Patent Publication Number: US-2023162899-A1

Title: Multilayer coil component

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit of priority to Japanese Patent Application No. 2021-188648, filed Nov. 19, 2021, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a multilayer coil component. 
     Background Art 
     Japanese Unexamined Patent Application Publication No. 2017-59749 describes a multilayer coil component including an element body containing a magnetic material, a coil including a plurality of internal conductors which are spaced apart from each other in a first direction and are electrically connected together inside the element body, and a plurality of stress-relaxing spaces which are in contact with surfaces of the internal conductors and in which powders are present. 
     SUMMARY 
     Japanese Unexamined Patent Application Publication No. 2017-59749 discloses a configuration in which a section in a width direction of a coil conductor is trapezoidal or rectangular. If a section in a width direction of a coil conductor is trapezoidal or rectangular, stress due to a differential shrinkage between the coil conductor and an element body at the time of firing is likely to concentrate on a corner portion of the coil conductor. As a result, a crack may appear inside a multilayer coil component. 
     Accordingly, the present disclosure provides a multiplayer coil component in which stress concentration in a coil conductor is inhibited. 
     A multilayer coil component according to the present disclosure is a multilayer coil component including a multilayer body which is composed of a plurality of insulating layers stacked and incorporates a coil, and an outer electrode which is provided on an outer surface of the multilayer body and is electrically connected to the coil. The coil is formed by a plurality of coil conductors which are stacked together with the insulating layers being electrically connected together. The coil conductors each include an inner end face and an outer end face which face each other in a direction orthogonal to a stacking direction of the multilayer body in a section in a width direction of the coil conductor. The inner end face is located on an inner side of the multilayer body in the section in the width direction of the coil conductor. The outer end face is located on an outer side of the multilayer body in the section in the width direction of the coil conductor. At least one end face of the inner end face and the outer end face in at least one of the coil conductors includes a first surface and a second surface which is continuous with the first surface and is different in an angle with respect to a plane perpendicular to the stacking direction from the first surface. 
     According to the present disclosure, it is possible to provide a multilayer coil component in which stress concentration in a coil conductor is inhibited. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view schematically showing an example of a multilayer coil component according to the present disclosure; 
         FIG.  2    is a schematic view transparently showing an interior of the multilayer coil component according to the present disclosure such that a structure of a coil is understandable; 
         FIG.  3    is an LT sectional view schematically showing an internal structure of the multilayer coil component according to the present disclosure; 
         FIG.  4    is a WT sectional view schematically showing the internal structure of the multilayer coil component according to the present disclosure; 
         FIG.  5    is a sectional view schematically showing an example of a shape of a section in a width direction of a coil conductor; 
         FIG.  6 A 1  to  6 D 4    are exploded views schematically showing a method for producing a multilayer body; 
         FIGS.  7 A- 7 C  are sectional views for explaining a process of stacking a conductive paste layer and a ferrite paste layer; 
         FIG.  8    is a sectional view schematically showing a coil conductor and an insulating layer after firing; 
         FIG.  9    is an LT sectional view schematically showing an internal structure of a different example of the multilayer coil component according to the present disclosure; 
         FIG.  10 - 1 A 1  to  10 - 1 B 4   ′ are exploded views schematically showing a method for producing a multilayer body of the different example of the multilayer coil component according to the present disclosure; 
         FIG.  10 - 2 C 1  to  10 - 2 D 4   ′ are exploded views schematically showing the method for producing the multilayer body of the different example of the multilayer coil component according to the present disclosure; and 
         FIGS.  11 A- 11 C  are views showing results of simulating stress applied to a corner portion of a coil conductor in a process of firing a multilayer coil component. 
     
    
    
     DETAILED DESCRIPTION 
     A multilayer coil component according to the present disclosure will be described below. 
     The present disclosure, however, is not limited to the configurations and aspects below, and can be appropriately changed and applied without departing from the spirit of the present disclosure. Note that a combination of two or more of individual preferred configurations and aspects of the present disclosure to be described below is also included in the present disclosure. 
       FIG.  1    is a perspective view schematically showing an example of the multilayer coil component according to the present disclosure. 
     A multilayer coil component  1  shown in  FIG.  1    includes a multilayer body  10  and a first outer electrode  21  and a second outer electrode  22  which are provided on an outer surface of the multilayer body  10 . The multilayer body  10  has a substantially rectangular parallelepiped shape having six surfaces. A configuration of the multilayer body  10  will be described later. 
     As for the multilayer coil component  1  and the multilayer body  10 , a direction in which the first outer electrode  21  and the second outer electrode  22  face each other will be referred to as a length direction L. A direction orthogonal to the length direction L will be referred to as a height direction T, and a direction orthogonal to the length direction L and the height direction T will be referred to as a width direction W. 
     In  FIG.  1   , the length direction L, the width direction W, and the height direction T in the multilayer coil component  1  and the multilayer body  10  are shown as directions of double-headed arrows L, W, and T. 
     The length direction L, the width direction W, and the height direction T are orthogonal to one another. 
     A surface to be mounted of the multilayer coil component  1  is a surface (LW surface) parallel to the length direction L and the width direction W. 
     The multilayer body  10  shown in  FIG.  1    has a first end face  11  and a second end face  12  which are opposed to each other in the length direction L, a first principal surface  13  and a second principal surface  14  which are opposed to each other in the height direction T orthogonal to the length direction L, and a first side surface  15  and a second side surface  16  which are opposed to each other in the width direction W orthogonal to the length direction L and the height direction T. 
     Although not shown in  FIG.  1   , a multilayer body is preferably rounded at corner portions and ridge line portions. A corner portion is a portion where three surfaces of the multilayer body intersect, and a ridge line portion is a portion where two surfaces of the multilayer body intersect. 
