Patent Publication Number: US-6906597-B2

Title: Non-reciprocal circuit device and resin casing used therefor

Description:
FIELD OF THE INVENTION 
     The present invention relates to a non-reciprocal circuit device having non-reciprocal transmission characteristics to high-frequency signals and a resin casing used therein, particularly to a non-reciprocal circuit device used in mobile communications systems such as cellular phones, etc., which are generally called an isolator or a circulator, and a resin casing therefor. 
     BACKGROUND OF THE INVENTION 
     There is a non-reciprocal circuit device such as an isolator, a circulator, etc. as one of transmitting and receiving circuit parts for cellular phones, automobile phones, etc. used in a microwave band and a UHF band. Generally, the isolator and the circulator have small insertion loss in a signal transmission direction and large loss in an opposite direction, so that they are used to prevent the breakage of amplifiers. 
       FIG. 19  shows one example of the conventional isolators. This isolator is constituted by upper and lower metal casings  1 ,  12  functioning as magnetic yokes, a permanent magnet  2 , an assembly  20 , flat capacitors  8 ,  9 ,  10 , a dummy resistor  11 , and a resin casing  7 . 
     The assembly  20  is constituted by a thin conductor plate having such a shape that three central conductors  4 ,  5 ,  6  are radially projecting from a circular shield plate, and a garnet-type ferrite (ferrimagnetic body)  3  disposed on a circular portion of the thin conductor plate, with the three central conductors  4 ,  5 ,  6  folded onto the garnet-type ferrite  3  and integrally overlapping via insulating layers therebetween. 
     The resin casing  7  has a circular recess  13   a  for the assembly  20  at center, and recesses  13   b ,  13   c ,  13   d  for flat capacitors around the circular recess  13   a . The bottom of each recess  13   a ,  13   b ,  13   c ,  13   d  is provided with a connecting electrode  14   a . The connecting electrode  14   a  is integrally formed by a thin conductor plate  14  of about 0.1 mm in thickness, and exposed from the sidewalls of the resin casing  7  as external terminals  15   a - 15   f . The external terminals  15   a - 15   c  and the external terminals  15   d - 15   f  are exposed symmetrically on opposing side surfaces. The resin casing  7  is provided with terminal electrodes  16   a ,  16   b  connected to the central conductors  4 ,  5 , and the terminal electrodes  16   a ,  16   b  are connected to the external terminals  15   a ,  15   d  on the side surfaces. The external terminal  15   a  and the terminal electrode  16   a  are formed by one integral thin conductor plate separate from the connecting electrode  14   a , and the external terminal  15   d  and the terminal electrode  16   b  are formed by the other integral thin conductor plate separate from the connecting electrode  14   a.    
     Each flat capacitor  8 ,  9 ,  10  is received in each recess  13   b ,  13   c ,  13   d  of the resin casing  7 . Each flat capacitor  8 ,  9 ,  10  is constituted by electrodes formed on upper and lower surfaces of a flat dielectric substrate, and the lower electrode of each flat capacitor is connected by soldering to the connecting electrode  14   a  appearing in each bottom of the recesses  13   b ,  13   c ,  13   d . Received in the recess  13   d  are the flat capacitor  10  and a dummy resistor  11 , whose one electrode is connected by soldering to the connecting electrode  14   a , and whose other electrode is connected to the central conductor  6 . 
     The assembly  20  is disposed in the recess  13   a  of the resin casing  7 . The circular shield plate of the central conductors  4 ,  5 ,  6  is connected to the connecting electrode  14   a  by soldering. With this structure the central conductors are grounded. One end of the central conductor  4  is connected to the upper electrode of the flat capacitor  8  and a terminal electrode  16   a , and one end of the central conductor  5  is connected to the upper electrode of the flat capacitor  9  and a terminal electrode  16   b.    
     The resin casing  7  is disposed on the lower casing  12 . The lower casing  12  has a shape complemental to that of a recess  21  in the bottom of the resin casing  7 . The connecting electrode  14   a  exposed from the recess  21  is connected to the lower casing  12  by soldering, so that the resin casing  7  is made integral with the lower casing  12 . The permanent magnet  2  for applying a DC magnetic field to the garnet-type ferrite  3  is fixed to an inner wall of the upper casing  1 . With the upper casing  1  mounted to the lower casing  12 , a surface-mountable isolator is obtained. Incidentally, ends of the central conductors can be connected to the capacitors and the terminal electrodes, for instance, by soldering or spot welding (for instance, see JP 10-135711 A). Also, a circulator can be obtained when the same terminal electrode as that for the other central conductor is used in place of the dummy resistor  11 . 
