Patent Publication Number: US-7907743-B2

Title: Electro-acoustic transducer

Description:
TECHNICAL FIELD 
     The present invention relates to an electro-acoustic transducer such as a microphone and, in particular, to an electro-acoustic transducer that is soldered using the surface mounting art using a reflow furnace, wherein the transducer&#39;s cylindrical capsule itself functions as a ground electrode. 
     BACKGROUND ART 
     In conventional microphones, a diaphragm ring, a diaphragm, a spacer, a back electrode, a holder, a gate ring, and a substrate, for example, are stacked in a cylindrical metal capsule having sound apertures and the components are fixed by caulking the end of the capsule toward the substrate (Japanese Patent Application Laid Open No. 2003-153392 (Patent Reference 1)). Electrodes are protruded from the substrate for conduction of electricity with an external object. The caulked part has a rounded portion (prominent portion) and the extent to which the portion is rounded (the height of the prominence) varies. That is, the amount of the protrusion of the electrodes with respect to the caulked part varies. Therefore, when such a microphone is soldered using a reflow furnace, the unevenness causes poor soldering in the reflow furnace or a faulty posture (tilt) of the microphone mounted on a wiring board. 
     To solve the problem, the applicant has previously proposed a structure in which the disposition of components in the cylindrical metal capsule is reversed (Japanese Patent Application No. 2005-121051 filed on Apr. 19, 2005).  FIG. 1  shows a cross-sectional view of the microphone previously proposed by the applicant. According to the related art, a ground electrode pattern  114  is formed on the side (bottom  121 ) in which opening  123  of a capsule  102  is provided. A built-in substrate  112  is provided on the ground electrode pattern  114 . The built-in substrate  112  has an output terminal electrode  111  and ground terminal electrode  115  on the same side on which the ground electrode  114  is provided. The terminal electrodes  111 ,  115  are longer than the thickness of the capsule  102  and protrude outward through the opening  123  of the capsule  102 . A conductor pattern  109  is formed on the upper surface of the built-in substrate  112  and an electronic circuit  110  is provided on it. Stacked on the upper surface of the built-in substrate  112  are a gate ring  108 , a holder  107 , a back electrode  106 , a spacer  105 , a diaphragm  104 , a diaphragm ring  103 , and a top plate  130  having sound apertures  131 . The end of the capsule is caulked to the top plate  130 , thereby fixing each of the components as well. The top plate  130  may be made of the same metal as the capsule  102  and may have the same thickness as the capsule  102 , for example. 
     In this microphone  100 , the terminal electrodes  111 ,  115  can be reliably protruded with respect to the thickness of the bottom  121  without being affected by unevenness of the caulked part  113 . Accordingly, defects in soldering using a reflow furnace can be prevented. 
     However, for example, if the microphone  100  is installed in a cell phone, the microphone  100  picks up touch noise generated when a user touches the cell phone, vibration noise generated by driving of a built-in motor and the like. This problem is unavoidable as long as the microphone is directly mounted on a wiring board. 
       FIG. 2  shows a circuit configuration of an analog microphone. Contained in a capsule  102  are an acoustic-electric converter  100 ′ and an electronic circuit  110 . The acoustic-electric converter  100 ′ is formed by the capsule  102  and internal components. The electronic circuit  110  consists of a field-effect transistor (FET) and a capacitor, for example. As can be seen from  FIG. 2 , the microphone  100  has two terminals: an output terminal and a ground terminal. It should be noted that, the terminal electrode (ground)  115  is shown in two positions in  FIG. 1  because  FIG. 1  is a cross-sectional view of a toroidal terminal. 
