Abstract:
A composite substrate has a rigid substrate that includes: a core layer, a first laminated layer on a first surface of the core layer, and a second laminated layer on a second surface of the core layer, the rigid substrate having a cutout in the core layer and the second laminated layer on one side face of the rigid substrate; and a flexible substrate inserted into the cutout in the rigid substrate on the one side face and laterally and externally protruding from the one side face of the rigid substrate, wherein the rigid substrate has opposing walls each constituted of the second laminated layer and the core layer erected on the first laminated layer to define inner side faces, respectively, of the cutout so as to accommodate the flexible substrate in a direction perpendicular to a direction in which the side face of the rigid substrate extends.

Description:
BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a composite substrate, in which a rigid substrate and a flexible substrate are joined together. 
     Background Art 
     A composite substrate formed by joining a rigid substrate to a flexible substrate that is flexible is widely used in various electronic devices. Many ways of joining a rigid substrate to a flexible substrate have been developed. 
     Patent Document 1 discloses a rigid-flexible printed circuit in which a flexible substrate is laminated on a rigid substrate, for example. Forming a cutout in the base substrate removes part of the base substrate, which makes it possible to prevent contamination by the residues from the base substrate. 
     Also, Patent Document 2 discloses a composite wiring substrate in which a flexible substrate is sandwiched between rigid substrates. Forming dummy vias in the rigid substrates allows equal pressure to be applied to the flexible substrate, thereby making it possible to prevent warping of the flexible substrate. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2012-134490 
     Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2005-011859 
     SUMMARY OF THE INVENTION 
     However, in the composite substrate according to Patent Document 1 or 2, it is difficult to reduce the thickness of the composite substrate because the flexible substrate and the rigid substrate are stacked together. The recent miniaturization of electronic devices has increased the demand for reducing the thickness of composite substrates. A composite substrate is generally divided into individual pieces after the rigid substrate and the flexible substrate are joined together. Part of the rigid substrate is wasted in this process, and it has therefore been difficult to bring down the manufacturing cost. 
     In view of the circumstances described above, a purpose of the present invention is to provide a composite substrate and a rigid substrate that are suitable for reducing thickness and have structures that yield excellent productivity. Accordingly, the present invention is directed to a scheme that substantially obviates one or more of the above-discussed and other problems due to limitations and disadvantages of the related art. 
     Additional or separate features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect, the present disclosure provides a composite substrate, including: a rigid substrate including a core layer, a first laminated layer on a first surface of the core layer, and a second laminated layer on a second surface of the core layer that is opposite to the first surface, the first laminated layer having at least one layer each of an insulating layer and a wiring layer laminated together on the first surface, the second laminated layer having at least one layer each of an insulating layer and a wiring layer laminated together on the second surface, the rigid substrate having a cutout in the core layer and the second laminated layer on one side face of the rigid substrate, exposing partially a surface of the first laminated layer as a bottom surface of the cutout, the partially exposed surface of the first laminated layer having connection terminals formed thereon; and a flexible substrate inserted into the cutout in the rigid substrate on the one side face and laterally and externally protruding from the one side face of the rigid substrate, the flexible substrate being electrically connected to the connection terminals, wherein the rigid substrate has opposing walls each constituted of the second laminated layer and the core layer erected on the first laminated layer to define inner side faces, respectively, of the cutout so as to accommodate the flexible substrate in a direction perpendicular to a direction in which the side face of the rigid substrate extends. 
     In another aspect, the present disclosure provides a composite substrate, including: a rigid substrate formed by laminating a core layer, an insulating layer, and a wiring layer together, the rigid substrate having a first thickness, a cutout in at least one of four sides thereof, and a first connection terminal exposed from the cutout; and a flexible substrate joined in the cutout, the flexible substrate having a second connection terminal that electrically connects to the first connection terminal and a second thickness that is thinner than the first thickness and shallower than a depth of the cutout. 
     In another aspect, the present disclosure provides a rigid substrate, including: a core layer having a first surface and a second surface opposite to the first surface; a first internal wiring layer that is formed by a first insulating layer and a first wiring layer and is laminated on the first surface; and a second internal wiring layer that is formed by a second insulating layer and a second wiring layer and is laminated on the second surface, wherein a cutout is created by removing the first insulating layer and the core layer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a composite substrate according to Embodiment 1 of the present invention. 
         FIG. 2  is an exploded perspective view of the composite substrate. 
         FIG. 3  is a perspective view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 4  is a perspective view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 5  is a plan view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 6  is a plan view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 7  is a cross-sectional view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 8  is a perspective view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 9  is a plan view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 10  is a cross-sectional view of a rigid substrate forming a portion of the composite substrate. 
         FIG. 11  is a perspective view of a flexible substrate forming a portion of the composite substrate. 
         FIG. 12  is a cross-sectional view of a flexible substrate forming a portion of the composite substrate. 
         FIG. 13  is a cross-sectional view of the composite substrate. 
         FIG. 14  is a diagram showing the relationship among various thicknesses of the composite substrate. 
         FIG. 15  is a cross-sectional view of the composite substrate according to a comparison example. 
         FIG. 16  is a cross-sectional view of the composite substrate according to a comparison example. 
         FIG. 17  shows a composite substrate according to Embodiment 1 of the present invention and is a cross-sectional view of a composite substrate on which a surface component is mounted. 
         FIGS. 18A to 18C  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 19A to 19C  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 20A and 20B  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 21A and 21B  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 22A and 22B  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 23A and 23B  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 24A and 24B  are diagrams showing a manufacturing process of the composite substrate. 
