electronic component; and a heat radiation member defining a flat surface opposed to the surface of the heat spreading member at a position adjacent to the contact piece.  
         [0008]     The printed circuit board unit allows concentration of an urging force at the contact area between the heat spreading member and the contact piece when the urging force is applied to the heat spreading member through the contact piece. The heat spreading member is thus reliably urged against the electronic component.  
         [0009]     Now, assume that the electronic component generates heat. The generated heat is transferred to the heat spreading member. The heat is allowed to spread over a larger or wider area of the heat spreading member since the heat spreading member extends larger than the predetermined surface of the electronic component. The spread heat is then transferred to the heat radiation member. The heat radiation member serves to radiate the heat into air. The electronic component is in this manner efficiently prevented from a rise in temperature.  
         [0010]     The heat of the electronic component simultaneously induces deformation of the electronic component package. Since the contact piece realizes concentration of the urging force on the heat spreading member as well as on the electronic component, the heat spreading member and the electronic component are thus reliably prevented from camber. Separation is thus avoided between the heat spreading member and the electronic component. The heat spreading member reliably keeps contacting with the electronic component, so that the electronic component package is allowed to enjoy improvement in heat radiation. This results in a sufficient suppression of a rise in temperature of the electronic component.  
         [0011]     In this case, the contact piece may contact with the surface of the heat spreading member in a section defined on the surface of the heat spreading member based on the projected image of the electronic component. This enables concentration of the urging force on the heat spreading member. The heat spreading member is thus reliably urged against the surface of the electronic component. The contact piece may be integral with the heat radiating member so as to establish a one-piece component. Alternatively, the contact piece may be made of a metallic mass separate from the heat radiation member.  
         [0012]     A heat conductive fluid may be interposed between the surface of the heat spreading member and the heat radiation member. The fluid may contain fine particles having heat conductivity and dispersed in the fluid, and a grain having a size larger than that of the fine particles so as to serve as the contact piece in the fluid. A solidified material having heat conductivity may be interposed between the surface of the electronic component and the heat spreading member.  
         [0013]     The printed circuit board unit may further comprise: a spring having one end supported on the surface of the heat radiation member, said spring having the other end distanced from the heat radiation member based on elasticity; and a restraint member holding the other end of the spring in a direction distanced form the printed wiring board. The spring and the restrain member in combination enable establishment of a sufficient urging force applied to the contact piece through the heat radiation member. The heat spreading member is allowed to enjoy concentration of the urging force through the contact piece.  
         [0014]     An electronic component package assembly of a specific type can be utilized for realization of the aforementioned printed circuit board unit. The electronic component package assembly may comprise: a heat spreading member received on a predetermined surface of an electronic component, said heat spreading member extending larger than the predetermined surface of the electronic component; and a contact piece contacting with the surface of the heat spreading member over a contact area smaller than the predetermined surface of the electronic component. The contact piece may contact with the surface of the heat spreading member in a section defined on the surface of the heat spreading member based on the projected image of the electronic component.  
         [0015]     The electronic component package assembly may further comprise a heat radiation member defining a flat surface opposed to the surface of the heat spreading member at a position adjacent to the contact piece. The contact piece may be integral with the heat radiation member so as to establish a one-piece component. Alternatively, the contact piece may be made of a metallic mass separate from the heat radiation member. A heat conductive fluid may be interposed between the surface of the heat spreading member and the heat radiation member. The fluid may contain fine particles dispersed in the fluid, and a grain having a size larger than that of the fine particles so as to serve as the contact piece in the fluid. The solidified material having heat conductivity may be interposed between the surface of the electronic component and the heat spreading member. The electronic component package assembly may further comprise an urging mechanism related to the contact piece. The urging mechanism is designed to apply an urging force to the contact piece toward the surface of the electronic component.  
