Abstract:
A method of producing an electric component-mounted substrate having a cavity structure is provided. Because a supporting substrate and electric components and are connected together to a first face of a base substrate, the number of steps for the connection is reduced to shorten the producing time. When electric components are to be connected to a second face, the base substrate is supported by the supporting substrate, and the electric components and connected to the first face do not contact a processing table and therefore they undergo no damage. Thus, an electric component-mounted substrate having high reliability can be produced in a short time.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of International Application No. PCT/JP2007/056173 filed Mar. 26, 2007, which claims priority to Japan Patent Application No. 2006-088441, filed on Mar. 28, 2006. The entire disclosures of the prior applications are hereby incorporated by references herein in their entireties. 
    
    
     BACKGROUND 
     The present disclosure relates to an electric component-mounted substrate. 
     Since electronic component modules such as communication modules, wireless LANs and tuners are mounted on electric devices including cell phones, the modules have been heretofore required to be small and thin. 
     The electric component module, which usually has a few or several ICs for processing signals and the like, and dozens of passive components such as C and R and other odd-form component, is mounted on a mother board. 
     As to the electric component module, there are electric component-mounted substrates in which electric components are mounted on a front face and a back face of a base substrate. In an example of such substrates, a ring-shaped substrate, which surrounds the electric components, is formed on the front face or the back face of the base substrate, and the electric components are connected in a space (cavity) inside the ring. 
     Because it is difficult to form such a cavity structure in a process for producing a buildup substrate, the cavity structure is generally formed by connecting the ring-shaped substrate onto the front face of the base substrate. The electric components are connected to the base substrate after the cavity structure is formed. 
     However, because, in this method, the cavity structure is formed on the base substrate before the connection of the electric components, there is a problem that because the cavity structure hinders feeding of a connecting material such as a solder, the connecting material could not be fed by a simple method such as printing with a metal mask or bonding of an anisotropic electroconductive film. Accordingly, because the method for feeding the connecting material is limited to a method using a dispenser, the connecting material is limited to a material having excellent dischargeability from the dispenser. In addition, it is difficult to feed the connecting material in the form of fine patterns. 
     See, Patent Documents JP-A H11-45904, JP-A 2000-105388, JP-A 2002-93933, JP-A 2002-359264, and JP-A 2005-32952. 
     SUMMARY 
     The present disclosure is directed to solving the above mentioned problems, and is aimed at providing a technique to produce a substrate having a cavity structure by a simple method. 
     In an embodiment, a method for producing an electric component-mounted substrate includes connecting electric components to a first face as one side of a base substrate and a second face as a side opposite to the first face, wherein the producing method includes disposing, on the first face of the base substrate, the electric component and a supporting substrate thicker than the electric component, connecting the electric component and the supporting substrate to the base substrate by pressing the electric component and the supporting substrate together, then placing the electric component on the second face in a state that the base substrate is supported by the supporting substrate, and connecting the electric component to the second face. 
     In an embodiment, the supporting substrate and the electric component are pressed by pressing a pressing rubber against the supporting substrate and the electric component. 
     In an embodiment, the electric component and the supporting substrate are disposed to locate the supporting substrate such that a through-hole of the base substrate is electrically connected to a through-hole of the supporting substrate. 
     In an embodiment, a planar shape of the supporting substrate is in the form of a ring, and the electric component is arranged inside the ring of the supporting substrate. 
     In an embodiment, and the electric component and the supporting substrate are connected to the base substrate by a connecting material such as an anisotropic electroconductive adhesive, an anisotropic electroconductive film, a paste-like electroconductive material, a low melting point metal (a solder) or the like. 
     Before the electric component is connected to the first face, the supporting substrate is not connected to the first face, either, so that there is nothing to hinder the disposition of the connecting material. Therefore, the method for disposing the connecting material is not limited to the method using the disposer. 
