Patent Publication Number: US-2022216591-A1

Title: Antenna substrate, antenna module, and method of manufacturing antenna substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Patent Application No. PCT/JP2020/027834, filed Jul. 17, 2020, which claims priority to Japanese Patent Application No. 2019-176991, filed Sep. 27, 2019, the entire contents of each of which being incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an antenna substrate having a flexible section, an antenna module including the antenna substrate, and a method of manufacturing the antenna substrate. 
     BACKGROUND ART 
     International Publication No. 2019/026595 discloses an antenna module including a power fed component (RFIC) and an antenna substrate. The antenna substrate includes a first flat section where the power fed component (RFIC) is mounted, a second flat section where an antenna element is mounted, a flexible section having flexibility and disposed between the first flat section and the second flat section, and a conductor wire that extends in an in-plane direction inside the first flat section, the second flat section, and the flexible section and connects the power fed component and the antenna element to each other. 
     In the antenna module, the first flat section and the second flat section are disposed at positions so as to be perpendicular to each other. The flexible section is disposed in a bent state so that the first flat section and the second flat section are connected to each other with the first flat section and the second flat section disposed at perpendicular positions to each other. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: International Publication No. 2019/026595 
     SUMMARY 
     Technical Problems 
     In general, a conductor wire used for antenna substrates is composed of electrolytic copper foil having a polycrystalline structure. Electrolytic copper foil is manufactured by immersing a polarized drum having a mirror-like surface in an electrolytic solution so as to cause copper ions contained in the electrolytic solution to be deposited on the surface of the polarized drum utilizing the principle of electroplating to form copper foil, and then removing the copper foil from the surface of the polarized drum and winding the copper foil up once the thickness of the copper foil has reached a target value. Electrolytic copper foil has high dimensional accuracy and is suitable for use in antenna substrates requiring impedance adjustment. 
     On the other hand, electrolytic copper foil is characterized as having isotropic grains and a small grain size. Therefore, as recognized by the present inventors, if electrolytic copper foil is used as the material of a conductor wire disposed in a flexible section of an antenna substrate, bending stress generated when bending the flexible section is likely to cause cracks to progress in the thickness direction of the conductor wire, and there is concern that the conductor wire may break in some cases. 
     The present disclosure has been made in order to solve the above-described problem, as well as other problems, and it is an object thereof to ensure bending resistance of a conductor wire (or more generally an electrical conductor, which in selected embodiments are also a wire) disposed in a flexible section while suppressing variations in antenna characteristics in an antenna substrate having a flat section where an antenna element is disposed and a flexible section where a conductor wire connected to the antenna element is disposed. 
     Solutions to Problems 
     An antenna substrate according to the present disclosure is an antenna substrate having an antenna element. The antenna substrate includes a flat section which has a plate shape and in which the antenna element is disposed, a flexible section having a substantially uniform thickness that is shorter in width than length, the flexible section disposed adjacent to the flat section, and having flexibility, being disposed adjacent to the flat section, and having flexibility, a first electrical conductor that extends along an in-plane direction of the flat section and inside the flat section, the first electrical conductor has one end portion thereof connected to the antenna element, and has a polycrystalline structure. The second electrical conductor that extends along an in-plane direction of the flexible section and is disposed inside the flexible section, the second electrical conductor has one end portion thereof connected to another end portion of the first electrical conductor, and has a polycrystalline structure. An average or median particle size in an extension direction of the second electrical conductor is larger than an average or median particle size in an extension direction of the first electrical conductor, and an average or median value of a ratio of a particle size in the extension direction of the second electrical conductor to a particle size in a thickness direction of the second electrical conductor is larger than an average or median value of a ratio of a particle size in the extension direction of the first electrical conductor to a particle size in a thickness direction of the first electrical conductor. 
     In the above-described antenna substrate, the average or median particle size in the extension direction of the second conductor wire in the flexible section is larger than the average or median particle size in the extension direction of the first conductor wire in the flat section. In addition, the average or median aspect ratio (ratio of particle size in extension direction to particle size in thickness direction) of the second conductor wire is larger than the average or median aspect ratio of the first conductor wire. With this configuration, even if minute cracks are generated in the second conductor wire when the flexible section is bent, the cracks are unlikely to progress in the thickness direction of the second conductor wire and preventing the second conductor wire from electrically breaking can be made easier. On the other hand, electrolytic copper foil, which has a particle structure that is small in size and isotropic and has good dimensional accuracy, can be used as the material for the first conductor wire of the flat section. As a result, since variations in the length of the first conductor wire can be suppressed, variations in the antenna characteristics (for example, shifting of the frequency band in the reflection characteristics) can be reduced. As a result, the bending resistance of the second conductor wire disposed in the flexible section can be ensured while suppressing variations in the antenna characteristics. 