     The first outer electrode  21  is arranged so as to cover the first end face  11  of the multilayer body  10 , extend from the first end face  11 , and cover a part of the first principal surface  13 , a part of the second principal surface  14 , a part of the first side surface  15 , and a part of the second side surface  16 , as shown in  FIG.  1   . The second outer electrode  22  is arranged so as to cover the second end face  12  of the multilayer body  10 , extend from the second end face  12 , and cover a part of the first principal surface  13 , a part of the second principal surface  14 , a part of the first side surface  15 , and a part of the second side surface  16 , as shown in  FIG.   1   . 
     The second principal surface  14  serves as the surface to be mounted. 
       FIG.  2    is a schematic view transparently showing an interior of the multilayer coil component according to the present disclosure such that a structure of a coil is understandable. 
     The multilayer body  10  includes a plurality of coil conductors  31 ,  32 ,  33 , and  34 , lead-out conductors  35  and  36 , and a plurality of via conductors  41 ,  42 , and  43 . 
     The coil conductors  31 ,  32 ,  33 , and  34  are arranged in order from a lower side in the height direction T. 
     The coil conductor  31  is continuous with the lead-out conductor  36 . The coil conductor  31  is electrically connected to the second outer electrode  22  at the second end face  12  via the lead-out conductor  36 . The coil conductor  34  is continuous with the lead-out conductor  35 . The coil conductor  34  is electrically connected to the first outer electrode  21  at the first end face  11  via the lead-out conductor  35 . 
     The coil conductors  31  to  34  are electrically connected together via the via conductors  41  to  43 . 
     The coil conductor  31  is electrically connected to the via conductor  41  at an end portion which is not continuous with the lead-out conductor  36 . The coil conductor  32  is electrically connected to the via conductor  41  at one end portion. The coil conductor  32  is electrically connected to the via conductor  42  at an end portion which is not electrically connected to the via conductor  41 . The coil conductor  33  is electrically connected to the via conductor  42  at one end portion. The coil conductor  33  is electrically connected to the via conductor  43  at an end portion which is not electrically connected to the via conductor  42 . The coil conductor  34  is electrically connected to the via conductor  43  at an end portion which is not continuous with the lead-out conductor  35 . 
       FIG.  3    is an LT sectional view schematically showing an internal structure of the multilayer coil component according to the present disclosure.  FIG.  3    is a sectional view taken along line A-A of the multilayer coil component in  FIG.  2   . 
       FIG.  4    is a WT sectional view schematically showing the internal structure of the multilayer coil component according to the present disclosure.  FIG.  4    is a sectional view taken along line B-B of the multilayer coil component in  FIG.  2   . 
     The multilayer body  10  is composed of a plurality of insulating layers  51 ,  52 ,  53 ,  54 , and  55  stacked and incorporates a coil  30 . 
     The coil  30  is formed by the plurality of coil conductors  31 ,  32 ,  33 , and  34  that are stacked together with the insulating layers  51 ,  52 ,  53 ,  54 , and  55  being electrically connected together. The first outer electrode  21  and the second outer electrode  22  are electrically connected to the coil  30 . In  FIGS.  2  to  4   , a conductor which leads the coil  30  to the first end face  11  is the lead-out conductor  35 , and a conductor which leads the coil  30  to the second end face  12  is the lead-out conductor  36 . A position where a coil conductor and an outer electrode are to be connected can be changed by changing a position where the coil conductor is to be led out of a multilayer body. The lead-out position may be changed, and the coil conductor and the outer electrode may be electrically connected at a principal surface or a side surface of the multilayer body. 
     The insulating layer  51  is present between the coil conductor  31  and the second principal surface  14  of the multilayer body  10 . The insulating layer  52  is present between the coil conductor  31  and the coil conductor  32 . The insulating layer  53  is present between the coil conductor  32  and the coil conductor  33 . The insulating layer  54  is present between the coil conductor  33  and the coil conductor  34 . The insulating layer  55  is present between the coil conductor  34  and the first principal surface  13  of the multilayer body  10 . 
     The via conductors  41 ,  42 , and  43  extend through the insulating layers  52 ,  53 , and  54 , respectively, in the height direction T. 
     Respective void portions  56  are preferably present between the coil conductors  31 ,  32 ,  33 , and  34  and the insulating layers  51 ,  52 ,  53 , and  54 . In  FIGS.  3  and  4   , the void portions  56  are present on lower sides in the height direction T of the coil conductors  31 ,  32 ,  33 , and  34 . The void portions  56  will be described later in detail. 
       FIG.  5    is a sectional view schematically showing an example of a shape of a section in a width direction of a coil conductor. 
     The coil conductor  31  has an inner end face  61  and an outer end face  62  which are two end faces facing each other in a direction orthogonal to a stacking direction of the multilayer body in a section in a width direction. 
     The coil conductor  31  preferably has a first principal surface  63  and a second principal surface  64  which are two principal surfaces facing each other in the stacking direction of the multilayer body in a section in the width direction. 
     The inner end face  61  is an end face which is present on an inner side of the multilayer body of the two end faces facing each other in the direction orthogonal to the stacking direction of the multilayer body. The outer end face  62  is an end face which is present on an outer side of the multilayer body of the two end faces facing each other in the direction orthogonal to the stacking direction of the multilayer body. 
     In the multilayer coil component according to the present disclosure, at least one end face of an inner end face and an outer end face in at least one coil conductor includes a first surface and a second surface which is continuous with the first surface and is different in an angle with respect to a plane perpendicular to the stacking direction from the first surface. 
     Note that “continuity” between surfaces in the present disclosure is not limited to a case where a first surface and a second surface are connected to have a flection and that the first surface and the second surface only need to be substantially continuous. For example, a case where a minute curved surface is interposed between the first surface and the second surface is also included. 
     If at least one end face of an inner end face and an outer end face includes a first surface and a second surface in a coil conductor, the following effect is produced. 