         FIG. 25  is a cross-sectional view of a rigid substrate forming a portion of a composite substrate according to Embodiment 2 of the present invention. 
         FIG. 26  is a cross-sectional view of a flexible substrate forming a portion of the composite substrate. 
         FIG. 27  is a cross-sectional view of the composite substrate. 
         FIGS. 28A and 28B  are diagrams showing a manufacturing process of the composite substrate. 
         FIGS. 29A and 29B  are diagrams showing a manufacturing process of the composite substrate. 
         FIG. 30  is a cross-sectional view of the composite substrate according to a modification example of the present invention. 
         FIG. 31  is a cross-sectional view of the composite substrate according to a modification example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A composite substrate according to one embodiment of the present invention includes a rigid substrate and a flexible substrate. 
     The rigid substrate is formed by laminating a core layer, an insulating layer, and a wiring layer together, and has a first thickness. The rigid substrate has a cutout in at least one of the four sides thereof, and a first connection terminal is exposed in the cutout. 
     The flexible substrate is joined to the cutout and has a second connection terminal that electrically connects to the first connection terminal. The flexible substrate has a second thickness that is thinner than the first thickness and shallower than the depth of the cutout. 
     According to this configuration, the flexible substrate joined to the rigid substrate is housed inside the cutout formed in the rigid substrate and does not protrude therefrom. Thus, the maximum thickness of the composite substrate does not exceed the thickness of the rigid substrate, and it is possible to prevent an increase in thickness of the composite substrate caused by joining the flexible substrate. 
     The first connection terminal may be formed by the wiring layer. 
     According to this configuration, the number of steps in the manufacturing process can be reduced because the wiring layer can be used as the connection terminal, thus eliminating the need for an additional connection terminal. 
     The core layer may be made of metal, and 
     the first connection terminal may be formed by the core layer. 
     According to this configuration, the number of steps in the manufacturing process can be reduced because the core layer can be used as the connection terminal, thus eliminating the need for an additional connection terminal. 
     The first connection terminal may be formed by the wiring layer and a conductive layer laminated on the wiring layer. 
     According to this configuration, the surface of the connection terminal can either be on the same level as the surface of the insulating layer or be made to protrude from the surrounding insulating layer. Thus, joining a rigid substrate and a flexible substrate becomes possible using NCP or NCF, which is described later. 
     The composite substrate may further include a bonding layer made of a conductive member, which is arranged between the rigid substrate and the flexible substrate and establishes an electrical connection between the first connection terminal and the second terminal. 
     According to this configuration, the bonding layer can join the rigid substrate and the flexible substrate, thereby making it possible to establish an electrical connection between the rigid substrate and the flexible substrate. 
     The conductive member may be ACP (Anisotropic Conductive Paste) or ACF (Anisotropic Conductive Film). 
     ACP and ACF are insulating materials containing conductive particles. When heat and pressure are applied, insulating materials are pushed out, and an electrical connection is established by the leftover conductive particles. Thus, even if the first connection terminal and the second connection terminal are separated in space, it becomes possible to establish an electrical connection between the first connection terminal and the second connection terminal by placing a bonding layer made of ACP or ACF between the terminals. 
     The conductive member may be NCP (Non-anisotropic Conductive Paste) or NCF (Non-anisotropic Conductive Film). 
     NCP and NCF are insulating materials not containing conductive particles. When using these materials, an electrical connection is established between the two connection terminals by fixing the terminals while allowing them to touch each other. According to the configuration above, a conductive layer that increases the thickness of the first connection terminal is formed, and it becomes possible to physically connect the first and the second connection terminals. Thus, it becomes possible to use NCP or NCF as the bonding layer. 
     The rigid substrate includes: a core layer having a first surface on one side and a second surface on the opposite side; a first internal wiring layer that is formed by a first insulating layer and a first wiring layer and is laminated on the first surface; and a second internal wiring layer that is formed by a second insulating layer and a second wiring layer and is laminated on the second surface. The cutout may be created by removing the first insulating layer and the core layer. 
     A rigid substrate according to one embodiment of the present invention includes: a core layer having a first surface on one side and a second surface on the opposite side; a first internal wiring layer that is formed by a first insulating layer and a first wiring layer and is laminated on the first surface; and a second internal wiring layer formed by a second insulating later and a second wiring layer and laminated on the second surface. A cutout is created by removing the first insulating layer and the core layer. 
     Embodiment 1 
     A composite substrate according to Embodiment 1 of the present invention will be described. 
       FIG. 1  is a perspective view of a composite substrate  100  according to Embodiment 1 of the present invention.  FIG. 2  is an exploded perspective view of the composite substrate  100 . As  FIGS. 1 and 2  show, the composite substrate  100  is formed by joining a rigid substrate  200  to a flexible substrate  300 . 
     &lt;Rigid Substrate&gt; 
     The rigid substrate  200  has a surface component (an integrated circuit, for example), which is described later, mounted thereon. Hereafter, the side of the rigid substrate  200  on which the flexible substrate  300  is joined is defined as the lower side, and the opposite side is defined as the upper side.  FIG. 3  is a perspective view of the rigid substrate  200  seen from the lower side.  FIG. 4  is a perspective view of the rigid substrate  200  seen from the upper side.  FIG. 5  is a plan view showing the lower side of the rigid substrate  200 .  FIG. 6  is a plan view showing the upper side of the rigid substrate  200 .  FIG. 7  is a cross-sectional view of the rigid substrate  200  along a line A in  FIGS. 3 to 6 . 
