Patent Publication Number: US-7911295-B2

Title: Common mode noise filter

Description:
TECHNICAL FIELD 
     The present invention relates to a common mode noise filter for suppressing common mode noises in an electronic device. 
     BACKGROUND ART 
     Common mode noise filters have large impedance for common mode signals to remove common mode noises. The common mode noise filters have small impedance for differential mode signals, necessary signals, to prevent the signal from being distorted. 
       FIG. 12  is an exploded perspective view of conventional common mode noise filter  180  disclosed in Japanese Patent Laid-Open Publication No. 2002-203718. Filter  180  includes insulating magnetic substrates  110 A and  110 B and insulator layers  120 A to  120 D made of nonmagnetic material. Insulator layers  120 A to  120 D have spiral coil patterns  130 ,  140 ,  150 , and  160  formed thereon. Insulator layers  120 A to  120 D are stacked to form insulating block  120  made of the nonmagnetic material. Coil patterns  130 ,  140 ,  150 , and  160  are embedded in insulating block  120 , and are sandwiched between magnetic substrates  110 A and  110 B, thus providing common mode noise filter  180 . Coil patterns  130 ,  140 ,  150 , and  160  provide two coils having terminals electrically connected with external edge electrodes, respectively. 
     Conventional common mode noise filter  180  has a small bonding strength to dielectric block  120  of the external edge electrodes due to decreasing of the area of the external edge electrodes according to reducing of its size. Filter  180  may have low reliability to be mounted on a portable electronic device. 
     SUMMARY OF THE INVENTION 
     A common mode noise filter includes a nonmagnetic layer, first and second magnetic layers sandwiching the nonmagnetic layer between the magnetic layers and contacting the nonmagnetic layer, a plane coil provided between the first and second magnetic layers and contacting the nonmagnetic layer, and an external electrode connected electrically with the plane coil. The first and second magnetic layers include a magnetic oxide layer and an insulator layer provided on the magnetic oxide layer. The insulator layer contains glass component. 
     This common mode noise filter has a large bonding strength between the external electrode and the insulator layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a common mode noise filter according to Exemplary Embodiments 1 and 2 of the present invention. 
         FIG. 2  is an exploded view of the common mode noise filter according to Embodiments 1 and 2. 
         FIG. 3  is a sectional view of the common mode noise filter at line  3 - 3  shown in  FIG. 1 . 
         FIG. 4  is a sectional view of another common mode noise filter according to Embodiment 1. 
         FIG. 5  is an exploded perspective view of still another common mode noise filter according to Embodiment 1. 
         FIG. 6  is a sectional view of the common mode noise filter shown in  FIG. 5 . 
         FIG. 7  is a sectional view of a further common mode noise filter according to Embodiment 1. 
         FIG. 8  is a perspective view of a common mode noise filter according to Exemplary Embodiment 3 of the invention. 
         FIG. 9  is a sectional view of the common mode noise filter at line  9 - 9  shown in  FIG. 8 . 
         FIG. 10A  is a sectional view of a common mode noise filter according to Exemplary Embodiment 5 of the invention. 
         FIG. 10B  is an enlarged sectional view of the common mode noise filter according to Embodiment 5. 
         FIG. 11  shows evaluation results of the common mode noise filters according to Embodiments 1 to 5. 
         FIG. 12  is an exploded perspective view of a conventional common mode noise filter. 
     
    
    
     REFERENCE NUMERALS 
     
         
           20  Nonmagnetic Layer 
           21 A Magnetic Layer (First Magnetic Layer) 
           21 B Magnetic Layer (Second Magnetic Layer) 
           22 A Plane Coil (First Plane Coil) 
           22 B Plane Coil (Second Plane Coil) 
           22 E,  22 F Plane Coil 
           25 A,  25 B External Electrode (First External Electrode) 
           25 C,  25 D External Electrode (Second External Electrode) 
           523 A Magnetic Oxide Layer (First Magnetic Oxide Layer) 
           523 B Magnetic Oxide Layer (Second Magnetic Oxide Layer) 
           623 A,  623 B Magnetic Oxide Layer 
           723 A Magnetic Oxide Layer (Third Magnetic Oxide Layer) 
           723 B Magnetic Oxide Layer (Fourth Magnetic Oxide Layer) 
           520 A Surface of Nonmagnetic Layer (First Surface) 
           520 B Surface of Nonmagnetic Layer (Second Surface) 
           524 A Insulator Layer (First Insulator Layer) 
           524 B Insulator Layer (Second Insulator Layer) 
           624 A,  624 B Insulator Layer 
           724 A Insulator Layer (Third Insulator Layer) 
           724 B Insulator Layer (Fourth Insulator Layer) 
       
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Exemplary Embodiment 1 
       FIG. 1  is a perspective view of common mode noise filter  1001  according to Exemplary Embodiment 1 of the present invention.  FIG. 2  is an exploded view of filter  1001 .  FIG. 3  is a sectional view of filter  1001  at line  3 - 3  shown in  FIG. 1 . 
