Patent Publication Number: US-11024451-B2

Title: Multilayer coil component

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2014-149360 filed on Jul. 23, 2014 and is a Continuation Application of PCT Application No. PCT/JP2015/067960 filed on Jun. 23, 2015. The entire contents of each application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multilayer coil component, and more particularly, to a multilayer coil component applied to a DC/DC converter and including a plurality of coil conductors, a plurality of planar conductors containing silver, and a plurality of ferrite layers containing copper and stacked with the plurality of coil conductors and the plurality of planar conductors interposed therebetween. The plurality of coil conductors define a portion of a coil with a winding axis extending in a lamination direction, and the plurality of planar conductors are arranged in the lamination direction at a position in an outer side portion of the coil in the lamination direction so that principal surfaces thereof face in the lamination direction and that specific regions on the principal surfaces overlap with the coil when viewed from the lamination direction. 
     2. Description of the Related Art 
     In such a type of multilayer coil component, the pressure for pressure-bonding a multilayer body increases in a region where a plurality of coil conductors overlap when viewed from the lamination direction. This decreases the distance between a plurality of planar conductors in the lamination direction. Here, in a typical DC/DC converter, each of a plurality of planar conductors serves as a ground electrode, a shield electrode, or a capacitance electrode, and a potential difference of the input voltage or the output voltage from the ground potential is generated between the layers. For this reason, some measures, for example, to secure insulation are necessary in the region where the distance between the plural planar conductors is short in the lamination direction. 
     Japanese Unexamined Patent Application Publication No. 2-224513 discloses that shield electrode layers are provided on surfaces of an LC composite component with sheet layers formed of a dielectric or insulating material interposed therebetween, circular, L-shaped, or linear cavities are provided, and the frequency characteristics are changed according to the shape of the cavities. Japanese Unexamined Patent Application Publication No. 11-340039 also discloses that a slit is provided in at least a part of each shield layer. However, these structures are both directed to problems different from the problem of accumulation of silver and copper between the planar conductors, and are based on greatly different premises. 
     SUMMARY OF THE INVENTION 
     Accordingly, preferred embodiments of the present invention provide a multilayer coil component that significantly reduces or prevents accumulation of silver and copper between planar conductors. 
     A multilayer coil component according to a preferred embodiment of the present invention includes a plurality of coil conductors, a plurality of planar conductors containing silver, and a plurality of ferrite layers containing copper and stacked with the plurality of coil conductors and the plurality of planar conductors interposed therebetween. The plurality of coil conductors define a portion of a coil with a winding axis extending in a lamination direction, and the plurality of planar conductors are arranged in the lamination direction at a position in an outer side portion of the coil in the lamination direction so that each of principal surfaces thereof faces in the lamination direction and a specific region of the principal surface overlaps with the coil when viewed from the lamination direction. Each of the plurality of planar conductors includes a plurality of first through holes penetrating the principal surface in the lamination direction in the specific region. 
     Preferably, the plurality of planar conductors have a plurality of different potentials. 
     Preferably, each of the plurality of planar conductors further includes at least one second through hole penetrating the principal surface in the lamination direction in a region different from the specific region. 
     Preferably, a multilayer body includes the plurality of ferrite layers includes one principal surface on which an electronic component is mounted, and the plurality of planar conductors are provided closer to the one principal surface than the coil. 
     Preferably, the plurality of coil conductors contain the silver. 
     When iron oxide serving as a major raw material of the ferrite layers is produced, a sulfur component is mixed therein. The sulfur component reacts with silver due to heat during firing of the multilayer body, and silver sulfide generated by the reaction diffuses inside the multilayer body. Accumulation of the diffusing silver sulfide between the planar conductors may cause electrochemical migration. 
     While copper contributes to low-temperature sintering of the ferrite layers, cuprous oxide formed by sintering shows the property of a semiconductor. Hence, accumulation of cuprous oxide between the planar conductors may deteriorate insulation between the planar conductors. 
     As a result, each of the principal surfaces of the plurality of planar conductors includes one, two, or more first through holes penetrating in the lamination direction in the specific region overlapping with the coil when viewed from the lamination direction. This structure significantly reduces or prevents accumulation of silver and copper between the planar conductors. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view illustrating a certain cross section (a cross section perpendicular or substantially perpendicular to a depth direction of a rectangular parallelepiped) of a multilayer coil component according to a preferred embodiment of the present invention. 
