Patent Publication Number: US-7223136-B2

Title: Condenser microphone and method for manufacturing substrate for the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a condenser microphone and a method for manufacturing a substrate used for the condenser microphone. 
   2. Description of the Related Art 
   A related art will be described below taking the case of what is called a back electret type electret condenser microphone (referred to as an ECM below).  FIG. 1  shows a sectional configuration of an ECM in accordance with the related technique, which is described in Japanese Patent Application Laid Open No. 2003-153392. In  FIG. 1 , the contour of the ECM is formed by a cylindrical capsule  61 . A sound wave passing opening  610  is formed in a front plate  61   a  of the capsule  61 . The following are incorporated into the capsule  61  and arranged in the following order from an inner surface of the front plate  61   a  toward the rear of the capsule  61 : a diaphragm  62 , an insulating spacer  63 , a rear pole  64 , a ring-like rear pole holder  65  consisting of an insulating material, a conductive cylinder  66 , and a circuit substrate  67 . In this case, the diaphragm  62  comprises a dielectric film which consists of for example, polyphenylene sulfide (also referred to as PPS) and in which a metal film such as Ni or Al is formed, as a conductive layer, on a surface of the film located closer to the rear pole. A diaphragm ring  62   a  is fixed to the periphery of a front surface of the diaphragm  62  and is in contact with the front plate  61   a . The rear pole  64  is placed behind the diaphragm  62  via the thickness of the insulating spacer  63  and supported by the ring-like rear pole holder  65 , consisting of an insulating material. A conductive cylinder  66  is interposed between the rear pole  64  and the circuit substrate  67  to electrically connect the rear pole  64  to wiring formed on a top surface (front surface) of the circuit substrate  67 . An electret layer  64   a  is formed on a front surface of the rear pole  64 , that is, the surface of the rear pole  64  located opposite the diaphragm  62 ; the electret layer  64   a  is obtained by converting a dielectric layer such as FEP (Fluorinated Ethylene Propylene) into an electret. A circuit device  68  such as an FET (Field Effect Transistor) is mounted on a top surface of the circuit substrate  67 . Solder bump electrodes  69   a  and  69   b  that are externally connected electrodes are projected from a bottom surface (rear surface) of the circuit substrate  67 . For example, such a circuit as shown in  FIG. 2  is formed on the circuit substrate  67 . In  FIG. 2 , a gate of the FET is connected to the rear pole  64  through the conductive cylinder  66 , shown in  FIG. 1 . A source of the FET is connected to the diaphragm  62  through the capsule  61 , shown in  FIG. 1 . Two capacitors C are connected to between a source and a drain of the FET in parallel with each other; the part between the source and drain of the FET operates as an impedance converting section. The drain of the FET is connected to an output terminal  72  (in  FIG. 1 , the solder bump electrode  69   b ) through a through-hole (not shown in the drawings) formed in the circuit substrate  67 . The drain of the FET then leads to a DC inhibiting capacitor Cp. The source of the FET is connected to a ground terminal  71  (in  FIG. 1 , the solder bump electrode  69   a ) through a through-hole (not shown in the drawings) formed in the circuit substrate  67 . Further, the drain of the FET is connected to a reference power source through a resistance element R. In  FIG. 1 , a rear (back face-side) end of the capsule  61  is caulked to the rear surface of the circuit substrate  67  as a caulking portion  611 . The caulking allows element parts housed in the capsule  61  to be fixed to one another. If a sound wave enters the capsule  61  through the sound wave passing opening  610 , it vibrates the diaphragm  62  to change the capacitance between the diaphragm  62  and the rear pole  64 . This converts the sound wave into an electric signal, which is output to the output terminal  72  (in  FIG. 1 , the solder bump electrode  69   b ). 
