CN114388216A - Soft magnetic powder and inductor - Google Patents

Abstract

The invention provides a soft magnetic powder which is a metal magnetic body used in an inductor and can ensure mechanical strength and realize high magnetic conductivity when the metal magnetic body is formed. A soft magnetic powder comprising soft magnetic particles composed of a particle core comprising a soft magnetic metal, and an insulating film located on the surface of the particle core; the insulating film contains Si and a hydrocarbon group having a linear portion having 8 or more carbon atoms, and the weight ratio of Si to C in the insulating film is 7.6 to 42.8.

Description

Soft magnetic powder and inductor

Technical Field

The present invention relates to a soft magnetic powder and an inductor using the same.

Background

Inductors (coil components) using metallic magnetic materials are widely used in various electronic devices such as smart phones, for example, as surface-mountable chip inductors. As a metallic magnetic material used for such an inductor, a compressed powder magnetic core or a unit body is known, which is obtained by adding a resin to a soft magnetic powder composed of particles of a metal as a soft magnetic material and compression-molding the mixture.

Patent document 1 describes magnetic particles in which an insulating film is formed on the surface of a core (particle core) of a fine magnetic material, the insulating film being formed of a sol-gel reaction product of an organic phosphoric acid having a hydrocarbon group with a carbon chain length of 5 atoms or more and a metal alkoxide. The magnetic particles thus constructed improve the smoothness during compression molding for forming the metallic magnetic body, and increase the filling rate of the magnetic particles in the metallic magnetic body, thereby increasing the permeability of the metallic magnetic body.

However, if the smoothness of the magnetic particles is improved, the bonding force between the magnetic particles and the resin existing around the magnetic particles may be reduced, which may result in a reduction in the strength of the magnetic metal body. That is, the conventional magnetic particles have room for improvement in the balance between the height of permeability and the mechanical strength of the metallic magnetic body formed of the magnetic particles.

Documents of the prior art

Patent document

Patent document 1 International publication No. 2018/131536

Disclosure of Invention

The invention provides a soft magnetic powder which can realize high magnetic permeability while ensuring mechanical strength of a metal magnetic body when the metal magnetic body is formed by compression molding.

One embodiment of the present invention is a soft magnetic powder including soft magnetic particles composed of a particle core including a soft magnetic metal and an insulating film located on a surface of the particle core; the insulating film contains Si and a hydrocarbon group having a linear portion having 8 or more carbon atoms, and the weight ratio of Si to C in the insulating film is 7.6 to 42.8.

According to the soft magnetic powder of the present invention, when the soft magnetic powder is compression-molded to form a metal magnetic body, high magnetic permeability can be achieved while ensuring mechanical strength of the metal magnetic body.

Drawings

Fig. 1 is a diagram schematically showing the configuration of an inductor according to an embodiment of the present invention, and is a perspective view seen from the top surface side of the inductor.

Fig. 2 is a view schematically showing the structure of the same inductor, and is a perspective view seen from the side of the mounting surface of the inductor.

Fig. 3 is a perspective view showing the internal structure of the same inductor in perspective.

Fig. 4 is a cross-sectional view of a section orthogonal to the longitudinal direction of a lead wire used for a coil.

Fig. 5 is a diagram showing an outline of a manufacturing process of the inductor.

Fig. 6 is a diagram showing a method of molding a unit body using a flat plate formed of mixed powder.

Fig. 7 is a schematic view showing the state of the core of the molded unit body.

Fig. 8 is a diagram showing the structure of the 1 st soft magnetic particle constituting the mixed powder.

Fig. 9 is an electron micrograph of the surface of an oxide film formed on the grain core of the Cr-free 1 st soft magnetic grain.

Fig. 10 is a graph showing the change in withstand voltage of the molded article with respect to the oxygen content for the 1 st soft magnetic particle.

Fig. 11 is a graph showing changes in the specific permeability and saturation magnetic flux density of the molded body with respect to the oxygen content in the 1 st soft magnetic particles.

Fig. 12 is a graph showing the change in the magnetic characteristic coefficient of the molded body with respect to the oxygen content in the 1 st soft magnetic particles.

Fig. 13 is a diagram showing the structure of the 2 nd soft magnetic particles constituting the mixed powder.

Fig. 14 is a view showing a flow pattern of the mixed powder in the coil peripheral portion in the unit body molding and curing step.

