Patent Publication Number: US-2019178828-A1

Title: Gas sensor

Description:
TECHNICAL FIELD 
     The present invention relates to a gas sensor that detects the concentration of a particular gas. 
     BACKGROUND ART 
     One known metal oxide sensor has a structure in which a sensor element is fixedly suspended within an opening of a wiring board by, for example, wire bonding (Patent Document 1). 
     By fixing the sensor element in a suspended manner, the thermal capacity of the sensor element and the thermal influence from the surroundings can be reduced, and the accuracy of gas detection and the responsiveness of the sensor element can thereby be improved. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2007-298508 (FIG. 1) 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the sensor described in Patent Document 1, a plurality of (five) conductive pads electrically connected to the wire bonding through lead traces are arranged at one end of the generally square wiring board, and an external circuit is connected to the conductive pads. 
     To connect the external circuit to the conductive pads arranged on the above-described large (wide) wiring board, a pin connector, for example, is conventionally used. A problem with this method is that, since the connection structure is large, the sensor cannot be reduced in size. Moreover, since the conductor pads are arranged in a row in a wide area, a pin connector or the like to which the wiring board is to be connected may come into uneven contact with the conductor pads, and this may cause unstable electrical connection. 
     Accordingly, it is an object of the present invention to provide a gas sensor in which a structure for connection between a ceramic wiring board including a sensor element fixed thereto and an external circuit is rendered compact and miniaturization and reliable electrical connection are realized. 
     Means for Solving the Problems 
     In order to solve the above-described problem, the present invention provides a gas sensor comprising: a plate-shaped ceramic wiring board extending in a longitudinal direction and having an opening penetrating a distal end portion of the ceramic wiring board in a thickness direction thereof; and a sensor element having a detection section for detecting the concentration of a particular gas and a heater section for heating the detection section, the sensor element being fixedly suspended within the opening and electrically connected to the ceramic wiring board by a plurality of electricity conducting members, wherein the distal end portion of the ceramic wiring board in which the opening is formed is accommodated in a casing through which the gas can flow, and a proximal end portion of the ceramic wiring board extends to the outside of the casing; some of a plurality of electrically conductive pads electrically connected to the plurality of electricity conducting members through lead traces are disposed on a front surface of the proximal end portion of the ceramic wiring board and the remaining electrically conductive pads are disposed on a back surface of the proximal end portion of the ceramic wiring board; a width of the distal end portion of the ceramic wiring board having the opening as measured in a direction orthogonal to the longitudinal direction and the thickness direction is larger than a width of a narrow region of the proximal end portion of the ceramic wiring board, the narrow region containing the electrically conductive pads and portions of the lead traces connecting with the electrically conductive pads; and the gas sensor further comprises a tubular insulator which circumferentially surrounds the narrow region of the ceramic wiring board and a plurality of spring terminals which are accommodated in the insulator and elastically come into contact with the electrically conductive pads. 
     In this gas sensor, the width of the narrow region of the proximal end portion of the ceramic wiring board is smaller than the width of the distal end portion, and the electrically conductive pads are disposed on the front and back surfaces of the proximal end portion. 
     In this case, the width of the region (narrow region) of the ceramic wiring board in which the electrically conductive pads are disposed can be smaller as compared with the case where all the electrically conductive pads are disposed on one side of the ceramic wiring board. As a result, the structure for connection with an external circuit can be made compact and the gas sensor can be miniaturized by electrically connecting the plurality of spring terminals included in the tubular insulator to the electrically conductive pads such that the narrow region is circumferentially surrounded by the tubular insulator. 
     Also, unlike the case where all the electrically conductive pads are disposed on one side of the ceramic wiring board, the pressing force of the spring terminals is applied to both sides of the ceramic wiring board, and therefore the occurrence of breakage of the ceramic wiring board can prevented. 
     In the gas sensor of the present invention, each of the plurality of spring terminals may have a main body portion extending in the longitudinal direction, a turnback portion extending from a distal end of the main body portion toward a proximal end thereof, and a contact point-forming portion connected to the turnback portion and elastically coming into contact with the corresponding electrically conductive pad. 
     In this gas sensor, the turnback portions are elastically pressed against the electrically conductive pads and come into contact with the electrically conductive pads at the contact point-forming portions connected to the turnback portions. This allows the spring terminals and the electrically conductive pads to be electrically connected to each other in a reliable manner. 