     A method of manufacturing an antenna substrate according to the present disclosure includes forming a first substrate with an electrical conductor therein, the electrical conductor comprising an electrolytic copper foil that has one end portion thereof connected to an antenna element, and producing a second substrate including performing an annealing process in which a specific part of the first substrate prepared is heated while being pressed so as to cause particles of the electrical conductor in the specific part to grow. In the second substrate, an average or median particle size in an extension direction of the electrical conductor in the specific part is larger than an average or median particle size in the extension direction of the electrical conductor in a part other than the specific part, and an average or median value of a ratio of a particle size in the extension direction of the electrical conductor in the specific part to a particle size in a thickness direction of the electrical conductor in the specific part is larger than an average or median value of a ratio of a particle size in the extension direction of the electrical conductor in the part other than the specific part to a particle size in the thickness direction of the electrical conductor in the part other than the specific part. 
     In the second substrate produced using the above-described manufacturing method, the average or median particle size in the extension direction of the conductor wire in the specific part (hereafter, “second conductor wire”) is larger than the average or median particle size in the extension direction of the conductor wire in a part other than the specific part (hereafter, “first conductor wire”). In addition, the average or median aspect ratio of the first conductor wire is larger than the average or median aspect ratio of the second conductor wire. Therefore, even if minute cracks are generated in the second conductor wire when the specific part is bent, the cracks are unlikely to progress in the thickness direction of the second conductor wire and preventing the second conductor wire from electrically breaking can be made easier. On the other hand, electrolytic copper foil, which has a particle structure that is small in size and isotropic and has good dimensional accuracy, can be used as the material for the first conductor wire. As a result, since variations in the length of the first conductor wire can be suppressed, variations in the antenna characteristics (for example, shifting of the frequency band in the reflection characteristics) can be reduced. As a result, the bending resistance of the second conductor wire disposed in the specific part can be ensured while suppressing variations in the antenna characteristics. 
     Advantageous Effects 
     According to the present disclosure, bending resistance of a conductor wire disposed in a flexible section can be ensured while suppressing variations in antenna characteristics in an antenna substrate having a flat section where an antenna element is disposed and a flexible section where a conductor wire connected to the antenna element is disposed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an example of a block diagram of a communication device in which an antenna substrate is used. 
         FIG. 2  is a diagram for describing the arrangement of an antenna substrate. 
         FIG. 3  is a transparent view (first view) of the inside of the antenna substrate. 
         FIG. 4  is a diagram illustrating a cross section of a conductor wire (rolled copper foil) of a flexible section. 
         FIG. 5  is a diagram illustrating a cross section of a conductor wire (electrolytic copper foil) of a flat section. 
         FIG. 6  is a diagram for describing a preparation step. 
         FIG. 7  is a diagram for describing an annealing process step. 
         FIG. 8  is a diagram illustrating an antenna substrate produced by the annealing process step. 
         FIG. 9  is a diagram illustrating an antenna substrate produced by the annealing process step and mounted on a mounting substrate. 
         FIG. 10  is a transparent view (second view) of the inside of an antenna substrate. 
         FIG. 11  is a transparent view (third view) of the inside of an antenna substrate. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereafter, embodiments of the present disclosure will be described in detail while referring to the drawings. In the figures, identical or equivalent parts are denoted by the same symbols and repeated description thereof is omitted. 
     Basic Configuration of Communication Device 
       FIG. 1  is an example of a block diagram of a communication device  10  in which an antenna substrate  120  according to this embodiment is used. The communication device  10  is, for example, a mobile terminal such as a mobile phone, a smartphone, or a tablet, a personal computer having a communication function, and so forth. 
     Referring to  FIG. 1 , the communication device  10  includes an antenna module  100  including the antenna substrate  120  and a BBIC  200  that forms a baseband signal processing circuit. In addition to the antenna substrate  120 , the antenna module  100  further includes an RFIC  110 , which is an example of a power fed component. The communication device  10  up-converts a signal transmitted to the antenna module  100  from the BBIC  200  into a radio-frequency signal and radiates the radio-frequency signal from the antenna substrate  120  and the communication device  10  down-converts a radio-frequency signal received by the antenna substrate  120  and subjects the down-converted signal to processing using the BBIC  200 . 
     The antenna substrate  120  includes a plurality of radiating elements  121 . In  FIG. 1 , for simplicity of explanation, only the configurations corresponding to four radiating elements  121  among the plurality of radiating elements  121  included in the antenna substrate  120  are illustrated and the configurations corresponding to the rest of the radiating elements  121 , which have the same configurations, are omitted. Note that although an example is illustrated in  FIG. 1  in which the antenna substrate  120  includes a plurality of radiating elements  121  disposed in a two-dimensional array pattern, the arrangement of the radiating elements  121  does not necessarily have to be an array pattern and there do not necessarily have to be a plurality of radiating elements  121 . In this embodiment, the radiating elements  121  are patch antennas substantially shaped like square flat plates. 