     Since a differential shrinkage occurs between a coil conductor and an insulating layer at the time of firing, stress due to the differential shrinkage is likely to concentrate on a corner portion of the coil conductor. If at least one end face of an inner end face and an outer end face includes a first surface and a second surface which is continuous with the first surface and is different in an angle with respect to a plane perpendicular to the above-described stacking direction from the first surface in the coil conductor, stress is also dispersed to a corner portion where the first surface and the second surface are in contact with each other. This results in inhibition of stress concentration on a corner portion of the coil conductor. For this reason, appearance of a crack inside a multilayer coil component is inhibited. 
     In  FIG.  5   , a first surface  61   a  and a second surface  61   b  are present in the inner end face  61  in the section in the width direction of the coil conductor  31 . 
     The first surface  61   a  of the inner end face  61  is in contact with the second surface  61   b  of the inner end face  61  at one end portion. The first surface  61   a  of the inner end face  61  and the second surface  61   b  of the inner end face  61  are continuous with each other. If a plane on a side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is regarded as a reference plane, an angle with respect to the reference plane at an end portion of the first surface  61   a  of the inner end face  61  which is not in contact with the second surface  61   b  of the inner end face  61  is a first angle θ1 of the inner end face  61 . Note that the end portion in the first surface  61   a  of the inner end face  61  that is not in contact with the second surface  61   b  of the inner end face  61  is one and the same as an end portion which is in contact with the first principal surface  63  in  FIG.  5   . The plane on the side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is one and the same as the first principal surface  63  in  FIG.  5   . 
     The second surface  61   b  of the inner end face  61  is in contact with the first surface  61   a  of the inner end face  61  at one end portion. In  FIG.  5   , the second surface  61   b  of the inner end face  61  is in contact with the second principal surface  64  that is one of the two principal surfaces facing each other in the stacking direction of the multilayer body at the other end portion. If the plane on the side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is regarded as a reference plane, an angle with respect to the reference plane at the end portion of the second surface  61   b  of the inner end face  61  that is in contact with the first surface  61   a  of the inner end face  61  is a second angle θ2 of the inner end face  61 . 
     In  FIG.  5   , a first surface  62   a  and a second surface  62   b  are present in the outer end face  62  in the section in the width direction of the coil conductor  31 . 
     The first surface  62   a  of the outer end face  62  is in contact with the second surface  62   b   of the outer end face  62  at one end portion. The first surface  62   a  of the outer end face  62  and the second surface  62   b  of the outer end face  62  are continuous with each other. If a plane on a side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is regarded as a reference plane, an angle with respect to the reference plane at an end portion of the first surface  62   a  of the outer end face  62  which is not in contact with the second surface  62   b  of the outer end face  62  is a first angle θ3 of the outer end face  62 . Note that the end portion in the first surface  62   a  of the outer end face  62  that is not in contact with the second surface  62   b  of the outer end face  62  is one and the same as an end portion which is in contact with the first principal surface  63  in  FIG.  5   . The plane on the side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is one and the same as the first principal surface  63  in  FIG.  5   . 
     The second surface  62   b  of the outer end face  62  is in contact with the first surface  62   a  of the outer end face  62  at one end portion. In  FIG.  5   , the second surface  62   b  of the outer end face  62  is in contact with the second principal surface  64  that is one of the two principal surfaces facing each other in the stacking direction of the multilayer body at the other end portion. If the plane on the side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is regarded as a reference plane, an angle with respect to the reference plane at the end portion of the second surface  62   b  of the outer end face  62  that is in contact with the first surface  62   a  of the outer end face  62  is a second angle θ4 of the outer end face  62 . 
     Although the coil conductor  31  is shown as an example in  FIG.  5   , it is only required that at least one end face of an inner end face and an outer end face in at least one coil conductor of a plurality of coil conductors constituting a coil includes a first surface and a second surface. In the multilayer coil component  1 , at least one end face of an inner end face and an outer end face of the coil conductor  31 ,  32 ,  33 , or  34  only needs to include a first surface and a second surface. All inner end faces and all outer end faces in a plurality of coil conductors constituting a coil may each include a first surface and a second surface. 
     It is preferable that, of the first angle θ1 of the inner end face  61  and the second angle θ2 of the inner end face  61 , one angle is not more than 90° and that the other angle is less than 90°. If the first angle θ1 of the inner end face  61  and the second angle θ2 of the inner end face  61  satisfy the above-described condition, stress concentration on a corner portion of the coil conductor is further inhibited. The first angle θ1 of the inner end face  61  may be not more than 90°, and the second angle θ2 of the inner end face  61  may be less than 90°. Alternatively, the first angle θ1 of the inner end face  61  may be less than 90°, and the second angle θ2 of the inner end face  61  may be not more than 90°. 
     It is more preferable that, of the first angle θ1 of the inner end face  61  and the second angle θ2 of the inner end face  61 , one angle is not less than 40° and not more than 85° (i.e., from 40° to 85°) and that the other angle is not less than 5° and not more than 30° (i.e., from 5° to 30°). If the first angle θ1 of the inner end face  61  and the second angle θ2 of the inner end face  61  satisfy the above-described condition, stress concentration on a corner portion of the coil conductor is further inhibited. The first angle θ1 of the inner end face  61  may be not less than 40° and not more than 85° (i.e., from 40° to 85°), and the second angle θ2 of the inner end face  61  may be not less than 5° and not more than 30° (i.e., from 5° to 30°). Alternatively, the first angle θ1 of the inner end face  61  may be not less than 5° and not more than 30° (i.e., from 5° to 30°), and the second angle θ2 of the inner end face  61  may be not less than 40° and not more than 85° (i.e., from 40° to 85°). 
     The first angle θ1 of the inner end face  61  is preferably larger than the second angle θ2 of the inner end face  61 . If the first angle θ1 of the inner end face  61  and the second angle θ2 of the inner end face  61  satisfy the above-described relation, stress concentration on a corner portion of the coil conductor is further inhibited. 