         FIG. 2A  is a plan view illustrating a ferrite layer of the multilayer coil component and a coil conductor provided on an upper surface of the ferrite layer,  FIG. 2B  is a plan view illustrating another ferrite layer of the multilayer coil component and another coil conductor provided on an upper surface of the ferrite layer,  FIG. 2C  is a plan view illustrating a further ferrite layer of the multilayer coil component and a further coil conductor provided on an upper surface of the ferrite layer, and  FIG. 2D  is a plan view illustrating a still further ferrite layer of the multilayer coil component and a still further coil conductor provided on an upper surface of the ferrite layer. 
         FIG. 3A  is a plan view illustrating a further ferrite layer of the multilayer coil component and a further coil conductor provided on an upper surface of the ferrite layer,  FIG. 3B  is a plan view illustrating a still further ferrite layer of the multilayer coil component and a still further coil conductor provided on an upper surface of the ferrite layer, and  FIG. 3C  is a plan view illustrating an even still further ferrite layer of the multilayer coil component and an even still further coil conductor provided on an upper surface of the ferrite layer. 
         FIG. 4A  is a plan view illustrating a ferrite layer of the multilayer coil component and a planar conductor provided on an upper surface of the ferrite layer, and  FIG. 4B  is a plan view illustrating another ferrite layer of the multilayer coil component and another planar conductor provided on an upper surface of the ferrite layer. 
         FIG. 5A  illustrates a state in which the planar conductor FC 2   a  is superposed on the coil conductor CP 5 , and  FIG. 5B  illustrates a state in which the planar conductor FC 2   b  is superposed on the coil conductor CP 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a multilayer coil component (multilayer inductor element)  10  according to a preferred embodiment of the present invention is a multilayer coil component of an LGA (Land Grid Array) type, and includes a multilayer body  12  preferably with a rectangular parallelepiped or substantially rectangular parallelepiped shape.  FIG. 1  illustrates a cross section perpendicular or substantially perpendicular to a depth direction of the rectangular parallelepiped or substantially rectangular parallelepiped shape. 
     A plurality of electronic components  16   a  and  16   b , such as an IC chip and a capacitor, are mounted on an upper surface (one principal surface) of the multilayer body  12 , and outer electrodes  14   a  and  14   b  are provided on a lower surface (the other principal surface) of the multilayer body  12 . The electronic components  16   a  and  16   b  are connected to wiring (not illustrated) provided on the upper surface of the multilayer body  12 , and this realizes a DC/DC converter. 
     The multilayer body  12  has a structure in which a nonmagnetic body portion  121 , a magnetic body portion  122 , and a nonmagnetic body portion  123  are stacked in this order (the nonmagnetic body portions  121  and  123  may be low-magnetic body portions). Planar conductors FC 1   a  and FC 1   b  are embedded in the nonmagnetic body portion  121 , a plurality of coil conductors CP 1  to CP 7  that define a coil CIL 1  are embedded in the magnetic body portion  122 , and planar conductors FC 2   a  and FC 2   b  are embedded in the nonmagnetic body portion  123 . Therefore, the planar conductors FC 1   a  and FC 1   b  are provided on a lower side of the coil CIL 1 , and the planar conductors FC 2   a  and FC 2   b  are provided on an upper side of the coil CIL 1  (on the side of the one principal surface of the multilayer body  12 ). In the coil CIL 1 , a magnetic field is generated in a manner shown by broken lines in  FIG. 1 . 
     The nonmagnetic body portion  121 , the magnetic body portion  122 , and the nonmagnetic body portion  123  are formed preferably by stacking a plurality of ferrite layers including ferrite layers Lcp 1  to Lcp 7 , Lfc 2   a,  and LFc 2   b  to be described later. Therefore, the planar conductors FC 1   a,  FC 1   b,  FC 2   a,  and FC 2   b  and the coil conductors CP 1  to CP 7  are held between the plural stacked ferrite layers. The ferrite layers that define the nonmagnetic body portions  121  and  123  are nonmagnetic, and the ferrite layers that define the magnetic body portion  122  are magnetic. 
     In the nonmagnetic body portion  121 , the planar conductors FC 1   a  and FC 1   b  are arranged in the lamination direction such that their principal surfaces face in the lamination direction. Similarly, in the nonmagnetic body portion  123 , the planar conductors FC 2   a  and FC 2   b  are arranged in the lamination direction such that their principal surfaces face in the lamination direction. Here, the planar conductor FC 1   a  is connected to the outer electrode  14   a,  and the planar conductor FC 1   b  is connected to the outer electrode  14   b.  Also, the planar conductor FC 2   a  is connected to the electronic component  16   a,  and the planar conductor FC 2   b  is connected to the electronic component  16   b.    