Fig. 15 is a view showing a state of a gap between the surface region and the central region of the 1 st plate in the unit body molding and curing step.

Fig. 16 is a diagram illustrating a reference surface of the inductor.

Fig. 17 is a diagram showing a resin filling method for a side surface of the unit body.

Fig. 18 is a graph showing the measurement results of the surface roughness of the cell bodies.

Fig. 19 is a diagram illustrating the spacing between the coil and the side surface of the unit body.

Fig. 20 is a graph illustrating a relationship between the amount of resin in the mixed powder and the density of the unit bodies.

Fig. 21(a) is an image showing a lower-stage winding portion of the coil together with the surrounding material, and (B) is an image showing an upper-stage winding portion of the coil together with the surrounding material.

Fig. 22 is a diagram for explaining the pressurizing force when the unit body is molded.

Fig. 23 is a characteristic graph showing a simulation result of the line-to-line magnetic powder structure.

Fig. 24 is an image in the case where an air gap as a magnetic gap is provided in the vicinity of the winding portion.

Fig. 25 is a characteristic graph showing a simulation result corresponding to the presence or absence of an air gap.

Fig. 26 is a view schematically showing an example of a grinding apparatus for grinding a unit body.

Fig. 27 is an explanatory view of the side gap.

Fig. 28 is a diagram schematically showing an example of a protective film forming apparatus for forming a cell protective film.

Fig. 29 is a graph showing the results of an experiment on the content of nano silica and the drying rate.

Fig. 30 is a graph showing the experimental results of the average particle diameter of silica particles of nano silica and the adhesion generation rate.

Fig. 31 is an image showing cracks generated in the cell protective film.

Fig. 32 is a view showing the result of measuring the number of "plating jumps" by changing the thickness of the cell protective film.

Description of the reference numerals

1 inductor

10 unit body

12 mounting surface

14 top surface

16 1 st side

18 nd side 2

20 external electrode

30 coil

32 winding part

32L winding part

34 extension part

36 copper wire

40 core

40K air gap (magnetic gap)

50 unit body protection film

60 insulating coating material

61 insulating coating layer

62 welding layer

81 st soft magnetic particle

81A particle core

81B oxide film

81C insulating film

82 nd 2 soft magnetic particles

82A particle core

82B insulating film

Thickness of T

Width W

Length of L

P pressure

Roll width of CW winding

Length of LS 2 nd soft magnetic particles

Length of KL air gap

Width of KW air gap

Detailed Description

[ integral constitution of inductor ]

Fig. 1 and 2 are views schematically showing the configuration of an inductor 1 according to the present embodiment, fig. 1 being a perspective view seen from a top surface 14 side of the inductor 1, and fig. 2 being a perspective view seen from a mounting surface 12 side of the inductor 1.

The inductor 1 of the present embodiment is configured as a surface-mount electronic component, and includes a unit body 10 having a substantially rectangular parallelepiped shape, and a pair of external electrodes 20 provided on the surface of the unit body 10, one surface of the unit body 10 being configured as a mounting surface 12 (fig. 2) to be mounted on the surface of a circuit board (not shown), and the unit body 10 being covered with a unit body protective film 50 except for the external electrodes 20.

Hereinafter, in the unit body 10, the opposite surface of the mounting surface 12 is referred to as a top surface 14 (fig. 1), a pair of side surfaces located at an extension 34, which will be described later, of the 4 side surfaces excluding the mounting surface 12 and the top surface 14 is referred to as a1 st side surface 16, and the remaining pair of side surfaces is referred to as a2 nd side surface 18. These 1 st and 2 nd side surfaces 16 and 18 are also surfaces of the unit body 10 located in the radial direction of the winding portion 32 of the coil 30 described later. Hereinafter, the mounting surface 12 and the top surface 14 facing each other are referred to as a pair of main surfaces.

As shown in fig. 1, the length from the mounting surface 12 to the top surface 14 is defined as the thickness T of the unit body 10, the length of the short side of the top surface 14 is defined as the width W of the unit body 10, and the length of the long side is defined as the length L of the unit body 10.

Fig. 3 is a perspective view showing an internal configuration of the inductor 1 according to the present embodiment.

The unit body 10 includes a coil 30 and a core 40 embedded in the coil 30, and is configured as a coil-enclosed magnetic component in which the coil 30 is enclosed in the core 40.