     It is preferable that, of the first angle θ3 of the outer end face  62  and the second angle θ4 of the outer end face  62 , one angle is not more than 90° and that the other angle is less than 90°. If the first angle θ3 of the outer end face  62  and the second angle θ4 of the outer end face  62  satisfy the above-described condition, stress concentration on a corner portion of the coil conductor is further inhibited. The first angle θ3 of the outer end face  62  may be not more than 90°, and the second angle θ4 of the outer end face  62  may be less than 90°. Alternatively, the first angle θ3 of the outer end face  62  may be less than 90°, and the second angle θ4 of the outer end face  62  may be not more than 90°. 
     It is more preferable that, of the first angle θ3 of the outer end face  62  and the second angle θ4 of the outer end face  62 , one angle is not less than 40° and not more than 85° (i.e., from 40° to 85°) and that the other angle is not less than 5° and not more than 30° (i.e., from 5° to 30°). If the first angle θ3 of the outer end face  62  and the second angle θ4 of the outer end face  62  satisfy the above-described condition, stress concentration on a corner portion of the coil conductor is further inhibited. The first angle θ3 of the outer end face  62  may be not less than 40° and not more than 85° (i.e., from 40° to 85°), and the second angle θ4 of the outer end face  62  may be not less than 5° and not more than 30° (i.e., from 5° to 30°). Alternatively, the first angle θ3 of the outer end face  62  may be not less than 5° and not more than 30° (i.e.., from 5° to 30°), and the second angle θ4 of the outer end face  62  may be not less than 40° and not more than 85° (i.e., from 40° to 85°). 
     The first angle θ3 of the outer end face  62  is preferably larger than the second angle θ4 of the outer end face  62 . If the first angle θ3 of the outer end face  62  and the second angle θ4 of the outer end face  62  satisfy the above-described relation, stress concentration on a corner portion of the coil conductor is further inhibited. 
     The first angle θ1 of the inner end face  61  and the first angle θ3 of the outer end face  62  may be the same or may be different. 
     The second angle θ2 of the inner end face  61  and the second angle θ4 of the outer end face  62  may be the same or may be different. 
     Although the first surface  61   a  of the inner end face  61  and the second surface  61   b  of the inner end face  61  are shown to be linear in  FIG.  5   , the first surface  61   a  and the second surface  61   b  may be curved. 
     A method for measuring the first angle θ1 of the inner end face  61  is as follows. A straight line is drawn between the end portion of the first surface  61   a  of the inner end face  61  that is in contact with the second surface  61   b  of the inner end face  61  and the end portion of the first surface  61   a  of the inner end face  61  that is not in contact with the second surface  61   b  of the inner end face  61 . An angle which the straight line forms with the plane on the side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is measured and the angle is set as the first angle θ1 of the inner end face  61 . 
     A method for measuring the second angle θ2 of the inner end face  61  is as follows. A straight line is drawn between the end portion of the second surface  61   b  of the inner end face  61  that is in contact with the first surface  61   a  of the inner end face  61  and the end portion of the second surface  61   b  of the inner end face  61  that is not in contact with the first surface  61   a  of the inner end face  61 . An angle which the straight line forms with the plane on the side of the coil conductor of planes perpendicular to the stacking direction of the multilayer body is measured and the angle is set as the second angle θ2 of the inner end face  61 . 
     Although the first surface  62   a  of the outer end face  62  and the second surface  62   b  of the outer end face  62  are shown to be linear in  FIG.  5   , the first surface  62   a  and the second surface  62   b  may be curved. 
     A method for measuring the first angle θ3 of the outer end face  62  is the same as that for the first angle θ1 of the inner end face  61 . A method for measuring the second angle θ4 of the outer end face  62  is the same as that for the second angle θ2 of the inner end face  61 . 
     In the present specification, a width direction of a coil conductor is a direction orthogonal to a length direction of the coil conductor on a plane where the coil conductor is present. The length direction of the coil conductor is a direction in which the coil conductor extends on the plane where the coil conductor is present. Thus, a width direction of a coil conductor does not necessarily coincide with the width direction W in  FIGS.  1  to  4   , and a length direction of the coil conductor does not necessarily coincide with the length direction L in  FIGS.  1  to  4   . 
     For example, sections of the coil conductors  31  and  34  shown in  FIG.  3    are all sections in length directions of the coil conductors. 
     Meanwhile, a section on a right side in  FIG.  3    of the coil conductor  32  is a section in a width direction of the coil conductor, and a section on a left side in  FIG.  3    of the coil conductor  32  is a section in a length direction of the coil conductor. A section on the left side in  FIG.  3    of the coil conductor  33  is a section in a width direction of the coil conductor, and a section on the right side in  FIG.  3    of the coil conductor  33  is a section in a length direction of the coil conductor. 
     Sections of the coil conductors  31 ,  32 ,  33 , and  34  shown in  FIG.  4    are all sections in width directions of the coil conductors. 
     A void portion is preferably present between at least one of coil conductors which includes a first surface and a second surface and an insulating layer. 
     For the coil conductor  31 , the void portion  56  is present between the coil conductor  31  and the insulating layer  51 . 
       FIG.  5    shows a configuration in which the coil conductor  31  is present on the insulating layer  51 , and the void portion  56  is present between the insulating layer  51  and the coil conductor  31 . The void portion  56  is formed at a position slightly inward from end portions of the coil conductor  31 . 
     Since a ceramic material (for example, a ferrite material), of which an insulating layer is composed, shrinks more than a metal material (for example, silver), of which a coil conductor is composed, at the time of sintering, an unnecessary force is applied between the ceramic material and the metal material. This causes degradation of inductance and impedance of a multilayer coil component. 