         [0016]     According to a second aspect of the present invention, there is provided a printed circuit board unit comprising: a printed wiring board; an electronic component package mounted on the surface of the printed wiring board, said electronic component package allowing a heat spreading member to cover over an electronic component, said heat spreading member extending larger than a predetermined surface of the electronic component; a contact piece contacting with the surface of the heat spreading member over a contact area smaller than the predetermined surface of the electronic component; and a heat radiation member defining a flat surface opposed to the surface of the heat spreading member at a position adjacent to the contact piece.  
         [0017]     The printed circuit board unit allows concentration of an urging force at the contact piece on the heat spreading member when the urging force is applied to the heat spreading member through the contact piece. The heat spreading member is thus reliably urged against the surface of the electronic component. The heat spreading member and the electronic component are thus reliably prevented from camber because of the concentrate durging force applied to the heat spreading member as well as the electronic component through the contact piece. Separation is thus avoided between the heat spreading member and the electronic component. The heat spreading member reliably keeps contacting with the electronic component, so that the electronic component package is allowed to enjoy improvement in heat radiation. This results in a sufficient suppression of a rise in temperature of the electronic component.  
         [0018]     In this case, the contact piece may have an extent smaller than the predetermined surface of the electronic component. The contact piece may be located in a section defined on the flat upward surface of the heat spreading member based on the projected image of the electronic component. The solidified material having heat conductivity may be interposed between the surface of the electronic component and the heat spreading member.  
         [0019]     An electronic component package of a specific type can be utilized for realization of the aforementioned printed circuit board unit. The electronic component package may comprise: a printed wiring board; an electronic component mounted on the surface of the printed wiring board; a heat spreading member received on the surface of the electronic component on the printed wiring board, said heat spreading member defining an upward surface having the outer periphery extending in an imaginary plane; and a contact piece integrally formed on the upward surface of the heat spreading member, said contact piece defining the upper end located above the imaginary plane.  
         [0020]     The upper end of the contact piece may have an extent smaller than the predetermined surface of the electronic component. The upper end of the contact piece may be located in a section defined on the flat upward surface of the heat spreading member based on the projected image of the electronic component. The heat spreading member may have a flat surface at a position adjacent to the contact piece. A solidified material having heat conductivity may be interposed between the surface of the electronic component and the heat spreading member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments in conjunction with the accompanying drawings, wherein:  
         [0022]      FIG. 1  is an enlarged vertical sectional view schematically illustrating the structure of a printed circuit board unit according to a first embodiment of the present invention;  
         [0023]      FIG. 2  is an enlarged sectional view taken along the line  2 - 2  in  FIG. 1  for schematically illustrating the location of a contact piece on the upper surface of a heat spreader;  
         [0024]      FIG. 3  is an enlarged partial vertical sectional view schematically illustrating the structure of a printed circuit board unit according to a second embodiment of the present invention;  
         [0025]      FIG. 4  is an enlarged partial vertical sectional view schematically illustrating the structure of a printed circuit board unit according to a third embodiment of the present invention; and  
         [0026]      FIG. 5  is an enlarged partial vertical sectional view schematically illustrating the structure of a printed circuit board unit according to a fourth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]      FIG. 1  schematically illustrates the structure of a printed circuit board unit  11  according to a first embodiment of the present invention. The printed circuit board unit  11  includes a printed wiring board  12 . The printed wiring board  12  may includes a substrate made of a resin material. An electronic component package or LSI (Large-Scale Integrated circuit) package  13  is mounted on the printed wiring board  12 . Terminal bumps  14  are employed to fix the LSI package  13  on the printed wiring board  12 . The terminal bumps  14  may be made of an electrically-conductive material such as a solder material, for example.  