     Although the rigid substrate  200  is not limited to a specific size, the length of the long side is 16 mm and the length of the short side is 10 mm, for example. The shape of the rigid substrate does not need to be rectangular and can be changed according to factors such as the layout of parts mounted on the rigid substrate  200 . 
     As  FIGS. 3 and 7  show, a cutout  201  is created in the rigid substrate  200 . The cutout  201  is created in at least one of the four sides of the rigid substrate  200  and corresponds to the part where the rigid substrate  200  is hollowed out in the shape of a step. The method of forming the cutout  201  is described later. Although the cutout  201  is not limited to a specific size, the cutout width (long side) can be set to 8.24 mm, for example. 
     As  FIG. 3  shows, walls  202  are disposed on both sides of the cutout  201 . On the side of the rigid substrate  200  where the cutout  201  is formed, the walls  202  correspond to the portion where the cutout  201  is not formed. Although the walls  202  do not need to be formed, the walls  202  make it possible to maintain the strength of the rigid substrate  200 . 
     As  FIGS. 3 and 5  show, a connection terminal  203  is formed in the cutout  201 . The connection terminal  203  can be formed on the surface in the cutout  201 . The connection terminal  203  can include a plurality of terminals, each of which is separated by an insulator. The length of each terminal in the connection terminal  203  can be set to 0.7 mm, the width to 0.1 mm, and the distance between adjacent terminals to 0.1 mm, for example. 
     The shape and the number of the connection terminal  203  are not limited.  FIGS. 8 and 9  are a perspective view and a plan view showing an alternative form of the connection terminals  203 . As these figures show, the connection terminal  203  may be configured in multiple rows. In this case, the length of each terminal in the connection terminal  203  can be set to 0.4 mm, the width to 0.1 mm, and the distance between adjacent terminals to 0.1 mm, for example. 
     As  FIGS. 5 and 6  show, connection pads  204  are disposed on the upper and the lower side of the rigid substrate  200 . The connection pads  204  are parts used to establish an electrical connection with a surface component (described later) that is mounted on the upper and the lower side of the rigid substrate  200 . The arrangement and the number of the connection pads  204  are not specifically limited, and the connection pads  204  are disposed according to the layout of the surface component. Alternatively, the connection pads  204  may be disposed on only the upper side or only the lower side of the rigid substrate  200 . 
       FIG. 10  is an enlarged view of  FIG. 7  showing a section around the cutout  201  in the rigid substrate  200 . As the figure shows, the rigid substrate  200  can be formed by laminating a core layer  205 , a first internal wiring layer  206 , a second internal wiring layer  207 , a first solder resist layer  208 , and a second solder resist layer  209 . 
     The core layer  205  is made of metal such as copper or copper alloy and supports the laminate structure of the rigid substrate  200 . The rigid substrate  200  can be formed by processing or forming a film on the core layer  205 , for example. The core layer  205  can also function as ground for the rigid substrate  200 . In addition, the core layer  205  can include a through hole  205   a.    
     An insulating portion  210  made of an insulating material is disposed at the end of the core layer  205  on the cutout  201  side. Also, the insulating portion  210  can be formed around the through hole  205   a . The insulating material can be epoxy resin, polyimide, bismaleimide triazine resin, and the like. An insulating material containing a reinforcing filler made of silicon dioxide, for example, may also be used. 
     The first internal wiring layer  206  is laminated on the lower side of the core layer  205  and formed by laminating a wiring layer  211  and an insulating layer  212 . The wiring layer  211  is made of a conductive material such as copper and partially separated off by the insulating layer  212 . Part of the wiring layer  211  is exposed from the first solder resist layer  208  and forms the connection pads  204 . The wiring layer  211  functions as a signal line for the surface component, which is joined to the connection pads  204 , or as a connection line for the surface component and ground (the core layer  205 ). 
     A plating layer M can be formed on the surface of the wiring layer  211  that forms the connection pads  204 . The plating layer M can be made of Au or Cu. The thickness of the plating layer M can be set greater than or equal to 0.05 μm but no more than 1 μm, for example. As  FIG. 10  shows, the wiring layer  211  may be made of a single layer or multiple layers. Also, as the figure shows, the wiring layer  211  can be electrically connected to other layers. 
     The insulating layer  212  is made of an insulating material. The insulating material can be epoxy resin, polyimide, bismaleimide triazine resin, and the like. An insulating material that contains a reinforcing filler made of silicon dioxide, for example, may also be used. The insulating material for the insulating layer  212  and that for the insulating portion  210  may be same or different. 
     The second internal wiring layer  207  is laminated on the upper side of the core layer  205  and formed by laminating a wiring layer  211  and an insulating layer  212  in a manner similar to the first internal wiring layer  206 . Part of the wiring layer  211  is exposed from the second solder resist layer  209  and forms the connection pads  204 . The wiring layer  211  functions as a signal line for the surface component, which is joined to the connection pads  204 , or as a connection line for a surface component and ground (the core layer  205 ). 
     The second internal wiring layer  207  protrudes from the core layer  205  and the first internal wiring layer  206  and defines the surface of the cutout  201 . Part of the wiring layer  211  in the second internal wiring layer  207  is exposed from the surface of the cutout  201  and forms the connection terminal  203  described above. A plating layer M can be formed on the surface of the wiring layer  211  that forms the connection terminal  203 . The material for the plating layer M can be chosen according to the method of joining the rigid substrate  200  and the flexible substrate  300 , which will be described later, and can be made of Au or Cu, for example. 