     If a void portion is provided between an insulating layer and a coil conductor, contact between a ceramic material and a metal material decreases, and a force applied between the ceramic material and the metal material at the time of sintering is relaxed. 
     For this reason, inductance and impedance of a multilayer coil component can be inhibited from degrading. 
     The coil conductor  31  includes the first principal surface  63  and the second principal surface  64  that are the two principal surfaces facing each other in the stacking direction of the multilayer body, and the void portion  56  may be present so as to be in contact with either one of the first principal surface  63  and the second principal surface  64 . 
     A width of the void portion  56  may be smaller than a width of the coil conductor  31 . 
     The width of the coil conductor  31  means a width at a portion where the coil conductor  31  has a largest width in a section in the stacking direction of the coil conductor  31 . In  FIG.  5   , the width of the coil conductor  31  is the same as a width of the first principal surface  63 . 
     An example of a method for manufacturing the multilayer coil component according to the present disclosure will next be described. 
     A ferrite sheet, a ferrite paste, a resin paste, and a conductive paste as materials are first prepared. 
     For the ferrite sheet, a ferrite material containing Fe, Zn, Cu, and Ni as main ingredients is preferably used. It is preferable that the ferrite material contains 40 mol% or more to 49.5 mol% or less (i.e., from 40 mol% to 49.5 mol%) of Fe in terms of Fe 2 O 3 , 5 mol% or more to 35 mol% or less (i.e., from 5 mol% to 35 mol%) of Zn in terms of ZnO, and 4 mol% or more to 12 mol% or less (i.e., from 4 mol% to 12 mol%) of Cu in terms of CuO. An Ni content of the ferrite material is not particularly limited, and what is left after Fe, Zn, and Cu that are the other main ingredients are taken away can be set as the Ni content. The material may be made to contain trace additives (including inevitable impurities), such as Bi, Sn, Mn, and Co. 
     As a method for producing the ferrite sheet, for example, the following method can be named. 
     Fe 2 O 3 , ZnO, CuO, NiO, and optionally additives are weighed so as to achieve a predetermined composition. The weighed substances, pure water, a dispersant, and PSZ media (partially stabilized zirconia) are put in a ball mill, mixed, and milled. After obtained slurry is dried, the slurry is calcined at a temperature not less than 700° C. and not more than 800° C. (i.e., from 700° C. to 800° C.), thereby obtaining calcined powder of a ferrite material. 
     The calcined powder of the ferrite material obtained in the above-described manner, an organic binder based on, for example, polyvinyl butyral, an organic solvent, such as ethanol or toluene, and PSZ media are put in a ball mill, mixed, and milled. After an obtained mixture is formed into a sheet of predetermined thickness by a doctor blade method, pieces of predetermined size are stamped out of the sheet. In this manner, a ferrite sheet can be produced. 
     For the ferrite paste, a ferrite material containing Fe, Zn, Cu, and Ni as main ingredients is preferably used. It is preferable that the ferrite material contains 40 mol% or more to 49.5 mol% or less (i.e., from 40 mol% to 49.5 mol%) of Fe in terms of Fe 2 O 3 , 5 mol% or more to 35 mol% or less (i.e., from 5 mol% to 35 mol%) of Zn in terms of ZnO, and 4 mol% or more to 12 mol% or less (i.e., from 4 mol% to 12 mol%) of Cu in terms of CuO. An Ni content of the ferrite material is not particularly limited, and what is left after Fe, Zn, and Cu that are the other main ingredients are taken away can be set as the Ni content. The material may be made to contain trace additives (including inevitable impurities), such as Bi, Sn, Mn, and Co. 
     As a method for producing the ferrite paste, for example, the following method can be named. 
     A predetermined amount of solvent, such as a ketones solvent, resin, such as polyvinyl acetal, and a plasticizer, such as an alkyd plasticizer, are added to calcined powder of a ferrite material which is obtained in the same manner as the above-described method for producing the ferrite sheet, and an obtained mixture is kneaded by a planetary mixer. After that, the mixture is further dispersed by a triple roll mill. In this manner, a ferrite paste can be produced. 
     The resin paste is a paste for forming a resin layer between a ceramic paste and the conductive paste, and a void portion is formed by burning off the resin layer after firing. 
     As a method for producing the resin paste, for example, the following method can be named. 
     A resin paste is produced by causing a solvent (for example, isophorone) to contain resin (for example, acrylic resin) to be burned off at the time of firing. 
     It is preferable to use, as the conductive paste, a paste containing silver as a conductive material. 
     As a method for producing the conductive paste, for example, the following method can be named. 
     Silver powder is prepared, a predetermined amount of solvent (for example, eugenol), resin (for example, ethylcellulose), and a dispersant are put. After an obtained mixture is kneaded by a planetary mixer, the mixture is dispersed by a triple roll mill. In this manner, a conductive paste is produced. 
     An example of a method for producing a multilayer body using the above-described materials will next be described. 
       FIG.  6 A 1 - 6 D 4    are exploded views schematically showing the method for producing a multilayer body. 
       FIGS.  7 A- 7 C  are sectional views for explaining a process of stacking a conductive paste layer and a ferrite paste layer. 
     First, a ferrite sheet  71  is prepared ( FIG.  6 A 1   ). 
     Then, a resin paste is applied to an area where a void portion is to be formed on the ferrite sheet  71 , and a resin paste layer  81  is formed ( FIG.  6 A 2   ). 
     Then, a conductive paste is applied to an area where a coil conductor is to be formed, and a conductive paste layer  91  and a lead-out conductor portion  96  are formed ( FIG.  6 A 3   ). In this case, a section in a width direction of the conductive paste layer  91  has a so-called dome shape, in which a surface is rounded and a thickness in the stacking direction at a middle portion is larger than thicknesses in the stacking direction at end portions ( FIG.  7 A ). 