The coil 30 is an air-core coil component wound with a lead wire 31.

The core 40 is a molded body formed by compression molding a mixed powder obtained by mixing a soft magnetic powder and a resin in a state in which the coil 30 is enclosed by the powder, into a substantially rectangular parallelepiped shape.

The coil 30 includes a winding portion 32 around which the lead wire 31 is wound, and a pair of extension portions 34 extending from the winding portion 32. The winding portion 32 is formed by winding the conductive wire 31 in a spiral shape such that both ends of the conductive wire 31 are positioned on the outer circumference and the inner circumferences are connected to each other. In the inside of the unit body 10, the coil 30 has a core 40 embedded therein such that the central axis K of the wound portion 32 extends in the direction of the thickness T of the unit body 10, and the extending portions 34 extend from the wound portion 32 to the pair of 1 st side surfaces 16, respectively.

Fig. 4 is a cross-sectional view of a section perpendicular to the longitudinal direction of the lead wire 31 used for the coil 30. The lead wire 31 used for forming the coil 30 is composed of a copper wire 36 and an insulating coating material 60 for coating the copper wire 36. The insulating coating material 60 has an insulating coating layer 61 having electrical insulation properties, and a fusion-bonded layer 62 formed on the insulating coating layer 61. In the coil forming step, the lead wire 31 is heated and wound to melt the fusion-bonded layer 62, whereby the lead wires 31 of the wound portion 32 are bonded to each other, and the shape of the wound portion 32 after the coil is formed can be suppressed from collapsing. Further, the insulating coating layer 61 can reliably insulate the coil 30 from the core 40.

The pair of external electrodes 20 are L-shaped members extending from the 1 st side surface 16 of the unit cell 10 across the mounting surface 12. The external electrodes 20 are connected to the extending portions 34 of the coil 30 on the 1 st side surface 16, and the portions 20A (fig. 2) extending from the mounting surface 12 are electrically connected to the wiring of the circuit board by an appropriate mounting means such as welding.

The inductor 1 having this configuration is a power inductor, for example, and is used as a choke coil for a DC-DC converter circuit, a power supply circuit, and the like through which a large current flows, and is used in electronic devices such as a personal computer, a DVD player, a digital camera, a TV, a mobile phone, a smart phone, an automotive electronic component, and medical and industrial machines. The use of the inductor 1 is not limited to this, and may be used in, for example, a tuning loop, a filter loop, a rectifying/smoothing loop, and the like.

[ outline of inductor production Process ]

Fig. 5 is a diagram showing an outline of a manufacturing process of the inductor 1.

As shown in the figure, the manufacturing process of the inductor 1 includes a granulation process, a coil forming process, a unit body molding and curing process, a unit body grinding process, a unit body protective film forming process, a unit body protective film removing process, and an external electrode forming process.

The granulation step is a step of granulating a mixed powder obtained by mixing the soft magnetic powder contained in the core 40 with a resin. The soft magnetic powder is formed of particles whose surfaces are covered with an insulating film.

The coil forming step is a step of forming the coil 30 from the conductive wire 31 covered with the insulating covering material 60. In this step, the coil 30 is formed into a shape having the winding portion 32 and the pair of extending portions 34 by winding the conductive wire 31 by a winding method called "α -winding". The α -coil is a state wound in a 2-step spiral shape so that the extending portions 34 of the lead and the tail of the conductive wire 31 functioning as a conductor are positioned on the outer periphery. The number of turns of the coil 30 is not particularly limited, and is, for example, 6.5 turns.

The unit body molding and curing step is a step of molding a molded body serving as a base of the unit body 10.

The mixed powder obtained in the granulation step is used as a molding material for the molded article.

In this step, the mixed powder is preformed to produce a flat plate (solid material of a predetermined shape), and the flat plate and the coil 30 are disposed in a cavity of a molding die. Next, the cavity is heated and pressurized by a punch to compress the molded body of the inner coil 30, and then the cured molded body is taken out from the cavity and polished. The polishing uses barrel polishing, so that the corner of the molded body can be chamfered.