     The wiring layers  211  in the first internal wiring layer  206  and the second internal wiring layer  207  can be connected to each other. As  FIG. 10  shows, by virtue of through hole wiring  211   a  placed inside the through hole  205   a , an electrical connection can be established between the wiring layers  211  in the first internal wiring layer  206  and the second internal wiring layer  207  via the through hole wiring  211   a.    
     The first solder resist layer  208  made of an insulating material is laminated on the first internal wiring layer  206 . The insulating material can be epoxy resin, acrylic resin, polyimide, bismaleimide triazine resin, or the like. An insulating material containing a reinforcing filler made of silicon dioxide, for example, may also be used. The thickness of the first solder resist layer  208  can be set greater than or equal to 5 μm but no more than 70 μm, for example. The first solder resist layer  208  has an opening in a portion thereof, and the wiring layer  211  exposed from the opening forms the connection pads  204 . When mounting the surface component on the connection pads  204 , the first solder resist layer  208  acts as a solder resist for the solder for joining the surface component on the connection pads  204 . 
     The second solder resist layer  209  made of an insulating material is laminated on the second internal wiring layer  207 . Here, the same insulating material used for the first solder resist layer  208  can be used. The thickness of the second solder resist layer  209  can be set greater than equal to 5 μm but no more than 70 μm, for example. The second solder resist layer  209  has an opening in a portion thereof, and the wiring layer  211  exposed from the opening forms the connection pads  204 . When mounting the surface component on the connection pads  204 , the second solder resist layer  209  acts as a solder resist for the solder for joining the surface component on the connection pads  204 . 
     The rigid substrate  200  is configured as above. The thickness of the rigid substrate  200  is described later. 
     &lt;Flexible Substrate&gt; 
     A flexible substrate  300  is a flexible substrate, which contains wiring and the like embedded therein and establishes an electrical connection between the rigid substrate  200  and other electronic components (display, for example).  FIG. 11  is a perspective view showing part of the flexible substrate  300  and the side that joins the rigid substrate  200 . 
     Although the flexible substrate  300  is not limited to a specific size, as  FIG. 2  shows, the width of the flexible substrate  300  can fit within the cutout  201 . As  FIG. 11  shows, a connection terminal  301  is formed on the flexible substrate  300 . Once the flexible substrate  300  and the rigid substrate  200  are joined together, the connection terminal  301  is connected electrically to the connection terminal  203  of the rigid substrate  200 . Thus, the connection terminal  301  is arranged according to the form and the number of the connection terminal  203 . 
       FIG. 12  is a cross-sectional view of the flexible substrate  300  along a line B-B in  FIG. 11 . As the figure shows, the flexible substrate  300  is formed by a base material  302 , a wiring layer  303 , a cover lay  304 , and an adhesive layer  305 . 
     The base material  302  is a base material for the flexible substrate  300  and is made of an insulating material such as polyimide. The thickness of the base material  302  can be such that the base material is flexible. The base material  302  can include a through hole  302   a.    
     The wiring layer  303  is made of a conductive material such as copper and laminated on the base material  302  with an adhesive layer  305  in between. Part of the wiring layer  303  is exposed from the cover lay  304  and forms the connection terminal  301 . A plating layer (not shown in the figure) may be formed on the surface of the wiring layer  303 . The material for the plating layer can be chosen according to the method of joining the rigid substrate  200  and the flexible substrate  300 , which will be described later, and can be made of Au, for example. The wiring layer  303  establishes an electrical connection via the through hole  302   a.    
     The cover lay  304  is made of an insulating material and covers the wiring layer  303 . The cover lay  304  is laminated on the wiring layer  303  with the adhesive layer  305  in between. 
     The adhesive layer  305  is a layer on which an adhesive material is hardened and serves as the bond between the base material  302  and the wiring layer  303  as well as the bond between the wiring layer  303  and the cover lay  304 . 
     The flexible substrate  300  is configured as above. The thickness of the flexible substrate  300  is described later. 
     &lt;Joining Rigid Substrate and Flexible Substrate&gt; 
     As described above, the composite substrate  100  is formed by joining the rigid substrate  200  and the flexible substrate  300 .  FIG. 13  is an enlarged cross-sectional view of the composite substrate  100 . 
     As the figure shows, the flexible substrate  300  and the rigid substrate  200  are joined together by a bonding layer  400 . The bonding layer  400  fixes the rigid substrate  200  and the flexible substrate  300  and establishes an electrical connection between the connection terminals  203  of the rigid substrate  200  and the connection terminals  301  of the flexible substrate  300 . 
     Specifically, the bonding layer  400  can be made of ACP (Anisotropic Conductive Paste). ACP is a material in a paste form, in which conductive particles are distributed in insulating resin. When the connection terminal  203  and the flexible substrate  300  press the ACP, the resin between the connection terminals  203  and  301  is squeezed out from the space between the terminals, and the conductive particles are left behind. In this circumstance, when the rigid substrate  200  and the flexible substrate  300  are further pressed toward each other, the conductive particles are crushed establishing an electrical connection between the terminals. The expelled resin fills the space between the terminals, thereby insulating adjacent terminals from each other. 
     Also, the bonding layer  400  may be made of ACF (Anisotropic Conductive Film). ACF is ACP in a film form and functions in a manner similar to ACP and establishes an electrical connection between the connection terminal  203  and the connection terminal  301 . 
     When the bonding layer  400  is either ACP or ACF, the surfaces of both terminals need to be covered by Au. Thus, the material of the plating layer M of the connection terminal  203  can be Au. The connection terminal  301  can also be Au plated. Yet, the connection terminal  203  may be plated with Cu instead of Au. 
