Abstract:
To provide a mounting substrate that requires a reduced amount of solder and reduces a thermal effect of solder on the interior of an electronic component, and a microphone to be mounted on the substrate. A mounting substrate according to the present invention includes: a solder part formed on a part of an electrode formed on the mounting substrate; a resist film formed to prevent the solder of the solder part from flowing out of a predetermined range; and a gas-escape groove that is constituted by the absence of the electrode and the resist film and allows gas produced during soldering to escape.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the invention 
   The present invention relates to a mounting substrate on which a component is mounted by soldering, and a microphone mounted thereon. 
   2. Description of the Related Art 
     FIG. 1  shows an exemplary configuration of a conventional microphone disclosed in Japanese Patent Application Laid Open No. 2003-153392 (Patent Reference 1). This microphone is an electret condenser microphone that comprises a capsule  1 , and a diaphragm ring  3 , a diaphragm  4 , a spacer  5 , a back electrode  6 , a holder  7 , a gate ring  8 , a circuit board  10  and an electric circuit  9  mounted on the circuit board  10 , which are stacked in the capsule  1  in this order from a sound aperture  2 . An edge of the capsule  1  is caulked onto the circuit board  10 , thereby fixing the incorporated components in the capsule  1 . Conductor parts (not shown in  FIG. 1 ) are formed on the outer surface of the circuit board  10 , and a solder bump electrode  11  to constitute an output terminal and a solder bump electrode  12  to constitute a grounding terminal are formed. 
     FIG. 2  shows exemplary electrodes of a mounting substrate on which the microphone shown in  FIG. 1  is to be mounted. As shown in  FIG. 2 , on a mounting substrate  13  on which the microphone is to be mounted, a circular solder part  14  corresponding to the solder bump electrode  11  of the microphone and a ring-shaped solder part  15  corresponding to the solder bump electrode  12  are formed. 
   When mounting the microphone on the mounting substrate  13 , the solder bump electrodes  11  and  12  of the microphone are placed on the solder parts  14  and  15  of the mounting substrate  13 , respectively. Then, the mounting substrate and the microphone are placed in a reflow oven to make the solder molten, thereby fusing the solder parts  14  and  15  with the solder bump electrodes  11  and  12 , respectively. 
   Here, the solder bump electrodes  11  and  12  of the microphone are formed in the procedure shown in  FIG. 3A ,  FIG. 3B , and  FIG. 3C . As shown in  FIG. 3A , a circular conductor part  10   a  corresponding to the output terminal and a ring-shaped conductor part  10   a  corresponding to the grounding terminal are formed on the outer surface of the circuit board  10  of the microphone. As shown in  FIG. 3B , a resist film  10   b  is formed on the conductor parts  10   a  excluding the areas on which the solder bump electrodes  11  and  12  are to be formed. As shown in  FIG. 3C , the solder bump electrodes  11  and  12  are formed on the areas in which no resist film  10   b  is formed in such a manner that the solder bump electrodes  11  and  12  protrude beyond a caulked part  1   a.    
   As described above, the reflow oven is used in the process of mounting the microphone on the mounting substrate and achieving soldering. According to the prior art, before soldering, the solder bump electrodes  11  and  12  protruding beyond the caulked part have to be formed on the microphone, and the solder parts  14  and  15  have to be formed on the mounting substrate  13 . In this case, the solder has to be applied over a wide area, and an increased amount of solder is required to form such raised solder bumps. Thus, there is a problem that a large amount of solder is required to achieve adequate soldering. 
   In addition, in the reflow oven, the solder bump electrodes  11  and  12  and the solder parts  14  and  15  are made molten, and the solder flows over a wide area. Thus, the resist film  10   b  also comes into contact with the molten solder. In this way, a wide area of the circuit board  10  comes into contact with the molten solder, so that the heat is likely to be conducted to the interior of the microphone through the circuit board  10 . The heat once conducted to the interior of the microphone causes a stress therein and adversely affects the same. For example, the sensitivity of the electret may be reduced. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a mounting substrate that requires a reduced amount of solder and reduces a thermal effect of solder on the interior of an electronic component (a microphone, for example), and a microphone to be mounted on the substrate. 
   A mounting substrate according to the present invention is a mounting substrate on which a component is mounted by soldering (in particular, a reflow process). 