As shown in fig. 6, the preformed flat plate may be 2 types of flat plates, i.e., a1 st flat plate 70 having an appropriate shape (e.g., E-shape) into which the groove 71 of the coil 30 is inserted, and a2 nd flat plate 72 having an appropriate shape (e.g., I-shape, plate-shape, etc.) covering the groove 71 of the 1 st flat plate 70. In the compression molding, the 1 st plate 70 and the 2 nd plate 72 are arranged in a cavity 75 of a molding die 74 in a superposed manner, and the coil 30 is fitted into the groove 71 in the 1 st plate 70. Then, the 1 st plate 70 and the 2 nd plate 72 are heated and pressed by a punch 76 from the 1 st plate 70 or/and the 2 nd plate 72 side (the 2 nd plate 72 side in the example of fig. 6) in the overlapping direction, so that the 1 st plate 70, the coil 30, and the 2 nd plate 72 are integrated.

Not only the pre-formed flat plate but also the mixed powder obtained in the granulation step may be directly charged into the cavity and compression-molded.

As shown in fig. 7, the pressure P during compression molding is preferably lower than the pressure before compression molding, so that the soft magnetic powder particles 80 formed after molding of the unit body 10 do not collapse and maintain the shape before molding. This pressure P can suppress damage to the insulating film on the surface of each particle 80 constituting the soft magnetic powder, and thus can suppress a decrease in insulating performance (i.e., suppress a decrease in withstand voltage performance).

As shown in fig. 7, the particles 80 constituting the soft magnetic powder preferably have a particle size of 2 or more (in the example of fig. 7, the 1 st soft magnetic particle 81 as a large particle having a large average particle size and the 2 nd soft magnetic particle 82 as a small particle having a small average particle size). According to this soft magnetic powder, as shown in fig. 7, the resin 90 and the 2 nd soft magnetic particles 82 as small particles are interposed between the 1 st soft magnetic particles 81 as large particles at the time of compression molding, and therefore, a molded body (unit cell 10) having a high filling rate of the particles 80 can be obtained. The embodiments of the 1 st and 2 nd soft magnetic particles 81 and 82 constituting the core 40 will be described later.

The unit body grinding step is a step of grinding (i.e., grinding) the 2 nd side surface 18 to a predetermined width W by causing abrasive grains to act on the 2 nd side surface 18 of the molded body obtained in the unit body molding/curing step.

By this step, the unit cell 10 in which the width W of the molded body is reduced to a predetermined width can be obtained. This reduction reduces the distance (also referred to as a side gap) between the coil 30 and the 2 nd side surface 18 in the unit cell 10, and therefore the occupancy of the coil 30 in the radial direction of the winding portion 32 of the coil 30 can be increased. Further, since the unit cell 10 is obtained by grinding the molded body obtained by compression molding to a predetermined size, it is possible to reduce the dimensional unevenness of the unit cell 10 as compared with the case where the unit cell 10 is controlled to a predetermined size by compression molding only.

In the unit body grinding step, polishing (for example, barrel polishing) for chamfering the corner generated by grinding the 2 nd side surface 18 may be performed.

The cell protective film forming process is a process of forming the cell protective film 50 on the entire surface of the cell 10 ground to a predetermined size in the cell grinding process.

As the material of the cell protective film 50, for example, a thermosetting resin such as an epoxy resin, a polyimide resin, or a phenol resin, or a thermoplastic resin such as a polyethylene resin or a polyamide resin can be used. These resins may further contain a filler containing silicon oxide, titanium oxide, or the like.

In this step, a material of the cell protective film 50 is applied to the entire surface of the cell 10 by an appropriate means such as coating or dipping, and is cured to form the cell protective film 50.

The unit body protective film removing step is a step of removing the unit body protective film 50 at the electrode forming position (in the present embodiment, a predetermined position in the 1 st side surface 16 and the mounting surface 12) where the external electrode 20 is formed and the insulating coating material 60 of the extension portion 34 of the coil 30 exposed at the electrode forming position by irradiating the unit body 10 whose entire surface is covered with the unit body protective film 50 with laser light.

In the unit protective film removing step, after the insulating coating material 60 is removed by laser light, etching treatment may be performed to clean the surface of the electrode formation site.

The external electrode forming step is a step of forming the external electrodes 20 by plating at the electrode forming positions where the cell protective film 50 is removed in the cell protective film removing step. The external electrode forming step may be performed before the cell protective film forming step.

In this step, the external electrode 20 is formed by plating the soft magnetic powder exposed on the surface of the unit body 10 and the extension portion 34 of the coil 30. In this plating treatment, a layer made of copper (Cu) is formed by plating growth, thereby forming the external electrodes 20.