     The RFIC  110  includes switches  111 A to  111 D,  113 A to  113 D, and  117 , power amplifiers  112 AT to  112 DT, low-noise amplifiers  112 AR to  112 DR, attenuators  114 A to  114 D, phase shifters  115 A to  115 D, a signal multiplexer/demultiplexer  116 , a mixer  118 , and an amplification circuit  119 . 
     In the case where a radio-frequency signal is to be transmitted, the switches  111 A to  111 D and  113 A to  113 D are switched to the power amplifiers  112 AT to  112 DT and the switch  117  is connected to a transmission-side amplifier of the amplification circuit  119 . In the case where a radio-frequency signal is to be received, the switches  111 A to  111 D and  113 A to  113 D are switched to the low-noise amplifiers  112 AR to  112 DR and the switch  117  is connected to a reception-side amplifier of the amplification circuit  119 . 
     A signal transmitted from the BBIC  200  is amplified by the amplification circuit  119  and up-converted by the mixer  118 . A transmission signal, which is the up-converted radio-frequency signal, is divided into four signals by the signal multiplexer/demultiplexer  116 , and the respective four signals pass along four signal paths and are supplied to different radiating elements  121 . In this case, the directivity of radio waves radiated from the antenna substrate  120  can be adjusted by the phases being individually adjusted of the phase shifters  115 A to  115 D disposed along the respective signal paths. 
     Reception signals, which are radio-frequency signals received by the radiating elements  121 , pass along four different signal paths and are multiplexed by the signal multiplexer/demultiplexer  116 . The multiplexed reception signal is down-converted by the mixer  118 , amplified by the amplification circuit  119 , and transmitted to the BBIC  200 . 
     The RFIC  110  is, for example, formed as a single chip integrated circuit component including the above-described circuit configuration. Alternatively, devices (switches, power amplifiers, low-noise amplifiers, attenuators, and phase shifters) of the RFIC  110  that correspond to the individual radiating elements  121  may be formed as a single integrated chip component for each corresponding radiating element  121 . 
     Arrangement and Configuration of Antenna Substrate 
       FIG. 2  is a diagram for describing the arrangement of the antenna substrate  120  in Embodiment 1. Referring to  FIG. 2 , the antenna substrate  120  includes plate-shaped flat sections  131  and  133  and a plate-shaped flexible section  132  that connects the flat section  131  and the flat section  133  to each other. The reference to “plate-shape” or “plate-shaped” as used herein corresponds with a structure that is has a substantially uniform thickness from one side to the other that is smaller than a length of the structure. Radiating elements  121   a  and  121   b  are respectively disposed in the flat sections  133  and  131 . 
     The flat section  131  is disposed on one main surface  21  of a mounting substrate  20  with the RFIC  110  interposed therebetween. The flat section  131  extends along the main surface  21  of the mounting substrate  20 . The flat section  133  extends along a side surface  22  of the mounting substrate  20 . In other words, the flat section  131  and the flat section  133  are disposed at positions so as to be perpendicular to each other. Hereafter, a direction normal to the main surface  21  of the mounting substrate  20  is also referred to as a “Z-axis direction”, a direction normal to the side surface  22  of the mounting substrate  20  is also referred to as an “X-axis direction”, and a direction perpendicular to both the Z-axis direction and the X-axis direction is also referred to as a “Y-axis direction”. 
     The radiating elements  121   b  of the flat section  131  are disposed so that radio waves are radiated therefrom in a direction normal to the main surface  21  (i.e., Z-axis direction). The radiating elements  121   a  of the flat section  133  are disposed so that radio waves are radiated therefrom in a direction normal to the side surface  22  (i.e., X-axis direction). 
     The flexible section  132  is disposed in a curved state in order to connect the first flat section  131  and the second flat section  133 , which are disposed at perpendicular positions, to each other. As a result of connecting the two flat sections  131  and  133  to each other using the curved flexible section  132 , radio waves can be radiated in two different directions. In view of the fact that the flexible section  132  is disposed in a curved state, the thickness of the flexible section  132  is set to a value smaller than the thicknesses of the flat sections  131  and  133 . 
     The antenna substrate  120  (flat sections  131  and  133  and flexible section  132 ) is composed of a resin multilayer substrate formed by stacking sheets of a liquid crystal polymer (LCP) having thermoplasticity. 
       FIG. 3  is a transparent view of the inside of the antenna substrate  120  from the positive Y-axis direction side in  FIG. 2 . The flat sections  131  and  133  and the flexible section  132  of the antenna substrate  120  all have a multilayer structure. 