     Each of the planar conductors FC 1   a,  FC 1   b,  FC 2   a,  and FC 2   b  thus provided defines and functions as any of a ground electrode, a shield electrode, and a capacitance electrode. Therefore, a potential difference is generated between the planar conductors, of the planar conductors FC 1   a  and FC 1   b  or the planar conductors FC 2   a  and FC 2   b,  except for the electrodes whose potentials are made equal, for example, by connection through a via-hole conductor. 
     In the magnetic body portion  122 , the coil CIL 1  is wound in, for example, seven turns in the lamination direction, and its winding axis extends in the lamination direction. When viewed from the lamination direction, the coil CIL 1  overlaps with the principal surfaces of the planar conductors FC 1   a,  FC 1   b,  FC 2   a,  and FC 2   b.  Hereinafter, a region overlapping with the coil CIL 1  when viewed from the lamination direction, of regions on each principal surface, is defined as a “specific region”. A region that does not overlap with the coil CIL 1  when viewed from the lamination direction, of the regions on the principal surfaces, is defined as a “non-overlapping region”. 
     Referring to  FIGS. 2A to 2D , the coil conductor CP 1  is provided on an upper surface of a magnetic ferrite layer LCp 1 , the coil conductor CP 2  is provided on an upper surface of a magnetic ferrite layer Lcp 2 , the coil conductor CP 3  is provided on an upper surface of a magnetic ferrite layer Lcp 3 , and the coil conductor CP 4  is provided on an upper surface of a magnetic ferrite layer Lcp 4 . Referring to  FIGS. 3A to 3C , the coil conductor CP 5  is provided on an upper surface of a magnetic ferrite layer Lcp 5 , the coil conductor CP 6  is provided on an upper surface of a magnetic ferrite layer Lcp 6 , and the coil conductor CP 7  is provided on an upper surface of a magnetic ferrite layer Lcp 7 . 
     All the coil conductors CP 1  to CP 7  define a C-shaped or substantially C-shaped pattern (a pattern shaped like a partly missing ring) when viewed from the lamination direction. However, the position where the conductor is missing is different among the coil conductors CP 1  to CP 7 . A plurality of black dots shown in  FIGS. 2A to 2D and 3A to 3C  show via-hole conductors, and the coil conductors CP 1  to CP 7  are connected in a helical shape by these via-hole conductors. Thus, the coil CIL 1  having the winding axis extending in the lamination direction is provided, and both ends of the coil CIL 1  appear on the lower surface of the magnetic body portion  122 . 
     The coil CIL 1  with the structure described above is appropriately connected to the planar conductors FC 1   a,  FC 1   b,  FC 2   a , and FC 2   b,  the outer electrodes  14   a  and  14   b,  or the electronic components  16   a  and  16   b  by via-hole conductors or side surface conductors that are not illustrated. 
     Referring to  FIGS. 4A and 4B , the planar conductor FC 2   a  is provided on an upper surface of a nonmagnetic ferrite layer Lfc 2   a,  and the planar conductor FC 2   b  is provided on an upper surface of a nonmagnetic ferrite layer Lfc 2   b.  The planar conductors FC 2   a  and FC 2   b  have different patterns according to the functions thereof. In principal surfaces of the planar conductor FC 2   a,  a plurality of first through holes HL 1   a  and a single second through hole HL 2   a  are provided. Similarly, in principal surfaces of the planar conductor FC 2   b,  a plurality of first through holes HL 1   b  and a single second through hole HL 2   b  are provided. 
       FIG. 5A  illustrates a state in which the planar conductor FC 2   a  is superposed on the coil conductor CP 5 , and  FIG. 5B  illustrates a state in which the planar conductor FC 2   b  is superposed on the coil conductor CP 5 . According to  FIGS. 5A and 5B , the first through holes HL 1   a  and HL 1   b  are provided in the specific region when viewed from the lamination direction (or at least one or some of the first through holes HL 1   a  and HL 1   b  are provided in the specific region), and the second through holes HL 2   a  and HL 2   b  are provided in the non-overlapping region when viewed from the lamination direction. 
     Here, while it does not matter how the first through holes HL 1   a  and HL 1   b  and the second through holes HL 2   a  and HL 2   b  are shaped, the through holes preferably have a ϕ shape to significantly reduce or prevent heat generation resulting from the decrease in cross-sectional area of the portion where the current flows. The hole diameter is adjusted to about 0.05 to about 1.0 mm, and as many holes as possible are formed, for example. The holes are the smallest possible holes to be formed by screen printing. Therefore, a plurality of second through holes HL 2   a  may be provided in the non-overlapping region of the planar conductor FC 2   a.  Similarly, a plurality of second through holes HL 2   b  may be provided in the non-overlapping region of the planar conductor FC 2   b.  Alternatively, the second through holes HL 2   a  and HL 2   b  may be omitted. 