A layer made of nickel (Ni) and a layer made of tin (Sn) were sequentially stacked on a copper (Cu) layer by plating growth. Instead of the copper (Cu) layer, an aluminum (Al), silver (Ag), gold (Au), or palladium (Pd) layer may be used.

The external electrode may be formed using sputtering, conductive resin, copper plate, or the like.

The external electrode 20 is not limited to the L-shape illustrated in the drawing, and may have a so-called 5-plane electrode structure or may be a bottom-plane electrode.

According to the inductor 1 manufactured as described above, it is possible to improve the specific resistance of the core 40 and the ratio of the soft magnetic metal portion while maintaining the mechanical strength of the core 40, and it is possible to realize high reliability and good withstand voltage, magnetic permeability, saturation magnetic flux density, and dc superposition characteristics.

Next, an embodiment of the inductor 1 is explained below.

In each example, unless otherwise specified, the inductor 1 has a length L of 2.0 ± 0.2mm, a width W of 1.2 ± 0.2mm, a thickness T of 0.7 ± 0.1mm, and a withstand voltage of about 20V.

The inductor 1 may be configured by using any of the embodiments described below as [ a-1-1. 1 st soft magnetic particle ], [ a-1-2. 2 nd soft magnetic particle ], [ a-2. resin ], [ b. coil ], [ c. magnetic circuit ], [ d. cell body grinding ], and [ e. cell body protective film ], in any combination of these embodiments.

[ A. Mixed powder ]

The mixed powder used in the formation of the core 40 includes a soft magnetic powder and a resin.

[ A-1. Soft magnetic powder ]

The soft magnetic powder contained in the mixed powder is composed of particles of soft magnetic metal. The soft magnetic powder includes, for example, 1 st soft magnetic particles 81 (large particles) and 2 nd soft magnetic particles 82 (small particles) having a smaller average particle size than the 1 st soft magnetic particles 81. In the present specification, the "average particle diameter" refers to a volume-based median particle diameter.

The average particle diameter of each of the 1 st soft magnetic particle 81 and the 2 nd soft magnetic particle 82 can be measured using a particle size distribution meter before these are mixed with each other. In addition, as the case of measurement in the state of the core 40 of the molded body obtained by compression molding the mixed powder, the measurement was performed by analyzing an electron microscope image of a cross section of the soft magnetic particles obtained by polishing the core 40. For example, the equivalent circle diameter of the cross section of each soft magnetic particle is obtained from the electron micrograph, the volume of each sphere is obtained assuming that each soft magnetic particle is a sphere having the equivalent circle diameter, and the average particle diameter is calculated from the median of the volume value distribution.

The 1 st soft magnetic particles 81 have an average particle diameter of 20 to 28 μm, preferably 21.4 to 27.4 μm. The 2 nd soft magnetic particles 82 have an average particle diameter of 1 to 6 μm, preferably 1.5 to 1.8 μm. By forming the mixed powder of the 1 st soft magnetic particles 81 and the 2 nd soft magnetic particles 82 having different average particle diameters in this manner, the saturation magnetic flux density of the cores 40 is increased by the 1 st soft magnetic particles 81 having a large average particle diameter, the dc bias characteristic is improved, the 2 nd soft magnetic particles 82 having a small average particle diameter are inserted into the gaps between the 1 st soft magnetic particles 81, the filling ratio of the soft magnetic particles in the cores 40 is increased, and the specific magnetic permeability is improved.

The amount of the 2 nd soft magnetic particles 82 contained in the mixed powder is 15 to 30 wt%, preferably 20 to 30 wt%, based on the total weight of the soft magnetic particles contained in the mixed powder. If the content of the 2 nd soft magnetic particles 82 in the soft magnetic powder is within the above range, the filling ratio of the soft magnetic particles in the core 40 as a compact of the mixed powder can be further increased.

The composition of the soft magnetic metal constituting the 2 nd soft magnetic particles 82 may be the same as that of the soft magnetic metal constituting the 1 st soft magnetic particles 81, but it is preferable that the compositions are different from each other and have almost the same hardness as each other. The hardness of the 1 st soft magnetic particle 81 and the 2 nd soft magnetic particle 82 can be measured by the nanoindentation method. For example, the 1 st soft magnetic particle 81 has a hardness of 600HV (kgf/mm)²) 1200HV, preferably 800HV to 1000 HV. The hardness of the 2 nd soft magnetic particles 82 was 900HV (kgf/mm)²) 1400HV, desirably 900HV to 1100 HV.