     The radiating elements  121   b,  conductor wires  141 , and a ground electrode GND are stacked in this order with prescribed intervals therebetween in the flat section  131 . The radiating elements  121   b  extend in plate-like shapes along an in-plane direction of the flat section  131 . The conductor wires  141  extend in line-like shapes along an in-plane direction of the flat section  131 . The ground electrode GND extends in a plate-like shape along an in-plane direction of the flat section  131 . 
     The radiating elements  121   a,  conductor wires  143 , and a ground electrode GND are stacked in this order with prescribed intervals therebetween in the flat section  133 . The radiating elements  121   a  extend in plate-like shapes along an in-plane direction of the flat section  133 . The conductor wires  143  extend in line-like shapes along an in-plane direction of the flat section  133 . The ground electrode GND extends in a plate-like shape along an in-plane direction of the flat section  133 . 
     Conductor wires  142  and a ground electrode GND are stacked in this order with a prescribed interval therebetween in the flexible section  132 . The conductor wires  142  extend in line-like shapes along an in-plane direction of the flexible section  132  such that a distance from an outer edge of the conductor wires  142  to an outer edge of the flexible section  132  is substantially the same along the curved portion of the flexible section  132 . The ground electrode GND also extends in a plate-like shape along an in-plane direction of the flexible section  132  and remains a substantially constant distance away from in inner edge of the flexible section  132  along its inner radial curved portion. The ground electrodes GND are formed so as to be integrated with each other across the flat sections  131  and  133  and the flexible section  132 . 
     In the flat section  131 , the radiating elements  121   b  are connected to the RFIC  110  by vias V 1 . A radio-frequency signal is supplied from the RFIC  110  to the radiating elements  121   b  through the vias V 1 , and as a result, radio waves are radiated from the radiating elements  121   b.  One end portions of the conductor wires  141  are connected to the RFIC  110  through vias V 2  and the other end portions of the conductor wires  141  are connected to one end portions of the conductor wires  142  through vias V 3 . The term “via” in this context is a vertical portion that extends in a normal direction from an edge surface. The via may be empty, or in the case of it serving as an electrical conductor, is at least partially occupied with an electrically conductive material. 
     The radiating elements  121   a  are connected to one end portions of the conductor wires  143  through vias V 5  in the flat section  133 . The other end portions of the conductor wires  143  are connected to the other end portions of the conductor wires  142  through vias V 4 . In other words, the radiating elements  121   a  of the flat section  133  are connected to the RFIC  110  through the conductor wires  141  to  143  and the vias V 2  to V 5 . A radio-frequency signal is supplied from the RFIC  110  to the radiating elements  121   a  through the conductor wires  141  to  143  and vias V 2  to V 5 , and as a result, radio waves are radiated from the radiating elements  121   a.    
     The flexible section  132  is disposed in a curved state as described above. Therefore, bending stress acts on the flexible section  132  when the flexible section  132  is bent from a flat state to a curved state. 
     Grain Structures of Conductor Wires 
     Generally, conductor wires used inside a multilayer substrate are composed of electrolytic copper foil having a polycrystalline structure. Electrolytic copper foil is manufactured by immersing and rotating a polarized drum having a mirror-like surface in an electrolytic solution so as to cause copper ions contained in the electrolytic solution to be deposited on the surface of the polarized drum utilizing the principle of electroplating to form copper foil, and then removing the copper foil from the surface of the polarized drum and winding the copper foil up once the thickness of the copper foil has reached a target value. Electrolytic copper foil has high dimensional accuracy due to the manufacturing principles thereof and is suitable for use in antenna substrates requiring impedance adjustment. 
     On the other hand, the grains (particles) of electrolytic copper foil have characteristics of being small in size and having an isotropic structure. Therefore, if electrolytic copper foil is used as the material of the conductor wires  142  disposed in the flexible section  132 , bending stress (in particular, tensile stress generated at the outer peripheral side of the bent part) generated when the flexible section  132  is bent is likely to cause cracks to progress in the thickness direction of the conductor wires  142 , and there is concern that the conductor wires  142  may break in some cases. 
     Accordingly, in the antenna substrate  120  according to this embodiment, electrolytic copper foil having high dimensional accuracy is used as the material of the conductor wires  141  and  143  in the flat sections  131  and  133  where bending stress does not act. In contrast, rolled copper foil, rather than electrolytic copper foil, is used as the material of the conductor wires  142  in the flexible section  132  where bending stress may act. 