     Common mode noise filter  1001  includes nonmagnetic layer  20 , magnetic layers  21 A and  21 B, plane coils  22 A and  22 B, and external electrodes  25 A to  25 D. Nonmagnetic layer  20  is made of nonmagnetic insulating material, such as glass ceramic, and has surface  520 A and surface  520 B opposite to surface  520 A. Magnetic layer  21 A is provided on surface  520 A of nonmagnetic layer  20 . Magnetic layer  21 B is provided on surface  520 B. Plane coils  22 A and  22 B are provided between magnetic layers  21 A and  21 B and contact nonmagnetic layer  20 . Coils  22 A and  22 B face each other. In filter  1001 , plane coils  22 A and  22 B are embedded in nonmagnetic layer  20 . Plane coil  22 A has ends  522 A and  622 A. Ends  522 A and  622 A are connected to external electrodes  25 A and  25 B via extraction electrodes  522 C and  622 C, respectively. Plane coil  22 B has ends  522 B and  622 B. Ends  522 B and  622 B are connected to external electrodes  25 C and  25 D via extraction electrodes  522 D and  622 D, respectively. Magnetic layer  21 A includes magnetic oxide layer  523 A provided on surface  520 A of nonmagnetic layer  20 , insulator layer  524 A on magnetic oxide layer  523 A, magnetic oxide layer  623 A on insulator layer  524 A, insulator layer  624 A on magnetic oxide layer  623 A, and magnetic oxide layer  723 A on insulator layer  624 A. Magnetic layer  21 B includes magnetic oxide layer  523 B provided on surface  520 B of nonmagnetic layer  20 , insulator layer  524 B on magnetic oxide layer  523 B, magnetic oxide layer  623 B on insulator layer  524 B, insulator layer  624 B on magnetic oxide layer  623 B, and magnetic oxide layer  723 B on insulator layer  624 B. Insulator layers  524 A,  624 A,  524 B, and  624 B contain glass component. Filter  1001  includes four insulator layers and six magnetic oxide layers, and the numbers of these layers may be changed according to the shape of filter  1001 . 
     Nonmagnetic layer  20  includes nonmagnetic segment layer  20 A having surface  520 A, nonmagnetic segment layer  20 B provided on nonmagnetic segment layer  20 A, and nonmagnetic segment layer  20 C which is provided on nonmagnetic segment layer  20 B and has surface  520 B. 
     A method of manufacturing common mode noise filter  1001  will be described below. First, Zn—Cu ferrite powder, material of nonmagnetic segment layers  20 A to  20 C of nonmagnetic layer  20  is mixed with solvent and binder component, thereby to producing ceramic slurry. Then, the ceramic slurry is molded by, for example, a doctor blade method, to produce ceramic green sheets having predetermined thicknesses of about 25 μm providing nonmagnetic segment layers  20 A to  20 C. 
     Similarly, powder non-borosilicate glass (SiO 2 —CaO—ZnO—MgO based glass) which can be fired at a temperature not higher than 920° C. is mixed with 9 wt % of Ni—Zn—Cu ferrite to produce ceramic green sheets with thicknesses of about 25 μm providing insulator layers  524 A,  524 B,  624 A, and  624 B. 
     Ceramic green sheets with thicknesses of about 100 μm for providing magnetic oxide layers  523 A,  523 B,  623 A,  623 B,  723 A, and  723 B are produced from magnetic powder of Ni—Zn—Cu ferrite oxide magnetic substance. 
     Then, as shown in  FIG. 2 , conductors having predetermined coil patterns and via-electrodes for electrical connection between layers are provided on these ceramic green sheets. These ceramic green sheets are stacked, and fired at a predetermined temperature, thus producing a laminated fired body. 
     A method of forming plane coils  22 A and  22 B and nonmagnetic layer  20  will be described below. 