     As described above, the bonding layer  400  joins the rigid substrate  200  and the flexible substrate  300  to form the composite substrate  100 . 
     &lt;Thicknesses of Rigid Substrate and Flexible Substrate&gt; 
     The thicknesses of the rigid substrate  200  and the flexible substrate  300  are described.  FIG. 14  is a schematic cross-sectional view of the rigid substrate  200  and the flexible substrate  300 . As the figure shows, the thickness of the rigid substrate  200  is defined as the first depth D 1  and the thickness of the flexible substrate  300  is defined as the second depth D 2 . The depth of the cutout  201  is defined as the cutout depth D 3 . 
     Here, the second depth D 2  is less than the first depth D 1 , and the second depth D 2  is less than the cutout depth D 3 . Given these dimensions, the flexible substrate  300  does not protrude from the cutout  201 , and the maximum thickness of the composite substrate  100  is no greater than the thickness of the rigid substrate  200 . 
       FIGS. 15 and 16  are cross-sectional views showing the structure of a composite substrate according to a comparison example. A composite substrate  500  shown in  FIG. 15  and a composite substrate  600  shown in  FIG. 16  are formed by a rigid substrate  510  and a flexible substrate  520  that are joined together by a bonding layer  530 . The rigid substrate  510  is formed by a connection terminal  511 , connection pads  512 , a core layer  513 , wiring layers  514 , insulating layers  515 , and a solder resist layer  516 . The flexible substrate  520  is formed by a connection terminal  521 , a base material  522 , a wiring layer  523 , a cover lay  524 , and an adhesive layer  525 . 
     In both of the structures shown in  FIGS. 15 and 16 , the maximum thickness of the composite substrate is the combined thickness of the rigid substrate  510  and the flexible substrate  520 . In contrast, the maximum thickness of the composite substrate  100  according to the embodiment of the present invention is the thickness of the rigid substrate  200  as described above. Joining the rigid substrate  200  and the flexible substrate  300  does not increase the thickness of the composite substrate  100 . In short, compared to the composite substrates having the structures shown in  FIGS. 15 and 16 , the thickness of the composite substrate  100  can be reduced. 
     &lt;Mounting Surface Component&gt; 
     A surface component is mounted on the composite substrate  100 .  FIG. 17  is a cross-sectional view showing the composite substrate  100  with a surface component  900  mounted thereon. As shown in the figure, the surface component  900  is joined to the connection pads  204  by solder H and electrically connected to the connection pads  204 . The first solder resist layer  208  and the second solder resist layer  209  prevent melted solder H from leaking from the connection pads  204 . 
     &lt;Method of Manufacturing Composite Substrate&gt; 
     A method of manufacturing the composite substrate  100  will be described.  FIGS. 18A to 25  are diagrams showing the manufacturing process of the composite substrate  100 . 
     As  FIG. 18A  shows, a core layer  205  is prepared, and as  FIG. 18B  shows, through holes  205   b  are formed in the core layer  205 . The through holes  205   b  can be formed by etching after covering the parts of the core layer  205  that do not correspond to the through holes  205   b  with an etch mask. 
     Then, as  FIG. 18C  shows, insulating portions  210  made of insulating material are formed inside the through holes  205   b . First, a film  221  that temporarily fixes the core layer  205  is put on the entire surfaces of the core layer  205 . Next, the through holes  205   b  are filled with curing agent (prepreg), which is an insulating material, and the insulating portions  210  are formed by hardening the curing agent by heating. The insulating materials can include epoxy resin, polyimide, bismaleimide triazine resin, or the like. Insulating materials containing a reinforcing filler made of silicon dioxide, for example, may also be used. The thickness of the insulating portions  210  can be set approximately between 30 μm and 200 μm, for example. 
     Next, as  FIG. 19A  shows, the temporary film  221  is removed. 
     Next, as  FIG. 19B  shows, insulating layers  212  made of insulating material are formed on the top and the bottom surfaces of the core layer  205 . The insulating materials can include epoxy resin, polyimide, bismaleimide triazine resin, or the like. Insulating materials containing a reinforcing filler made of silicon dioxide, for example, may also be used. The insulating material used here may be same or different as the insulating material used for the insulating portions  210 . The thickness of the insulating layers  212  can be set approximately between 10 μm and 30 μm, for example. The insulating layers  212  can be formed by first placing sheets made of a curing agent (prepreg), which is an insulating material, on the top and the bottom surfaces of the core layer  205  and hardening the sheets by heating. 
     Then, as  FIG. 19C  shows, the wiring layers  211  are formed on the top and the bottom surfaces of the core layer  205 . The wiring layers  211  can be made of conductive material such as copper or copper alloy. Specifically, the wiring layers  211  can be formed by first drilling a hole in the insulating layer  212  by lasers and the like, then performing electrolytic plating using the core layer  205  as the base material, and finally creating patterns on that plating layer. Patterns can be created by etching techniques such as photo etching, which uses etchants (like ferric chloride) on the copper or copper alloy, for example. Such etchants do not chemically react with the insulating layer  212 . Thus, such etchants do not alter or roughen the surface of the insulating layers  212  during photo etching. The part of the wiring layers  211  connected to the core layer  205  functions as ground wiring, and those parts unconnected to the core layer  205  function as signal wiring. 