     In the gas sensor of the present invention, the plurality of spring terminals may be connected to a plurality of lead wires extending from a connector for external circuit connection. 
     In this gas sensor, the ceramic wiring board can be electrically connected to the external circuit through the connector for external circuit connection in an easy and reliable manner. 
     Effects of the Invention 
     According to the present invention, there can be obtained a gas sensor in which a structure for connection between a ceramic wiring board including a sensor element fixed thereto and an external circuit is rendered compact and miniaturization and reliable electrical connection are realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [ FIG. 1 ] Exploded perspective view of a gas sensor according to an embodiment of the present invention. 
       [ FIG. 2 ] Top view of the gas sensor according to the embodiment of the present invention. 
       [ FIG. 3 ] Bottom view of the gas sensor according to the embodiment of the present invention. 
       [ FIG. 4 ] Perspective view of a ceramic wiring board accommodated in a casing. 
       [ FIG. 5 ] Top view showing a manner of connecting the ceramic wiring board to a cassette connector. 
       [ FIG. 6 ] Cross-sectional view taken along line A-A in  FIG. 1 . 
       [ FIG. 7 ] Side view of a spring terminal. 
       [ FIG. 8 ] Top view showing a modification of the ceramic wiring board. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     The present invention will next be described in detail with reference to the drawings.  FIG. 1  is an exploded perspective view of a gas sensor  1  of an embodiment of the present invention, and  FIGS. 2 and 3  are top and bottom views, respectively, of the gas sensor  1 .  FIG. 4  is a perspective view showing a ceramic wiring board  50  accommodated in a casing  10 , and  FIG. 5  is a top view showing a manner of connecting the ceramic wiring board  50  to a cassette connector  70 .  FIG. 6  is a cross-sectional view taken along line A-A in  FIG. 1 , and  FIG. 7  is a side view of a spring terminal  79 . 
     In  FIG. 1 , the gas sensor  1  includes: a lower case  10  having a generally rectangular box shape with an opening on its upper surface (an upward surface in  FIG. 1 ); a lid  20  that covers the opening of the lower case  10 ; the ceramic wiring board  50  accommodated in the lower case  10 ; rectangular frame-shaped seal members (gaskets)  31  and  32 ; a sensor element  40  disposed within an opening  50   h  of the ceramic wiring board  50 ; electricity conducting members  61  and  63  that fixedly suspend the sensor element  40  within the opening  50   h ; and the cassette connector  70 . 
     The lower case  10  and the lid  20  correspond to the “casing” in the claims. 
     The lower case  10  has an inner space  10   r , a rectangular notch  10   n  extending downward from the upper edge of the lower case  10 , and pipe-shaped introduction and discharge tubes  10   a  and  10   b  used as connection ports for pipes. The introduction tube  10   a  and the discharge tube  10   b  protrude rightward from one side surface (the right side-surface in  FIG. 1 ) of the lower case  10 , and the bores of the tubes  10   a  and  10   b  are in communication with the inner space  10   r , so that gas G can flow into and from the lower case  10 . 
     The ceramic wiring board  50  includes a rectangular frame-shaped distal end portion  50   a  and a proximal end portion  50   e  that has a narrower width than the distal end portion  50   a  and extends outward (leftward in  FIG. 1 ) from one edge of the distal end portion  50   a.    
     Each of the seal members  31  and  32  and the distal end portion  50   a  of the ceramic wiring board  50  is sized so as to be exactly fittable into the inner space lOr of the lower case  10 . The seal member  31 , the distal end portion  50   a , and the seal member  32  stacked in this order from the lower case  10  side are accommodated in the inner space  10   r . The proximal end portion  50   e  of the ceramic wiring board  50  protrudes from the notch  10   n  to the outside of the lower case  10 . 
     Then the lid  20  is placed on the seal member  32  and fastened to the lower case  10  with bolts  25 . The seal members  31  and  32  are thereby pressed between the lower case  10  and the lid  20 , and the gap between the lower case  10  and the ceramic wiring board  50  is hermetically sealed. 