     The applicant has also proposed previously, in another application, an electret condenser microphone that can be soldered using a reflow furnace and outputs a digital signal (Japanese Patent Application No. 2005-320815 filed on Nov. 14, 2005).  FIG. 3  is a cross-sectional view of an exemplary electret condenser microphone outputting a digital signal proposed by the present applicant. The front type electret condenser microphone  200  has an electret polymer film made of a heat-resistant material within an electrically conductive capsule  201 . An electrically conductive diaphragm  207 , an electrically conductive ring  208 , a gate ring  209 , and a wiring substrate  202  are provided and are separated from the electret polymer film by a spacer  206  made of an heat-resistant insulator. The end of the electrically conductive capsule  201  is caulked to the wiring substrate  202  and fixes the internal components. An IC device  210  is mounted on the interior side of the wiring substrate  202 . Four terminals  204 ( a - d ) are provided on the exterior side of the wiring substrate  202 . The terminals  204 ( a - d ) are protruded through an opening  223  of the front type electret condenser microphone  200  for conduction of electricity with an external object. With this configuration, a digital electret condenser microphone capable of resisting high temperatures generated by soldering in a reflow furnace can be implemented. 
       FIG. 4  shows a circuit configuration of a digital microphone. Provided in an electrically conductive capsule  201  are an acoustic-electric converter  200 ′ and an IC device  210 . The acoustic-electric converter  200 ′ is formed by the capsule  201  and internal components. The IC device  210  includes an impedance converter/amplifier  210   a  and a digital sigma modulator  210   b . As can be seen from  FIG. 4 , four terminals, a power supply terminal  204   a , a clock input terminal  204   b , a digital data output terminal  204   c , and a ground terminal  204   d , are provided. A problem with this digital microphone is that it is susceptible to high-frequency noise from nearby components because its ground terminal does not have a toroidal shape. 
     An approach to reducing the number of components of both analog and digital microphones may be to solder the bottom of the capsule directly to a wiring board, thereby omitting the ground terminal. In this case, if a ground electrode can be formed into a toroidal shape, the microphone would be less susceptible to high-frequency noise. However, some measures must be taken against heat transferred to the interior of the microphone during soldering in a reflow furnace. Furthermore, the vibration pickup problem cannot be solved by using the bottom itself as the ground electrode. 
     BRIEF SUMMARY OF THE INVENTION 
     Thus, there are various problems with mounting an electro-acoustic transducer directly on a wiring board, and it has been impossible to solve all of those problems at the same time. An object of the present invention is to provide a structure that achieves the following four objects at the same time: a first object is to make the structure resistant to vibration from a wiring board; a second object is to make the structure resistant to high-frequency noise; a third object is to reduce the number of components, and a fourth object is to make the structure resistant to heat generated during soldering in a reflow furnace. 
     An electro-acoustic transducer (such as a microphone) according to the present invention includes: an electrically conductive capsule having an opening for electrically connecting internal circuitry to an external object; terminals which protrude from the opening to the outside; and a raised part which is a portion of the capsule on the opening side and is spaced with a gap from the internal structure of the capsule. The raised part and the terminals are arranged in such a manner that the raised part and all of the terminals are able to be directly soldered to a wiring board. The raised part may extend toward the terminals in such a manner that the opening is narrowed. Furthermore, the raised part may have a slit extending to the boundary between the raised part and the other part of the capsule. 
     According to the present invention, there is a gap between the raised part to be soldered to a wiring board and the main structure of the electro-acoustic transducer (such as a microphone). The gap makes the transducer resistive to vibration. Also, a ground electrode of the present invention may be toroidal so that it is not affected by any high-frequency noise. Furthermore, the number of components of the transducer can be reduced because the capsule itself functions as a ground electrode. Moreover, the gap between the raised part and the main structure of the electro-acoustic transducer makes the transducer resistive to heat generated during soldering in a reflow furnace. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-section of a microphone previously proposed by the applicant; 
         FIG. 2  shows a circuit configuration of an analog microphone; 
         FIG. 3  is a cross-sectional view of an exemplary electret condenser microphone outputting a digital signal proposed previously by the applicant; 
         FIG. 4  shows a circuit configuration of a digital microphone; 
         FIG. 5  is a cross-sectional view showing a structure of a microphone according to a first embodiment; 
         FIG. 6  is an external perspective view of the microphone  1  in  FIG. 5  viewed from the bottom  21 ; 
         FIG. 7  is an external perspective view of the microphone  1  in  FIG. 5  viewed from the bottom  21 ; 
         FIG. 8  is a cross-sectional view of a digital front type electret condenser microphone to which the present invention is applied; 
         FIG. 9  is a cross-sectional view of a digital back type electret condenser microphone to which the present invention is applied; 
         FIG. 10  is a cross-sectional view of another digital back type electret condenser microphone to which the present invention is applied; 
         FIG. 11  is a cross-sectional view of a digital foil type electret condenser microphone to which the present invention is applied; 
         FIG. 12A  is an external perspective view of a digital electret condenser microphone having a front plate with three small sound apertures, viewed from the front-plate side; 
         FIG. 12B  is an external perspective view of a digital electret condenser microphone having a raised part raised near a caulked part, viewed from the opening side; 
         FIG. 13A  is an external perspective view of a digital electret condenser microphone having a front plate with a large circular sound aperture, viewed from the front-plate side; 
         FIG. 13B  is an external perspective view of a digital electret condenser microphone having a raised part extending toward terminals to narrow the opening, viewed from the opening side; 
         FIG. 14A  is an external perspective view of a digital electret condenser microphone having a front plate with a large square sound aperture, viewed from the front-plate side; and 
         FIG. 14B  is an external perspective view of a digital electret condenser microphone having a raised part extending toward terminals to narrow the opening, viewed from the opening side. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following description, components having like functions are labeled like reference numerals and redundant description of which will be omitted. 