     Magnetic oxide layer  523 A has surface  2523 A contacting surface  520 A of nonmagnetic layer  20 . Magnetic oxide layer  523 B has surface  1523 B contacting surface  520 B of nonmagnetic layer  20 . Extraction electrodes  522 C and  622 C are formed on surface  2523 A of magnetic oxide layer  523 A. Then, magnetic oxide layers  523 A,  623 A, and  723 A and insulator layers  524 A, and  624 A are stacked to produce magnetic layer  21 A. 
     Plane coil  22 A is formed on surface  620 A of nonmagnetic segment layer  20 A opposite to surface  520 A. Via-conductor  1522 A communicating with surface  520 A and surface  620 A are formed in nonmagnetic segment layer  20 A at a position contacting end  522 A of plane coil  22 A and extraction electrode  522 C. Via-conductor  2522 A communicating with surface  520 A and surface  620 A is formed in nonmagnetic segment layer  20 A at a position contacting end  622 A of plane coil  22 A and extraction electrode  622 C. Via-conductor  1522 A connects end  522 A of plane coil  22 A electrically with extraction electrode  522 C. Via-conductor  2522 A connects end  622 A of plane coil  22 A electrically with extraction electrode  622 C. 
     Plane coil  22 B is formed on surface  620 B of nonmagnetic segment layer  20 C opposite to surface  520 B. Via-conductor  1522 B communicating with surface  520 B and surface  620 B is formed in nonmagnetic segment layer  20 C at a position contacting end  522 B of plane coil  22 B and extraction electrode  522 D. Via-conductor  2522 B communicating surface  520 B and surface  620 B is formed in nonmagnetic segment layer  20 C at a position contacting end  622 B of plane coil  22 B and extraction electrode  622 D. Via-conductor  1522 B electrically connects end  522 B of plane coil  22 B electrically with extraction electrode  522 D. Via-conductor  2522 B connects end  622 B of plane coil  22 B electrically with extraction electrode  622 D. 
     Then, nonmagnetic segment layer  20 A is stacked on magnetic layer  21 A so that surface  520 A of nonmagnetic segment layer  20 A contacts surface  2523 A of magnetic layer  21 A. Then, nonmagnetic segment layers  20 B and  20 C are stacked to produce nonmagnetic layer  20  that has plane coils  22 A and  22 B and via-conductors  1522 A,  1522 B,  2522 A, and  2522 B all embedded in nonmagnetic layer  20 . 
     Next, magnetic oxide layer  523 B is stacked on surface  520 B of nonmagnetic layer  20  so that surface  520 B of nonmagnetic layer  20  contacts surface  1523 B of magnetic oxide layer  523 B. Then, insulator layer  624 B, magnetic oxide layer  623 B, insulator layer  624 B, and magnetic oxide layer  723 B are stacked in this order on magnetic oxide layer  523 B to produce a green-sheet-laminated body including magnetic layers  21 A and  21 B and nonmagnetic layer  20 . This green-sheet-laminated body is fired at a temperature lower than the melting point of the material of plane coils  22 A and  22 B, thus providing laminated fired body having plane coils  22 A and  22 B embedded therein. 
     The laminated fired body has edge surfaces  1001 A and  1001 B. Ends  1522 C and  1522 D of extraction electrodes  522 C and  522 D expose at edge surface  1001 A. Ends  1622 C and  1622 D of extraction electrodes  622 C and  622 D expose at edge surface  1001 B. External electrode  25 C electrically connected with end  1522 D of extraction electrode  522 D is formed on edge surface  1001 A by the following method. Ag paste containing glass frit as glass component is applied onto edge surface  1001 A as to contact end  1522 D of extraction electrode  522 D, thus providing base electrode layer  125 C, an Ag-metallized layer connected with end  1522 D. Then, Ni-plated layer  225 C is formed on base electrode layer  125 C by Ni plating, and Sn-plated layer  325 C is formed on Ni-plated layer  225 C, thus producing external electrode  25 C. Similarly, external electrode  25 D connected electrically with end  1622 D of extraction electrode  622 D is formed on edge surface  1001 B by the following method. Ag paste is applied onto edge surface  1001 B as to contact end  1622 D of extraction electrode  622 D thus providing base electrode layer  125 D, an Ag metallized layer connected with end  1622 D. Base electrode layer  125 D of external electrode  25 D contacts insulator layers  524 A,  524 B,  624 A, and  624 B, nonmagnetic layer  20 , and oxidization magnetic layers  523 A,  523 B,  623 A,  623 B,  723 A, and  723 B. Then, Ni-plated layer  225 D is formed on base electrode layer  125 D by Ni plating, and Sn-plated layer  325 D is formed on Ni-plated layer  225 D thus producing external electrode  25 D. Similarly, external electrode  25 A connected with end  1522 C of extraction electrode  522 C is formed on edge surface  1001 A to form external electrode  25 B which is connected with end  1622 C of extraction electrode  622 C and located on edge surface  1001 B. External electrodes  25 A to  25 D may be produced by other methods for forming terminals of ceramic electronic components. 