The ratio of the hardness of the 2 nd soft magnetic particles 82 to the hardness of the 1 st soft magnetic particles 81 is preferably 0.7 to 1.2. Thus, when the core 40 is formed by compression molding the mixed powder containing these soft magnetic particles, the soft magnetic particles having a lower hardness, out of the 1 st soft magnetic particles 81 or the 2 nd soft magnetic particles 82, can be prevented from being deformed, and the insulation resistance as the core 40 can be prevented from being lowered.

[ A-1-1. 1 st Soft magnetic particles ]

[ A-1-1-1 ] 1 st embodiment of Soft magnetic particles ]

Fig. 8 is a diagram showing the structure of the 1 st soft magnetic particle 81. The 1 st soft magnetic particle 81 is composed of a particle core 81A including a soft magnetic metal, and an insulating film 81C formed on the surface of the particle core 81A. The particle core 81A has an oxide film 81B, and the oxide film 81B is formed by oxidizing the soft magnetic metal constituting the particle core 81A on the surface of the particle core 81A.

In order to stably realize high withstand voltage in the core 40, the insulating film 81C needs to secure bonding strength with the oxide film 81B as an underlayer to such an extent that the insulating film 81C does not peel off from the oxide film 81B. When the insulating film 81C is peeled off from the oxide film 81B, the insulation resistance of the core 40 is lowered, and the voltage resistance as an inductor is lowered. On the other hand, the amount of the soft magnetic metal in the particle core 81A is reduced by the formation of the oxide film 81B, and the specific permeability of the core 40 formed using the particle core 81A is reduced. Therefore, from the viewpoint of specific magnetic permeability, the thickness of the oxide film 81B is preferably as thin as possible.

The present inventors have found that, when the particle core 81A is made of a soft magnetic metal containing Cr, the oxide film 81B formed on the surface of the particle core 81A becomes thin, the surface tends to become smooth, and the bonding strength of the insulating film 81C in the oxide film 81B may not be sufficiently obtained. In addition, the present inventors have found that, as a solution to this problem, by limiting the Cr content in the particle cores 81A and setting the film thickness of the oxide film 81B formed on the surface of the particle cores 81A to a predetermined range, the bonding strength of the insulating film 81C in the oxide film 81B can be secured, and the decrease in the specific permeability of the cores 40 formed using the particle cores 81A can be suppressed within a certain range.

Specifically, the soft magnetic metal constituting the grain core 81A is an iron-based soft magnetic metal having a Cr content of 1.5 wt% or less. When the Cr content is in this range, the iron content is increased to increase the specific magnetic permeability of the particle cores 81A, and thus a non-uniform passivation film is formed on the surfaces of the particle cores 81A, so that the oxide film 81B becomes non-uniform, the contact surface area between the oxide film 81B and the insulating film 81C is increased, and the bonding strength between the insulating film 81C and the oxide film 81B can be increased.

The particle core 81A may be a Cr-free (Cr-free) iron-based soft magnetic metal. Here, "Cr-free" means that the material does not substantially contain Cr, and even if Cr is contained, the amount is a minute amount (for example, 500ppm or less) to the extent that the material can be mixed from the environment in the production process of the particle core 81A.

More specifically, the particle core 81A is an Fe-Si-Cr alloy or an amorphous or crystalline metallic magnetic body of an Fe-Si alloy having a Cr content within the above numerical range. In the Fe-Si-Cr alloy or the Fe-Si alloy, for example, 87 wt% or more of Fe and 3 wt% or more of Si may contain B (boron).

The particle cores 81A of the 1 st soft magnetic particles 81 are not limited to the above-described Fe-Si-Cr alloy or Fe-Si alloy, as long as they are formed using an iron-based soft magnetic metal. Such an iron-based soft magnetic metal may be, for example, Fe-Si-Cr-Al or an amorphous or crystalline alloy of Fe-Si-Al with the Cr content in the above numerical range.

When the Cr-free alloy is used as the particle core 81A of the 1 st soft magnetic particle 81, the weight ratio of Fe in the particle core 81A can be increased, the saturation magnetic flux density of the core 40 produced using the particle core 81A can be further increased, and a more favorable dc superposition characteristic can be obtained as an inductor.