   To mount the above ECM on a mounting substrate (not shown in the drawings), the solder bump electrodes  69   a  and  69   b  are soldered to the corresponding electrodes on the mounting substrate. That is, the ECM placed on the entire mounting substrate is passed through a reflow bath and then heated. The heating melts the solder bump electrodes  69   a  and  69   b  to achieve soldering. In this case, as shown particularly in  FIG. 1 , the solder bump electrodes  69   a  and  69   b  are projected from the bottom surface of the circuit substrate  67 , with the caulking portion  611  present on the bottom surface of the circuit substrate  67  at an end of the capsule  61 . This configuration presents the problem described below. When the solder is heated and melted in the reflow bath, solder melting heat distorts the caulking portion  611 . This may relax the caulking or cause the molten solder and fluxes to advance between the caulking portion  611  and the circuit substrate  67 . This may make the electric connection between the rear pole  64  and the wiring on the circuit substrate  67  unstable; the conductive cylinder  66  is interposed between the rear pole  64  and the wiring. The electret layer  64   a  of the rear pole  64  may be degraded to reduce the voltage applied to between the diaphragm  62  and the rear pole  64 . Further, the sensitivity of the ECM may decrease. 
   With the reflow type ECM for which soldering is carried out using a reflow bath, the measure described below is taken to prevent solder or fluxes from advancing between the caulking portion  611  and the circuit substrate  67 . If the mounting substrate is directly soldered, solder paste is accumulated between the caulking portion  611  and the mounting substrate. The caulking portion  611  is thus separated from the mounting substrate before soldering. However, this measure is not reliable. 
   Another measure involves applying a second substrate to the bottom surface of the circuit substrate  67  to form such a step as projects beyond the thickness of the caulking portion  611 . A solder bump electrode is then projected from the substrate. Then, the solder is connected to the mounting substrate in the reflow bath. This amounts to the application of the substrate to the circuit substrate  67  resulting in the formation of a step. The application of the substrate to the circuit substrate  67  requires alignment at a predetermined accuracy and the formation of a through-hole for electric connection followed by an attachment operation. However, these operations preclude inexpensive circuit substrate from being obtained. Further, even if a circuit substrate is obtained by using a router to carry out machining to form a step, disadvantageously the resulting circuit substrate is not inexpensive. That is, structures with steps are expensive. 
   Moreover, conventional circuit substrates are mostly pattern wired substrates. Fabrication of a pattern wired substrate requires production of conductor electrodes, glass, multilayer wiring, through-holes, and the like using various materials and various printing processes. Consequently, the fabrication process is complicated and expensive. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a condenser microphone which minimizes the adverse effect of heating in a reflow bath to preclude relaxing of a caulking portion and thus the entry of solder and fluxes into the caulking portion, thus prevent electric instability and a decrease in the sensitivity, as well as a method for manufacturing a substrate for the condenser microphone. It is another object of the present invention to provide a condenser microphone which is made reliable by separating the caulking portion from a mounting substrate by using a step rather than accumulating solder paste, as well as a method for manufacturing a substrate for the condenser microphone. It is another object of the present invention to provide a condenser microphone which allows an inexpensive substrate to be obtained without using a router to carry out machining to form a step, as well as a method for manufacturing a substrate for the condenser microphone. It is another object of the present invention to provide a condenser microphone which allows a substrate to be obtained without using various materials or various printing processes, that is, without executing a complicated and expensive manufacturing process. 