         [0028]     The LSI package  13  includes a substrate  15  mounted on the front or upper surface of the printed wiring board  12 . The substrate  15  may be made of a resin material, for example. An electronic component or LSI chip  16  is mounted on the front or upper surface of the substrate  15 . Terminal bumps  17  are employed to fix the LSI chip  16  on the substrate  15 . The terminal bumps  17  may be made of an electrically-conductive material such as a solder material, for example. Capacitors  18  may be mounted on the front or upper surface of the substrate  15  in the vicinity of the LSI chip  16 . A ceramic substrate may alternatively be employed as the substrate  15 .  
         [0029]     The LSI package  13  includes a heat spreading member or heat spreader  19  received on the upper surface of the LSI chip  16 . The heat spreader  19  comprises a surrounding wall  19   a  and a flat ceiling plate  19   b . The surrounding wall  19   a  stands upright in the vertical direction from the upper surface of the substrate  15 . The surrounding wall  19   a  is designed to surround the LSI chip  16  and the capacitors  18 . The flat ceiling plate  19   b  is coupled to the upper end of the surrounding wall  19   a . The lower end of the surrounding wall  19   a  is firmly fixed to the upper surface of the substrate  15 . An adhesive may be employed to fix the surrounding wall  19   a . The flat ceiling plate  19   b  defines a flat upward surface  19   c . The surrounding wall  19   a  and the flat ceiling plate  19   b  cooperate to define an inner space.  
         [0030]     The flat ceiling plate  19   b  of the heat spreader  19  has an extent larger than the upper surface of the LSI chip  16  on the substrate  15 . Specifically, the heat spreader  19  completely covers over the LSI chip  16 . The heat spreader  19  may be made of a heat conductive material such as copper, aluminum, aluminum carbide, silicon carbide, aluminum silicon carbide (AlSiC), or the like.  
         [0031]     As is apparent from  FIG. 1 , a solidified material or mass  21  having a predetermined heat conductivity is interposed between the upper surface of the LSI chip  16  and the flat ceiling plate  19   b  of the heat spreader  19 . Silver paste, diamond paste, or the like, may be employed to form the solidified material  21 , for example. In this case, a swell  19   d  may be formed on the lower surface of the flat ceiling plate  19   b . The swell  19   d  allows the downward surface of the flat ceiling plate  19   b  to sufficiently approach the LSI chip  16  without contacting with the capacitors  18 .  
         [0032]     A heat radiation member or heat sink  22  is received on the flat upper surface of the heat spreader  19  or LSI package  13 . The heat sink  22  includes a plate-shaped main body or heat plate  22   a  and fins  22   b  standing upright in the vertical direction from the heat plate  22   a . The heat plate  22   a  is designed to oppose a flat downward surface or opposed surface  22   c  to the flat upper surface  19   c  of the heat spreader  19 . Air passages  23  are defined between the individual adjacent pairs of the fins  22   b . The air passages  23  extend in parallel with one another. The heat sink  22  may be molded out of a metallic material such as aluminum, copper, or the like.  
         [0033]     Coil springs  24  are coupled to the heat sink  22 . The coil springs  24  are designed to shrink and elongate in the vertical direction based on its own elasticity. The lower ends of the coil springs  24  are supported on the upper surface of the heat sink  22 . The elasticity of the coil springs  24  serves to distance the upper ends of the coil springs  24  from the heat sink  22 . A restraint member  25  is coupled to the upper ends of the individual coil springs  24  so as to restrain the movement of the upper end of the coil spring  24  in the direction distanced from the printed wiring board  12 . The elasticity of the coil springs  24  serves to urge the heat sink  22  in the direction toward the printed wiring board  12 . In this case, the restraint member  25  may include four attachment shafts  25   a  penetrating through the printed wiring board  12  as well as the heat sink  22 , for example. A nut  25   b  is coupled to the tip end of the individual attachment shaft  25   a . The individual nut  25   b  defines a downward surface opposed to the heat sink  22 . The downward surface of the nut  25   b  receives the upper end of the corresponding coil spring  24 . The attachment shafts  25   a  are coupled to an anchoring member or frame  25   c  located at the back or lower surface of the printed wiring board  12 . The heat sink  22 , the coil springs  24  and the restraint member  25  in combination establish an urging mechanism according to the present invention.  