     According to an embodiment, because the electric components and the supporting substrate can be connected at a time and the number of the connecting steps is smaller than in the prior art references, the time for producing the electric component-mounted substrate is shortened. Further, if heating is necessary in a step or steps for connecting the electric component and the supporting substrate, reduction in the number of the connecting steps decreases the number of heatings of the base substrate and the electric components, so that damages caused by heating are lessened. When the electric component is connected to the face opposite to the supporting substrate, the supporting substrate contacts a processing table, the electric component connected to the same face on which the supporting substrate is present does not contact the processing table. Consequently, the electric component is free from damage without a special backup jig. When the pressing rubber is deformed, the flow of the pressing rubber is dammed up by the supporting substrate. Thus, the electric component is not positionally misaligned, so that the electric component-mounted substrate having high reliability can be obtained. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1(   a ) is a sectional view of a base substrate,  FIG. 1(   b ) is a sectional view of a supporting substrate,  FIG. 1(   c ) is a sectional view of an electric component, and  FIG. 1(   d ) is a side view of an electric component. 
         FIGS. 2(   a ) to  2 ( c ) are sectional views illustrating steps of producing an electric component-mounted substrate. 
         FIGS. 3(   a ) to  3 ( c ) are sectional views illustrating steps of producing an electric component-mounted substrate. 
         FIGS. 4(   a ) and  4 ( b ) are sectional views for illustrating steps of producing an electric component-mounted substrate. 
         FIG. 5  is a plan view for illustrating a state in which L-letter shaped supporting substrates are connected to a base substrate. 
         FIG. 6  is a plan view for illustrating a state in which linear or columnar supporting substrates are connected to a base substrate. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 4(   b ), a reference numeral  1  shows an embodiment of an electric component-mounted substrate produced by a producing method. The electric component-mounting substrate  1  has a base substrate  10  and a supporting substrate  20 . 
     The base substrate  10  has a substrate body  11 , and wiring films  12 , which are formed by patterning and disposed on a front face and a back face of the substrate body  11 . 
     The wiring film  12  has connecting portions  16  formed wide by patterning and wiring portions  17  formed long and thin by patterning, and the connecting portions  16  are connected to each other by the wiring portions  17 . 
     Electric components  32  and in this embodiment, an odd-form component  33 , are disposed on the front face of the substrate body  11 , while an electric component  31  and an electric component  32  being thinner than the electric component  31  are disposed on the back face of the substrate body  11 . 
     The electric components  31  to  33  have connection terminals  36  and  37 , and the electric components  32  on the front face of the substrate body  11  are fixed with a solidified solder  18  with the connection terminals  37  contacting with the connecting portions  16 . The odd-form component  33  is also fixed onto the connecting portions  16  with the solidified solder. 
     The electric components  31  and  32  on the back face of the substrate body  11  are fixed with a cured adhesive layer  46  in the state that the connecting terminals  36  and  37  are connected to the connecting portions  16  directly or via electroconductive particles mentioned later. Therefore, while the electric components  31  to  33  are electrically connected to the wiring film  12 , they are also mechanically connected to the base substrate  10 . 
     In the substrate body  11  are provided through-holes  15  which penetrate it from the front face to the back face, and electric conductors that are not shown in the Figures are formed inside the through-holes  15 . The connecting portions  16  are connected to both ends of the through-holes  15  respectively, so that the wiring film  12  on the front face and the wiring film  12  on the back face of the substrate body  11  are electrically connected via the electric conductors in the through-holes  15 . As mentioned above, because the electric components  31  to  33  are connected to the wiring films  12 , the electric components  31  to  33  are also electrically connected to the through-holes  15 . 
     The supporting substrate  20  has a substrate body  21 , through-holes  25  penetrating the substrate body  21  from the front face to the back face, and wiring films (lands)  22  formed at one and the other ends of the through-holes  25 . 