     With demand for miniaturization, higher performance and lower prices increasingly mounting not only for isolators but also for mobile communications equipment, present objectives are miniaturization by the level of several hundreds microns, increase in performance by the level of one-tenth of dB, and further reduction of cost. Though widely used at present are 5-mm-square isolators, there is demand for further miniaturization. Thus, the inventors have been developing isolators with a target of providing 4-mm-square isolators (nearly 40% reduction of mounting area). 
     When an isolator comprising flat capacitors is further miniaturized, the flat capacitors  8 ,  9 ,  10  and/or the garnet-type ferrite  3  should be reduced in size. However, because the capacitance C of the flat capacitor is represented by C=ε r ·ε 0 ·S/d, wherein ε r  is a dielectric constant of a dielectric body, ε 0  is a dielectric constant of vacuum, S is the area of an electrode, and d is the thickness of a dielectric body, the reduction of the flat capacitor in a planar size makes it necessary to use a dielectric body having a large dielectric constant ε r  or make the dielectric body thinner, to obtain the same capacitance. However, a dielectric body having a large dielectric constant ε r  generally tends to have a large dielectric loss. Thus, the larger the dielectric loss, the larger the insertion loss of the resultant isolator. In addition, the reduction of a dielectric body in thickness results in decrease in the mechanical strength of a flat capacitor, making it likely that breakage, cracking, etc. occur at the time of assembling a non-reciprocal circuit device. 
     The miniaturization of a garnet-type ferrite is accompanied by the problem that a frequency band, in which the desired insertion loss is obtained, is narrowed. Further, the reduction of a garnet-type ferrite in diameter results in decrease in inductance obtained by the central conductors and the garnet-type ferrite. Therefore, the capacitance of flat capacitors should be increased to obtain a necessary operation frequency, resulting in increase in the size of the flat capacitors. In the conventional non-reciprocal circuit device, the reduction of a garnet-type ferrite is at most 2.2 mm in diameter, and the use of a smaller garnet-type ferrite than this size results not only in the deterioration of electric characteristics but also in the necessity of using larger flat capacitors. Accordingly, in the conventional structure in which flat capacitors are arranged around an assembly, it has been difficult to provide a small non-reciprocal circuit device with practically acceptable characteristics. 
     In the conventional isolator, flat capacitors  8 ,  9 ,  10  are arranged around the assembly  20  in a U-shaped pattern. The flat capacitor  10  connected in parallel to the dummy resistor  11  is disposed perpendicularly to a row of external terminals  15   a - 15   f  mounted onto the resin casing  7 . Accordingly, when a bending force is applied to the non-reciprocal circuit device by the flexure of the circuit board, onto which the non-reciprocal circuit device is mounted, etc., the resin casing  7  is deformed with the external terminals  15   a - 15   f  as a fulcrum. As a result, the flat capacitor  10  connected in parallel to the dummy resistor is broken. 
     As another example, JP 10-303607 A discloses an isolator having a structure, in which erect capacitors are arranged in a resin casing such that the electrode surfaces of capacitors are substantially parallel to the center axis of a garnet-type ferrite, a garnet-type ferrite being disposed in a space encircled by the capacitors. However, such an isolator comprises a resin casing with a complicated structure, and it is thus difficult to handle the capacitors during assembling. 
     OBJECT OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a resin casing that can serve to miniaturize a non-reciprocal circuit device and prevent the breakage of flat capacitors by a force applied from outside to the non-reciprocal circuit device, and a small and highly reliable non-reciprocal circuit device comprising such a resin casing. 
     DISCLOSURE OF THE INVENTION 
     The first resin casing of the present invention used in a non-reciprocal circuit device comprising an assembly of a ferrimagnetic body and a plurality of central conductors enclosing the ferrimagnetic body, and a plurality of elongated flat capacitors connected to ends of the central conductors comprises a thin conductor plate having a substantially flat bottom portion, and a resin frame injection-molded integrally with the thin conductor plate; the resin frame having sidewalls and partition walls inside the sidewalls for defining a plurality of recesses for receiving the flat capacitors; a pair of opposing sidewalls being provided with external terminals constituted by the thin conductor plate; any bottoms of a plurality of the recesses being integrally formed by a substantially flat bottom portion of the thin conductor plate; the recesses being formed such that they receive all of the flat capacitors substantially in parallel with sidewalls having the external terminals; and the assembly being disposed on the flat capacitors received in the recesses. 
     The second resin casing of the present invention used in a non-reciprocal circuit device comprising an assembly of a ferrimagnetic body and a plurality of central conductors enclosing the ferrimagnetic body, and a plurality of elongated flat capacitors connected to ends of the central conductors comprises a conductor plate constituting a magnetic yoke for the non-reciprocal circuit device and having a substantially flat bottom surface, and a resin frame injection-molded integrally with the conductor plate; the resin frame having sidewalls and partition walls inside the sidewalls for defining a plurality of recesses for receiving the flat capacitors; a pair of opposing sidewalls being provided with external terminals constituted by the conductor plate; any bottoms of a plurality of the recesses being integrally formed by a substantially flat bottom portion of the conductor plate; the recesses being formed such that they receive all of the flat capacitors substantially in parallel with sidewalls having the external terminals; and the assembly being disposed on the flat capacitors received in the recesses. 