     First Embodiment 
       FIG. 5  is a cross-sectional view showing a structure of a microphone according to a fist embodiment. An electrically conductive capsule  2  has, on the bottom face, a bottom  21  with which internal components are in contact, an opening  23  through which a terminal electrode is exposed, and raised parts  21   b  raised from the bottom  21 . The capsule  2  may be made of albata or aluminum. A built-in substrate  112  is in contact with the bottom  21 . The built-in substrate  112  has a ground electrode pattern  114  electrically connected to the bottom  21 , and a conductor pattern  109  provided on the side opposite to the bottom  21 . A terminal electrode (output)  11  for providing electrical contact with an external object through an opening  23  is provided on the surface of the built-in substrate  112  on the bottom  21  side. An electronic circuit  110  is mounted on the surface of the built-in substrate  112  on the side opposite to the bottom  21 . The terminal electrode  11  may be formed as an integral part of the built-in substrate  112  or may be formed by plating or the like on the built-in substrate  112 . Stacked on the built-in substrate  112  on the side opposite to the bottom  21  are a gate ring  108 , a holder  107 , a back electrode  106 , a spacer  105 , a diaphragm  104 , a diaphragm ring  103 , and a top plate  130  having sound apertures  131 . The end of the capsule  2  is caulked to the top plate  130 , thereby fixing the internal components. The lower end of the raised part  21   b  is substantially in the same plane as the lower end of the terminal electrode (output)  11 . The purpose of this is to ensure that the terminal electrode (output)  11  and the raised part  21   b  are evenly soldered when the microphone is soldered to a wiring board and that the microphone is firmly mounted on the wiring board without tilting with respect to the wiring board. 
     With this configuration, a gap of approximately 50 μm-100 μm is created between the raised part  21   b  and the built-in substrate  112 . The size of the gap depends on the size of the microphone in practice. Because of the gap between the raised part  21   b  and the built-in substrate  112 , the raised part  21   b  functions as a member that absorbs vibration from an external vibration source. Accordingly, vibration transferred to the microphone  1  can be reduced. Furthermore, because only the raised part  21   b , rather than the entire bottom  21 , is in contact with the wiring board, the contact area is reduced and therefore less vibration is transferred to the microphone  1 . In addition, the gap can prevent heat conduction to the interior of the microphone even when the portion (raised part  21   b ) to be soldered is exposed to a high temperature, for example 260° C., in a reflow furnace. It should be noted that if the raised part  21   b  is reduced in length in the radial direction, heat transferred from the raised part  21   b  to the built-in substrate  112  can also be reduced because the area in contact with solder (heated area) is reduced. Furthermore, the need for the terminal electrode (ground)  115  shown in  FIG. 1  can be eliminated because the raised part  21   b  functions as a ground electrode. Moreover, the raised part  21   b  can be formed into a toroidal shape, thereby resolving the high-frequency noise problem. 