     The planar shape of the supporting substrate  20  is in the form of a ring, and the size of the inner periphery of the ring is set to be smaller than the planar shape of the substrate body  11 . Thus, the supporting substrate  20  is fixed on the back face of the substrate body  11  with the cured adhesive layer  46  such that at least one electric component  31 ,  32  is surrounded by the inner periphery of the ring. 
     The land  22  is located immediately above the through-hole  25 , and the connecting portion  16  immediately above the through-hole  15  of the base substrate  10  is connected to the land  22 . Therefore, the through-hole  15  of the base substrate  10  and the through-hole  25  of the supporting substrate  20  are connected to each other, such that they are aligned along a straight line. 
     However, the through-holes  15  and  25  are not limited to the case where they are aligned linearly, but it may be a case where the through-hole  15  of the base substrate  10  is not located immediately above the through-hole  25  of the supporting substrate  20 , but they are located away from each other. 
     As mentioned above, because the electric components  31  and  32  connected to the base substrate  10  are connected to the through-holes  15  of the base substrate  10 , the electric components  31  and  32  are connected to the through-holes  25  of the supporting substrate  20 . 
     In this embodiment, solder balls  27  are disposed on the land  22  which is positioned on a side of the supporting substrate  20  opposite to the base substrate  10  so that the solder balls may connect the through-hole  25  to another wiring board or electric device. 
     When the face on the side of the base substrate  10  on which the supporting substrate  20  is connected is taken as a first face  13  and the face on the side opposite to the first face  13  is taken as a second face  14 , the supporting substrate  20  is made thicker than the electric components  31  and  32  disposed on the first face  13 . 
     Therefore, the supporting substrate  20  is projected from the first face  13  higher than the electric components  31  and  32 , so that when this electric component-mounted substrate  1  is to be connected to another wiring board, it is possible that the lands  22  of the supporting substrate  20  are connected to terminals of the wiring board, and the wiring board and the electric component-mounted substrate  1  are connected electrically. 
     Next, steps for producing the above-mentioned electric component-mounted substrate  1  will be explained.  FIG. 1(   a ) shows the base substrate  10  before the electric components  31  and  32  and the supporting substrate  20  are connected. In this embodiment, the base substrate  10  is a rigid substrate. 
     The adhesive layer  45  is disposed on the connecting portions  16  of the first face  13 , and the first face  13  is set in a flush plane free from unevenness when the base substrate  10  is placed on a mounting table  48  and the wiring film  12  of the second face  14  is closely contacted with a flat mounting face  49  of the mounting table  48  ( FIG. 2(   a )). 
       FIGS. 1(   b ) to  1 ( d ) show the supporting substrate  20  and the electric components  31  and  32 , respectively, before they are connected to the base substrate  10 . In this embodiment, the supporting substrate  20  is a rigid substrate. 
     Positions of the base substrate  10  to which the electric components  31  and  32  are connected are preliminarily determined for the electric components  31  and  32 , respectively. When the thicker electric component  31  and the thinner electric component  32  are disposed on the first face  13  at the predetermined positions, respectively, the connecting terminals  36  of the thicker electric component  31  and terminals  37  of the thinner electric component  32  are placed on the determined connecting portions  16  via the adhesive layer  45 . 
     The relative position between the supporting substrate  20  and the base substrate  10  is preliminarily determined. The supporting substrate  20  thicker than the electric components  31  and  32  is positionally aligned with the base substrate  10  so that alignment marks, that are not shown in Figures, may overlap each other. After that, the supporting substrate  20  is placed on the first face  13 . Then the lands  22  of the supporting substrate  20  are put on the predetermined connecting portions  16  via the adhesive layer  45  ( FIG. 2(   b )). 
     As mentioned above, because the supporting substrate  20  is thicker than the electric components  31  and  32  on the first face  13 , the front edge of the supporting substrate  20  is projected from the first face  13  higher than the front ends of the electric components  31  and  32  in the state that the supporting substrate  20  and the electric components  31  and  32  are placed on the first face  13 . 