     In a preferred embodiment of the present invention, the resin casing comprises three recesses for receiving three elongated flat capacitors. The electrode surfaces of the flat capacitors connected to ends of the central conductors preferably have substantially the same height. 
     It is preferable that the partition walls are not higher than the electrode surfaces of the flat capacitors connected to ends of the central conductors, and that the sidewalls are higher than the electrode surfaces of the flat capacitors. This prevents the partition walls from interfering with the assembly disposed on the flat capacitors. In addition, because the resin casing has sidewalls higher than the electrode surfaces of the flat capacitors connected to ends of the central conductors, the rigidity of the resin casing can be kept. 
     It is preferable that the resin frame has second partition walls along the short side surfaces of the recesses adjacent to the sidewalls having the external terminals, and that the second partition walls are higher than the partition walls between the recesses and equal to or lower than the sidewalls. 
     The first non-reciprocal circuit device of the present invention comprises a resin casing, to which an assembly of a ferrimagnetic body and a plurality of central conductors enclosing the ferrimagnetic body, a permanent magnet for applying a DC magnetic field to the ferrimagnetic body, and a plurality of elongated flat capacitors connected to ends of the central conductors are assembled; the resin casing comprising a thin conductor plate having a substantially flat bottom portion, and a resin frame injection-molded integrally with the thin conductor plate; the resin frame having sidewalls and partition walls inside the sidewalls for defining a plurality of recesses for receiving the flat capacitors; a pair of opposing sidewalls being provided with external terminals constituted by the thin conductor plate; any bottoms of a plurality of the recesses being integrally formed by a substantially flat bottom portion of the thin conductor plate; the recesses being formed such that they receive all of the flat capacitors substantially in parallel with sidewalls having the external terminals; and the assembly being disposed on the flat capacitors received in the recesses. 
     The second non-reciprocal circuit device of the present invention comprises a resin casing, to which an assembly of a ferrimagnetic body and a plurality of central conductors enclosing the ferrimagnetic body, a permanent magnet for applying a DC magnetic field to the ferrimagnetic body, and a plurality of elongated flat capacitors connected to ends of the central conductors are assembled; the resin casing comprising a conductor plate constituting a magnetic yoke for the non-reciprocal circuit device and having a substantially flat bottom surface, and a resin frame injection-molded integrally with the conductor plate; the resin frame having side walls and partition walls inside the sidewalls for defining a plurality of recesses for receiving the flat capacitors; a pair of opposing sidewalls being provided with external terminals constituted by the thin conductor plate; any bottoms of a plurality of the recesses being integrally formed by a substantially flat bottom portion of the thin conductor plate; the recesses being formed such that they receive all of the flat capacitors substantially in parallel with sidewalls having the external terminals; and the assembly being disposed on the flat capacitors received in the recesses. 
     In both non-reciprocal circuit devices, the other ends of the central conductors are preferably at substantially the same potential but not grounded. 
     When the non-reciprocal circuit device comprises a resistor connected in parallel to one of the flat capacitors, it is operated as an isolator. This resistor is preferably a chip resistor from the aspect of withstand-voltage. This resistor is preferably disposed in a recess enclosed by the sidewalls and a second partition wall formed along a short side surface of a recess adjacent to a sidewall having the external terminals. 