     As a result, the gas G introduced from the introduction tube  10   a  into the inner space  10   r  comes into contact with the sensor element  40 , and the concentration of a particular gas is detected. Then the gas is discharged to the outside through the discharge tube  10   b.    
     Notably, the sensor element  40  has a generally rectangular plate shape. A heater section  42  is disposed on the upper surface side (the upward side in  FIG. 1 ) of a base  41 , and a detection section  43  is disposed on the lower surface side of the base  41 . In this structure, the detection section  43  and the heater section  42  are stacked on the upper and lower sides, respectively, of the base  41  and integrated together. 
     The electrical properties of the detection section  43  change in proportion to the concentration of the particular gas component, and the concentration of the particular gas component is detected by detecting an electric signal corresponding to the changing electrical properties. When the heater section  42  is energized, the heater section  42  generates heat, thereby heating the detection section  43  to its operating temperature. Output terminals of the detection section  43  and energization terminals of the heater section  42  are fixed and electrically connected to the ceramic wiring board  50  through the electricity conducting members  61  and  63  in a suspended manner. 
     The base  41  may be, for example, a ceramic substrate. The detection section  43  may be formed using, for example, an oxide semiconductor. The heater section  42  may be, for example, a circuit formed on a surface of the base  41  and serving as a heat-generating resistor. 
     A plurality of electrically conductive pads  50   p  electrically connected to the detection section  43  and the heater section  42  through the electricity conducting members  61  and  63  and lead traces  50 L are disposed on the front and back surfaces of the proximal end portion  50   e  of the ceramic wiring board  50 . The electric signal outputted from the detection section  43  is outputted to the outside through the electrically conductive pads  50   p  of the ceramic wiring board  50 , and the heater section  42  is energized by electric power supplied from the outside through electrically conductive pads  50   p  and thereby generates heat. 
     As shown in  FIG. 2 , the sensor element  40  is formed into a rectangular shape in plan view, and two electricity conducting members  61   a  and  61   b  extending across a first side  40   a  (the left side in  FIG. 2 ) of the sensor element  40  are joined to the ceramic wiring board  50  and to opposite side edges of the sensor element  40 . Similarly, other two electricity conducting members  61   c  and  61   d  extending across a second side  40   b  (the right side in  FIG. 2 ) opposed to the first side  40   a  are joined to the ceramic wiring board  50  and to the opposite side edges of the sensor element  40 . 
     A plurality of (four in  FIG. 2 ) element peripheral pads  50   s  are formed on the front surface of the ceramic wiring board  50  so as to surround the opening  50   h . These element peripheral pads  50   s  are connected to the respective electrically conductive pads  50   p  (three pads in  FIG. 2 ) through lead traces  50 L. 
     Energizing pads  42   p  are formed at opposite ends of the heat-generating resistor forming the heater section  42 , and two element peripheral pads  50   s  are disposed so as to be opposed to the respective energizing pads  42   p . The electricity conducting members  61   a  and  61   d  establish connection between the energizing pads  42   p  and the respective element peripheral pads  50   s  opposed to the energizing pads  42   p.    
     A temperature sensor  44  is disposed along the heater section  42 , and energizing pads  44   p  are formed at opposite ends of the temperature sensor  44 . Two element peripheral pads  50   s  are disposed so as to be opposed to the respective energizing pads  44   p , and the electricity conducting members  61   b  and  61   c  establish connection between the energizing pads  44   p  and the respective element peripheral pads  50   s  opposed to the energizing pads  44   p.    
     Notably, a U-shaped conductive member  55  is connected to the lower-right element peripheral pad  50   s  in  FIG. 2 . This conductive member  55  is connected to an element peripheral pad  50   s  on the opposite surface of the ceramic wiring board  50  (see  FIG. 3 ). 
     As shown in  FIG. 3 , on the opposite surface of the sensor element  40  as well, a plurality of (three in  FIG. 3 ) element peripheral pads  50   s  are formed on the back surface of the sensor element  40  so as to surround the opening  50   h . These element peripheral pads  50   s  are connected to the respective electrically conductive pads  50   p  (three pads in  FIG. 3 ) through lead traces  50 L. 
     Two electricity conducting members  63   a  and  63   b  are joined to two energizing pads  43   p  connected to the detection section  43  of the sensor element  40  and are joined to the element peripheral pads  50   s  of the ceramic wiring board  50  opposed to these energizing pads  43   p.    