     Then, the conductive paste layer  91  is pressurized using a calendering roll  100 , thereby planarizing a surface of the conductive paste layer  91  ( FIG.  7 B ). With this operation, corner portions are formed at the surface of the conductive paste layer  91 . 
     Then, a ferrite paste is applied to a region where the conductive paste layer  91  is not formed, and a ferrite paste layer  72  is formed ( FIG.  6 A 4   ). The ferrite paste layer  72  serves as the insulating layer  51  surrounding a coil conductor after firing. Here, the application is performed such that the ferrite paste layer  72  overlaps with a part of each end portion of the conductive paste layer  91  by a predetermined length (a length indicated by a double-headed arrow X in  FIG.  7 C ) and a predetermined thickness (a thickness indicated by a double-headed arrow Y in  FIG.  7 C ) ( FIG.  7 C ). 
     With the above-described steps, pattern sheet  1  on which the resin paste layer  81 , the lead-out conductor portion  96 , the conductive paste layer  91 , and the ferrite paste layer  72  are printed is formed. 
     The ferrite sheet  71  is separately prepared, and laser is applied to an area which is to be connected to the conductive paste layer  91  formed on pattern sheet  1 , and a via hole  44  is formed ( FIG.  6 B 1   ). 
     Then, the resin paste is applied to an area where a void portion is to be formed, and the resin paste layer  81  is formed ( FIG.  6 B 2   ). 
     Then, the conductive paste is applied to an area where a coil conductor is to be formed, a conductive paste layer  92  is formed, and the via hole  44  is filled with the conductive paste ( FIG.  6 B 3   ). 
     Then, the ferrite paste is applied to a region where the conductive paste layer  92  is not formed, and the ferrite paste layer  72  is formed. Here, as in pattern sheet  1 , the application is performed such that the ferrite paste layer  72  overlaps with a part of each end portion of the conductive paste layer  92  ( FIG.  6 B 4   ). 
     With the above-described steps, pattern sheet  2  on which the resin paste layer  81 , the conductive paste layer  92 , and the ferrite paste layer  72  are printed is formed. 
     Pattern sheet  3  and pattern sheet  4  are produced by the same procedures as pattern sheet  2  ( FIG.  6 C 1  to  6 C 4    and  FIG.  6 D 1  to  6 D 4   ). Note that a conductive paste layer  94  and a lead-out conductor portion  95  are formed on pattern sheet  4  ( FIG.  6 D 3   ). 
     Pattern sheet  1 , pattern sheet  2 , pattern sheet  3 , and pattern sheet  4  produced in the above-described manner are stacked in order, and a predetermined number of unprinted ferrite sheets are stacked on the top and bottom. The stacked sheets are subjected to pressurization and pressure bonding processing under a condition with a temperature not less than 70° C. and not more than 90° C. (i.e., from 70° C. to 90° C.) and a pressure not less than 60 MPa and not more than 100 MPa (i.e., from 60 MPa to 100 MPa), thereby obtaining a multilayer body block (element assembly). 
     Then, elements are obtained by cutting the multilayer body block with a dicer or the like into individual pieces. After that, the obtained elements are fired in a firing furnace at a temperature not less than 880° C. and not more than 950° C. (i.e., from 880° C. to 950° C.) for one hour or more to eight hours or less (i.e., from one hour to eight hours). 
     It is preferable to round ridge line portions and corner portions of each fired element by putting the element in a rotating barrel machine together with a medium and rotating the rotating barrel machine. With the above-described steps, electronic component element bodies are obtained. 
     Then, a conductive paste containing silver and glass is applied to end faces which are side surfaces of each element and to which a coil is led. An underlying electrode of each outer electrode is formed by baking the conductive paste at a temperature not less than 600° C. and not more than 850° C. (i.e., from 600° C. to 850° C.). After that, the outer electrode is formed by sequentially forming an Ni coating and an Sn coating on the underlying electrode by electrolytic plating, thereby obtaining the multilayer coil component  1  as shown in  FIG.  1   . The thicknesses of the Ni coating and the Sn coating may be, for example, about 3 µm. A multilayer coil component produced by the above-described method may have, for example, the following dimensions: L (length) = 1.0 mm, W (width) = 0.5 mm, and T (height) = 0.5 mm. 
     Note that the ferrite sheets  71  and the ferrite paste layers  72  in  FIG.  6 A 1 - 6 D 4    serve as the insulating layers  51 ,  52 ,  53 , and  54  after firing. The resin paste layers  81  in  FIG.  6 A 1 - 6 D 4    serve as the void portions  56  after firing. The conductive paste layers  91  and  92 , a conductive paste layer  93 , and the conductive paste layer  94  in  FIG.  6 A 1 - 6 D 4    serve as the coil conductors  31 ,  32 ,  33 , and  34 , respectively, after firing. The lead-out conductor portions  95  and  96  in  FIG.  6 A 1 - 6 D 4    serve as the lead-out conductors  35  and  36 , respectively, after firing. The conductive pastes, with which the via hole  44  and via holes  45  and  46  in  FIG.  6 A 1 - 6 D 4    are filled, serve as the via conductors  41 ,  42 , and  43 , respectively, after firing. 
       FIG.  8    is a sectional view schematically showing a coil conductor and an insulating layer after firing. 
     A method for forming a first surface and a second surface at each end face in a section in a width direction of a coil conductor will be described with reference to  FIGS.  7 A- 7 C and  8   . 