   The present invention relates to a substrate comprising a planar periphery portion, a central step portion which is projected from the planar periphery portion toward a mounted surface side and which comprises an external terminal located in a part of the central step portion and a ground terminal located in another part of the central step portion and insulated from the external terminal, an apparatus connection terminal provided on a parts mounted surface side of the planar periphery portion and comprising a connection terminal connected to the external terminal of the central step portion and another connection terminal insulated from the above connection terminal and connected to a ground terminal, and a resin mold portion formed by exposing mounted surface sides of the external terminal of the central step portion and the ground terminal, exposing surfaces of the apparatus connection terminal and another external terminal which are located opposite their mounted surfaces, and then filling resin into the exposed portions. The substrate simply comprises the resin mold portion obtained by using resin to mold the planar periphery portion, central step portion, the apparatus connection portion, all of which consist of for example, a metal plate for a lead frame; these portions are used as a skeleton. Thus, the present substrate requires a simpler manufacturing process and is more inexpensive than the conventional pattern wired substrate. Further, the present substrate consists only of the metal and resin and thus contributes to environmental protection. Moreover, in the central step portion, the step portion is formed which projects from the bottom surface. Consequently, when the caulking portion is located on the bottom surface of the substrate, the step enables the caulking portion to float from the mounted surface. It is thus possible to hinder the caulking portion from being adversely affected by heat resulting from reflow and to prevent the flow-in of solder and fluxes, without accumulating solder paste, which is conventionally unreliable, stacking substrates, or performing an expensive step forming operation such as one using a router. Therefore, a microphone can be obtained the sensitivity of which is subject to few variations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a related ECM; 
       FIG. 2  is a circuit diagram of the ECM; 
       FIG. 3A  is a perspective view of an example of a structure in accordance with an embodiment of the present invention as viewed from obliquely above; 
       FIG. 3B  is a perspective view of an example of the structure in accordance with the embodiment of the present invention as viewed from obliquely below; 
       FIG. 4  is a plan view of an example of punching of a metal plate; 
       FIG. 5  is a plan view illustrating a planar structure and a three-dimensional structure in accordance with the embodiment of the present invention; 
       FIG. 6A  is a perspective view of an example of a substrate in accordance with the embodiment of the present invention as viewed from obliquely above; 
       FIG. 6B  is a perspective view of an example of the substrate in accordance with the embodiment of the present invention as viewed from obliquely below; 
       FIG. 6C  is a perspective view of a notch portion of the example of the substrate in accordance with the embodiment of the present invention; 
       FIG. 7  is a sectional view taken along line V—V in  FIG. 3A ; 
       FIG. 8  is a sectional view taken along line VI—VI in  FIG. 3A ; and 
       FIG. 9  is an exploded perspective view of an ECM. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
   An embodiment of the present invention will be described with reference to the drawings. Description will be given by taking the case of a back electret type electret condenser microphone (ECM). However, the present invention is applicable to what is called a front electret ECM. 
   In the present embodiment, a substrate is obtained by using resin to fix a structure constructed by connecting a planar periphery portion, a central step portion, and an apparatus connection terminal which are formed by punching and folding metal plate for a lead frame. 
     FIG. 3  shows a structure  1  obtained by punching and folding a metal plate for a lead frame which consists of brass or phosphor bronze pre-plated with tin or silver.  FIG. 3A  is a perspective view of the structure  1  as viewed from obliquely above.  FIG. 3B  is a perspective view of the structure  1  as viewed from obliquely below. As shown in  FIG. 3A , on a mounted surface side of the structure  1  which corresponds to its front surface, the structure  1  has a planar central step portion  1   a  projecting toward the mounted surface side and a planar periphery portion  1   b  located around the periphery of the central step portion  1   a  which corresponds to the root of the central step portion  1   a ; the planar periphery portion  1   b  is one step lower than the central step portion  1   a . The central step portion  1   a  has an external terminal  10  formed in its center like a disk and a ground (GND) terminal  11  located around the external terminal  10  and separated (insulated) from the external terminal  10 . The planar periphery portion  1   b  is a ground terminal integrated with the ground terminal  11  of the central step portion  1   a  via a step  13 . Further, as shown in  FIG. 3B , five connection terminals  12  ( 12   S ,  12   G ) are formed on a parts mounted surface side of the structure  1  which corresponds to its back surface. These connection terminals  12  are located substantially between the external terminal  10  and the ground terminal  11  and project from the planar periphery portion  1   b  toward the parts mounted surface side. A part  12   S  of the five connection terminals  12  is connected, inside the structure  1 , to the external terminal  10  via a bent portion  14 . The other part  12   G  of the connection terminals  12  is connected to the planar peripheral portion  1   b  via a bent portion  15 . In this manner, the structure  1  is configured so that the external terminal  10  and the ground terminal  11  project toward the mounted surface side and so that the connection terminal ( 12   S ,  12   G ) project toward the parts mounted surface side, with reference to the position of the planar periphery portion  1   b.    
   Further, as shown in  FIG. 3A , the step  13  is formed so that the center of the structure  1  rises from the planar periphery portion  1   b , thus constituting a central step portion  1   a . The planar periphery portion  1   b  constitutes a donut-shaped ground terminal. In this case, the step  13  is formed by punching and folding a metal plate for a lead frame. Consequently, the height of the step  13  can be freely increased or reduced. 