       FIGS. 6 and 7  are perspective views of the microphone  1  shown in  FIG. 5 , viewed from the bottom  21 . While both  FIGS. 6 and 7  show examples in which the raised part  21   b  is split into three, the raised part  21   b  may be split into any other number of sections. The difference between the examples in  FIGS. 6 and 7  lies in the width of the slit  24 . With this configuration, the elasticity of the raised part  21   b  can be controlled by adjusting the width of the raised part  21   b . That is, the ability of the raised part  21   b  to absorb vibration can be controlled by adjusting the number of sections into which the raised part  21   b  is split and by adjusting the width of the slit  24 . Heat conduction can also be controlled by adjusting the width of the raised part  21   b . However, if the slit  24  is too wide, the raised part  21   b  which also functions as a ground electrode would lose the shape of toroid and would become susceptible to high-frequency noise. 
     As has been described, the provision of the raised part  21   b  allows for the effects of absorbing vibration and high-frequency noise, reducing the number of components, and preventing heat conduction. The number of sections of the raised part  21   b , the radial length of the raised part  21   b , and the width of the slit  24  should be chosen to be appropriate to the environment in which the microphone  1  is used because the effects of absorbing vibration and high-frequency noise and preventing heat conduction can be in a trade-off relationship with one another. 
     It should be noted that the position of the terminal electrode (output)  11  does not change even if the microphone is rotated because the electrode  11  is positioned in the center of the built-in substrate  112  and the raised part  21   b  is provided around it in toroidal form. Therefore, when mounting the microphone, the microphone can be positioned in place merely by aligning the terminal electrode (output)  11 . Furthermore, the slit  24  dividing the raised part  21   b  extends to the boundary  21   c  between the raised part  21   b  and a marginal portion  21   a . Accordingly, the opening is not completely sealed when the microphone is soldered on a wiring board. That is, the slit  24  at the boundary  21   c  let the gas escape during soldering. The slit  24  must have a sufficient width for releasing gas. 
     Second Embodiment 
       FIG. 8  is a cross-sectional view of a digital front type electret condenser microphone to which the present invention is applied. The differences of the microphone in  FIG. 8  from that in  FIG. 3  lie in the shape of the electrically conductive capsule and the number of the terminals  204 . The electrically conductive capsule  41  of the present invention has a raised part  41   c  on the opening  42  side. Accordingly, a caulked part  43  is not an end of the electrically conductive capsule  41 . The raised part  41   c  acts as a ground terminal and therefore eliminates the need for the ground terminal  204   d  shown in  FIG. 3 . 
       FIG. 9  is a cross-sectional view of a digital back type electret condenser microphone to which the present invention is applied. The electrically conductive capsule  51  has a raised part  51   c  on the opening  52  side. A heat-resistive cylindrical synthetic-resin molded member  211  is provided on the internal sidewall of the electrically conductive capsule  51 . Stacked inside the electrically conductive capsule  51  are a front plate  51   a , an electrically conductive ring  208 , an electrically conductive diaphragm  207 , a spacer  206 , an electret polymer film  205 , a fixed electrode  212  having sound apertures  212   a , a gate ring  209 , and a wiring substrate  202  having an IC device  210  and terminals  204   a - 204   c , in this order. 
       FIG. 10  is a cross-sectional view of another digital back type electret condenser microphone to which the present invention is applied. The electrically conductive capsule  61  has a raised part  61   c  on the opening  62  side. A heat-resistive cylindrical synthetic-resin molded member  211  is provided on the internal sidewall of the electrically conductive capsule  61 . Stacked inside the electrically conductive capsule  61  are a front plate  61   a , a dust-preventive metallic mesh  213  having pores  213   b , a fixed electrode  212  having sound apertures  212   a , an electret polymer film  205 , a spacer  206 , an electrically conductive diaphragm  207 , a gate ring  209 , an electrically conductive ring  208 , and a wiring substrate  202  having an IC device  210  and terminals  204   a - 204   c , in this order. 
       FIG. 11  is a cross-sectional view of a digital foil type electret condenser microphone to which the present invention is applied. The electrically conductive capsule  71  has a raised potion  71   c  on the opening  72  side. A heat-resistive cylindrical synthetic-resin molded member  211  is provided on the internal sidewall of the electrically conductive capsule  71 . Stacked inside the electrically conductive capsule  71  are a front plate  71   a , an electrically conductive ring  208 , an electrically conductive diaphragm  207 , a spacer  206 , a fixed electrode  212  having sound apertures  212   a , a gate ring  209 , and a wiring substrate  202  having an IC device  210  and terminals  204   a - 204   c , in this order. 