   The mounting substrate according to the present invention comprises: a solder part formed on a part of an electrode formed on the mounting substrate; a resist film formed to prevent the solder of the solder part from flowing out of a range required for soldering; and a gas-escape groove that is constituted by the absence of the electrode and the resist film and allows gas produced during soldering to escape. In the case where a component (a microphone, for example) having an electrode (referred to as central terminal hereinafter) and another electrode (referred to as peripheral terminal hereinafter) formed around the central terminal is mounted, each part of the mounting substrate has characteristics described below. That is, the electrode formed on the mounting substrate includes a central electrode part that faces the central terminal, a plurality of outer electrode parts that face part of the peripheral terminal, and a linkage electrode part that interconnects the outer electrode parts. The solder part is formed on each of the central electrode part and the outer electrode parts. The gas-escape groove is configured to allow the gas inside the peripheral terminal to escape to the outside. 
   In addition, in a microphone according to the present invention, a central terminal and a peripheral terminal formed on an outer surface of a circuit board protrude beyond an outer housing (a capsule and a caulked part of the capsule) of the microphone, and the peripheral terminal is ring-shaped. 
   According to the present invention, only an area of the electrode formed on the mounting substrate in which the resist film is not formed is covered with solder, and the solder does not flow to the other areas. Therefore, the amount of solder can be reduced. In addition, since the area of the solder is limited, the thermal effect of the solder on the interior of the electronic component can be reduced. Furthermore, gas produced during soldering can escape to the outside without being confined in the gap between the electronic component and the mounting substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a conventional microphone; 
       FIG. 2  is a plan view of a conventional mounting substrate; 
       FIG. 3A  is a plan view of a circuit board on which conductor parts are formed in a process of forming a central terminal and a peripheral terminal; 
       FIG. 3B  is a plan view of the circuit board on which a resist film is formed in the process of forming the central terminal and the peripheral terminal; 
       FIG. 3C  is a plan view of the circuit board on which solder bump electrodes are formed in the process of forming the central terminal and the peripheral terminal; 
       FIG. 4A  is a perspective view showing the appearance of a microphone; 
       FIG. 4B  is a plan view of a circuit board of the microphone; 
       FIG. 4C  is a cross-sectional view of the microphone showing a relationship among a central terminal, a peripheral terminal and conductor parts on the circuit board; 
       FIG. 5A  is a plan view of a mounting substrate on which electrodes are formed; 
       FIG. 5B  is a cross-sectional view of the mounting substrate on which the electrodes are formed; 
       FIG. 6A  is a plan view of the mounting substrate on which a resist film is formed; 
       FIG. 6B  is a cross-sectional view of the mounting substrate on which the resist film is formed; 
       FIG. 7A  is a plan view of the mounting substrate on which solder parts are formed; 
       FIG. 7B  is a cross-sectional view of the mounting substrate on which the solder parts are formed; 
       FIG. 7C  is another cross-sectional view of the mounting substrate on which the solder parts are formed; 
       FIG. 7D  is a cross-sectional view of a mounting substrate having a through hole on which solder parts are formed; 
       FIG. 8  is a cross-sectional view showing an embodiment of the present invention; 
       FIG. 9  is another cross-sectional view showing the embodiment of the present invention; and 
       FIG. 10  is a plan view showing another example of the resist film formed on the mounting substrate. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following, an embodiment of the present invention will be described. The same parts are denoted by the same reference numerals, and redundant descriptions thereof will be omitted. 
   Embodiment 
     FIG. 4A  is a perspective view showing the appearance of a microphone,  FIG. 4B  is a plan view of a circuit board  10  of the microphone, and  FIG. 4C  is a cross-sectional view of the microphone showing a relationship among a central terminal  20 , a peripheral terminal  21  and conductor parts  10   a  on the circuit board  10 . The central terminal  20  has a circular shape and is formed on an outer surface of the circuit board  10 . The peripheral terminal  21  is ring-shaped and is formed around the central terminal  20 . Typically, the central terminal  20  constitutes an output terminal, and the peripheral terminal  21  constitutes a grounding terminal. In addition, the central terminal  20  and the peripheral terminal  21  protrude beyond an outer housing (a capsule  1  and a caulked part  1   a ) of the microphone. Specifically, the central terminal  20  and the peripheral terminal  21  are higher than the caulked part  1   a  at the edge of the capsule  1 . For example, if the caulked part  1   a  has a height of 0.1 mm, the central terminal  20  and the peripheral terminal  21  have a height of 0.3 mm. The cross-sectional view of  FIG. 4C  is taken along the line B-B in  FIG. 4B . The central terminal  20  and the peripheral terminal  21  are formed on the conductor parts  10   a  formed on the circuit board  10 . For example, the central terminal  20  and the peripheral terminal  21  may be composed of a copper foil covered with an antioxidant film of nickel or gold. Furthermore, the central terminal  20  and the peripheral terminal  21  can be formed simultaneously with the conductor parts  10   a  by changing the plating condition of the conductor parts  10   a . The conductor parts  10   a  may be formed by trimming a copper foil covering the circuit board  10 . 