     A reference numeral  40  of  FIG. 2(   c ) denotes a pressing head in which a pressing rubber  42  is attached to a metallic pressing plate  41 . 
     The pressing rubber  42  is made of an elastic material such as an elastomer. When the pressing head  40  is descended, the pressing rubber  42  first contacts the front edge of the supporting substrate  20 . When the pressing head is further descended, the pressing rubber  42  is pressed against the supporting substrate  20 . Even if a lateral force is applied to the pressing rubber  42 , a portion of the pressing rubber  42  pressed against the supporting substrate  20  does not move but becomes a stationary condition relative to the supporting substrate  20 . 
     As mentioned above, because the electric components  31  and  32  differ in thickness, the heights from the first face  13  to the leading ends of the electric components  31  and  32  differ. 
     As the pressing head  40  is further descended, the pressing rubber  42  is contacted with the thickest electric component  31  and next with the thinner electric component  32 , and the electric components  31  and  32  are pressed with the pressing rubber  42 . 
     At this time, the pressing rubber  42  is depressed with the electric components  31  and  32 , whereas the other portion is swelled accordingly to move into a gap between the electric components  31  and  32  and a gap between the electric components  31 ,  32  and the supporting substrate  20 . However, because the portion of the pressing rubber  42  closely contacted with the supporting substrate  20  does not move, the pressing rubber  42  does not flow outwardly beyond the supporting substrate  20  from the interior of the ring, even if a force acts outwardly from the center of the ring, following the swelling of the pressing rubber  42 . Thus, a lateral force is not applied to the electric components  31  and  32 . When the electric components  31  and  32  are pressed with the pressing rubber  42 , the pressing rubber  42  flows outside if no supporting substrate  20  is provided around the electric components  31  and  32 . So, the outwardly moving force would act upon the electric components  31  and  32 , and cause the positional misalignment. 
     In this embodiment, because the electric components  31  and  32  are positioned inside the ring of the supporting substrate  20 , no lateral force is apparently applied to the electric components  31  and  32 . Accordingly, when the electric components  31  and  32  and the supporting substrate  20  are pressed together by the pressing rubber  42 , the electric component  31  or  32  is not laterally displaced, and no positional misalignment of the electric components  31  and  32  occurs. 
     The adhesive layer  45  is composed of an anisotropic electroconductive adhesive in which electroconductive particles are dispersed in a binder. When the supporting substrate  20  and the electric components  31  and  32  are pressed, the adhesive layer  45  is pushed away by pressing. Thereby, the connecting terminals  36  and  37  of the electric components  31  and  32  and the lands  22  of the supporting substrate  20  are brought into contact with the connecting portions  16  directly or via the electroconductive particles, so that the supporting substrate  20  and the electric components  31  and  32  are electrically connected to the base substrate  10 . 
     In this embodiment, the mounting table  48  is provided with a heater  47 , which preliminarily heats the mounting table  48 , and heats the adhesive layer  45  through heat conduction. The binder of the adhesive layer  45  includes a thermosetting resin, and when the adhesive layer  45  is heated to a predetermined temperature or higher, the thermosetting resin is polymerized to cure the adhesive layer  45 . 
       FIG. 3(   a ) shows a state in which the pressing head  40  is moved away from the electric components  31  and  32  and the supporting substrate  20  after the adhesive layer  45  is cured. 
     The supporting substrate  20  and the electric components  31  and  32  are electrically connected and then fixed to the base substrate  10  with the adhesive layer  45 , and thus the supporting substrate  20  and the electric components  31  and  32  are mechanically and electrically connected to the base substrate  10 . 
     In this embodiment, the wiring film  12  of the first face  13  is positioned in the same plane, and the supporting substrate  20  is thicker than the electric components  31  and  32  as mentioned above. Therefore, the height of the leading edge of the supporting substrate  20  from the first face  13  is higher than the ends of the electric components  31  and  32  from the first face  13  in the state that the supporting substrate  20  and the electric components  31  and  32  are electrically and mechanically connected to the base substrate  10 . 