     Rolled copper foil is manufactured by repeatedly performing a rolling process in which the thickness of copper is reduced by passing copper material between rolling rollers and stretching the copper material until the thickness of the copper reaches a target value. Repeated performance of the rolling process causes the copper particles to grow in an extension direction, and therefore the grains of the rolled copper foil have characteristics of being larger in size and having an anisotropic structure that is longer in the extension direction compared to those before the rolling process. 
       FIG. 4  is a diagram illustrating a cross section of the conductor wires  142  (rolled copper foil) of the flexible section  132 .  FIG. 5  is a diagram illustrating a cross section of the conductor wires  141  or  143  (electrolytic copper foil) of the flat section  131  or  133 . In  FIGS. 4 and 5 , the horizontal direction of the paper corresponds to the extension direction of the conductor wires and the vertical direction of the paper corresponds to the thickness direction of the conductor wires. 
     Rolled copper foil and electrolytic copper foil both have polycrystalline structures, but have different grain sizes and have different ratios of the grain size in the extension direction to the grain size in the thickness direction (hereafter, also referred to as “aspect ratio”). 
     The grains of the conductor wires  142  (rolled copper foil) illustrated in  FIG. 4  are large in size and have an anisotropic structure in which the grains are longer in the extension direction due to the rolling process causing the copper particles to grow in the extension direction. In contrast, the grains of the conductor wires  141  and  143  (electrolytic copper foil) illustrated in  FIG. 5  are smaller in size and have an isotropic structure when compared to the grains of the conductor wires  142  (rolled copper foil) illustrated in  FIG. 4 . 
     The average or median grain size in the extension direction of the conductor wires  142  (rolled copper foil) illustrated in  FIG. 4  is larger than the average or median grain size in the extension direction of the conductor wires  141  and  143  (electrolytic copper foil) illustrated in  FIG. 5 . In addition, the average or median aspect ratio of the conductor wires  142  (rolled copper foil) illustrated in  FIG. 4  is larger than the average or median aspect ratio of the conductor wires  141  and  143  (electrolytic copper foil) illustrated in  FIG. 5 . 
     The size of each grain can be obtained by, for example, analyzing an image of the cross section of each conductor wire, identifying as each grain a part enclosed by the same boundary surface, and measuring the size of each identified grain in the thickness direction and extension direction. The average and median grain sizes and the average and median aspect ratios can be obtained by statistically processing the results of a plurality of grain size calculations. 
     In the conductor wires  142  (rolled copper foil) illustrated in  FIG. 4 , the average grain size in the extension direction is generally 2.0 μm to 4.0 μm and the average grain size in the thickness direction is generally 0.5 μm to 1.5 μm. In contrast, in the conductor wires  141  and  143  (electrolytic copper foil) illustrated in  FIG. 5 , the average grain size in the extension direction is generally 0.1 μm to 0.5 μm and the average grain size in the thickness direction is generally 0.1 μm to 0.5 μm. 
     By using rolled copper foil, which has a larger grain size and a larger aspect ratio than electrolytic copper foil, as the material for the conductor wires  142  of the flexible section  132 , cracks can be made to progress more easily in the extension direction and less easily in the thickness direction even when the same bending stress is applied compared to the case where electrolytic copper foil is used. In other words, even if minute cracks are generated in the conductor wires  142  when bending the flexible section  132 , progress of the cracks in the thickness direction of the conductor wires  142  can be suppressed and it can be made easier to prevent the conductor wires  142  from electrically breaking. As a result, bending resistance of the conductor wires  142  of the flexible section  132  can be ensured. 
     On the other hand, electrolytic copper foil, which has higher dimensional accuracy than rolled copper foil, is used as the material for the conductor wires  141  and  143  of the flat sections  131  and  133 . Therefore, variations in the lengths of the conductor wires  141  and  143  can be suppressed compared to a case where rolled copper foil is used as the material for the conductor wires  141  and  143 . As a result, since the dimensional accuracy of the total lengths of the conductor wires  141 ,  142 , and  143  from the RFIC  110  to the radiating elements  121   a  is increased, variations in the antenna characteristics of the radiating elements  121   a  (for example, shifting of the frequency band in the reflection characteristics) can be reduced. 
     Furthermore, adhesion between the conductor wires  141  and  143  and the surrounding liquid crystal polymer substrate can be improved as a result of using electrolytic copper foil rather than rolled copper foil as the material of the conductor wires  141  and  143 . In other words, since rolled copper foil is manufactured by stretching copper using rolling rollers, the surfaces of rolled copper foil tend to be rougher than the surfaces of electrolytic copper foil. In contrast, electrolytic copper foil is manufactured by deposition on the surface of a polarized drum having a mirror-like finish, and therefore the surfaces of electrolytic copper foil are smooth. Therefore, the strength of adhesion between the conductor wires  141  and  143  and the surrounding liquid crystal polymer substrate can be improved as a result of using electrolytic copper foil rather than rolled copper foil as the material of the conductor wires  141  and  143 . 