     In common mode noise filter  1001 , external electrodes  25 A to  25 D include the base electrode layers made of Ag paste containing glass frit tightly jointed with insulator layers  524 A,  524 B,  624 A, and  624 B including the glass component, and thus have strong bonding strength to edge surfaces  1001 A and  1001 B. Magnetic oxide layers  523 A,  523 B,  623 A,  623 B,  723 A, and  723 B having excellent magnetic properties couples plane coils  22 A and  22 B tightly with each other magnetically. 
     Fifty pieces of samples of common mode noise filter  1001  of Embodiment 1 were produced, and were measured in the bonding strength of edge surfaces  1001 A,  1001 B of external electrodes  25 A to  25 D. The samples according to Embodiment 1 have thicknesses of 0.5 mm, widths of 1.0 mm, and lengths of 1.2 mm. Conductive wires having diameters of 0.20 mm were soldered to external electrodes  25 A and  25 B which are positioned opposite to each other; and were pulled by a tensile testing machine until the electrodes broke.  FIG. 11  shows average, maximum, and minimum values of tensile forces when the wires broke.  FIG. 11  further shows the bonding strength of edge electrode  25  of samples of comparative examples including magnetic layers made of only oxide magnetic material, instead of magnetic layers  21 A and  21 B. 
     As shown in  FIG. 11 , external electrodes  25 A to  25 D according to example 1 have stronger bonding strength and smaller variation than the comparative examples. Thus, magnetic layers  21 A and  21 B include magnetic oxide layers and insulator layers including glass which are stacked, and provides reliable common mode noise filter  1001  without depressing its electrical characteristics. 
     Magnetic oxide layers  523 A,  523 B,  623 A,  623 B,  723 A, and  723 B contain Ni—Zn—Cu ferrite. These layers may be made of other magnetic oxide material which can be fired together with Ag, the material of plane coils  22 A and  22 B, at a temperature not higher than 920° C., and which has a magnetic permeability not smaller than 20 for providing electrical characteristics as a common mode noise filter. 
     The thicknesses of magnetic oxide layers  523 A,  523 B,  623 A,  623 B,  723 A, and  723 B range preferably from about 50 μm to 150 μm, while the thicknesses depend on the size of the common mode noise filter. Thicknesses smaller than 50 μm do not provide adequate electrical characteristics as a common mode noise filter. Thicknesses larger than 150 μm decrease the number of insulator layers containing glass component, thereby hardly providing external electrodes  25 A to  25 D with large bonding strength. 
     Insulator layers  524 A,  524 B,  624 A, and  624 B containing the glass component is made of mixture of borosilicate glass powder and Ni—Zn—Cu ferrite powder. The mixture ratio of the borosilicate glass powder to the Ni—Zn—Cu ferrite powder may be changed to control the characteristic of the common mode noise filter, while the mixture ratio of the Ni—Zn—Cu ferrite ranges preferably from 0 wt % to 15 wt %. A mixture ratio not less than 15 wt % causes the green-sheet-laminated body to be sintered sufficiently at 920° C. and decreases the mechanical strength of common mode noise filter  1001 , resulting in defects, such as chipping during a mounting process. Instead of borosilicate glass powder, other glass powder, such as borosilicate alkali glass, that can be fired at a temperature not higher than 920° C. and additionally has a linear expansion coefficient ranging from 80×10 −7 /° C. to 110×10 −7 /° C. Glass powder having a linear expansion coefficient out of this range may cause defects, such as a crack, due to the difference between linear expansion coefficients of the glass powder and the oxide magnetic material. 
     Instead of Zn—Cu ferrite, other nonmagnetic insulating material that is substantially nonmagnetic and can be fired at 920° C., and that has a linear expansion coefficient ranging from 80×10 −7 /° C. to 110×10 −7 /° C. can be used for nonmagnetic layer  20 . 
