Abstract:
A method of designing and manufacturing a piezoelectric component which includes a case, a piezoelectric resonator in the case, a terminal including an external connecting portion extending from an opening of the case, and a spring member to contact the piezoelectric resonator and the terminal in the thickness direction, includes the step of designing the spring member such that the piezoelectric resonator and the terminal are held by a holding pressure in range not larger than Fmax when a spring height is Smax, where a thickness range of the piezoelectric resonator, the terminal and the spring member is not smaller than the minimum value of Tmin and not larger than the maximum value of Tmax, a dimension, range of the case interior is not smaller than the minimum value of Wmin and not larger the maximum value of Wmax.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of designing a piezoelectric component used as a ladder filter, an oscillator, a discriminator, or other electronic component. More particularly, the present invention relates to a method of designing a piezoelectric component in which a piezoelectric element and a terminal are secured in position by a holding pressure applied in the thickness direction. 
     2. Related Art 
     Typically, in such a piezoelectric component, a piezoelectric resonator, a terminal, and a spring member are contained in a case. The piezoelectric resonator and the terminal are held by a holding pressure which is applied in the thickness direction and generated by the spring member. Further, the interior of the case is sealed by providing a lid including a resin permeation prevention sheet and a resin at the opening of the case. The piezoelectric resonator and the terminal are electrically connected to each other by the holding pressure applied by the spring member. 
     Such a piezoelectric component inevitably experiences variation in the thickness of the piezoelectric resonator, the terminal and the spring member and the variation of the interior-dimension of the case. 
     Therefore, if the variation of the dimensions of each of the piezoelectric resonator, the terminal and the spring member is small, a contact pressure (holding pressure) of the piezoelectric resonator and the terminal becomes small. As a result, poor connection occurs and the reliability of the shock resistance is greatly decreased. 
     On the other hand, if the variation of the dimensions of each of the piezoelectric resonator, the terminal and the spring member is large, a contact pressure (holding pressure) of the piezoelectric resonator and the terminal becomes large, and thereby the assembly of the component becomes very difficult. Further, there is a problem that an electric characteristic is deteriorated because of too much damping of the piezoelectric resonator. 
     To overcome the above described problems, conventionally, the contact pressure is stabilized by making uniform the thickness of the components contained in the case via lap polishing of the piezoelectric resonator or aligning the thickness of the terminal when forming thereof. 
     However, in the above described methods, the alignment process is very expensive and requires great effort. 
     SUMMARY OF THE INVENTION 
     To overcome the above described problems, preferred embodiments of the present invention provide a method of manufacturing a piezoelectric component which reliably and consistently achieves an appropriate contact pressure even if the variation of the thickness of the components constituting the piezoelectric component occurs. 
     According to a preferred embodiment of the present invention, a method of manufacturing a piezoelectric component includes the steps of providing a case having an opening, placing a piezoelectric resonator in the case, arranging a terminal including a external connecting portion to extend from the opening of the case toward outside of the case, and arranging a spring member in the case so as to maintain contact with the piezoelectric resonator and the terminal in the thickness direction, and designing the spring member such that the piezoelectric resonator and the terminal are held by a holding pressure in a range not larger than F max  when a spring height is S min  and not smaller than F min  when a spring height is S max , where a thickness range of the piezoelectric resonator, the terminal and the spring member is not smaller than the minimum value of T min  and not larger than the maximum value of T max , a dimension range of the case interior is not smaller than the minimum value of W min  and not larger than the maximum value of W max , a permissible holding pressure range of the piezoelectric resonator and the terminal is not smaller than the minimum value of F min  and not larger than the maximum value of F max , S min  is the spring height when the thickness of the piezoelectric resonator, the terminal and the spring member is the maximum value of T max  and the dimension range of the case interior is the minimum value of W min , and S max  is the spring height when the thickness of the piezoelectric resonator, the terminal and the spring member is the minimum value of T min  and the dimension range of the case interior is the maximum value of W max . 