     When the base substrate  10  to which the supporting substrate  20  and the electric components  31  and  32  are connected is placed on a flat processing face  52  of a processing table  51  with the supporting substrate  20  being on the downside, the supporting substrate  20  contacts the processing face  52 , whereas the electric components  31  and  32  are suspended from the first face  13  to form gaps between the electric components  31  and  32  and the processing face  52  ( FIG. 3(   b )). 
     Low melting point metal layers  18  are formed by placing a solder paste on the connecting portions  16  of the second face  14  in the state that the second face  14  is almost horizontally disposed on the processing table  51 , and then the electric components  32  and the odd-form component  33  are placed at determined positions on the second face  14 . Thereby, the connecting terminals  37  of the electric components  32  are opposed to the connecting portions  16  via the low melting point metal layer  18 , and the odd-form component  33  is also placed on connecting portions  16  ( FIG. 3(   c )). 
     When the base substrate  10 , for example, is put into a reflow furnace together with the processing table  51  and the low melting point metal layers  18  are heated to the melting point or higher with the second face  14  being kept almost horizontally, the low melting point metal layer  18  is softened. This makes the connecting terminals  37  of the electric components  32  to be brought into contact with the connecting portions  16 , and also the odd-form component  33  to be contacted with the connecting portion  16 . Thus, the electric components  32  and the odd-form component  33  are electrically connected to the base substrate  10  ( FIG. 4(   a )). 
     When the low melting point metal layers  18  are heated, the cured adhesive layer  46  is also heated. However, the polymer of the thermosetting resin is not softened at least the melting point of the low melting point metal layer  18 , and the adhesive layer  46  is kept cured. Accordingly, the electric components  31  and  32  on the side of the first face  13  are kept suspended without falling down. 
     Then, when the entire assembly is cooled to solidify the low melting point metal layer  18 , the electric components  32  and the odd-form components  33  are fixed with the solidified low melting point metal layer  18 , while being kept connected to the base substrate  10 . Therefore, the electric components  32  and the odd-form component  33  are electrically and mechanically connected to the base substrate  10  with the solidified low melting point metal layer  18 . 
     Thereafter, the electric component-mounted substrate  1  shown in  FIG. 4(   b ) is obtained by disposing the solder balls  27  on the lands  22  on the side opposite to the base substrate  10 . 
     The above examples are not limited to the case where the base substrate  10  and the supporting substrate  20  are rigid substrates (glass epoxy substrates), and flexible wiring boards can be used, too. When a flexible wiring board is used as the supporting substrate  20 , the board which may not be deformed by compression when it supports the base substrate  10  is preferable. 
     When a flexible wiring board having flexibility is used as the base substrate  10 , the base substrate  10  should be prevented from bending when the base substrate  10  is placed on the mounting table  48 . Also, the supporting substrate  20  and the electric components  31  and  32  should be connected to the base substrate with the first face  13  being positioned in plane. Then, when the base substrate  10  to which the supporting substrate  20  and the electric components  31  and  32  are connected is moved onto the processing table  51 , the base substrate  10  is not bent, so that the electric component  31 ,  32  does not contact the processing table  51 . 
     The ring shape of the supporting substrate may be a rectangular shape, a circular shape or the like, not particularly limited. Any shape and any size of the ring may suffice so long as the supporting substrate  20  can be disposed on the base substrate  10  without falling down from the first face  13 . For example, a part of the ring may protrude from the edge of the base substrate  10 . 
     Further, the supporting substrate  20  is not limited to the ring-like shape. So long as the supporting substrate  20  can support the base substrate  10 , one having an U-letter shape, a L-letter shape, a linear shape or a columnar shape can be used. However, when the supporting substrate  20  is in the ring-like shape, the positional misalignment of the electric components  31  and  32  can be prevented as mentioned above. 