         [0034]     Fluid having a predetermined heat conductivity such as thermal grease  26  is interposed between the flat upper surface  19   c  of the heat spreader  19  and the opposed surface  22   c  of the heat sink  22 . The thermal grease  26  includes silicone grease and fine particles dispersed in the silicone grease. The fine particles have a predetermined heat conductivity. The fine particles thus serve as heat conductive filler  32 . Ceramic particles, metallic particles, or the like, may be employed as the heat conductive filler  32 , for example. Alternatively, a heat conductive sheet, heat conductive gel, or the like, may be interposed between the flat upper surface  19   c  of the heat spreader  19  and the opposed surface  22   c  of the heat sink  22 .  
         [0035]     A contact piece  27  is also interposed between the flat upper surface  19   c  of the heat spreader  19  and the opposed surface  22   c  of the heat sink  22 . The opposed surface  22   c  of the heat sink  22  gets opposed to the flat upper surface  19   c  of the heat spreader  19  at a position around the contact piece  27 . The contact piece  27  may contact with the flat upper surface  19   c  of the heat spreader  19  over a contact area smaller than the upper surface of the LSI chip  16 . The contact piece  27  may likewise contact with the opposed surface  22   c  of the heat sink  22  over a contact area smaller than the upper surface of the LSI chip  16 .  
         [0036]     A metallic mass may be employed as the contact piece  27 . The metallic mass may be a thin plate of 1 mm square having the thickness of 100 mμ approximately, for example. Alternatively, the metallic mass may be a disk, a ball, or the like. The metallic mass may be made of a metallic material such as aluminum, copper, or the like. The assembly of the LSI package  13  and the contact piece  27  form an electronic component package assembly according to the present invention.  
         [0037]     As shown in  FIG. 2 , the contact piece  27  may contact with the flat upper surface  19   c  of the heat spreader  19  at a section  28  defined on the upper surface of the heat spreader  19  based on the projected image of the LSI chip  16 . In this case, the projected image of the LSI chip  16  is established based on a projection of the LSI chip  16  in the direction vertical to the upper surface of the heat spreader  19 .  
         [0038]     As described above, when the urging force is applied to the heat sink  22 , the urging force acts on the heat spreader  19  through the contact piece  27 . The urging force concentrates at the contact area between the flat upper surface  19   b  of the heat spreader  19  and the contact piece  27 . Since the urging force concentrates on the flat ceiling plate  19   b  at a position sufficiently remoter from the surrounding wall  19   a , the heat spreader  19  is reliably urged against the upper surface of the LSI chip  16 .  
         [0039]     The LSI chip  16  generates heat during the operation. The generated heat is transferred to the heat spreader  19  through the solidified material  21 . The heat is allowed to spread over a larger or wider area of the heat spreader  19 . The spread heat is then transferred to the heat sink  22  through the thermal grease  26 . The heat sink  22  enables radiation of the heat into air from a larger exposed surface. The LSI chip  16  is in this manner effectively prevented from a rise in temperature.  
         [0040]     The heat of the LSI chip  16  is simultaneously transferred to the substrate  15 . The substrate  15  exhibits a larger thermal expansion as compared with the LSI chip  16  and the heat spreader  19 . Since the urging force is applied to the heat spreader  19  and the LSI chip  16  over a smaller area because of the contact piece  27 , the heat spreader  19  and the LSI chip  16  are reliably prevented from suffering from camber. Separation is in this manner reliably avoided between the heat spreader  19  and the solidified material  21 . The heat spreader  19  and the solidified material  21  reliably keep contacting with each other, so that the LSI chip  16  is sufficiently suppressed from a rise in temperature. In the case where the flat upper surface  19   c  of the heat spreader  19  is simply opposed to the opposed surface  22   c  of the heat sink  22  without interposal of the contact piece  27  in a conventional manner, the heat spreader  19  and the LSI chip  16  tend to suffer from camber in response to a rise in temperature of the LSI chip  16 . Such camber leads to separation between the heat spreader  19  and the solidified material  21 . The inventor has revealed that the separation of the heat spreader  19  off the solidified material  21  due to the camber in this manner results in a rapid rise in temperature of the LSI chip  16  beyond the permissible operating temperature range.  