     The magnetic oxide layer including magnetic layers  21 A and  21 B made of Ni—Zn—Cu ferrite can be fired simultaneously together with material, such as silver, having a large conductivity. The insulator layer may be made of glass ceramic, or mixture of oxide magnetic material and the glass ceramic, that can be fired simultaneously together with the magnetic oxide layer. 
       FIG. 4  is a sectional view of another common mode noise filter  1002  according to Embodiment 1. In  FIG. 4 , components identical to those shown in  FIGS. 1 to 3  are denoted by the same reference numerals, and their description is omitted. In filter  1002 , plane coil  22 A is provided at the boundary between nonmagnetic layer  20  and magnetic layer  21 A, namely, between surface  520 A of nonmagnetic layer  20  and surface  2523 A of magnetic layer  21 A (magnetic oxide layer  523 A). Plane coil  22 B is provided at the boundary between nonmagnetic layer  20  and magnetic layer  21 B, namely, between surface  520 B of nonmagnetic layer  20  and surface  1523 B of magnetic layer  21 B (magnetic oxide layer  523 B). Plane coils  22 A and  22 B approximate more closely to magnetic layers  21 A and  21 B, respectively, than those of common mode noise filter  1001  shown in  FIG. 3 , accordingly allowing filter  1002  to have higher impedance against common mode signals. 
       FIG. 5  is an exploded perspective view of still another common mode noise filter  1003  according to Embodiment 1.  FIG. 6  is a sectional view of filter  1003 . In  FIG. 5 , components identical to those shown in  FIGS. 1 to 3  are denoted by the same reference numerals, and their description is omitted. Filter  1003  includes plane coils  22 E and  22 F embedded in nonmagnetic layer  20  instead of plane coils  22 A and  22 B of common mode noise filter  1001  shown in  FIG. 1 . Plane coils  22 E and  22 F form a double-spiral shape. Plane coil  22 E includes spiral plane coil  122 E provided on surface  620 A of nonmagnetic segment layer  20 A, spiral plane coil  222 E provided on surface  620 B of nonmagnetic segment layer  20 C, and via-conductor  322 E which is provided in nonmagnetic segment layer  20 B and which connects plane coil  122 E electrically with plane coil  222 E. Plane coil  22 F includes spiral plane coil  122 F provided on surface  620 A of nonmagnetic segment layer  20 A, spiral plane coil  222 F provided on surface  620 B of nonmagnetic segment layer  20 C, and via-conductor  322 F which is provided in nonmagnetic segment layer  20 B and connects plane coil  122 F electrically with plane coil  222 F. Plane coils  122 E and  122 F form a double-spiral shape, and plane coils  222 E and  222 F form a double-spiral shape. Extraction electrodes  722 D and  822 D are connected with both ends of plane coil  22 E, respectively. Extraction electrodes  722 C and  822 C are connected with ends of plane coil  22 F, respectively. Extraction electrodes  722 D and  822 D are connected to external electrodes  25 A and  25 B, respectively. Extraction electrodes  722 C and  822 C are connected to external electrodes  25 C and  25 D, respectively. 
     Common mode noise filters  1001  and  1002  shown in  FIGS. 3 ,  4  require at least four layers in order to form plane coils  22 A and  22 B. In filter  1003  shown in  FIG. 5 , plane coils  22 E and  22 F forming the double-spiral shapes can be formed on two layers, thus allowing common mode noise filter  1003  to be manufacture with high productivity. 
       FIG. 7  is a sectional view of further common mode noise filter  1004  according to Embodiment 1. In  FIG. 7 , components identical to those shown in  FIGS. 5 and 6  are denoted by the same reference numerals, and their description is omitted. In filter  1004 , plane coils  22 E and  22 F are provided at the boundary between nonmagnetic layer  20  and magnetic layer  21 A and at the boundary between nonmagnetic layer  20  and magnetic layer  21 B. In other words, plane coils  122 E and  122 F are provided between surface  520 A of nonmagnetic layer  20  and surface  2523 A of magnetic layer  21 A (magnetic oxide layer  523 A). Plane coil  222 E and  222 F are provided at the boundary between nonmagnetic layer  20  and magnetic layer  21 B, namely between surface  520 B of nonmagnetic layer  20  and surface  1523 B of magnetic layer  21 B (magnetic oxide layer  523 B). Plane coils  22 E and  22 F approximate more closely to magnetic layers  21 A,  21 B, respectively, than those of common mode noise filter  1003  shown in  FIG. 4 , accordingly allowing filter  1004  to have higher impedance against common mode signals. 