     The electricity conducting members  61  and the conductive member  55  may be formed of, for example, Pt, and the electricity conducting members  63  may be formed of, for example, Au. They can be electrically connected to the respective energizing pads and element peripheral pads by, for example, welding. The energizing pads can be formed by applying, for example, a Pt paste to the base  41  of the sensor element  40  by printing and then firing the printed Pt paste. The element peripheral pads  50   s , the lead traces  50 L, and the electrically conductive pads  50   p  can be formed by printing, for example, a Au paste and then firing the printed Au paste. 
     Referring next to  FIGS. 4 to 7 , a manner of connecting the ceramic wiring board  50  to the cassette connector  70  will be described. 
     As shown in  FIG. 4 , the distal end portion  50   a  of the ceramic wiring board  50  is accommodated in the casing (case)  10 , and the proximal end portion  50   e  extends outward from the casing (case)  10 . The width W 1  of the distal end portion  50   a  having the opening  50   h  formed therein, as measured in a direction orthogonal to the longitudinal direction L and the thickness direction T, is larger than the width W 2  of a narrow region  50 R of the proximal end portion  50   e  that includes the electrically conductive pads  50   p  and portions of the lead traces  50 L connected to the electrically conductive pads  50   p.    
     The “portions of the lead traces  50 L” are portions of the lead traces  50 L on the electrically conductive pad  50   p  side and include at least portions connected to the electrically conductive pads  50   p.    
     As shown in  FIG. 5 , part of the narrow region  50 R (on its proximal end portion  50   e  side) is inserted into a thorough hole  71   h  of a first separator  71  of the cassette connector  70 , and the ceramic wiring board  50  is thereby electrically connected to the cassette connector  70  as described later. 
     The first separator  71  corresponds to the “insulator” in the claims. 
     As shown in  FIG. 6 , the cassette connector  70  includes: the tubular ceramic-made first separator  71  having the thorough hole  71   h  at its center; a tubular ceramic-made second separator  72 ; a rubber-made grommet  74 ; lead wires  76 ; and a connector  78  for external circuit connection that is a female connector connected to a male connector of an external circuit (not shown). Notably, the maximum outer diameter of the first separator  71  is smaller than the width of the casing including the lower case  10  and the lid  20  (see  FIG. 5 ) . 
     The second separator  72  and the grommet  74  are coaxially connected in this order to the proximal end side of the first separator  71 . A plurality of (six in this example) prescribed insertion holes are formed in each of the first separator  71  and the second separator  72 , and spring terminals  79  are accommodated and held in the insertion holes. 
     The spring terminals  79  are in elastic contact at contact points P with the electrically conductive pads  50   p  of the proximal end portion  50   e  inserted into the thorough hole  71   h  and are electrically connected to the electrically conductive pads  50   p . By means of crimping, exposed distal ends of the core conductors of the lead wires  76  are fixed to crimp terminal portions  79 c of the spring terminals  79 , which portions are located on the proximal end side, and the lead wires  76  are led outward through the thorough holes of the grommet  74 , and the rear ends of the lead wires are connected to the female connector  78 . 
     Notably, the proximal end portion  50   e  is held between the spring terminals  79  and thereby prevented from coming off the thorough hole  71   h . The second separator  72  is held between the proximal end portion  50   e  and the grommet  74 . 
     As shown in  FIG. 7 , each of the spring terminals  79  includes: a main body portion  79   a  extending in a longitudinal direction L; a turnback portion  79   b  extending from a distal end of the main body portion  79   a  toward a proximal end side (the upper side in  FIG. 7 ); a contact point-forming portion  79   p  connected to the turnback portion  79   b  and to be in elastic contact with one of the electrically conductive pads  50   p ; and the above-described crimp terminal portion  79   c.    
     The spring terminals  79  can be produced, for example, by pressing a refractory metal (Inconel) plate. 
     As described above, the width W 2  of the narrow region  50 R of the proximal end portion  50   e  of the ceramic wiring board  50 —which region includes the electrically conductive pads  50   p  and portions of the lead traces  50 L connected to the electrically conductive pads  50   p —is smaller than the width W 1  of the distal end portion  50   a . Further, some of the electrically conductive pads  50   p  are disposed on the front surface of the proximal end portion  50   e , and the remaining electrically conductive pads  50   p  are disposed on the back surface of the proximal end portion  50   e.    