     Then, as  FIG. 20A  shows, the insulating layers  212  are formed further on the wiring layers  211 . As described above, the insulating layers  212  can be formed by placing sheets made of insulating material and heating the sheets. Furthermore, as the figure shows, the wiring layers  211  are formed on the insulating layers  212 . The wiring layers  211  can be formed by first drilling a hole in the insulating layer  212 , then performing electrolytic plating, and finally creating patterns on that plating layer. Hereafter, the wiring layer  211  and the insulating layer  212  are alternately laminated in the same manner, and the number of the wiring layers  211  and the insulating layers  212  are set as desired. Out of all the wiring layers  211 , the outermost layer forms the connection pads  204 . 
     Next, as  FIG. 20B  shows, the first solder resist layer  208  and the second solder resist layer  209  are formed on the insulating layer  212  and the connection pads  204 . Materials such as epoxy resin, acrylic resin, polyimide, and bismaleimide triazine resin can be used for the first solder resist layer  208  and the second solder resist layer  209 . Insulating materials containing reinforcing filler made of silicon dioxide, for example, may also be used. 
     The thickness of the first solder resist layer  208  and the second solder resist layer  209  can be set greater than or equal to 5 μm but no more than 70 μm, for example. The first solder resist layer  208  and the second solder resist layer  209  can be formed by laminating materials on the insulating layer  212  and the connection pads  204  and creating patterns such that there is an opening on the connection pads  204 . Methods of laminating materials can include vacuum lamination, for example. Patterns can be created by photo etching, for example. 
     Next, as  FIG. 21A  shows, protective resist layers  222  are formed on the first solder resist layer  208  and the second solder resist layer  209 . The protective resist layers  222  can be made of epoxy resin, acrylic resin, polyimide, bismaleimide triazine resin, and the like. Insulating materials containing a reinforcing filler made of silicon dioxide, for example, may also be used. The thickness of the protective resist layers  222  can be set greater than or equal to 5 μm but no more than 75 μm, for example. 
     The protective resist layers  222  can be formed by laminating materials on the first solder resist layer  208  and the second solder resist layer  209  and creating patterns. Methods of laminating materials can include vacuum lamination, for example. Patterns can be created by photo etching, for example. 
     Next, as  FIG. 21B  shows, the insulating layer  212  is partly removed to expose the core layer  205 . The insulating layer  212  can be removed by cutting the layer using a router (cutting tool). As the figure shows, it is preferable that the insulating layer  212  be cut such that the cut roughly reaches the surface of the core layer  205 . 
     Next, as  FIG. 22A  shows, the cutout  201  is formed. The cutout  201  can be formed by removing the core layer  205  by etching. Etchants such as ferric chloride do not chemically react with the insulating portions  210 , the insulating layer  212 , and the protective resist layer  222 . Also, the connection pads  204  are protected from etchants by the protective resist layer  222 . This setup makes it possible to remove only the portion of the core layer  205  located in between the insulating portions  210 . 
     Next, as  FIG. 22B  shows, the protective resist layer  222  is removed. The protective resist layer  222  can be removed using resist removal solution. Sodium hydroxide or amine solutions can be used for the resist removal solution, for example. 
     Next, as  FIG. 23A  shows, the insulating layer  212  corresponding to the bottom surface of the cutout  201  is removed to expose the wiring layer  211  located below the insulating layer. The insulating layer  212  can be removed by irradiating the insulating layer  212  with a laser. The wiring layer  211  exposed from the insulating layer  212  forms the connection terminal  203 . 
     Next, as  FIG. 23B  shows, the plating layer M is formed on the surface of the wiring layer  211 , which forms the connection pads  204  and the connection terminal  203 . The plating layer M can be made of Au. 
     Next, as  FIG. 24A  shows, the insulating layer  212  and the second solder resist layer  209  are cut at a point between the insulating portion  210  and the connection terminal  203 . The cut can be made by a dicer. From this, the rigid substrate  200  shown in  FIG. 10  is formed. 
     Next, as  FIG. 24B  shows, the bonding layer  400  is arranged on the connection terminal  203  and the insulating layer  212  located around the connection terminal. The bonding layer  400  is arranged by applying ACP using a dispenser or by placing an ACF. 
     Next, the flexible substrate  300  is placed on the bonding layer  400  to join the rigid substrate  200  and the flexible substrate  300  by heating and to establish an electrical connection between the connection terminal  203  and the connection terminal  301 . From this, the composite substrate  100  shown in  FIG. 13  is formed. 
     The composite substrate  100  can be produced by the procedure described above. 
     Embodiment 2 
     A composite substrate according to Embodiment 2 of the present invention is described. In the present embodiment, the same reference characters are used to designate configurations that are the same as those in Embodiment 1, and a description thereof is omitted. 
     In a manner similar to Embodiment 1, the composite substrate according to the present embodiment is formed by joining a rigid substrate and a flexible substrate. 
     &lt;Composite Substrate&gt; 
       FIG. 25  is an enlarged cross-sectional view of a rigid substrate  700  according to the present embodiment. In a manner similar to the rigid substrate  200  according to Embodiment 1, the rigid substrate  700  is an inflexible substrate on which a surface component is mounted. A cutout  201  is formed in the rigid substrate  700 . The shape and the depth of the cutout  201  is the same as those in Embodiment 1. 
     In a manner similar to Embodiment 1, the rigid substrate  700  is formed by laminating a core layer  205 , a first internal wiring layer  206 , a second internal wiring layer  207 , a first solder resist layer  208 , and a second solder resist layer  209 . The first internal wiring layer  206  and the second internal wiring layer  207  are formed by laminating a wiring layer  211  and an insulating layer  212 . Also, part of the wiring layer  211  forms connection pads  204 . 