     The electric components  31  and  32  are not limited to the case where they are disposed only inside the ring of the supporting substrate  20 . It may be that electric components  31  and  32  arranged outside the ring of the supporting substrate  20  are connected to the base substrate  10  together with the supporting substrate  20  and the electric components  31  and  32  positioned inside the ring of the supporting substrate  20 . 
     When the supporting substrate  20  has the L-letter shape, desirably two or more supporting substrates  20  are connected to the side of the first face  13  of the base substrate  10  as shown in  FIG. 5  so as to stably support the base substrate  10 . Meanwhile, when the supporting substrate  20  is linear or columnar, it is desirable that three or more supporting substrates  20  are connected to the side of the first face  13  of the base substrate  10  at intervals as shown in  FIG. 6 . 
     When the supporting substrates  20  are linear or columnar, and if fixing members are attached to the supporting substrates  20  for connection to the first face  13 , the relative position among the supporting substrates  20  is fixed. Thereby, the base substrate  10  is not bent when it is supported by the supporting substrates  20 , even if the base substrate  10  is a flexible wiring board having flexibility. 
     Although the case where the supporting substrate  20  is connected to the first face  13  only has been explained above, the embodiment is not limited thereto. The supporting substrates  20  can be connected to both the first and second faces  13  and  14 . In this case, after the supporting substrate  20  is connected to the first face  13 , the supporting substrate  20  is connected to the second face  14  in the state that the base substrate  10  is supported by that supporting substrate  20 . 
     In the above, the case where the low melting point metal (solder) is used as the connecting material for connecting the electric components  31  and  32  to the second face  14  and the connection is performed by melting the low melting point metal has been explained. However, the embodiment is not limited thereto. 
     For example, in the case where the base substrate  10  is composed of a rigid substrate or the like which is not deformed even by pressing, the base substrate  10  is not bent when the electric component  32  or the odd-form component  33  is pressed in connecting the electric component  32  or the odd-form component  33  onto the second face  14 . Consequently, the electric components  31  and  32  are kept suspended from the first face  13 . 
     Therefore, a thermosetting adhesive, a thermoplastic adhesive or the like can be used, besides the low melting point metal, as the connecting material for connecting the electric components  32  and the odd-form component  33  to the second face  14 . 
     When heating is performed to connect the electric components  32  and the odd-form component  33  to the second face  14 , the heating temperature is set not to melt or break the connecting material for fixing the electric components  31  and  32  to the first face  13 . 
     For example, when the electric components  31  and  32  are to be connected to the first face  13 , it is possible to use a low melting point metal having a melting point higher than the heating temperature at which the electric components  32  and the odd-form component  33  are connected to the second face  14 , or a thermoplastic adhesive having a softening temperature lower than the above heating temperature. 
     The thermosetting adhesive to be used in the present embodiment is an adhesive in which the electroconductive particles are dispersed in the binder including the thermosetting resin. The thermoplastic adhesive is an adhesive in which the electroconductive particles are dispersed in the binder including the thermoplastic resin. Further, when the thermoplastic resin is incorporated into the thermosetting adhesive, the adhesion property of the thermosetting adhesive is improved. 
     The method for disposing the connecting material on the first face  13  and second face  14  is not particularly limited, and it is possible to adopt various methods such as a mask printing method in which the connecting material is coated in the predetermined patterns by using a mask formed with the predetermined pattern openings, a dispenser method in which the connecting material is discharged by using a dispenser, and a method in which a film of the connecting material (adhesive film) is bonded, etc. 
     Furthermore, because the method for disposing the connecting material is not limited, the kind of the connecting material is not limited. Therefore, the connecting material can be selected to meet the characteristics of the electric components  31 ,  32  and  33  and the supporting substrate  20 . 
     As the electric components  31  and  32  to be connected to the base substrate  10 , a semiconductor tip, a resistance element, etc. are recited, for example. The kinds thereof are not particularly limited, either. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.