     Therefore, the width of the portion of the ceramic wiring board  50  in which the electrically conductive pads  50   p  are disposed (the narrow region  50 R of the proximal end portion  50   e ) can be narrower as compared with the case where all the electrically conductive pads  50   p  are disposed on one side of the ceramic wiring board  50 . 
     As a result, the structure for connection with an external circuit can be made compact and the gas sensor  1  can be miniaturized by electrically connecting the plurality of spring terminals  79  included in the tubular insulator  71  to the electrically conductive pads  50   p  such that the narrow region  50 R is circumferentially surrounded by the tubular insulator  71 . 
     Also, unlike the case where all the electrically conductive pads  50   p  are disposed on one side of the ceramic wiring board  50 , the pressing force of the spring terminals  79  is applied to opposite sides of the ceramic wiring board  50 , and therefore breakage of the ceramic wiring board  50  can be prevented. 
     In the above embodiment, each of the spring terminals  79  includes the turnback portion  79   b  extending from the main body portion  79   a  and the contact point-forming portion  79   p . In this case, the elastic force of each spring terminal  79  presses its turnback portion  79   b  toward a corresponding electrically conductive pad  50   p  of the proximal end portion  50   e , and the contact point-forming portion  79   p  connected to the turnback portion  79   b  comes into contact with the electrically conductive pad  50   p , so that the spring terminal  79  and the electrically conductive pad  50   p  can be electrically connected to each other in a reliable manner. 
     In the above embodiment, the spring terminals  79  are connected to the respective lead wires  76  extending from the connector  78  for external circuit connection, so that the external circuit and the ceramic wiring board  50  can be electrically connected to each other through the connector  78  for external circuit connection in an easy and reliable manner. 
     It will be appreciated that the present invention is not limited to the embodiment described above and encompasses various modifications and equivalents within the spirit and scope of the present invention. 
     For example, as shown in  FIG. 8 , a ceramic wiring board  150  having an intermediate portion  150   c  may be employed. A distal end portion  150   a  of the ceramic wiring board  150  having an opening  150   h  for fixedly suspending the sensor element  40  is accommodated in a case (casing)  110 . The intermediate portion  150   c  is provided between the distal end portion  150   a  and a narrow region  150 R. The intermediate portion  150   c  extends outward through a notch  110   n  of the casing  110  while maintaining the same width W 1  as the distal end portion  150   a , is tapered toward the narrow region  150 R such that its width decreases to W 2 , and is integrally connected to the narrow region  150 R. 
     In this case, the narrow region  150 R which extends from the intermediate portion  150   c , has the reduced width W 2 , and includes the electrically conductive pads and portions of the lead traces connected to the electrically conductive pads is partially accommodated in the thorough hole  71   h  of the insulator  71 . Also in this embodiment, the structure for connection with the external circuit can be made compact, and the gas sensor can be reduced in size. 
     The shapes of the ceramic wiring board and the casing, the shapes and numbers of the electrically conductive pads and the spring terminals, and the shapes, etc. of the sensor element and the insulator are not limited to those in the above embodiments. The structures, types, etc. of the heater section and the detection section are not limited to those in the above embodiments. A circuit board including a circuit that processes the electric signal outputted from the detection section  43  and a circuit that energizes and controls the heater section  42  may be accommodated in the connector  78  for external circuit connection. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1  gas sensor 
       10 ,  20 ,  110  casing 
       40  sensor element 
       42  heater section 
       43  detection section 
       50 ,  150  ceramic wiring board 
       50   a ,  150   a  distal end portion 
       50   e ,  150   e  proximal end portion 
       50   h ,  150   h  opening 
       50 L lead trace 
       50   p  electrically conductive pad 
       50 R,  150 R narrow region 
       61 ,  63  electricity conducting member 
       71  insulator 
       76  lead wire 
       78  connector for external circuit connection 
       79  spring terminal 
       79   a  main body portion 
       79   b  turnback portion 
       79   p  contact point-forming portion 
     G gas 
     L longitudinal direction 
     T thickness direction of ceramic wiring board 
     W 1  width of distal end portion 
     W 2  width of narrow region