     In a preferred embodiment of the present invention, the non-reciprocal circuit device comprises three flat capacitors, one flat capacitor connected in parallel to the resistor being disposed between other two flat capacitors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG.  1 ( a ) is a plan view showing a resin casing for a non-reciprocal circuit according to one embodiment of the present invention; 
       FIG.  1 ( b ) is a side view showing a sidewall provided with external terminals, among the sidewalls of the resin casing of FIG.  1 ( a ); 
       FIG.  1 ( c ) is a bottom view showing the resin casing of FIG.  1 ( a ); 
       FIG.  2 ( a ) is a cross-sectional view taken along the line A-A′ in FIG.  1 ( a ); 
       FIG.  2 ( b ) is a cross-sectional view taken along the line B-B′ in FIG.  1 ( a ); 
       FIG.  2 ( c ) is a cross-sectional view taken along the line C-C′ in FIG.  1 ( a ); 
       FIG.  3 ( a ) is a cross-sectional view taken along the line D-D′ in FIG.  1 ( a ); 
       FIG.  3 ( b ) is a cross-sectional view taken along the line E-E′ in FIG.  1 ( a ); 
         FIG. 4  is a perspective view showing a thin conductor plate used in the resin casing of FIG.  1 ( a ); 
       FIG.  5 ( a ) in a partial plan view showing a leadframe for forming the thin conductor plate of  FIG. 4 ; 
       FIG.  5 ( b ) is a plan view showing the leadframe of FIG.  5 ( a ), whose portions corresponding to electrodes are bent; 
       FIG.  6 ( a ) is a plan view showing the leadframe of FIG.  5 ( a ), with which a resin frame is injection-molded; 
       FIG.  6 ( b ) is a plan view showing the injection-molded product of FIG.  6 ( a ), whose external terminals are bent; 
         FIG. 7  is an exploded perspective view showing one example of the non-reciprocal circuit devices of the present invention; 
         FIG. 8  is a plan view showing the resin casing of FIG.  1 ( a ), in which flat capacitors are disposed; 
       FIG.  9 ( a ) is a plan view showing a resin casing for a non-reciprocal circuit device according to one embodiment of the present invention on the mounting surface side; 
       FIG.  9 ( b ) is a cross-sectional view taken along the line F-F′ in FIG.  9 ( a ); 
         FIG. 10  is a view showing one example of the equivalent circuit of the non-reciprocal circuit device of the present invention; 
       FIG.  11 ( a ) is a graph showing the relation between insertion loss and frequency, and between return loss and frequency in the non-reciprocal circuit device of the first embodiment; 
       FIG.  11 ( b ) is a graph showing the relation between isolation and frequency, and between return loss and frequency in the non-reciprocal circuit device of the first embodiment; 
       FIG.  12 ( a ) is a plan view showing a resin casing for a non-reciprocal circuit device according to another embodiment of the present invention; 
       FIG.  12 ( b ) is a side view showing a sidewall provided with external terminals, among the sidewalls of the resin casing of FIG.  12 ( a ); 
       FIG.  12 ( c ) is a bottom view showing the resin casing of FIG.  12 ( a ); 
       FIG.  13 ( a ) is a cross-sectional view taken along the line G-G′ in FIG.  12 ( a ); 
       FIG.  13 ( b ) is a cross-sectional view taken along the line H-H′ in FIG.  12 ( a ); 
       FIG.  13 ( c ) is a cross-sectional view taken along the line I-I′ in FIG.  12 ( a ); 
       FIG.  14 ( a ) is a cross-sectional view taken along the line J-J′ in FIG.  12 ( a ); 
       FIG.  14 ( b ) is a cross-sectional view taken along the line K-K′ in FIG.  12 ( a ); 
       FIG.  15 ( a ) is a plan view showing a leadframe for forming a metal plate used in the resin casing of FIG.  12 ( a ); 
       FIG.  15 ( b ) is a plan view showing the leadframe of FIG.  15 ( a ), to which bending operation has been carried out; 
         FIG. 16  is a plan view showing the leadframe of FIG.  15 ( b ), to which a resin frame has been injection-molded; 
         FIG. 17  is a partial cross-sectional view showing a portion of an external terminal, which is to be cut; 
         FIG. 18  is an exploded perspective view showing a non-reciprocal circuit device according to a further embodiment of the present invention; and 
         FIG. 19  is an exploded perspective view showing a conventional non-reciprocal circuit device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The non-reciprocal circuit device of the present invention will be explained in detail referring to the attached drawings, taking an isolator for example, though the present invention is of course not restricted to the isolator. 
     [1] First Embodiment 
     A resin casing for a non-reciprocal circuit device according to the first embodiment of the present invention and a non-reciprocal circuit device comprising it will be explained in detail below referring to  FIGS. 1-9 . 
     The resin casing  7  comprises a thin conductor plate  14  comprising a connecting electrode  14   a  and external terminals  15   a - 15   f , and a resin frame  18  molded integrally therewith. The overall structure of the thin conductor plate  14  is shown in FIG.  4 . The resin casing  7  is provided with recesses  13   b ,  13   c ,  13   d  for receiving flat capacitors  8 ,  9 ,  10 , and a recess  23  in which a dummy resistor  11  connected in parallel to the flat capacitor  10  is disposed. The resin casing  7  may be integrally formed by injection-molding high-heat-resistance thermoplastic engineering plastics such as liquid crystal polymers, polyphenylene sulfide, etc. in a die in which a thin conductor plate  14 , for instance, an about 0.1-mm-thick copper plate, is placed. 
     A thin copper plate  14  may be coated with a high-conductivity metal layer made of at least one of silver, gold, copper and aluminum or its alloy, which has electric resistivity of 5.5 μΩ cm or less, preferably 3.0 μΩ cm or less, more preferably 1.8 μΩ cm or less, to increase the transmission efficiency of microwave signals, and to reduce loss by suppressing interference with the outside. The thickness of the high-conductivity metal layer is 0.5-25 μm, preferably 0.5-10 μm, more preferably 1-8 μm. 