     As for the distribution of the first through holes HL 1   a  and HL 1   b  and the second through holes HL 2   a  and HL 2   b,  the distribution density in the specific region is higher than the distribution density in the non-overlapping region. 
     Specifically, it is preferable that the ratio of the area of the first through holes HL 1   a  and the second through hole HL 2   a  to the area of the region surrounded by the outline of the planar conductor FC 2   a  is within a range of about 5% to about 30% and that not less than about 70% of the total area of the first through holes HL 1   a  and the second through hole HL 2   a  is in the specific region, for example. 
     Similarly, it is preferable that the ratio of the area of the first through holes HL 1   b  and the second through hole HL 2   b  to the area of the region surrounded by the outline of the planar conductor FC 2   b  is within a range of about 5% to about 30% and that not less than about 70% of the total area of the first through holes HL 1   b  and the second through hole HL 2   b  is within the specific region, for example. However, the positions of the first through holes HL 1   a  and HL 1   b  provided in the specific region do not need to be aligned with each other when viewed from the lamination direction. 
     The upper limit of “about 30%” is set to prevent deterioration of the functions of the planar conductors FC 2   a  and FC 2   b  (that is, the shield function in the case of the shield electrode) and to prevent heat generation of the planar conductors FC 2   a  and FC 2   b  resulting from the decrease in cross-sectional area of the portion where the current flows. That is, the original functions of the planar conductors FC 2   a  and FC 2   b  are set as a premise, and the first through holes HL 1   a  and HL 1   b  and the second through holes HL 2   a  and HL 2   b  are within a range such as not to impair the functions. 
     Although not illustrated, the planar conductors FC 1   a  and FC 1   b  have different patterns according to the functions thereof, similarly to the planar conductors FC 2   a  and FC 2   b.  Also, first through holes and a second through hole are provided in principal surfaces of each of the planar conductors FC 1   a  and FC 1   b.  Similarly to the above, the first through holes are provided in the specific region, and the second through hole is provided in the non-overlapping region. 
     The planar conductors FC 1   a,  FC 1   b,  FC 2   a,  and FC 2   b  and the coil conductors CP 1  to CP 7  contain silver, and a plurality of ferrite layers including the ferrite layers Lcp 1  to Lcp 7 , Lfc 2   a , and Lfc 2   b  contain copper. 
     More specifically, the planar conductors FC 1   a,  FC 1   b , FC 2   a,  and FC 2   b  and the coil conductors CP 1  to CP 7  preferably are mainly composed of conductive paste containing silver such as Ag, Ag—Pd, or Ag—Pt. The ferrite layers that define the nonmagnetic body portions  121  and  123  preferably are mainly composed of ferrite powder containing copper such as Zn—Cu-based ferrite powder, and the ferrite layers that define the magnetic body portion  122  preferably are mainly composed of ferrite powder containing copper such as Ni—Zn—Cu-based or Ni—Mn—Cu-based ferrite powder. 
     The multilayer body  12  is preferably produced as follows. First, green sheets serving as bases of the above-described ferrite layers are formed by applying slurry containing the above-described ferrite powder in the sheet form. Next, via holes are formed at predetermined positions in the green sheets by using a laser beam machine, and the via holes are filled with the above-described conductive paste. Further, the above-described conductive paste is applied on the green sheets by screen printing to form patterns serving as bases of planar conductors FC 1   a,  FC 1   b , FC 2   a,  and FC 2   b  or coil conductors CP 1  to CP 7 . 
     As a result of screen printing, holes corresponding to first through holes HL 1   a  and a second through hole HL 2   a  are formed in the pattern serving as the base of the planar conductor FC 2   a , and holes corresponding to first through holes HL 1   b  and a second through hole HL 2   b  are formed in the pattern serving as the base of the planar conductor FC 2   b.  In each of the planar conductors FC 1   a  and FC 1   b,  similar holes corresponding to first through holes and a second through hole are formed. 
     The plural green sheets in which the conductive paste is filled or applied by printing are stacked and pressure-bonded so that a coil CIL 1  having a winding axis extending in the lamination direction is formed, the planar conductors FC 1   a  and FC 1   b  are provided on a lower side of the coil CIL 1 , and the planar conductors FC 2   a  and FC 2   b  are provided on an upper side of the coil CIL 1 . By subjecting a green multilayer body thus produced to firing and plating, the above-described multilayer body  12  is obtained. 