     According to another preferred embodiment of the present invention, a method of designing a piezoelectric component which includes a substantially box-shaped case having an opening, a piezoelectric resonator contained in the case, a terminal including a external connecting portion extending from the opening of the case toward an outside of the case, and a spring member contained in the case and arranged to maintain contact with the piezoelectric resonator and the terminal in the thickness direction, the method including the step of designing the spring member such that the piezoelectric resonator and the terminal are held by a holding pressure in a range not larger than F max  when a spring height is S min  and not smaller than F min  when a spring height is S max , where a thickness range of the piezoelectric resonator, the terminal and the spring member is not smaller than the minimum value of T min  and not larger than the maximum value of T max , a dimension range of the case interior is not smaller than the minimum value of W min  and not larger than the maximum value of W max , a permissible holding pressure range of the piezoelectric resonator and the terminal is not smaller than the minimum value of F min  and not larger than the maximum value of F max , S min  is the spring height when the thickness of the piezoelectric resonator, the terminal and the spring member is the maximum value of T max  and the dimension range of the case interior is the minimum value of W min , and S max  is the spring height when the thickness of the piezoelectric resonator, the terminal and the spring member is the minimum value of T min  and the dimension range of the case interior is the maximum value of W max . 
     In preferred embodiments of the present invention, the spring may have linear spring characteristics or non-linear spring characteristics. 
     Also, it is preferred that the variation range of the dimension of the component and the interior of the case, in other words an admissible range of the spring-height S min -S max  should be wide. 
     In addition, the variation of the pressure applied by the spring member when the spring height is low is preferably small. 
     Further, the original spring height should be low in view of the assembly and the size of the product. 
     In preferred embodiments of the present invention, the spring can be made of material which is different from that of the terminal. Alternatively, the same material can be used for the spring member and the terminal. For material having a spring characteristic, elastic bodies such as rubber can be used instead of a plate-shape metal spring. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a sectional view of an example of a piezoelectric component relating to preferred embodiments of the present invention. 
     FIG. 2 is a sectional view taken along the line II—II of FIG.  1 . 
     FIG. 3 is a diagram showing a range of the spring height and an admissible pressure range in preferred embodiments of the present invention. 
     FIG. 4 is a characteristic view of the spring member which has an appropriate spring characteristic. 
     FIG. 5 is a view showing one example of a method of adjusting the spring characteristic. 
     FIG. 6 is a view showing another example of a method of adjusting the spring characteristic. 
     FIG. 7 is a view showing a spring characteristic when a spring has non-linear characteristics. 
     FIG. 8 is a view showing an ideal spring characteristic. 
     FIG. 9 is an exploded perspective view of an example which preferred embodiments of the present invention are applied to a ladder filter. 
     FIG. 10 is a sectional view of the ladder filter shown in FIG.  9 . 
     FIG. 11 shows a top view, a front view, right and left side views, a bottom view and a back view of the ladder filter shown in FIG.  9 . 
     FIG. 12 is a circuit diagram of the ladder filter shown in FIG.  9 . 
     FIG. 13 is a characteristic view of the spring member of the ladder filter shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The above described method will be explained hereinafter with reference to FIGS. 1 and 2 which show an example of the piezoelectric component relating to preferred embodiments of the present invention. 
     Here, the reference numeral  1  denotes a piezoelectric resonator which has electrodes on both front and back surfaces thereof. The reference numerals  2  and  3  denote terminals. The reference numerals  4 ,  5 ,  6  and  7  denote a spring member, a case, a resin permeation prevention sheet and a sealing resin, respectively. 
     The spring member  4  is press-fitted between terminal  3  and the inner-wall of the case  5 . The pair of electrodes of the piezoelectric resonator  1  and the terminals  2  and  3  are contacted and electrically connected to each other by a holding pressure applied by the spring member  4 . External connecting portions  2   a  and  3   a  are integral with the terminals  2  and  3  and arranged to extend outwardly from the case  5 . 
     Suppose that the interior-dimension of the case  5  is W, the total thickness of the components is T, and the spring height of the spring member  4  is S. Here, the spring height S is the height or dimension to which the spring member can deform in the thickness direction and which does not include the thickness of the spring member. 
     As shown in FIG. 3, an area indicated by slash lines in the x-y axes in which the horizontal axis indicates the holding pressure and the vertical axis indicates the spring height is set. This area is surrounded by the range of the spring height (S min -S max ) and the permissible range of the holding pressure (F min -F max ). In preferred embodiments of the present invention, the spring member is designed such that the piezoelectric resonator and the terminal are held by a holding pressure in a range not larger than F max  when a spring height is S min  and not smaller than F min  when a spring height is S max . For example, when Y=aX+b (a&lt;0, b&gt;0) in which Y is the spring height and X is a spring pressure, the values of a and b are determined such that the line indicating the equation Y=aX+b in the x-y coordinates passes through the upper and lower sides of the above described area, as shown in FIG.  4 . 
     Specifically, the values of a and b preferably satisfy the following two equations; 
     