     A flat thin conductor plate  14  is exposed as a common connecting electrode  14   a  from the bottoms of three recesses  13   b ,  13   c ,  13   d  for receiving flat capacitors  8 ,  9 ,  10 . The flat capacitors  8 ,  9 ,  10  are disposed in the same plane in the recesses  13   b ,  13   c ,  13   d , with the lower electrodes of the flat capacitors  8 ,  9 ,  10  connected by soldering to the connecting electrode  14   a  exposed from the bottoms of the recesses  13   b ,  13   c ,  13   d . A dummy resistor  11  is disposed in a recess  23 , and one electrode of the dummy resistor  11  is connected by soldering to the connecting electrode  14   a.    
     The assembly  20  per se may be the same as a conventional one. For instance, the assembly  20  comprises a thin conductor plate having such a shape that three central conductors  4 ,  5 ,  6  are radially projecting from a circular shield plate, and a garnet-type ferrite  3  disposed on a circular portion of the thin conductor plate, with three central conductors  4 ,  5 ,  6  folded to integrally overlap the garnet-type ferrite  3 . The assembly  20  is disposed on the flat capacitors  8 ,  9 ,  10  via an insulating member  22 , for instance, an insulating sheet made of a polyimide resin, etc. One end of the central conductor  4  is connected to an upper electrode of the flat capacitor  8  and a terminal electrode  16   a , one end of the central conductor  5  is connected to an upper electrode of the flat capacitor  9  and a terminal electrode  16   b , and one end of the central conductor  6  is connected to an upper electrode of the flat capacitor  10  and a dummy resistor  11 . 
     In the resin casing  7  of the present invention, the electrode surfaces of the flat capacitors  8 ,  9 ,  10  have substantially the same height, and partition walls  50  between the recesses  13   b ,  13   c ,  13   d  for receiving the flat capacitors  8 ,  9 ,  10  are not higher than the electrode surfaces of the flat capacitors  8 ,  9 ,  10 . Accordingly, the assembly  20  can be disposed on the flat capacitors  8 ,  9 ,  10  stably without inclination. Also, because the three flat capacitors  8 ,  9 ,  10  are connected to the same electrode  14 , the electrode surfaces of the flat capacitors  8 ,  9 ,  10  having an equal thickness have substantially the same height. Further, the flat capacitors  8 ,  9  disposed on both sides of the flat capacitor  10  may have substantially the same thickness, and the flat capacitor  10  may be thinner than the flat capacitors  8 ,  9 , to achieve the stable arrangement of the assembly  20 . 
     The sidewalls  51  of the resin casing  7  are higher than the electrode surfaces of the flat capacitors  8 ,  9 ,  10  connected to ends of the central conductors  4 ,  5 ,  6 , and a pair of opposing sidewalls are provided with external terminals  15   a - 15   f  constituted by the thin copper plate. The external terminals  15   b ,  15   c ,  15   e ,  15   f  are connected to the connecting electrode  14   a . The resin casing  7  comprises partition walls  52  opposing the shorter sidewalls (end surfaces) of the flat capacitors  8 ,  9 ,  10 , in addition to the sidewalls  51  and the partition walls  50  between the flat capacitors  8 ,  9 ,  10 . The partition walls  52  are higher than the partition walls  50  between the recesses  13   b ,  13   c ,  13   d  but equal to or lower than the sidewalls  51 . The dummy resistor  11  connected in parallel to the flat capacitor  10  is received in a recess  23  defined by the sidewalls  51  and one partition wall  52 . The sidewalls  51  integrally have portions  53  opposing the partition walls  52  via the flat capacitors  8 ,  9 , which project toward the assembly  20 , and the assembly  20  is positioned in the resin casing  7  by the inwardly projecting portions  53  and the partition walls  52 . 
     In the resin casing  7 , each flat capacitor  8 ,  9 ,  10  is disposed in each recess  13   b ,  13   c ,  13   d  with the longitudinal direction of each flat capacitor  8 ,  9 ,  10  being substantially the same as that of each recess  13   b ,  13   c ,  13   d . Each recess  13   b ,  13   c ,  13   d  is formed such that the longer sidewalls of all flat capacitors  8 ,  9 ,  10  are substantially in parallel with opposing sidewalls  51  of the resin casing  7  provided with external terminals  15   a - 15   f  constituted by the thin conductor plate  14 . The resin casing  7  has a recess on the bottom side, from which the connecting electrode  14   a  is exposed. The lower casing  12  of the non-reciprocal circuit device is fitted in the recess  21  and integrally soldered to the connecting electrode  14   a.    
     When a force is applied from outside to the resin casing  7  having a such shape in a thickness direction, flexure may occur with the external terminals  15   a - 15   f  as a fulcrum. Even so, the influence of flexure is extremely small on the flat capacitor  10  connected in parallel to the dummy resistor  11 , whereby the flat capacitor  10  is not likely to be destroyed. 