   Now, a production method for and an arrangement of the mounting substrate according to the present invention will be described with reference to the drawings.  FIG. 5A  is a plan view of the mounting substrate  13  on which electrodes are formed.  FIG. 5B  is a cross-sectional view taken along the line C-C in  FIG. 5A . The electrodes are a central electrode part  24  intended to face the central terminal  20  of the microphone, a plurality of outer electrode parts  25  intended to face parts of the peripheral terminal  21  of the microphone, and a linkage electrode part  25   a  that links the outer electrode parts with each other. The central electrode part  24  is surrounded by the outer electrode parts  25  and the linkage electrode part  25   a . The central electrode part is in communication with the outside via a leading electrode part  24   a . Instead of the leading electrode part  24   a , a through hole may be formed in the mounting substrate  13  to bring the central electrode part into communication with the backside of the mounting substrate  13 . 
     FIG. 6A  is a plan view of the mounting substrate  13  on which a resist film  26  is formed.  FIG. 6B  is a cross-sectional view taken along the line C-C in  FIG. 6A . On the central electrode part  24  and the outer electrode parts  25 , there are formed areas in which the resist film is not formed (referred to as resist-film-free area  27 ). In addition, resist-film-free areas  27  are also formed between the outer electrode parts so that the resist-film-free areas  27  interconnect the areas on the inner and outer sides of the peripheral terminal  21  of the microphone. In this way, the resist-film-free areas  27  are formed at the positions of the electrodes on the mounting substrate  13  and at positions where no electrode is formed. The resist-film-free areas  27  formed at the positions of the electrodes are intended for soldering of the electrodes to the microphone (or other component to be mounted on the mounting substrate  13 ). The resist-film-free areas  27  formed at positions where no electrode is formed are intended for formation of grooves for the escape of gas produced during soldering. 
     FIG. 7A  is a plan view of the mounting substrate  13  on which solder parts  22  and  23  are formed.  FIG. 7B  is a cross-sectional view taken along the line C-C in  FIG. 7A .  FIG. 7C  is a cross-sectional view taken along the line D-D in  FIG. 7A . The solder parts  22  and  23  are formed on the central electrode parts  24  and the outer electrode parts  25 , which are not covered with the resists film  26 . As shown in  FIG. 7C , the resist-film-free areas  27  at positions where no electrode is formed constitute grooves that interconnect the areas on the inner side (that is, the side closer to the central terminal) and on the outer side of the peripheral terminal  21  of the microphone. The grooves are gas-escape grooves that allow gas produced on the inner side of the peripheral terminal  21  of the microphone (that is, the side closer to the central terminal) to escape during soldering.  FIG. 7D  is a cross-sectional view taken along the line C-C in  FIG. 7A , which shows an arrangement in which a through hole is formed to bring an outer electrode part  25  into communication with the backside of the mounting substrate  13 . 
     FIGS. 8 and 9  are cross-sectional views of the microphone mounted on the mounting substrate according to the present invention. The cross-sectional view of  FIG. 8  is taken along the line C-C in  FIG. 7A  (which shows the mounting substrate) and the line A-A in  FIG. 4B  (which shows the microphone), and the cross-sectional view of  FIG. 9  is taken along the line E-E in  FIG. 7A  (which shows the mounting substrate) and the line B-B in FIG.  4 B (which shows the microphone). The circuit board  10  is housed in the capsule  1 , and an edge of the capsule  1  is caulked to secure the circuit board  10  together with other incorporated components. Even if the solder is made molten in a reflow oven, the solder of the solder parts  23  on the outer electrode parts  25  is confined by the resist film  26  and thus does not flow to the outside of the outer electrode parts  25 . In addition, the solder of the solder part  22  on the central electrode part  24  is also confined by the resist film  26  and thus does not flow to the outside of the central electrode part  24 . Since the flow of the solder is limited in this way, heat conduction to the microphone can be reduced. 
   In the example shown in  FIG. 7A , most area of the mounting substrate  13  is covered with the resist film. However, the resist film  26  can be formed arbitrarily as far as the resist film  26  prevents the solder from flowing to the outside of the central electrode part  24  and the outer electrode parts  25 . For example, a resist-film-free area  29  may be formed that constitutes an integral gas-escape groove as shown in  FIG. 10 . Alternatively, a resist-film-free area may be formed on the outer side of the outer electrode parts  25 . Furthermore, the resist film  26  may be formed only along the outer peripheries of the central electrode part  24  and the outer electrode parts  25  and on the linkage electrode part  25   a  and the leading electrode part  24   a.    
   In the embodiment described above, the component to be mounted on the mounting substrate  13  is a microphone. However, the mounting substrate according to the present invention can be used for any other components with the same advantages.