       
           a ≦( S   max   −S   min )/( F   min   −F   max )  (1), and 
       
     
     
       
           S   max   −aF   min   ≦b≦S   min   −aF   max   (2). 
       
     
     By designing the spring member as described above, an appropriate contact pressure F can be obtained even if the variation in the thickness of the piezoelectric resonator, the terminal and the spring member and the variation of the interior-dimension of the case occur. Therefore, a piezoelectric component with high electric reliability is achieved with this structure and arrangement. That is, the problems such as the poor connection, deterioration of the shock resistance, difficulty with assembling and manufacturing, damping of the piezoelectric resonator, can be overcome with preferred embodiments of the present invention. 
     The spring constant of the spring member (=1/a) indicated as an oblique line shown in FIG. 5 can be adjusted by a material, a thickness, a shape and other characteristics of the spring. When changing the holding pressure without changing the spring constant, the oblique line can be adjusted by changing the original spring height (=b) as shown in FIG.  6 . In such a way, it is possible to obtain the appropriate spring characteristic by setting a suitable combination of the amounts of the spring constant (oblique) and the original spring heights. 
     In addition, in FIG.  5  and FIG. 6, the line “G” is an appropriate spring characteristic, and the line “NG” is an inappropriate spring characteristic. 
     Further, it is noted that the present invention is not restricted to the above-described linear spring characteristics. The spring member which has a non-linear spring characteristic may also be used. That is, if the spring characteristic has a substantially convex-shape as shown in FIG. 7, the original spring height can be made low which is advantageous for assembly. Moreover, the variation of the pressure applied by the spring member when the spring height is low is small. It is suitable for actual usage, and the interior-dimension of the case can be made small, and thereby, the size of the case can be greatly reduced. 
     On the other hand, if the spring member has a substantially concave-shape spring characteristic B as shown in FIG. 7, the original spring height becomes very high. In this case, the assembly becomes difficult, and the variation of the pressure by the spring member when spring height is low is large which is not suitable for actual usage. The component may be used when the spring height is high, but this causes a disadvantage that the case itself becomes large. 
     The outline of the preferred characteristics of the spring member which holds a piezoelectric resonator and a terminal is as follows: 
     Firstly, the variation range of the dimension of the component and the interior of the case, in other words a permissible range of the spring-height S min -S max  should be wide. 
     Secondly, the variation of the pressure by the spring member when the spring height is low is preferably small. 
     Thirdly, the original spring height should be low in view of the assembly and the size of the product. 
     In view of the above, the spring characteristic having a substantially convex shape is preferable. In order to make the size of the product as small as possible, when the thickness of the component is the maximum and the interior of the case is minimum, the spring height should be set to 0. That is, as shown in FIG. 8, it is ideal that S min =0. 
     In preferred embodiments of the present invention, it is not necessary to form the spring member of a material which is different from that of the terminal. The same material can be used for the spring member and the terminal. For material having a spring characteristic, elastic bodies such as rubber can be used instead of a plate-shape metal spring. 
     FIGS. 9-11 show a surface-mount type ladder filter which is an example of a piezoelectric component of preferred embodiments of the present invention. 
     This ladder filter preferably has a plurality of elements and constitutes a ladder circuit shown in FIG.  12 . The interior of a case  10  is divided into two chambers  10   b  and  10   c  by a partition-wall  10   a . In one chamber  10   b , series-resonators  11  and  12 , a parallel-resonator  14 , an input terminal  17 , one electrode-plate portion  18   a  of a connecting terminal  18 , one electrode-plate portion  19   a  of a connecting terminal  19  and a grounding terminal  20  are provided. In the other chamber  10   c , a series-resonator  13 , parallel-resonators  15  and  16 , an output terminal  21 , the other electrode-plate portion  18   b  of a connecting terminal  18 , the other electrode-plate portion  19   b  of a connecting terminal  19  and a grounding terminal  22  are provided. 
     Both of the series-resonators  11 - 13  and the parallel-resonators  14 - 16  are preferably piezoelectric resonators which are adapted to be vibrated in a square type vibration mode, and electrodes are provided on front and back surfaces thereof. 
     One side of the case  10  is opened via an opening. An external connecting portion  17   a  of the input terminal  17  and an external connecting portion  21   a  of the output terminal  21  are respectively extended out from the opening. A sheet  23  for resin penetration prevention is fitted to the opening of the case  10 , and a sealing resin  24  is filled thereon. In addition, slits  23   a  and  23   b  (see FIG. 9) for deriving the external connecting portions  17   a  and  21   a  of the input terminal  17  and the output terminal  21  are provided on the sheet  23 . Two terminal draw-through holes  10   d  and  10   e  are provided on the opposite side of opening of the case  10 . The external connecting portions  20   a  and  22   a  of the grounding terminals  20  and  22  are respectively drawn from these draw-through holes  10   d  and  10   e . The terminal draw-through holes  10   d  and  10   e  of case  10  are also filled with a sealing resin  24 . Accordingly, the full sealing of the interior of the case  10  is achieved. 
     After filling the resin  24 , external connecting portions  17   a ,  20   a ,  21   a  and  22   a  which are drawn out from the case  10  are bent downwardly along the side-wall of the case  10  and further bent inwardly along the bottom surface of the case  10 , as shown in FIG.  11 . 
     In the ladder filter of this preferred embodiment, the input terminal  17  and the grounding terminal  20  are preferably defined by a curved plate-shape spring and function as a spring member. The connecting terminals  18  and  19  have a structure which respectively integrally connects the two electrode-plate portions  18   a  and  18   b , and  19   a  and  19   b  in the shape of a crank via the narrow connectors  18   c  and  19   c . Further, the electrode-plate portions  18   b  and  19   b  are curved respectively to thereby allow the connecting terminals  18  and  19  to function as a spring member. Thus, since the spring member is not used as a discrete and independent element, the number of elements and parts can be reduced, and cost reduction and size-reduction are achieved. 
     Hereinafter, a method of designing the input terminal  17 , the grounding terminal  20  and the electrode-plate portions  18   b  and  19   b  of the connecting terminals  18  and  19  which are spring members in the above described ladder filter will be explained. 
     First, the thickness variation range of the piezoelectric resonators  11 - 16  and the terminals  17 - 22  is preferably set to about 2120+/−180 (1940-2300) micrometers, and the variation range of the interior size of the case  10  is preferably set to about 2400+/−20 (2380-2420) micrometers. In this case, the range of the spring height is about 80-480 micrometers since 2380−2300=80 micrometers and 2420−1940=480 micrometers. 
     In the above described ladder filter, since two spring members are disposed in each of the chambers  10   b  and  10   c , the permissible range of the spring height is about 40 micrometers to about 240 micrometers. 
     On the other hand, in view of damping of the piezoelectric resonator, shock resistance, assembly, and other factors, the range of the holding pressure (contact pressure) of the piezoelectric resonators  11 - 16  and the terminals  17 - 22  is preferably about 100 g to about 300 g. This range is known as a proper range of the holding pressure of the piezoelectric resonator from experience. 
     Therefore, a permissible range of the holding pressure is not larger than about 300 g when the spring height is about 40 micrometers and not smaller than about 100 g when the spring height is about 240 micrometers. 
     The above relationship will be expressed by the equation (1). Since S min =40 micrometers, S max =240 micrometers, F max =300 g and F min =100 g, 
     a≦(240−40)/(100-300) 
     i.e., a≦−1.0 (micrometers/g). 
     A spring made of an appropriate material and having an appropriate thickness and shape which satisfies the above described condition is preferred. For example, when the material is phosphor bronze, the shape is a curved plate shape, the value of “a” is determined as shown in Table 1 in accordance with the thickness 70, 100 and 150 micrometers. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Thickness 
                 70 micrometers 
                 100 micrometers 
                 150 micrometers 
               