     Exemplary Embodiment 2 
     A common mode noise filter according to Exemplary Embodiment 2 has the same structure as common mode noise filter  1001  shown in  FIGS. 1 and 2 . Nonmagnetic layer  20  of the common mode noise filter according to Embodiment 2 contains glass component. 
     Ceramic green sheet with thicknesses of about 50 μm to be nonmagnetic segment layers  20 A to  20 C of nonmagnetic layer  20  were produced from non-borosilicate glass (SiO 2 —CaO—ZnO—MgO based glass) powder containing crystal as filler that can be fired at a temperature not higher than 920° C. and has a linear expansion coefficient of about 100×10 −7 /° C. Fifty samples according to Embodiment 2 each including nonmagnetic layer  20  were produced by stacking nonmagnetic segment layers  20 A to  20 C.  FIG. 11  shows the bonding strength of external electrodes  25 A to  25 D of these samples which were measured by the same method as filter  1001  according to Embodiment 1. 
     As shown in  FIG. 11 , nonmagnetic layer  20  containing the glass material provides a large bonding strength between nonmagnetic layer  20  and external electrodes  25 A to  25 D and decreases variation of the strength. Thus, a common mode noise filter with higher mounting reliability is provided. 
     The glass material added into nonmagnetic layer  20  decreases the dielectric constant of nonmagnetic layer  20 , accordingly allowing the common mode noise filter according to Embodiment 2 to be used in a high-frequency band. 
     The glass powder to form nonmagnetic layer  20  of the filter according to Embodiment 2 may be other glass ceramic powder, such as dielectric-material-based glass-crystal, glass-alumina, or glass-forsterite, that can be fired at a temperature not higher than 920° C. and has a linear expansion coefficient ranging from about 80×10 −7 /° C. to 110×10 −7 /° C. This decreases the dielectric constant of nonmagnetic layer  20 , accordingly providing a common mode noise filter that has superior electrical characteristics in up to a high-frequency band. 
     Exemplary Embodiment 3 
       FIG. 8  is a perspective view of common mode noise filter  3001  according to Exemplary Embodiment 3 of the present invention.  FIG. 9  is a sectional view of filter  3001  at line  9 - 9  shown in  FIG. 8 . Component identical to those of the common mode noise filter according to Embodiments 1 and 2 shown in  FIG. 1  are denoted by the same reference numerals, and their description is omitted. 
     Common mode noise filter  3001  includes magnetic layers  1021 A and  1021 B instead of magnetic layers  21 A and  21 B of common mode noise filter  1001  according to Embodiment 1. Magnetic layer  1021 A further includes insulator layer  724 A containing glass component provided on magnetic oxide layer  723 A of magnetic layer  21 A of filter  1001 . Magnetic layer  1021 B further includes insulator layer  724 B containing glass component provided on magnetic oxide layer  723 B of magnetic layer  21 B of filter  1001 . That is, the respective outermost layers of magnetic layers  1021 A and  1021 B are insulator layers  724 A are  724 B containing the glass component, while insulator layers  724 A and  724 B expose outside magnetic layers  1021 A and  1021 B, respectively. 
     Ceramic green sheets with thicknesses of about 25 μm to be insulator layers  724 A and  724 B were produced from powder mixture of non-borosilicate glass (SiO 2 —CaO—ZnO—MgO-based glass) that can be fired at a temperature not higher than 920° C. and 9 wt % of Ni—Zn—Cu ferrite. Insulator layers  724 A and  724 B including the glass component are formed by stacking these ceramic green sheets on green sheets to be magnetic oxide layers  723 A and  723 B, respectively. Fifty samples according to Embodiment 3 each including magnetic layers  1021 A and  1021 B and nonmagnetic layer  20  made of non-borosilicate glass containing crystal as inorganic filler were produced.  FIG. 11  shows the bonding strength of external electrodes  25 A to  25 D of these samples which were measured by the same method as filter  1001  according to Embodiment 1. 
     As shown in  FIG. 11 , insulator layer  724 A and  724 B containing the glass component as the outermost layers increases the bonding strength of external electrodes  25 A to  25 D and decreases variation of the strength. Thus, common mode noise filter  3001  with high mounting reliability is provided. 