     The rigid substrate  700  includes a connection terminal  701 . The connection terminal  701  is formed by the wiring layer  211  and a conductive layer  702  laminated on the wiring layer  211 . The conductive layer  702  is made of conductive material and can be Ni plating, for example. Other than Ni plating, the conductive layer  702  can also be made of a metal or a conductive resin. The thickness of the conductive layer  702  can be set greater than or equal to 3 μm but no more than 30 μm, for example. 
     A plating layer M can be formed on the surface of the conductive layer  702 . The material for the plating layer M can be chosen according to the method of joining the rigid substrate  200  and the flexible substrate  300 , which will be described later, and can be made of Au or Cu, for example. It is preferable that the combined thickness of the conductive layer  702  and the plating layer M be such that the top surface of the plating layer M is either the same as that of the insulating layer  212  or protruding a few micrometers from the top of insulating layer  212 . 
     The rigid substrate  700  is configured as above. The shape and the thickness of the rigid substrate  700  are same as the rigid substrate  200  according to Embodiment 1. In short, the rigid substrate  700  has a first thickness D 1  and the cutout  201  has a depth D 3  (see  FIG. 14 ). 
     &lt;Flexible Substrate&gt; 
       FIG. 26  is an enlarged cross-sectional view of a flexible substrate  800  according to the present embodiment. The flexible substrate  800  is a flexible substrate containing wiring and the like embedded therein and establishes an electrical connection between the rigid substrate  200  and other electronic components (display, for example). 
     In a manner similar to the flexible substrate  300  according to Embodiment 1, the flexible substrate  800  is formed by laminating a base material  302 , a wiring layer  303 , a cover lay  304 , and an adhesive layer  305 . In the flexible substrate  800 , part of the wiring layer  303  protrudes from the adhesive layer  305  forming a connection terminal  301 . 
     The flexible substrate  800  is configured as above. The shape and the thickness of the flexible substrate  800  are same as the flexible substrate  300  according to Embodiment 1. In short, the flexible substrate  300  has a second thickness D 2  (See  FIG. 14 ). 
     &lt;Joining Rigid Substrate and Flexible Substrate&gt; 
     As described above, the flexible substrate  800  is joined to the rigid substrate  700  to form a composite substrate  1100 .  FIG. 27  is an enlarged cross-sectional view of the composite substrate  1100 . 
     As the figure shows, the flexible substrate  800  and the rigid substrate  700  are joined together by a bonding layer  1200 . The bonding layer  1200  fixes the rigid substrate  700  and the flexible substrate  800  and establishes an electrical connection between the connection terminals  701  of the rigid substrate  700  and the connection terminals  301  of the flexible substrate  800 . 
     The bonding layer  1200  can be made of NCP (Non-anisotropic Conductive Paste). NCP made of an insulating resin fixes the flexible substrate  800  and the rigid substrate  700 , thereby allowing the connection terminals  701  and  301  to touch each other and establishing an electrical connection between them. In the rigid substrate  700 , because the conductive layer  702  is laminated on the connection terminal  701 , the connection terminal  701  physically touches the connection terminal  301 . Thus, NCP can be used for the bonding layer  1200 . 
     Also, the bonding layer  1200  may be made of NCF (Non-anisotropic Conductive Film). NCF is NCP in a film form and functions in a manner similar to NCP to establish an electrical connection between the connection terminal  701  and the connection terminal  301 . 
     ACP or ACF can also be used for the bonding layer  1200 . In this case, conductive particles contained in ACP or ACF remain in the space between the connection terminals  701  and  301 , thereby establishing an electrical connection between them. 
     When NCP or NCF is used for the bonding layer  1200 , the surface of the terminals to be connected electrically need to be covered by Cu. Thus, the material of the plating layer M of the connection terminal  701  can be Cu. Since the connection terminal  301  can be made of copper, a plating layer is unnecessary for the connection terminal  301 . When ACP or ACF is used for the bonding layer  1200 , Au can be used for the plating layer M of the connection terminal  701 , and the surface of the connection terminal  301  can also be Au plated. 
     As described above, the bonding layer  1200  joins the rigid substrate  700  and the flexible substrate  800  to form the composite substrate  1100 . In a manner similar to Embodiment 1, a surface component can be mounted on the connection pads  204  of the composite substrate  1100 . 
     &lt;Method of Manufacturing Composite Substrate&gt; 
     The method of manufacturing the composite substrate  1100  is described.  FIGS. 28A to 29B  are diagrams showing the manufacturing process of the composite substrate  1100 . Since the manufacturing process from the point at which the cutout  201  is formed to the point at which the insulating layer corresponding to the bottom of the cutout  201  is removed ( FIG. 23A ) is the same as in Embodiment 1, the description thereof is omitted. 
     Then, as  FIG. 28A  shows, the conductive layer  702  is formed on the exposed wiring layer  211 . The conductive layer  702  can be formed by Ni plating the wiring layer  211 . The conductive layer  702  may also be formed by filling the space above the wiring layer  211  with conductive paste or by filling the space above the wiring layer  211  with conductive paste and then Ni plating the formed surface. Furthermore, the conductive layer  702  may be formed first by Cu plating the wiring layer  211  and then Ni plating over that. 
     Next, as  FIG. 28B  shows, the plating layer M is formed on the surface of the wiring layer  211 , which forms the connection pads  204  and the connection terminal  701 . For the plating layer M, Au or Cu plating can be used when the bonding layer  1200  uses ACP or ACF, and Cu plating can be used when the bonding layer  1200  uses NCP or NCF. When the bonding layer  1200  uses NCP or NCF, the surface of the connection terminal  701  can be coated with solder instead of the plated layer M. 