     As shown in  FIG.  7 C , in pattern sheet  1  before firing, the ferrite paste layer  72  overlaps with a part of each end portion of the conductive paste layer  91  by the predetermined length X and the predetermined thickness Y. In the pressurization and pressure bonding step, a higher pressure is applied to a portion where the ferrite paste layer  72  overlaps with the conductive paste layer  91  than in a portion where the ferrite paste layer  72  does not overlap. As a result, in the coil conductor  31 , at least one end face of the inner end face  61  and the outer end face  62  includes the first surface  61   a  or  62   a  and the second surface  61   b  or  62   b  that is continuous with the first surface  61   a  or  62   a  and is different in an angle with respect to a plane perpendicular to the stacking direction of the multilayer body from the first surface  61   a  or  62   a , in a section in the width direction ( FIG.  8   ). Note that if the planarization of the surface of the conductive paste layer  91  by the calendering roll  100  as shown in  FIG.  7 B  is not performed, no corner portion is present at the conductive paste layer  91 , and the ferrite paste layer  72  may flow transversely. For this reason, a formed coil conductor does not include a first surface and a second surface in a section in a width direction. 
     The second angle θ2 of the inner end face  61  and the second angle θ4 of the outer end face  62  in a section in a width direction of a coil conductor can be appropriately changed by changing the length X of a portion where the ferrite paste layer  72  overlaps with a part of each end portion of the conductive paste layer  91  and the thickness Y of the portion where the ferrite paste layer  72  overlaps with the part of the end portion of the conductive paste layer  91 . 
     In the multilayer coil component according to the present disclosure, the coil is preferably composed of two or more layers of coil conductors which are electrically connected in parallel via a via conductor and are present at different layers. 
     Since if a coil is composed of two or more layers of coil conductors which are electrically connected in parallel via a via conductor and are present at different layers, a DC resistance of the coil decreases, an even larger current can be added. 
       FIG.  9    is an LT sectional view schematically showing an internal structure of a different example of the multilayer coil component according to the present disclosure. 
     In a multilayer coil component  2 , the multilayer body  10  includes a plurality of coil conductors  31   a ,  31   b ,  32   a ,  32   b ,  33   a ,  33   b ,  34   a , and  34   b  and a plurality of via conductors  41   a ,  41   b ,  41   c ,  42   a ,  42   b ,  42   c ,  43   a ,  43   b , and  43   c . In  FIG.  9   , the coil conductors  31   a ,  31   b ,  32   a ,  32   b ,  33   a ,  33   b ,  34   a , and  34   b  are arranged in order from a lower side in a height direction T. The coil conductor  31   a  and the coil conductor  31   b  are the same in coil conductor pattern. Similarly, the coil conductor  32   a  and the coil conductor  32   b , the coil conductor  33   a  and the coil conductor  33   b , and the coil conductor  34   a  and the coil conductor  34   b  are each the same in coil conductor pattern. 
     The coil conductors  31   a  and  31   b  are electrically connected together via the via conductor  41   a . The coil conductors  31   b  and  32   a  are electrically connected together via the via conductor  41   b . The coil conductors  32   a  and  32   b  are electrically connected together via the via conductor  41   c . Note that the via conductor  41   a , the via conductor  41   b , and the via conductor  41   c  are present so as to overlap as viewed from a stacking direction of the multilayer body. 
     Similarly, the coil conductors  32   a  and  32   b  are electrically connected together via the via conductor  42   a . The coil conductors  32   b  and  33   a  are electrically connected together via the via conductor  42   b . The coil conductors  33   a  and  33   b  are electrically connected together via the via conductor  42   c . Note that the via conductor  42   a , the via conductor  42   b , and the via conductor  42   c  are present so as to overlap as viewed from a stacking direction of the multilayer body. 
     Similarly, the coil conductors  33   a  and  33   b  are electrically connected together via the via conductor  43   a . The coil conductors  33   b  and  34   a  are electrically connected together via the via conductor  43   b . The coil conductors  34   a  and  34   b  are electrically connected together via the via conductor  43   c . Note that the via conductor  43   a , the via conductor  43   b , and the via conductor  43   c  are present so as to overlap as viewed from a stacking direction of the multilayer body. 
     With the configuration as in  FIG.  9   , the coil conductor  31   a  and the coil conductor  31   b  are electrically connected in parallel. Similarly, the coil conductor  32   a  and the coil conductor  32   b , the coil conductor  33   a  and the coil conductor  33   b , and the coil conductor  34   a  and the coil conductor  34   b  are each electrically connected in parallel. 
     A method for producing the multilayer coil component  2  will next be described with reference to  FIGS.  10 - 1 A 1  to  10 - 1 B 4 ′ and  10 - 2 C 1  to  10 - 2 D 4   ′. 
       FIGS.  10 - 1 A 1  to  10 - 1 B 4 ′ and  10 - 2 C 1  to  10 - 2 D 4   ′ are exploded views schematically showing a method for producing a multilayer body of the different example of the multilayer coil component according to the present disclosure. 
       FIGS.  10 - 1 A 1  to  10 - 1 B 4 ′ and  10 - 2 C 1  to  10 - 2 D 4   ′ show a method for producing the multilayer body  10  in the multilayer coil component 2. 
     Pattern sheet  1  is produced by the same method as in  FIG.  6 A 1  to  6 A 4    ( FIG.  10 - 1 A 1  to  10 - 1 A 4   ). 
     Then, pattern sheet  1 ′ that is the same as pattern sheet  1  except that a via hole  44   a  is present is produced ( FIG.  10 - 1 A 1  to  10 - 1 A 4   ′). A conductive paste, with which the via hole  44   a  of pattern sheet  1 ′ is filled, serves as the via conductor  41   a  after firing. 
     Then, pattern sheet  2  is produced by the same method as in  FIG.  6 B 1  to  6 B 4    ( FIG.  10 - 1 B 1  to  10 - 1 B 4   ). Note that a conductive paste, with which a via hole  44   b  of pattern sheet  2  is filled, serves as the via conductor  41   b  after firing. 
     Then, pattern sheet  2 ′ that is the same as pattern sheet  2  except that a via hole  45   a  is present is produced ( FIG.  10 - 1 B 1 ′ to  10 - 1 B 4 ′ ). Note that a conductive paste, with which a via hole  44   c  of pattern sheet  2 ′ is filled, serves as the via conductor  41   c  after firing. A conductive paste, with which the via hole  45   a  of pattern sheet  2 ′ is filled, serves as the via conductor  42   a  after firing. 