     As shown in  FIGS. 5 and 6 , the thin conductor plate  14  may be formed from a leadframe  100  made of a conductive material such as copper, etc. The leadframe  100  has sprocket holes  101  in both edge portions (hoop portion) at a constant interval, and in each center portion encircled by four sprocket holes  101  a thin conductor plate portion  114  corresponding to the thin conductor plate  14  is integrally connected to a plurality of supports  102 ,  103  extending from the hoop portion. Each support  102 ,  103  is provided with a cutting line  105  having a V-shaped notch. 
     In the production of the thin conductor plate  14  from the leadframe  100  shown in FIG.  5 ( a ), the leadframe  100  is first subjected to necessary bending along lines  110  as shown in FIG.  5 ( b ). Next, as shown in FIG.  6 ( a ), a thermoplastic resin frame  18  is formed integrally with the thin conductor plate portion  114  by an injection-molding method using a high-heat-resistance thermoplastic engineering plastic such as a liquid crystal polymer, polyphenylene sulfide, etc. In this case, injection molding may be carried out while the thin conductor plate portion  114  is still connected to the leadframe  100 , or after it is cut from the leadframe  100 . At the time of injection molding, a plurality of pins abut a rear surface of the thin conductor plate portion  114  to fix it. Accordingly, the resin casing  7  is provided with a plurality of holes  45  on a rear surface. After injection molding, as shown in FIG.  6 ( b ), the external terminals  15   a - 15   f  are bent, and finally the support  102  is cut at the cutting line  105 . 
     The overall structure of the non-reciprocal circuit device of the present invention is shown in FIG.  7 . As shown in  FIG. 8 , each flat capacitor  8 ,  9 ,  10  is received in each recess  13   b ,  13   c ,  13   d  of the resin casing  7 . In the case of this embodiment, because the partition walls  50  are set to be as high as the flat capacitors  8 ,  9 ,  10  received in the recesses  13   b ,  13   c ,  13   d , a flat surface is formed by the flat capacitors  8 ,  9 ,  10  and the partition walls  50 . Incidentally, as long as there is no problem in fixing the flat capacitors  8 ,  9 ,  10 , the upper surfaces of the partition walls  50  may be lower than the upper surfaces of the flat capacitors  8 ,  9 ,  10 . 
     As shown in FIGS.  9 ( a ) and ( b ), the assembly  20  is disposed on the upper surfaces of the flat capacitors  8 ,  9 ,  10  and the partition walls  50 , the dummy resistor  11  is connected to the connecting electrode  14   a  in the recess  23 , and the central conductors  4 ,  5 ,  6  of the assembly  20  is connected to the electrodes  16   a ,  16   b  and the dummy resistor  11 . Thus obtained is a 4-mm-square isolator adapted to 800 MHz (D-AMPS, frequency: 824-849 MHz). Like a conventional isolator, this isolator comprises upper and lower metal casings  1 ,  12  functioning as magnetic yokes, a permanent magnet  2 , the assembly  20 , the flat capacitors  8 ,  9 ,  10 , the dummy resistor  11  and the resin casing  7 . 
       FIG. 10  shows the equivalent circuit of an isolator according to one embodiment of the present invention. The garnet-type ferrite  3  wound with the central conductors  4 ,  5 ,  6  constitute inductance L 1 , L 2 , L 3 , and the flat capacitors  8 ,  9 ,  10  constitute capacitors C 1 , C 2 , C 3 . With inductance L 1 , L 2 , L 3  and capacitance C 1 , C 2 , C 3  controlled by the dummy resistor  11  (Rt) connected in parallel to the capacitor C 3 , an isolator operated at the desired center frequency f 0  is provided. 
     An important feature of this isolator is that the assembly  20  is disposed on the flat capacitors  8 ,  9 ,  10  via the insulating member  22 ; in other words, the assembly  20  and the flat capacitors  8 ,  9 ,  10  are disposed on different planes. This makes it possible not only to dispose the flat capacitors  8 ,  9 ,  10  on the same plane, but also to dispose the assembly  20  on a flat surface constituted by the upper surfaces of the flat capacitors  8 ,  9 ,  10 . Accordingly, it is easy to mount the garnet and the capacitors in the desired sizes in the non-reciprocal circuit device. 
       FIG. 11  shows the electric characteristics of the isolator according to the first embodiment. Specifically, FIG.  11 ( a ) shows the frequency characteristics of insertion loss and return loss between an input port P 1  (corresponding to an external terminal  15   a ) and an output port P 2  (corresponding to an external terminal  15   d ), and FIG.  11 ( b ) shows the frequency characteristics of isolation and return loss between the output port P 2  and the input port P 1 . 
     In the isolator of the first embodiment, the insertion loss was 0.55 dB or less (peak value is 0.5 dB or less), and the isolation was 20 dB or more, both in a pass band. This proves that the present invention provides a small isolator with excellent electric characteristics. 