               
                   
               
               
                 a 
                 −1.4 
                 −1.0 
                 −0.7 
               
               
                   
               
             
          
         
       
     
     As apparent from Table 1, if the thickness is not larger than about 100 micrometers, the above condition of “a” is satisfied. In this case, the spring having the thickness of 70 micrometers (a=−1.4) is selected. 
     Next, the value of “b” will be calculated. When substituting the above-mentioned values (S min =40 micrometer, S max =240 micrometer, F max =300 g, F min =100 g), the equation (2) becomes as follows: 
     
       
         240−(−1.4)×100≦b≦40−(−1.4)×300 
       
     
     i.e., 380≦b≦460 (micrometers) 
     Here, “b” is determined to be 450 micrometers. 
     As described above, the spring member of the above ladder filter is determined as follows: Material: Phosphor bronze, Thickness: 70 micrometer, Shape: curved plate shape, Original height: 450 micrometers. 
     FIG. 13 shows the spring characteristic of the spring member determined as above in the X-Y coordinates, in which the horizontal-axis indicates holding pressure and the vertical-axis indicates the spring height. 
     As it is apparent from FIG. 13, the spring characteristic satisfies the initial setting range which is determined as described above. In other words, the spring characteristic satisfies an appropriate range. 
     In this preferred embodiment, a curved shape plate spring is preferably used as a spring member. However, the present invention is not restricted to this type of spring, and plate springs having various other shapes such as a shape of a cross-dome, may also be used. 
     The shape of the case and the shape of the piezoelectric resonator are not restricted to those provided in preferred embodiments described above. Further, in the preferred embodiments, the terminal is preferably used as the spring member. However, it is possible to provide a spring member that is separate from the terminal. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the forgoing and other changes in form and details may be made therein without departing from the spirit of the invention.