     FIG. 4  shows how a metal plate is cut using a mold to form a structure when a substrate is manufactured in accordance with the present embodiment. In this case, the structure  1  is arranged and formed along a longitudinal direction of a band-like metal plate  100  for a lead frame which consists of brass or phosphor bronze. That is, in a part of the metal plate  100  in which the metal plate  100  is to be formed, a square slot  101  is punched with the planar periphery portion  1   b , ground terminal  11 , external terminal  10 , connection terminals  12   S  and  12   G , and bent portions  14  and  15  left inside the square slot  101  in its central portion. The planar periphery portion  1   b  is connected to a lead frame  102  through thin connection pieces  103  extended in the opposite directions. The connection terminal  12   S  is connected to the lead frame  102  through thin connection pieces  104  extended in the opposite directions. Simultaneously with the formation of these connection pieces  104 , slits  1   b   1  (see  FIG. 3 ) are formed in the ground terminal  11  and planar periphery portion  1   b . After the punching, as is apparent from  FIGS. 7 and 8 , the central step portion  1   a , consisting of the external terminal  10  and ground terminal  11 , is projected from the planar periphery portion  1   b  connected to the ground terminal  11 , toward the mounted surface side. Moreover, the connection terminal  12   G  is projected, together with the bent portion  15 , from the planar periphery portion  1   b  toward the surface side of the structure lying opposite the mounted surface side. At the same time, the connection terminal  12   S  connected to the external terminal  10  is projected, via the bent portion  14 , toward the surface side of the structure lying opposite the mounted surface side. In this case, the connection pieces  104  are cut off the planar periphery portion  1   b  and projected toward the surface side of the structure lying opposite the mounted surface side, as in the case of the connection terminal  12   S . This operation can be performed by for example, molding with a force piston. A common forcing operation involves setting the band-like metal plate  100  flush with the planar periphery portion  1   b  and forcing the external terminal  10  and the ground terminal  11  in one direction, while forcing the connection terminals  12   S  and  12   G  in the other direction. This allows the external terminal  10 , ground terminal  11 , and planar periphery portion  1   b  to face the mounted surface side, while allowing the connection terminal  12   S  and the other connection terminal  12   G  to face the parts mounted surface side. 
     FIG. 5  shows the planar structure of the structure  1  formed by punching and folding the metal plate  100  for a lead frame as described above.  FIG. 5  also shows the three-dimensional positions of the terminals and the like except the bent portion  14  on the basis of the direction and width of hatching; the bent portion  14  is present across the thickness of the band-like metal plate. That is, the external terminal  10  and the ground terminal  11  are positioned at a three-dimensional position (I). Then, the planar periphery portion  1   b  is positioned at a three-dimensional position (II) separated by the step  13  from the position (I). Then, mainly the bent portion (connection portion)  15  between the planar periphery portion  1   b  and the connection terminal  12   G  and the connection piece  104  are positioned at a three-dimensional position (III). The connection terminals  12   S  and  12   G , connected to the external terminal  10  and planar periphery portion  1   b , are positioned at a three-dimensional position (IV) on an exposed surface on the parts mounted surface side. The bent portion  14  is positioned at an oblique three-dimensional position and thus does not correspond to any of the three-dimensional positions (I) to (IV). The dashed line in  FIG. 5  shows how an FET and a capacitor C are arranged and connected. 