         [0041]     As shown in  FIG. 3 , the aforementioned contact piece  27  may be formed integral with the heat sink  22  as a one-piece component in the printed circuit board unit  11 . Like reference numerals are attached to structure or components equivalent to those of the aforementioned first embodiment. Here, the contact piece  27  is formed on the opposed surface  22   c  of the heat sink  22 . The contact piece  27  contacts with the flat upper surface  19   c  of the heat spreader  19  over a contact area smaller than the upper surface of the LSI chip  16 . The contact piece  27  may contact with the flat upper surface  19   c  of the heat spreader  19  at a section defined on the flat upper surface  19   c  of the heat spreader  19  based on the projected image of the LSI chip  16 . Machining may be employed to form the contact piece  27  on the heat sink  22 , for example. Alternatively, etching process may be employed to configure the contact piece  27 .  
         [0042]     As shown in  FIG. 4 , the contact piece  27  may be formed integral with the heat spreader  19  into a one-piece component. Like reference numerals are attached to structure or components equivalent to those of the aforementioned first embodiment. In this case, the contact piece  27  is formed on the flat upper surface  19   c  of the heat spreader  19 . The flat upper surface  19   c  provides an upward surface having an outer periphery extending in an imaginary plane. The contact piece  27  is designed to define an upper end  27   a  located above the imaginary plane. The upper end  27   a  swells from the flat upper surface  19   c . The extent of the upper end  27   a  is set smaller than the upper surface of the LSI chip  16 . The upper end  27   a  may be located in a section defined on the flat upper surface  19   c  of the heat spreader  19  based on the projected image of the LSI chip  16 . The upper end  27   a  likewise contacts with the opposed surface  22   c  of the heat sink  22  in a section defined on the opposed surface  22   c  of the heat sink  22  based on the projected image of the LSI chip  16 . Machining may be employed to form the contact piece  27  on the heat spreader  19 , for example. Alternatively, etching process may be employed to form the contact piece  27 .  
         [0043]     As shown in  FIG. 5 , grains  31  may be employed as the aforementioned contact piece  27 . Like reference numerals are attached to structure or components equivalent to those of the aforementioned first embodiment. The grains  31  may have a size larger than those of the fine particles as the heat conductive filler  32 . Ceramic grains, metallic grains, or the like, may be employed as the grains  31 , for example. The grains  31  contact with the flat upper surface  19   c  of the heat spreader  19  over a contact area smaller than the upper surface of the LSI chip  16 . The grains  31  may contact with the flat upper surface  19   c  of the heat spreader  19  in a section defined on the flat upper surface  19   c  of the heat spreader  19  based on the projected image of the LSI chip  16 , for example. The grains  31  may likewise contact with the opposed surface  22   c  of the heat sink  22  over a contact area smaller than the upper surface of the LSI chip  16 .
A heat spreading member is received on a predetermined surface of an electronic component. The heat spreading member extends larger than the predetermined surface. A contact piece is contacted with the heat spreading member over a contact area smaller than the predetermined surface. The contact piece serves to realize concentration of an urging force applied to the heat spreading member. The heat spreading member is thus reliably urged against the electronic component. The concentration of the urging force serves to prevent the heat spreading member and the electronic component from camber even if heat is applied to the heat spreading member and the electronic component. Separation is thus avoided between the heat spreading member and the electronic component. The heat spreading member reliably keeps contacting with the electronic component, so that the electronic component package is allowed to enjoy improvement in heat radiation.