     In addition, since electrolytic copper foil can generally be manufactured more cheaply than rolled copper foil, the cost of the antenna substrate  120  can also be reduced. In other words, by using rolled copper foil for the conductor wires  142  of the flexible section  132  where bending stress may act in order to ensure bending resistance and by using electrolytic copper foil, which is cheaper than rolled copper foil, for the conductor wires  141  and  143  of the flat sections  131  and  133  where bending stress will not act, the cost can be reduced compared to the case of using rolled copper foil for all of the conductor wires  141 ,  142 , and  143 . 
     As described above, the antenna substrate  120  according to this embodiment includes the plate-shaped flat section  133  where the radiating elements  121   a  are disposed, the plate-shaped flexible section  132  disposed adjacent to the flat section  133 , the conductor wires  143  having one end portions connected to the radiating elements  121   a  inside the flat section  133 , and the conductor wires  142  having one end portions connected to the other end portions of the conductor wires  143  inside the flexible section  132 . Electrolytic copper foil is used as the material of the conductor wires  143  and rolled copper foil is used as the material of the conductor wires  142 . The average or median grain size in the extension direction of the conductor wires  142  is larger than the average or median grain size in the extension direction of the conductor wires  143 , and the average or median aspect ratio of the conductor wires  142  is larger than the average or median aspect ratio of the conductor wires  143 . 
     Thus, bending resistance of the conductor wires  142  when bending the flexible section  132  can be ensured by using rolled copper foil, which has a larger grain size and a larger aspect ratio than electrolytic copper foil, as the material of the conductor wires  142  of the flexible section  132 . On the other hand, since the dimensional accuracy of the total lengths of the conductor wires  141 ,  142 , and  143  from the RFIC  110  to the radiating elements  121   a  is increased by using electrolytic copper foil, which has higher dimensional accuracy than rolled copper foil, as the material of the conductor wires  143  of the flat section  133 , variations in the antenna characteristics of the radiating elements  121   a  can be reduced. As a result, in the antenna substrate  120  including the flat section  133  where the radiating elements  121   a  are disposed and the flexible section  132  where the conductor wires  142  connected to the radiating elements  121   a  are disposed, the bending resistance of the conductor wires  142  disposed in the flexible section  132  can be ensured while suppressing variations in antenna characteristics. 
     The radiating elements  121   a,  the flat section  133 , the flexible section  132 , the conductor wires  143 , and the conductor wires  142  of this embodiment may respectively correspond to an “antenna element”, a “flat section”, a “flexible section”, a “first conductor wire”, and a “second conductor wire” of the present disclosure. 
     Regarding the grain structure of the ground electrodes GND, rolled copper foil is preferably used as the material of the ground electrode GND of the flexible section  132  and electrolytic copper foil is preferably used as the material of the ground electrodes GND of the flat sections  131  and  133  from the viewpoint of preventing cracks. On the other hand, electrolytic copper foil may be used as the materials of all the ground electrodes GND from the viewpoint of reducing cost. 
     Modifications 
     Modification 1 
     The antenna substrate  120  according to the above-described embodiment has a configuration (hereafter, also referred to as “a characteristic configuration of the present disclosure”) in which the average or median grain size and the average or median aspect ratio of the conductor wires  142  of the flexible section  132  are larger than the average or median grain size and the average or median aspect ratio of the conductor wires  141  and  143  of the flat sections  131  and  133 . In the above-described embodiment, an example is described in which the antenna substrate  120  having the characteristic configuration of the present disclosure is manufactured by connecting the conductor wires  142  composed of rolled copper foil and the conductor wires  141  and  143  composed of electrolytic copper foil to each other using the vias V 3  and V 4 . 
     In contrast, in this Modification 1, an antenna substrate  120 B having the characteristic configuration of the present disclosure is manufactured by performing a preparation step and an annealing process (heating process) step described below in this order. 
       FIG. 6  is a diagram for describing a preparation step in Modification 1.  FIG. 7  is a diagram for describing an annealing process step in Modification 1.  FIG. 8  is a diagram illustrating the antenna substrate  120 B produced by the annealing process step. 
     As illustrated in  FIG. 6 , an antenna substrate  120 A is prepared in the preparation step. The antenna substrate  120 A has a configuration in which the conductor wires  141 ,  142 , and  143  and the vias V 3  and V 4  of the antenna substrate  120  described above are replaced with single conductor wires  140  composed of electrolytic copper foil. 
     As illustrated in  FIG. 7 , in the annealing process step, an annealing process is performed in which the flexible section  132  of the antenna substrate  120 A prepared in the preparation step is heated while being pressed in the thickness direction by using manufacturing devices  200   a  and  200   b,  and as a result, the grains in the part of the conductor wirings  140  contained in the flexible section  132  are made to grow in the extension direction. 