   The structure  1  is formed by punching and bending the metal plate  100  for a lead frame (see  FIG. 4 ) as previously described. If fine-pitch machining is required to form a fine structure, the metal plate may be bent by etching or cut.  FIG. 6  shows a substrate on which a resin mold portion  2  has been formed by using a heat-resistant resin to mold the structure  1  shown in  FIG. 3 . In  FIG. 6 , as in the case of  FIG. 3 ,  FIG. 6A  is a perspective view as viewed from obliquely above, and  FIG. 6B  is a perspective view as viewed from obliquely below. In the molded substrate, the surface from which the external terminal  10 , the ground terminal  11 , and the ground terminal corresponding to the planar peripheral portion  1   b  project is exposed from the mounted surface side of the structure  1 , which corresponds to its front surface. The connection terminals  12  ( 12   S  and  12   G ) are exposed from the parts mounted surface side of the structure  1 , which corresponds to its back surface. In this case, the heat-resistant resin of the resin mold portion  2  offers heat resistance enough to resist heating in for example, a reflow bath. Specific examples of the heat-resistant resin include PA6T (polyamide 6T), PPS (polyphenylene sulfide), and LCP (liquid crystal polymer). The formation of the resin mold portion  2  is carried out with the structure  1  connected to the lead frame  102 , shown in  FIG. 4 . After the resin mold portion  2  is formed, the connection piece  103 , shown in  FIG. 4 , is cut at an outer peripheral position of the planar periphery portion  1   b . The connection piece  104  is cut at position lying slightly inside of the outer periphery of the planar periphery portion  1   b . The substrate  3  is thus taken out. A notch portion  2   a  is formed in a part of the outer periphery of the planar periphery portion  1   b  to constitute the resin mold portion  2  so that the connection piece  104  can be cut inside the structure as previously described. This is shown in a partly enlarged view in  FIG. 6C . The spacing between the connection piece  104 , facing the notch portion  2   a , and the capsule  61  should be minimized to the extent that the insulation between them does not present any problem. This more effectively shields noise. 
   Thus, the substrate  3  is obtained by molding resin to form a resin mold portion  2  using, as a skeleton, the structure  1  formed by pressing or etching. This eliminates a substrate to which a circuit pattern is applied, such as the one described in the description of the related art. This makes the manufacturing process simple and inexpensive. Further, the materials are only the metal and resin. Accordingly, the present invention contributes to environmental protection. 
   For example, the substrate  3  is used in place of the circuit substrate  67  in the ECM shown in  FIG. 1 .  FIGS. 7 and 8  show the relationship between the substrate  3  and the caulking portion  611  of the capsule  61  in that case.  FIG. 7  shows a cross section taken along line V—V in  FIG. 6A  and corresponding to the cross section taken along line V—V in  FIG. 3A .  FIG. 7  shows how the external terminal  10  is connected to the connection terminal  12   S .  FIG. 7  also shows that the caulking portion  611  of the capsule  61  is caulked to an outer side of the planar periphery portion  1   b  so as to connect the capsule  61  to the ground terminal  11 . Here, the step  13  has a dimension larger than the thickness of the caulking portion  611  of the capsule  61 . The connection piece  104  is partly formed to be shorter than the periphery of the planar periphery portion  1   b  so as not to contact the capsule  61  for insulation. A solder layer  10   a  and  11   a  are formed on the front surfaces of the external terminal  10  and ground terminal  11 , respectively. The solder layers  10   a  and  11   a  have a thickness of for example, 100 μm. Since the solder layers  10   a  and  11   a  are thus formed, the thickness of the step  13  has only to be equal to or larger than that of the caulking portion  611 . The excessively large thickness of the step  13  results in the large height of the mounting substrate on which an ECM has been mounted. To reduce the size of the substrate, the thickness of the step  13  should be minimized. When the caulking portion  611  is 0.15 mm in thickness, the step  13  should be about 0.15 to 0.2 mm in thickness. That is, the distance between the caulking portion  611  and the terminals  10  and  11  in the thickwise direction of the substrate should be about 0 to 0.05 mm. Further, solder bumps  69   a  and  69   b  may be used in place of the solder layers  10   a  and  11   a  as shown by dashed lines. 