     [2] Second Embodiment 
       FIGS. 12-18  show a resin casing for a non-reciprocal circuit device according to the second embodiment. Because the resin casing of the second embodiment has parts common to the first embodiment, the same reference numerals are assigned to parts having the same functions. Detailed explanation will be made below only with respect to parts different from those of the first embodiment. As shown in  FIG. 12 , the non-reciprocal circuit device of the second embodiment is characterized by using a lower casing as a thin conductor plate for a resin casing  17 . 
     In the non-reciprocal circuit device of the second embodiment, too, partition walls  50  are lower than sidewalls  51 , inner projecting portions  53  and partition walls  52 , so that the flat capacitors  8 ,  9 ,  10  do not protrude from the upper surfaces of the partition walls  50  when each flat capacitor  8 ,  9 ,  10  is disposed on each recess  13   b ,  13   c ,  13   d . Accordingly, the assembly  20  can stably be disposed on flat upper surfaces after mounting the flat capacitors  8 ,  9 ,  10 . The overall structure of the non-reciprocal circuit device is shown in FIG.  18 . 
     The conductor plate  24  serving not only as a lower casing but also as a magnetic yoke is bent in both opposing side portions to form sidewalls  24   a ,  24   a . A thermoplastic resin is molded integrally with the conductor plate  24  bent in a rectangular U shape by an injection-molding method, to constitute a resin casing  17 . 
     A recess  13   d  substantially at a center of the resin casing  17  may be different in depth from recesses  13   b ,  13   c  in which the other flat capacitors  8 ,  9  are disposed. In this case, what is necessary is to draw the conductor plate  24  to a desired extent. A rear surface portion of the conductor plate  24  corresponding to the recess  13   d  is protruded, and it is preferable to control the depth of the recess  13   d  properly, such that there is no interference with mounting surface. If a ground electrode is formed on a circuit board in a portion corresponding to the rear surface projection of the conductor plate  24 , stable ground potential can be obtained, and bonding strength is improved between the circuit board and the non-reciprocal circuit device. 
     As shown in FIG.  12 ( c ), input and output terminals  15   a ,  15   d  and ground terminals  15   b ,  15   c ,  15   e ,  15   f  are exposed from the bottom surface of the resin casing  17 . Because there is a resin between the conductor plate  24  exposed from the bottom surface of the resin casing  17  and the input and output terminals  15   a ,  15   d  and the ground terminals  15   b ,  15   c ,  15   e ,  15   f , no solder does not flow into the bottom surface of the lower casing from the input and output terminals  15   a ,  15   d  and the ground terminals  15   b ,  15   c ,  15   e ,  15   f  at the time of mounting the non-reciprocal circuit device onto the circuit board of an electronic equipment. As a result, the input and output terminals  15   a ,  15   d  and the ground terminals  15   b ,  15   c ,  15   e ,  15   f  can be soldered to the circuit board surely without short-circuiting, etc. 
     As shown in FIG.  15 ( a ), to form the conductor plate  24 , a long metal sheet is first continuously punched by a press machine to form a leadframe  120 . In the second embodiment, because the conductor plate  24  serves as a lower casing (magnetic yoke), too, metal materials for the leadframe  120  are preferably SPCC, 42Ni—Fe alloys, 45Ni—Fe alloys, Fe—Co alloys, etc. with excellent magnetic properties, which are cold- or hot-rolled to about 100-300 μm. The magnetic properties of the metal materials for the leadframe  120  are preferably a maximum permeability of 5,000 or more and a saturation magnetic flux density of 1.4 Tesla or more. 
     The leadframe  120  is preferably coated with a high-conductivity metal layer made of at least one metal selected from the group consisting of silver, copper, gold and aluminum or its alloy, which has an electric resistivity of 5.5 μΩ cm or less, preferably 3.0 μΩ cm or less, more preferably 1.8 μΩ cm or less. The thickness of the high-conductivity metal layer is 0.5-25 μm, preferably 0.5-10 μm, more preferably 1-8 μm. When formed into the non-reciprocal circuit device, this metal layer functions to increase the transmission efficiency of high-frequency signals, with loss reduced by suppressing interference with the outside. 
     When the leadframe  120  is worked to magnetic yokes (conductor plate  24 , upper casing  1 ), its metal layer may be destroyed so that its metal substrate is exposed. In such a case, metal materials for the leadframe  120  are preferably 42Ni—Fe alloys, 45Ni—Fe alloys, etc. with excellent oxidation resistance. 