     In the annealing process, the flexible section  132  of the antenna substrate  120 A is heated at a prescribed temperature (for example, temperature greater than or equal to 230° C. and less than 300° C.) while being pressed at a prescribed pressure (for example, pressure less than or equal to 8.4 MPa) for a prescribed holding time (for example, time from 30 minutes to 3 hours). Although it is desirable to set the heating temperature to 230° C. or higher in order to grow (enlarge) the grain size, since melting and decomposition of the resin multilayer substrate, which is the base material of the antenna substrate  120 A, occur at around 300° C., it is desirable to perform the annealing process after setting the holding time, pressure value, and heating temperature as appropriate. The heating is desirably performed for 1 hour at 250° C., more desirably for 30 minutes at 280° C., or for 2 to 3 hours at 230° C. in order to suppress damage to mounted components and so forth. If the antenna substrate  120 A is a collective board manufactured by batch press stacking, the value of the pressure is preferably 8.4 MPa or less, as described above, taking into consideration the conditions during manufacturing and the fact that a local heating press is used. 
     The characteristic configuration of the present disclosure can be obtained by performing the above-described process on the flexible section  132  of the antenna substrate  120 A. In other words, the average or median grain size and aspect ratio in the parts of the conductor wires  140  in the flexible section  132  are larger than the average or median grain size and aspect ratio in the parts of the conductor wires  140  in the flat sections  131  and  133 . As a result, as illustrated in  FIG. 8 , in the antenna substrate  1208  produced using the annealing process step, the parts of the conductor wires  140  in the flat sections  131  and  133  form conductor wires  141 B and  143 B composed of electrolytic copper foil and the parts of the conductor wires  140  in the flexible section  132  form conductor wires  142 B having the same grain structure as rolled copper foil. 
     An example is illustrated in  FIG. 8  in which a boundary part B 1  between the conductor wires  142 B and the conductor wires  141 B and a boundary part between the flexible section  132  and the flat section  131  substantially coincide with each other, but these boundary parts may instead be shifted from each other to some extent. Similarly, an example is illustrated in  FIG. 8  in which a boundary part B 1  between the conductor wires  142 B and the conductor wires  143 B and a boundary part between the flexible section  132  and the flat section  133  substantially coincide with each other, but these boundary parts may instead be shifted from each other to some extent. In addition, the grain structures of the boundary parts B 1  and B 2  may gradually change from a grain structure equivalent to that of rolled copper foil to the grain structure of electrolytic copper foil from the region near the conductor wires  142 B to the sides near the conductor wires  141 B and  143 B. 
       FIG. 9  is a diagram illustrating the antenna substrate  120 B produced by the annealing process step according to Modification 1 mounted on the mounting substrate  20 . In the antenna substrate  120 B, as illustrated in  FIG. 9 , the conductor wires  142 B in the flexible section  132  are formed so as to be integrated with the conductor wires  141 B and  143 B in the flat sections  131  and  133 . 
     The grain structures of the conductor wires  141 B and  142 B of the flat sections  131  and  133  are the same as the grain structure of the electrolytic copper foil illustrated in  FIG. 5 . In contrast, the grain structure of the conductor wires  142 B of the flexible section  132  is equivalent to the grain structure of the rolled copper foil illustrated in  FIG. 4  as a result of the grains having been caused to grow in the extension direction by the annealing process. Thus, the bending resistance of the conductor wires  142 B disposed in the flexible section  132  can be ensured while suppressing variations in antenna characteristics similarly to as in the above-described embodiment. 
     The “antenna substrate  120 A”, the “preparation step”, the “annealing process”, the “antenna substrate  120 B”, and the “annealing process step” of Modification 1 may respectively correspond to a “first substrate”, a “step of preparing a first substrate”, an “annealing process”, a “second substrate”, and a “step of producing a second substrate” of the present disclosure. 
     Modification 2 
     In the above-described antenna substrate  120  illustrated in  FIG. 3 , the flat sections  131  and  133  and the flexible section  132  are all composed solely of flexible substrates having thermoplasticity. However, at least either of the flat sections  131  and  133  may have a structure in which a flat substrate is stacked on a flexible substrate. In this case, the flat substrate is connected to the flexible substrate using, for example, solder mounting, pressure bonding, or an adhesive layer. 
       FIG. 10  is a transparent view of the inside of an antenna substrate  120 C according to Modification 2 from the positive Y-axis direction side. The antenna substrate  120 C includes a flexible substrate  122 , which extends across the entirety of the flat sections  131  and  133  and the flexible section  132 , and flat substrates  123   a  and  123   b,  which are respectively stacked on regions of the flexible substrate  122  located in the flat sections  131  and  133 . 