     FIG. 8  shows the relationship between the caulking potion  611  of the capsule  61  and a cross section of the substrate corresponding to the cross sections taken along line VI—VI in  FIGS. 3A and 6A . The figure shows that the caulking portion  611  is connected to the planar periphery portion  1   b , to which the connection terminal  12   G  is connected. If the structure  1  is used as a skeleton for a circuit substrate for an ECM, the caulking portion  611  of capsule  61  of the ECM is located in the planar periphery portion  1   b  integrally connected to the ground terminal  11  as shown in  FIG. 3A . The caulking portion  611  is located on the mounted surface side of the structure  1  in  FIG. 3A , which corresponds to its front surface. Thus, the step  13  is higher than the thickness of an end of the capsule  61  which is caulked to the planar periphery portion  1   b , that is, the thickness of the caulking portion  611 . By projecting the solder layers from the external terminal  10  and ground terminal  11  toward the mounted surface side, it is possible to reduce the adverse effect of heating on the caulking portion  611  during reflow or to prevent the flow-in of solder and fluxes. The circuit substrate  67  of the ECM in  FIG. 1  and the structure  1  in  FIG. 3  have a positionally opposite relationship. The external terminal  10  of the central step portion  1   a  is located so as to project from the bottom of the ECM shown in  FIG. 1 , that is, the bottom surface of the caulking portion  611 . The planar periphery portion  1   b , ground terminal  11 , connection terminal  12   G , and the like constitute a first metal member  200  (see  FIG. 3B ) formed by folding one plate-like metal. The external terminal  10 , connection terminal  12   S , and the like constitute a second metal member  201  (see  FIG. 3B ) formed by folding other plate-like metal. In the figures, the relative dimensions may be inappropriately illustrated in order to show each portion in an easy-to-understand manner. For example, the substrate  3  has a thickness of about 0.2 mm. The metal plate constituting the planar periphery portion  1   b  and the like has a thickness of about 0.15 mm. 
     FIG. 9  is an exploded perspective view of an ECM formed using the substrate  3 , shown in  FIGS. 6 to 8 . The structure in  FIG. 9  is different from that in  FIG. 1 . Accordingly, even the same portions are denoted by different reference numerals in order to distinguish the structure in  FIG. 9  from the structure in  FIG. 1 . In this exploded perspective view, a rear pole  53  and a coil spring  52  can be housed in a holder  54 . The coil spring  52  is interposed between the substrate  3  and the rear pole  53 . One of those rings of the coil spring  52  which are closer to the substrate  3  is contacted with a gate terminal T G  of an FET mounted on the connection terminal  12   S  on the substrate  3 . Thus, when element parts are sealed in the capsule  57 , a spring force is exerted to ensure the contact between the gate terminal TG of the FET and the rear pole  53 . The coil spring  52  and the rear pole  53  are located in the holder  54 . A diaphragm  56  is placed on the holder  54  via a spacer  55 . That is, the substrate  3  to which the diaphragm  56 , spacer  55 , rear pole  53 , and coil spring  52  are connected is sequentially incorporated into the capsule  57 . An end of the capsule  57  is then caulked to and integrated with the rear surface of the substrate  3 . In this case, the diameter of the rear pole  53  is smaller than that of the holder  54 . However, the diameter of the rear pole  53  should be increased as long as the rear pole  53  can be easily inserted into the holder  54 . This is preferable for the positioning of the rear pole  53 . Further, although not shown in  FIG. 9 , the bottom surface of the substrate  3  has the step  13 , described above. Accordingly, in connection with the melting of solder on the bottom surface of the substrate  3  in a reflow bath, the formation of a step projecting from the caulking portion reduces the adverse effect heat on the caulking portion. Consequently, solder and fluxes are prevented from flowing in between the caulking portion and the bottom surface of the substrate  3 . If the coil spring  52  is used to press and contact the gate terminal TG with the rear pole  53  to connect them together, a stable spacing is established between the diaphragm and the rear pole without tilting the rear pole  53 . In  FIG. 9 , the FET and the two capacitors C are mounted on the substrate to provide such a circuit configuration as shown in  FIG. 2 . An output terminal  72  of this circuit is connected to the connection terminal  12   S , which leads to the external terminal  10 . The ground terminal  71  of circuit is connected to the connection terminal  12   G , which leads to the ground terminal  11 . The holder  54  electrically connects the coil spring  52  and rear pole  53  to the capsule  57 . 
   The above description is based on the ECM. However, since the height of the step portion can be freely set, the present invention is applicable to a front electret ECM or condenser microphone. 
   Further, a simple and inexpensive substrate can be easily obtained using only metal and resin as a material, simply by punching and folding a metal plate for a lead frame to form a structure and then using the resin to mold the structure. Therefore, a substrate can be obtained which is totally different from the conventional pattern wired substrate.