Patent Publication Number: US-2022219968-A1

Title: Substrate and microphone unit

Description:
TECHNICAL FIELD 
     The present invention relates to a substrate protected against electrostatic discharge (ESD), and a microphone unit. 
     BACKGROUND ART 
     Patent literature 1 and 2 are examples of technologies of the related art regarding a method of implementing a micro-electro-mechanical systems (MEMS) microphone. In Patent literature 1 and 2, a substrate sound hole that communicates with a sound hole of a MEMS microphone is provided on a substrate on which a reverse sound hole type MEMS microphone is mounted. 
     PRIOR ART LITERATURE 
     Patent Literature 
     
         
         Patent literature 1: Japanese Patent Application Laid-Open No. 2015-029182 
         Patent literature 2: Japanese Patent Application Laid-Open No. 2007-060661 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     MEMS microphones are devices that are vulnerable to ESD by static electricity, and reverse sound hole type MEMS microphones in particular are structured such that the sound collector is exposed through the sound hole, and if the sound hole is blocked, the sound collecting function is no longer achieved. However, the substrates disclosed in Patent literature 1 and 2 are not provided with any countermeasures against static electricity. Note that in Patent literature 1 and 2, the area around the sound hole of the MEMS microphone and the substrate sound hole are connected by a ground land and a ceiling terminal, but this is for the purpose of keeping the sound hole of the MEMS microphone close to the substrate sound hole, and these components do not provide protection against static electricity. 
     In the related art, when using a reverse sound hole type MEMS microphone, it is necessary to provide a structure that does not receive static electricity directly. For example, it is necessary to use a sound conduit to provide the MEMS microphone at a position that does not receive static electricity directly or provide a mesh (garnish/grill) on top of the MEMS microphone, and such measures may impose constraints on design or lead to increased costs due to the additional parts. 
     Accordingly, an object of the present invention is to provide a substrate that is highly resistant to ESD, on which a reverse sound hole type MEMS microphone can be mounted. 
     Means to Solve the Problems 
     A substrate according to the present invention has one surface connected to a MEMS microphone, and comprises a substrate sound hole and a GND pad. The substrate sound hole penetrates through the substrate and communicates with a sound hole of the MEMS microphone. The GND pad is disposed around the substrate sound hole on another surface of the substrate. 
     Effects of the Invention 
     The substrate according to the present invention is highly resistant to ESD, and a reverse sound hole type MEMS microphone can be mounted thereon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a microphone unit according to a first embodiment; 
         FIG. 2  is a schematic view of a substrate according to the first embodiment; and 
         FIG. 3  is a schematic view of a substrate according to a first modification. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described in detail. Note that structural elements having the same function are denoted with the same signs, and duplicate description of such elements is omitted. 
     First Embodiment 
     Hereinafter, a configuration of a microphone unit according to the first embodiment will be described with reference to  FIG. 1 . As illustrated in the diagram, a microphone unit  10  according to the present embodiment comprises a reverse sound hole type MEMS microphone  1  and a printed substrate  2 . The MEMS microphone  1  includes a sound hole  11 . The MEMS microphone  1  is connected to one surface (referred to as the microphone connection surface  21 ) of the printed substrate  2 . 
     Hereinafter, the structure of the printed substrate  2  will be described with reference to  FIG. 2 . As illustrated in the diagram, the substrate  2  includes a substrate sound hole  23  and a GND pad  24 . The substrate sound hole  23  penetrates through the substrate  2  and communicates with the sound hole  11  of the MEMS microphone  1 . The GND pad  24  is provided around the substrate sound hole  23  on another surface (referred to as the ESD protection surface  22 ) of the substrate  2  so as to surround the substrate sound hole  23 . In the example illustrated in the diagram, the GND pad  24  has a ring (circular) shape that surrounds the substrate sound hole  23 , but the present invention is not limited thereto. For example, the GND pad  24  may also have a square frame shape provided so as to surround the substrate sound hole  23 . In addition, the GND pad  24  may have any other polygonal frame shape (such as a triangular, hexagonal, octagonal, . . . frame shape) provided so as to surround the substrate sound hole  23 . Note that excess solder may also be provided on the GND pad  24 . By providing excess solder, the GND volume is increased, thereby enhancing the effect of sending static electricity to GND. Note that the substrate sound hole  23  may also be configured as a through-hole or a via hole. By configuring the substrate sound hole  23  as a through-hole or a via hole, the GND surface area is increased, thereby enhancing the effect of sending static electricity to GND. 
     &lt;Advantageous Effects of the First Embodiment&gt; 
     According to the substrate  2  and the microphone unit  10  of the present embodiment, because static electricity is sent to the GND pad  24 , the diffusion of static electricity to the substrate GND can be prevented without needing to provide additional parts such as a sound conduit or a mesh (garnish/grill), and constraints on the design or increased costs due to additional parts do not occur. Also, if a structure that covers the space in the front part (anterior chamber) of the sound hole with a housing, mesh, or the like is provided, the problem of an unwanted dip in the upper range of the frequency characteristics of the microphone has also occurred in the past, but according to the substrate  2  and the microphone unit  10  of the present embodiment, it is not necessary to cover the space in the front part (anterior chamber) of the sound hole, and consequently, more stable microphone acoustic characteristics can be achieved. 
     [First Modification] 
     As illustrated in  FIG. 3 , one or multiple chip components  25  having the GND potential may also be provided on the ESD protection surface  22  side of the substrate  2 , near the substrate sound hole  23  and the GND pad  24 . By disposing the chip components  25  having the GND potential near the substrate sound hole  23  and the GND pad  24 , a lightning rod effect is obtained, and the ESD resistance can be raised. 
     The substrate disclosed in the foregoing embodiment and modification is not limited to a MEMS microphone, and is applicable in general to substrates on which electronic components vulnerable to static electricity are mounted.