     Pattern sheet  3  and pattern sheet  4  are produced by the same procedures as pattern sheet  2  ( FIG.  10 - 2 C 1  to  10 - 2 C 4    and  FIG.  10 - 2 D 1  to  10 - 2 D 4   ). Pattern sheet  3 ′ and pattern sheet  4 ′ are produced by the same procedures as pattern sheet  2 ′ ( FIG.  10 - 2 C 1 ′ to  10 - 2 C 4 ′  and  FIG.  10 - 2 D 1 ′ to  10 - 2 D 4 ′ ). 
     Pattern sheet  1 , pattern sheet  1 ′, pattern sheet  2 , pattern sheet  2 ′, pattern sheet  3 , pattern sheet  3 ′, pattern sheet  4 , and pattern sheet  4 ′ produced in the above-described manner are stacked in order, and a predetermined number of unprinted ferrite sheets are stacked on the top and bottom. The stacked sheets are subjected to pressurization and pressure bonding processing under a condition with a temperature not less than 70° C. and not more than 90° C. (i.e., from 70° C. to 90° C.) and a pressure not less than 60 MPa and not more than 100 MPa (i.e., from 60 MPa to 100 MPa), thereby obtaining a multilayer body block (element assembly). 
     In terms of steps other than the above-described ones, the same steps as for the multilayer coil component  1  are performed, thereby obtaining the multilayer coil component  2  as shown in  FIG.  9   . 
     Conductive paste layers  91   a ,  91   b ,  92   a ,  92   b ,  93   a ,  93   b ,  94   a , and  94   b  in  FIG.  10 - 1 A 1  to  10 - 1 B 4 ′  or  FIG.  10 - 2 C 1  to  10 - 2 D 4 ′  serve as the coil conductors  31   a ,  31   b ,  32   a ,  32   b ,  33   a ,  33   b ,  34   a , and  34   b , respectively, after firing. Conductive pastes, with which the via holes  44   a ,  44   b ,  44   c , and  45   a  and via holes  45   b ,  45   c ,  46   a ,  46   b , and  46   c  in  FIG.  10 - 1 A 1  to  10 - 1 B 4 ′  or  FIG.  10 - 2 C 1  to  10 - 2 D 4 ′ are filled, serve as the via conductors  41   a ,  41   b ,  41   c ,  42   a ,  42   b ,  42   c ,  43   a ,  43   b , and  43   c , respectively, after firing. 
       FIGS.  11 A- 11 C  are views showing results of simulating stress applied to a corner portion of a coil conductor in a process of firing a multilayer coil component. 
     For simulation of stress applied to a corner portion of a coil conductor, a multilayer coil component according to Example 1 with a coil conductor having a shape including a first surface and a second surface at an end face, a multilayer coil component according to Comparative Example 1 with a coil conductor having a rectangular shape, and a multilayer coil component according to Comparative Example 2 with a coil conductor having a trapezoidal shape were used. 
     Note that the stress simulation was performed using Femtet® from Murata Software Co., Ltd. 
       FIG.  11 A  illustrates a result of simulating stresses applied to a coil conductor portion and an insulating layer in the multilayer coil component according to Example 1 with the coil conductor having the shape including the first surface and the second surface at the end face. 
       FIG.  11 B  illustrates a result of simulating stress applied to a coil conductor portion and an insulating layer in the multilayer coil component according to Comparative Example 1 with the coil conductor having the rectangular shape. 
       FIG.  11 C  illustrates a result of simulating stress applied to a coil conductor portion and an insulating layer in the multilayer coil component according to Comparative Example 2 with the coil conductor having the trapezoidal shape. 
     Comparison between a stress applied to a corner portion (A1) where the first surface and the second surface of the coil conductor are in contact with each other in  FIG.  11 A  and a stress applied to an upper left corner portion (B) of the coil conductor in  FIG.  11 B  has found that the stress applied to the corner portion (A1) was 0.70 times the stress applied to the corner portion (B). 
     Comparison between a stress applied to a corner portion (A2) which is an end portion not in contact with the first surface in the second surface of the coil conductor in  FIG.  11 A  and the stress applied to the upper left corner portion (B) of the coil conductor in  FIG.  11 B  has found that the stress applied to the corner portion (A2) was 0.66 times the stress applied to the corner portion (B). 
     It is apparent from this that concentration of stress on a corner portion of a coil conductor is lower by 30% or more to 34% or less (i.e., from 30% to 34%) in the multilayer coil component according to Example 1 than in the multilayer coil component according to Comparative Example 1. 
     Comparison between the stress applied to the corner portion (A1) where the first surface and the second surface of the coil conductor are in contact with each other in  FIG.  11 A  and a stress applied to an upper left corner portion (C) of the coil conductor in  FIG.  11 C  has found that the stress applied to the corner portion (A1) was 0.89 times the stress applied to the corner portion (C). 
     Comparison between the stress applied to the corner portion (A2) that is the end portion not in contact with the first surface in the second surface in  FIG.  11 A  and the stress applied to the upper left corner portion (C) of the coil conductor in  FIG.  11 C  has found that the stress applied to the corner portion (A2) was 0.83 times the stress applied to the corner portion (C). 
     It is apparent from this that concentration of stress on a corner portion of a coil conductor is lower by 11% or more to 17% or less (i.e., from 11% to 17%) in the multilayer coil component according to Example 1 than in the multilayer coil component according to Comparative Example 2. 
     The above-described results show that concentration of stress on a corner portion of a coil conductor is inhibited in a multilayer coil component according to the present disclosure, as compared with a multilayer coil component with a coil conductor having a rectangular shape or a trapezoidal shape.