     Because Fe—Co alloys, which have a maximum permeability of 15,000 and a saturation magnetic flux density of 2.25 Tesla, can be worked to extremely thin plates of about 100 μm in thickness, they are suitable for a magnetic yoke. A specific example of the composition of the Fe—Co alloy comprises 49% by mass of Co, 2% by mass of V, 0.015% by mass or less of C, 0.10% by mass or less of Si, and 0.15% by mass or less of Mn, the balance being substantially Fe and inevitable impurities. From the correlation between a frequency band in which the non-reciprocal circuit device is used and magnetic properties, Co is preferably 40-60% by mass, more preferably about 50% by mass. V is added to improve cold-workability, though it affects the magnetic properties of the resultant leadframe. Therefore, V is preferably 5% by mass or less, for instance, 2% by mass. When the Fe—Co alloys are used for the magnetic yoke, the leak of magnetic flux from the magnetic circuit can be sufficiently suppressed even if the magnetic yoke is made thin, making it possible to make the isolator further thinner and smaller. 
     The long leadframe  120  has sprocket holes  120  at a constant interval in a hoop portion, and a metal sheet portion  124  for forming the conductor plate  24  is formed by punching between four sprocket holes  121 . A plurality of supports  122 ,  123  extending from the hoop portion are connected to metal sheet portion  124 , and each support  122 ,  123  is provided with a cutting line  125  formed by a V-shaped notch  125   a  as shown in FIG.  17 . With such V-shaped notch  125   a , there is no burr generated by cutting, thereby achieving sure soldering at the time of mounting the non-reciprocal circuit device onto the circuit board. 
     As shown in FIG.  15 ( b ), input and output terminals  15   a ,  15   d , ground terminals  15   b ,  15   c ,  15   e ,  15   f , and sidewalls  24   a ,  24   a  provided in the metal sheet portion  124  for the conductor plate  24  are bent. Next, the metal sheet portion  124  is placed in a cavity of an injection-molding machine, and the resin casing  17  is produced by an insert molding method using high-heat-resistance thermoplastic engineering plastics such as liquid crystal polymers and polyphenylene sulfide, as shown in FIG.  16 . 
     A series of steps for producing the resin casing  17  can be carried out successively on a leadframe  120  having sprocket holes  121 . The flat capacitors  8 ,  9 ,  10  and the resistor  11  are disposed in the recesses  13   b ,  13   c ,  13   d  encircled by the partition walls in the resin casing  17  formed by injection-molding a resin integrally with the conductor plate  24 , and the assembly  20  is disposed on the flat capacitors  8 ,  9 ,  10  via an insulating sheet  22  made of a polyimide resin. One end of the central conductor  4  is connected to the upper electrode of the flat capacitor  8  and the terminal electrode  16   a ; one end of the central conductor  5  is connected to the upper electrode of the flat capacitor  9  and the terminal electrode  16   b ; and one end of the central conductor  6  is connected to the upper electrode of the flat capacitor  10  and the dummy resistor  11 . The permanent magnet  2  for applying a DC magnetic field to the assembly  20  is disposed on the assembly  20 , and an upper casing  1  for a magnetic yoke is mounted onto the resin casing  17 . To connect the upper casing  1 , through which high-frequency current flows, to the conductor plate  24  of the resin casing  17  electrically surely and mechanically fix them, it is preferable to solder them. 
     The non-reciprocal circuit device connected to the hoop portion of the leadframe  120  is thus obtained. After soldering by a reflow process, the non-reciprocal circuit device is cut away from the supports  122 ,  123  of the leadframe  120  at the V-shaped notches  125   a.    
     In the second embodiment, because the lower casing is used as a conductor plate, it is possible to reduce the height of the non-reciprocal circuit device. Because flat capacitors are directly disposed on the conductor plate  24  serving as a lower casing, even when a force is applied from outside to the non-reciprocal circuit device, a force directly applied to the flat capacitors  8 ,  9 ,  10  can be reduced by the rigidity of a metal material constituting the conductor plate  24 , resulting in decrease in the breakage of the flat capacitors  8 ,  9 ,  10 . 
     As described above, with the resin casing of the present invention, the miniaturization of the non-reciprocal circuit device can be easily achieved, and the breakage of the flat capacitors by an outside force applied to the non-reciprocal circuit device can be effectively prevented. Accordingly, the non-reciprocal circuit device comprising such resin casing can be made small with high reliability. 
     Because flat capacitors having relatively large area can be disposed in the resin casing having the above structure, it is not necessary to make the flat capacitors extremely thin to achieve the desired capacitance level, resulting in securing sufficient mechanical strength in the flat capacitors. Accordingly, in place of using three separate flat capacitors as in the above embodiments, three pairs of electrodes may be formed on both sides of one dielectric substrate to provide an integral capacitor assembly, or a laminate capacitor may be constituted. In this case, the dummy resistor may be formed by printing on the dielectric substrate having capacitors. The dielectric substrate having a plurality of capacitors formed thereon may be mounted onto, for instance, the resin casing shown in  FIG. 1  or  12 , from which the partition walls  50 ,  52  are removed.