     Insulator layers  724 A and  724 B may be made of other glass ceramic, such as dielectric-material-based glass-crystal, glass-alumina, or glass-forsterite, that can be fired at a temperature not higher than 920° C. and has a linear expansion coefficient ranging from about 80×10 −7 /° C. to 110×10 −7 /° C. 
     A sample including nonmagnetic layer  20  containing Zn—Cu ferrite provided the same effects. 
     Exemplary Embodiment 4 
     A common mode noise filter according to Exemplary Embodiment 4 has the same structure as that of common mode noise filter  1001  shown in  FIGS. 1 to 3 . 
     In a common mode noise filter according to Embodiment 4, Ag paste to be applied on edge surfaces  1001 A and  1001 B to form base electrode layers  125 C and  125 D of external electrodes contains the same glass powder as that of at least one of glass component contained in nonmagnetic layer  20  and glass component contained in magnetic layers  21 A and  21 B (insulator layers  524 A,  524 B,  624 A, and  624 B). In other words, the glass component contained in nonmagnetic layer  20  may be the same as that in magnetic layers  21 A and  21 B (insulator layers  524 A,  524 B,  624 A, and  624 B). Ni-plated layers  225 C and  225 D are formed on base electrode layers  125 C and  125 D, respectively. Sn-plated layers  325 C and  325 D are formed on Ni-plated layers  225 C and  225 D, respectively. 
     Nonmagnetic layer  20  is made of glass ceramic. The Ag paste is produced by mixing and kneading 5 wt % of non-borosilicate glass and binder, such as ethyl cellulose, α-terpineol, or carbitol acetate, with Ag powder. Fifty samples of the common mode noise filters according to Embodiment 4 were produced by applying the Ag paste onto edge surfaces  1001 A and  1001 B to form base electrode layers  125 C and  125 D.  FIG. 11  shows the bonding strength of external electrodes  25 A to  25 D of these samples which were measured by the same method as filter  1001  according to Embodiment 1. 
     As shown in  FIG. 11 , the common mode noise filter according to Embodiment 4 causes continuity between the glass component of nonmagnetic layer  20  and magnetic layers  21 A and  21 B, and the glass component of base electrode layers  125 C and  125 D of external electrodes  25 C and  25 D. This continuity further increases the bonding strength between edge surfaces  1001 A and  1001 B and the external electrodes, accordingly providing the common mode noise filter with high mounting reliability. 
     Ag paste containing less than 1 wt % of glass powder mixed therein for base electrode layers  125 C and  125 D provides small effects in increasing the bonding strength. Ag paste containing more than 5 wt % of the glass component decreases the bonding strength between base electrode layer  125 C and Ni-plated layer  225 C and the bonding strength between base electrode layer  125 D and Ni-plated layer  225 D. Thus, the amount of glass powder to be mixed into the Ag paste for base electrode layers  125 C and  125 D ranges preferably from 1 wt % to 5 wt %. Even if Pt or Pd is contained in the Ag paste, glass powder mixed into the Ag paste provided the same effects. The amount of the binder is determined mainly by a specific surface area of the powder, and was adjusted so that the Ag paste did not make thin spots or drips when being applied onto edge surfaces  1001 A and  1001 B. 
     Common mode noise filter  3001  which includes nonmagnetic layer  20  using Zn—Cu ferrite according to Embodiment 3 shown in  FIG. 9  provided the same effects by forming the base electrode layer with the Ag paste according to Embodiment 4. 
     Exemplary Embodiment 5 
       FIG. 10A  is a sectional view of common mode noise filter  5001  according to Exemplary Embodiment 5.  FIG. 10B  is an enlarged sectional view of common mode noise filter  5001 . In  FIG. 10A , Components identical to those of common mode noise filter  3001  according to Embodiment 3 shown in  FIG. 9  are denoted by the same reference numerals, and their description is omitted. 
     Common mode noise filter  5001  includes magnetic layers  2021 A and  2021 B instead of magnetic layers  1021 A and  1021 B of common mode noise filter  3001  shown in  FIG. 9 . Magnetic layer  2021 A includes magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B having widths smaller than those of nonmagnetic layer  20  and insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B instead of magnetic oxide layers  523 A,  523 B,  623 A,  623 B,  723 A, and  723 B shown in  FIG. 9 . In other words, edge surfaces  8523 A,  8523 B,  8623 A,  8623 B,  8723 A, and  8723 B of magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B sink below edge surfaces  1524 A,  1524 B,  1624 A,  1624 B,  1724 A, and  1724 B of insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B at edge surfaces  5001 A and  5001 B. 