     The flat substrates  123   a  and  123   b  are, for example, each formed of a low-temperature co-fired ceramic (LTCC) multilayer substrate, a multilayer resin substrate formed by stacking a plurality of resin layers composed of a resin such as epoxy or polyimide, a multilayer resin substrate formed by stacking a plurality of resin layers composed of a liquid crystal polymer, a multilayer resin substrate formed by stacking a plurality of resin layers composed of a fluorine-based resin, a multilayer resin substrate formed by stacking a plurality of resin layers composed of a polyethylene terephthalate (PET) material, or a ceramic multilayer substrate other than LTCC. The flat substrates  123   a  and  123   b  do not necessarily have to have a multilayer structure and may instead be single layer substrates. Furthermore, the flat substrates  123   a  and  123   b  may be disposed on the side of the communication device  10  near the casing (including a display panel such as a liquid crystal panel). 
     Furthermore, in the flexible substrate  122 , the conductor wires  142 , which are composed of rolled copper foil, extend from the flexible section  132  to the flat section  131  and the flat section  133 . 
     The radiating elements  121   a,  the conductor wires  143  composed of electrolytic copper foil, and the vias V 5 , which connect the radiating elements  121   a  and one end portions of the conductor wires  143  to each other, are disposed in the flat substrate  123   b  of the flat section  133 . The other end portions of the conductor wires  143  are connected to one end portions of the conductor wires  142  disposed in the flexible substrate  122  by the vias V 4 . The other end portions of the conductor wires  142  are connected to the RFIC  110  by the vias V 2  in the flat section  131 . Thus, the radiating elements  121   a  are electrically connected to the RFIC  110 . 
     The radiating elements  121   b,  the conductor wires  144  composed of electrolytic copper foil, and vias V 6 , which connect the radiating elements  121   b  and one end portions of the conductor wires  144  to each other, are disposed in the flat substrate  123   a  of the flat section  131 . The other end portions of the conductor wires  144  are connected to the RFIC  110  by the vias V 1 . Thus, the radiating elements  121   b  are electrically connected to the RFIC  110 . 
       FIG. 11  is a transparent view of the inside of another antenna substrate  120 D according to Modification 2 from the positive Y-axis direction side. In the antenna substrate  120 D, part of the flat substrate  123   a  is disposed so as to be shifted outward (negative X-axis direction side in  FIG. 11 ) from one end portion of the flexible substrate  122  and part of the flat substrate  123   b  is disposed so as to be shifted outward (negative Z-axis direction side in  FIG. 11 ) from the other end portion of the flexible substrate  122  with respect to the above-described antenna substrate  120 C illustrated in  FIG. 10 . The flexible substrate  122  is connected to the flat surfaces of the flat substrates  123   a  and  123   b  using pressure bonding, adhesion, or connectors. 
     Furthermore, in the antenna substrate  120 D, the conductor wires  144  in the flat section  131  are disposed in the flexible substrate  122  rather than in the flat substrate  123   a  and the radiating elements  121   b  and the vias V 6  are connected to each other by conductor wires  145  composed of electrolytic copper foil inside the flat substrate  123   a . The rest of the configuration of the antenna substrate  120 D is the same as that of the antenna substrate  120 C described above. 
     As described above, the flexible section  132  may be formed of a flexible substrate and the flat sections  131  and  133  may each have a multilayer structure consisting of a flexible substrate and a flat substrate. The “flexible substrate  122 ” and the “flat substrate  123   b ” of Modification 2 may respectively correspond to a “flexible substrate” and a “flat substrate” of the present disclosure. 
     The presently disclosed embodiments are illustrative in all points and should not be considered as limiting. The scope of the present disclosure is not defined by the above description of the embodiments but rather by the scope of the claims and it is intended that equivalents to the scope of the claims and all modifications within the scope of the claims be included within the scope of the disclosure. 
     REFERENCE SIGNS LIST 
       10  communication device,  20  mounting substrate,  21  main surface,  22  side surface,  100  antenna module,  111 A to  113 D,  117  switch,  112 AR to  112 DR low-noise amplifier,  112 AT to  112 DT power amplifier,  114 A to  114 D attenuator,  115 A to  115 D phase shifter,  116  signal multiplexer/demultiplexer,  118  mixer,  119  amplification circuit,  120 ,  120 A,  120 B antenna substrate,  121 ,  121   a,    121   b  radiating element,  131 ,  133  flat section,  132  flexible section,  140 ,  141 ,  141 B,  142 ,  142 B,  143 ,  143 B conductor wire,  200   a,    200   b  manufacturing device, GND ground electrode, V 1  to V 5  vias.