       FIGS. 12A ,  13 A, and  14 A are external perspective view of digital electret condenser microphones viewed from their front-plate side.  FIG. 12A  shows a microphone with a front plate  41   a ,  51   a ,  71   a  having three small sound apertures  41   b ,  51   b ,  71   b .  FIG. 13A  shows a microphone with a front plate  61   a  having a large circular sound aperture  61   b .  FIG. 14A  shows a microphone with a front plate  61   a  having a large square sound aperture  61   b .  FIGS. 12B ,  13 B, and  14 B are external perspective view of the digital electret condenser microphones viewed from the opening side. The digital electret condenser microphones have only three terminals, a power supply terminal  204 , a clock input terminal  204   b , and a digital data output terminal  204   c , because their raised part  41   c ,  51   c ,  61   c ,  71   c  also functions as a ground terminal. In  FIG. 12B , the raised part  41   c ,  51   c ,  71   c  is raised near the caulked part  43 ,  53 ,  73 . The internal structure may be any of the structures shown in  FIGS. 8 ,  9 , and  11 . In  FIGS. 13B and 14B , the raised part  61   c  extends toward the terminals to narrow the opening  62 . The internal structure is as shown in  FIG. 10 . Microphones having the structures shown in  FIGS. 8 ,  9 , and  11  also can be modified to have any of the exterior appearances shown in  FIGS. 13A and 14A  by attaching a metallic mesh  213  on the front plate  41   a ,  51   a ,  71   a . While the front plate of the three microphones is generally square, it may be a circle as shown in  FIGS. 6 and 7 . 
     The height of the raised parts  41   c ,  51   c ,  61   c ,  71   c  is substantially the same as the height of the protruded portion of the terminals  204   a - 204   c . The purpose of this is to ensure that the terminals  204   a - 204   c  and the raised part  41   c ,  51   c ,  61   c ,  71   c  are evenly soldered when the microphone is soldered to a wiring board and that the microphone is firmly mounted on the wiring board without tilting with respect to the wiring board. 
     With this configuration, a gap of approximately 50 μm-100 μm is created between the raised part  41   c ,  51   c ,  61   c ,  71   c  and the wiring substrate  202 . The size of the gap depends on the size of the microphone in practice. Because of the gap, the raised part  41   c ,  51   c ,  61   c ,  71   c  functions as a member that absorbs vibration from an external vibration source. Accordingly, vibration transferred to the electret condenser microphone  40 ,  50 ,  60 ,  70  can be reduced. In addition, the gap can prevent heat conduction to the interior of the microphone even when the portion (raised part  41   c ,  51   c ,  61   c ,  71   c ) to be soldered is exposed to a high temperature, for example 260° C., in a reflow furnace. It should be noted that if the area of the raised part is reduced, heat transferred to the wiring substrate  202  can also be reduced because the area in contact with solder (heated area) is reduced. Furthermore, because the raised part  41   c ,  51   c ,  61   c ,  71   c  surrounds the terminals  204   a - 204   c , the high-frequency noise problem is eliminated. 
     In addition, the elasticity and heat conduction of the raised part can be controlled by adjusting the width of the raised part  41   c ,  51   c ,  61   c ,  71   c . However, if the width of the raised part  41   c ,  51   c ,  61   c ,  71   c  is too small, the raised part would no longer surround the terminals and the microphone would become susceptible to high-frequency noise. 
     As has been described, the provision of the raised part  41   c ,  51   c ,  61   c ,  71   c  allows for the effects of absorbing vibration and high-frequency noise, reducing the number of components, and preventing heat conduction. The width of the raised part  41   c ,  51   c ,  61   c ,  71   c  and the length of its extension toward the terminals should be chosen to be appropriate to the environment in which the microphone is used because the effects of absorbing vibration and high-frequency noise and preventing heat conduction can be in a trade-off relationship with one another.