     The conductive paste serving as the base of the planar conductors FC 1   a,  FC 1   b,  FC 2   a,  and FC 2   b  and the coil conductors CP 1  to CP 7  is fired simultaneously with firing of the green sheets (co-fire). On the other hand, outer electrodes  14   a  and  14   b  may be formed by co-firing similarly to the conductive paste, or may be formed by application and baking on the multilayer body  12  obtained by sintering (post-fire). The firing atmosphere is not particularly limited, and may be, for example, an oxidation atmosphere or a reducing atmosphere in both cases of co-firing and post-firing. 
     A multilayer coil component  10  is completed by mounting electronic components  16   a  and  16   b  on an upper surface of the multilayer body  12  thus produced. 
     Silver contained in the conductive paste is fired near approximately 900° C., for example, whereas copper is contained in the ferrite powder so that the green sheets are sintered at a low temperature in accordance with the firing temperature of silver. When copper is included, the sintering temperature decreases to a temperature that enables co-firing with silver. 
     However, when the green multilayer body is produced by stacking and pressure-bonding, the pressure in pressure-bonding increases in the region where the coil conductors CP 1  to CP 7  overlap together when viewed from the lamination direction, that is, in the specific region. This decreases the distance between the planar conductors FC 1   a  and FC 1   b  or the distance between the planar conductors FC 2   a  and FC 2   b  in the lamination direction. 
     Here, when iron oxide serving as the major raw material of ferrite is produced, a sulfur component is mixed therein. The sulfur component reacts with silver due to heat generated in firing of the green multilayer body, and silver sulfide generated by the reaction diffuses inside the multilayer body. Accumulation of the diffusing silver sulfide between the planar conductors FC 1   a  and FC 1   b  or between the planar conductors FC 2   a  and FC 2   b  may cause electrochemical migration. Further, when the coil conductors CP 1  to CP 7  also contain silver, the silver diffuses in the multilayer body during firing. Therefore, the risk of electrochemical migration is increased further. 
     While copper contributes to low-temperature sintering of the ferrite material, cuprous oxide generated by sintering has the property of a semiconductor. Hence, accumulation of cuprous oxide between the planar conductors FC 1   a  and FC 1   b  or between the planar conductors FC 2   a  and FC 2   b  may deteriorate insulation between the planar conductors FC 1   a  and FC 1   b  or between the planar conductors FC 2   a  and FC 2   b.    
     In any case, the distance between the planar conductors FC 2   a  and FC 2   b  is decreased in the lamination direction in the specific region. Hence, the above-described problems are likely to arise. 
     Accordingly, in this preferred embodiment, the first through holes HL 1   a  and HL 1   b  penetrating the principal surfaces in the specific region are provided in the planar conductors FC 2   a  and FC 2   b,  and similar first through holes are provided in the planar conductors FC 1   a  and FC 1   b.  Thus, the first through holes function as holes to significantly reduce or prevent accumulation of silver and copper (for diffusion silver and copper to the outside), and significantly reduce or prevent substances that lower reliability (especially AgS and Cu 2 O) from accumulating between the layers between which the withstand voltage is applied. For this reason, it is unnecessary to provide the ferrite layers with a thickness more than or equal to the precisely required thickness, and it is also unnecessary to evaluate performance of each multilayer coil component  10 . 
     For comparison, a multilayer coil component  10 ′ having a structure similar to that of the multilayer coil component  10  of this preferred embodiment of the present invention except that the first through holes and the second through holes were not provided was prepared, and was subjected to reliability evaluation (PCBT: thickness between layers with different potentials; 25 μm, withstand voltage; 20 V, number of samples; 200 for each component). While the reliability NG rate of the multilayer coil component  10 ′ in the comparative example was 5%, the multilayer coil component  10  of this preferred embodiment did not have such a reliability NG rate. This result shows that the multilayer coil component  10  of this preferred embodiment of the present invention has higher reliability. 
     In the multilayer coil component  10  of the present preferred embodiment of the present invention, not less than about 70% of the total area of the first through holes and the second through hole is in the specific region. When the area ratio was about 60%, the reliability NG rate was about 2%. Therefore, it is considered that not less than about 70% of the total area of the first through holes and the second through hole is preferably provided in the specific region. 
     This preferred embodiment is intended to the multilayer coil component  10  of a closed magnetic circuit type in which the magnetic body portion  122  is provided all between the nonmagnetic body portions  121  and  123 . However, the present invention can also be applied to a multilayer coil component of an open magnetic circuit type in which one or some of a plurality of ferrite layers that define the magnetic body portion  122  are nonmagnetic layers. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.