     Next, as  FIG. 29A  shows, the insulating layer  212  and the second solder resist layer  209  are cut at a point between the insulating portion  210  and the connection terminal  701 . The cut can be made by a dicer. From this, the rigid substrate  700  shown in  FIG. 25  is made. 
     Next, as  FIG. 29B  shows, the bonding layer  1200  is arranged on the connection terminal  701  and the insulating layer  212 , which is located around the connection terminal. The bonding layer  1200  can be arranged by applying ACP or NCP using a dispenser or by placing an ACF or NCF. 
     Next, the flexible substrate  800  is placed on the bonding layer  1200  to join the rigid substrate  700  and the flexible substrate  800  by heating and to establish an electrical connection between the connection terminal  701  and the connection terminal  301 . From this, the composite substrate  1100  shown in  FIG. 27  is made. 
     Modification Example 
     A modification example of the composite substrate according to the present invention is described.  FIG. 30  is a cross-sectional view of the composite substrate  100  according to the modification example. Instead of the wiring layer  211 , part of the core layer  205  may form the connection terminal  203 . In the step where the core layer  205  is removed and the cutout  201  is created in the manufacturing process described above, the composite substrate  100  can be manufactured by digging the core layer  205  using a router, not an etchant, and preserving a predetermined thickness in the core layer  205 . 
       FIG. 31  is a cross-sectional view of the composite substrate  100  according to another modification example. The core layer  205  can be made of synthetic resin instead of metal. Synthetic resin such as glass epoxy resin can be used. In the step where the core layer  205  is removed and the cutout  201  is created in the manufacturing process described above, the composite substrate  100  can be manufactured by digging and removing the core layer  205  using a router, not an etchant. 
       FIG. 3  shows a plan view of a polygonal rigid substrate. This rigid substrate  200  has the core layer  205  and the first and second internal wiring layers  206  and  207 . Hereinafter, the first internal wiring layer is referred to as the “second laminated layer” and second internal wiring layer is referred to as the “first laminated layer.” These laminated layers are respectively provided on the first surface and second surface of this core layer. Each of these laminated layers  206  and  207  is constituted by at least one wiring layer made of an insulating layer and conductive pattern. In this example, the laminated layer on the first surface of the core layer on the top in  FIG. 3  is the second laminated layer  206 , and the laminated layer on the second surface of the core layer on the bottom in  FIG. 3  is the first laminated layer  207 . 
     Next, the cutout  201  is formed by the removal of the second laminated layer  206  and the core layer  205  of the rigid substrate  200 , and the resulting shape is theoretically a hexahedron, and is a cuboid in this example. In  FIG. 3 , the area corresponding to the bottom of the cutout  201  is the first surface, and the opposing surface (removed in  FIG. 3 ) is the second surface. In addition, the four side faces in the cutout (one of which is removed), clockwise from the area where the connection terminals  203  are located, are the first side face (removed), second side face, third side face, and fourth side face. As described before, the first side face and the second surface have been removed in  FIG. 3 , and thus the portions of the cavity where the first side face and the second surface were removed are now exposed to the outside atmosphere. 
     The long side of the first side face of the cutout is the length of the cutout, and the walls  202  are formed by being less than the length of the corresponding rigid substrate. It should be noted that “providing the cutout  201  in one side” means that the first side face of the cutout  201 , i.e., as defined by the gap between the walls, aligns with a side face of the rigid substrate  200 . 
     These walls  202  are located at both sides of the cutout  201  and respectively contact the second side face and the fourth side face of the cutout. It should be noted that, during the processing of the cutout  201 , these walls  202  are portions that are left over after the second laminated layer  206  and the core layer  205  are removed. The walls  202  have a thickness that is the sum of the thickness of the second laminated layer  206  and the core layer  205 , and are rectangular in a plan view. The first laminated layer  207  is provided so as to cover these two walls  202  and the cutout  201 . 
     The first laminated layer  207  has connection pads  204  on the top thereof, and connection terminals  203  on the bottom. As described above, the “laminated layers” are each constituted by at least one wiring layer made of an insulating layer and conductive pattern. Due to this, as shown in  FIG. 17  and the like, wiring lines, through-holes, etc. are provided between the connection terminals  203  and the connection pads  204 . 
     As can be understood from  FIG. 1 , the first laminated layer  207  is secured to the flexible substrate  300  despite the area of the first laminated layer being thinner than the rigid substrate  200 . As shown in  FIG. 17 , the top of the rigid substrate  200  corresponding to the cutout  201  includes the surface components  900 . Due to the difference in the thermal expansion coefficient between the terminals and the surface component  900  and due to the cutout  201  in the substrate, there is a risk of deflection of the first laminated layer  207 . The walls  202  respectively provided on each side of the cutout  201 , however, make it possible to prevent such deflection. Furthermore, the surface components  900  can be provided on the connection pads  204 , which enhances mounting efficiency and allows for a stable connection. 
     There is also the following merit when attaching the surface components  900 : In  FIG. 4 , consider a scenario in which the bottom surface of the substrate is placed on a work table and the surface components  900  are attached. At such time, as shown in  FIG. 1 , the cutout  201  is deeper than the flexible substrate  300  is tall, and thus the walls  202  respectively located on both sides can contact the work table. Thus, the rigid substrate  200  itself is stabilized on the work table, which allows for stable mounting of the surface components  900 . 
     It will be apparent to those skilled in the art that various modification and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents. In particular, it is explicitly contemplated that any part or whole of any two or more of the embodiments and their modifications described above can be combined and regarded within the scope of the present invention.