     A method of manufacturing common mode noise filter  5001  will be described below. 
     Ceramic green sheet with thicknesses of 25 μm to be insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B are produced from non-borosilicate glass powder with a firing-contraction rate having its maximum value at about 750° C. 
     Ceramic green sheets with thicknesses of about 100 μm to be magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B are produced from Ni—Zn—Cu ferrite oxide magnetic powder with a firing contraction rate having its maximum value at about 850° C. 
     These ceramic green sheets are stacked to produce a green-sheet-laminated body similarly to that of Embodiment 1. 
     This green-sheet-laminated body are fired at about 900° C., which is lower than the melting point of material of plane coils  22 A and  22 B, thus providing a laminated fired body including plane coils  22 A and  22 B embedded therein. During this firing process, insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B contacting magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B which are hardly sintered at a temperature lower than 800° C. are prevented from contracting in direction  5001 C in parallel with surfaces  520 A and  520 B, but contract and become dense in thickness direction  5001 D orthogonal to direction  5001 C. Then, the temperature is raised to higher than 800° C. to cause magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B to sinter. Peripheries  7523 A,  7523 B,  7623 A,  7623 B,  7723 A, and  7723 B of edge surfaces  8523 A,  8523 B,  8623 A,  8623 B,  8723 A, and  8723 B of magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B are restrained on insulator layer  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B which have become dense, and do not contract in direction  5001 C at their interfaces. Respective centers  6523 A,  6523 B,  6623 A,  6623 B,  6723 A, and  6723 B and their vicinities of edge surfaces  8523 A,  8523 B,  8623 A,  8623 B,  8723 A, and  8723 B of magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B are distanced from the interfaces in the thickness direction, and contract in direction  5001 C. Thus, edge surfaces  8523 A,  8523 B,  8623 A,  8623 B,  8723 A, and  8723 B of magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B which are sandwiched with insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B containing glass component sink below edge surfaces  1524 A,  1524 B,  1624 A,  1624 B,  1724 A, and  1724 B of insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B. Edge surface  1020  of nonmagnetic layer  20  and edge surfaces  1524 A,  1524 B,  1624 A,  1624 B,  1724 A, and  1724 B of insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B project from edge surfaces  8523 A,  8523 B,  8623 A,  8623 B,  8723 A, and  8723 B of magnetic oxide layers  5523 A,  5523 B,  5623 A,  5623 B,  5723 A, and  5723 B. 
     Extraction electrode  522 C,  522 D,  622 C, and  622 D from plane coils  22 A and  22 B expose at edge surfaces  5001 A and  5001 B from which edge surface  1020  of nonmagnetic layer  20  and edge surfaces  1524 A,  1524 B,  1624 A,  1624 B,  1724 A, and  1724 B of insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B project. Ag paste is applied onto edge surfaces  5001 A and  5001 B so as to be connected electrically with extraction electrodes  522 C,  522 D,  622 C, and  622 D, thereby forming base electrode layers  125 C and  125 D to form external electrodes  25 A to  25 D. Fifty samples of common mode noise filter  5001  according to Embodiment 5 were produced.  FIG. 11  shows the bonding strength of external electrodes  25 A to  25 D of these samples which were measured by the same method as filter  1001  according to Embodiment 1. 
     As shown in  FIG. 11 , the bonding strength between insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B and external electrodes  25 A to  25 D of the samples of embodiment 5. The samples of common mode noise filter  5001  has a larger average bonding strength and smaller variation of the strength than samples of example 3 of Embodiment 3, and thus common mode noise filter  5001  has high mounting reliability. 
     A sample including nonmagnetic layer  20  containing Zn—Cu ferrite has the same effects. The Ag paste forming base electrode layers  125 C and  125 D may contain glass component of nonmagnetic layer  20  or glass component of insulator layers  524 A,  524 B,  624 A,  624 B,  724 A, and  724 B. Samples using such Ag paste have the same effects. 
     INDUSTRIAL APPLICABILITY 
     A common mode noise filter according to the present invention has a large bonding strength between an external electrode and an insulator layer and is useful as a small common mode noise filter required to have mounting reliability so that the filter may be used in an electronic device, particularly a portable electronic device.