The oxide film 81B can be formed by oxidizing the soft magnetic metal on the surface of the particle core 81A in the process of producing the particle core 81A. For example, in the process of producing the particle nuclei 81A, the oxide film 81B may be formed by providing an active oxidation step of exposing the particle nuclei 81A to a water or oxygen atmosphere, and/or exposing the particle nuclei 81A to a high-temperature oxygen atmosphere, or the like.

As the oxidation of the soft magnetic metal proceeds in the surface of the particle core 81A, the oxide film 81B increases in thickness, and the surface roughness of the surface increases, so that the bonding strength between the oxide film 81B and the insulating film 81C formed on the surface increases. On the other hand, as the oxidation of the soft magnetic metal proceeds, the thickness of the oxide film 81B increases, the amount of metal contained in the particle core 81A decreases, and the specific permeability of the core 40 when the core 40 is formed from the particle core 81A decreases. From the viewpoint of securing the bonding strength of the insulating film 81C and suppressing the decrease in specific magnetic permeability to a certain extent, the oxygen content of the particle cores 81A is preferably 900ppm to 2800 ppm.

The insulating film 81C formed on the oxide film 81B is, for example, an inorganic glass film formed by a mechanochemical method. The inorganic glass coating is, for example, phosphate glass such as zinc phosphate or manganese phosphate, or glass. Instead, the insulating film 81C may be formed of an organic polymer film, an organic-inorganic hybrid film, or an inorganic insulating film. The insulating film 81C can be formed by a mechanochemical method, a sol-gel reaction of a metal alkoxide, or the like, depending on the material.

The thickness of the insulating film 81C is 10nm to 50 nm. The specific resistance of the 1 st soft magnetic particle 81 can be increased by setting the thickness of the insulating film 81C to 10nm or more. Further, by setting the thickness of the insulating film 81C to 50nm or less, the ratio of the metal to the 1 st soft magnetic particles 81 can be increased, and favorable magnetic characteristics can be obtained in the core 40 using the 1 st soft magnetic particles 81.

The 1 st soft magnetic particle 81 having the above configuration can ensure the bonding strength of the insulating film 81C formed on the oxide film 81B of the particle core 81A, stably realize high withstand voltage in the core 40, and maintain the specific permeability of the core 40 high.

[ A-1-1-2 ] 1 st Process for producing Soft magnetic particles ]

Next, a method for producing the 1 st soft magnetic particle 81 according to one embodiment of the present invention will be described. The method described below is merely an example, and the method for producing the 1 st soft magnetic particles 81 according to one embodiment of the present invention is not limited to the method described below.

The particle cores 81A of the 1 st soft magnetic particles 81 can be obtained by, for example, a gas atomization method. That is, each metal serving as a base of the particle core 81A is heated and melted in an induction furnace to prepare a molten metal, and the obtained molten metal is ejected from the ejection hole together with an ejection gas flow of argon gas serving as an inert gas to obtain metal fine particles. Then, the obtained fine particles are cooled in water and dried to obtain the particle cores 81A of the 1 st soft magnetic particles 81. The average particle diameter of the particle cores 81A can be adjusted by adjusting, for example, the velocity of the jet flow of argon gas used for jetting the molten metal in the above-described gas atomization method and/or the diameter of the jetting hole.

For example, when the particle cores 81A having an average particle diameter of 20 μm or more are formed of an amorphous metal, the SWAP method (Spinning Water Atomization Process) in which metal fine particles formed of the molten metal are rapidly cooled by a Water stream rotating at a high speed can be used as the particle cores 81A.

In the cooling in water and the subsequent drying, the oxide film 81B can be formed on the surface of the particle core 81A by exposing the particle core 81A to a water and/or oxygen atmosphere. The thickness of the oxide film 81B can be set to a desired thickness by controlling the exposure time in the water or oxygen atmosphere and/or controlling the oxygen concentration in the production environment of the particle nuclei 81A. In addition, by exposing the dried particle cores 81A to a high-temperature oxygen atmosphere, a thicker oxide film 81B can be formed on the surfaces of the particle cores 81A. It is considered that the average particle diameter of the particle cores 81A does not substantially change before and after the formation of the oxide film 81B and the formation of the insulating film 81C described later.

The oxide film 81B formed on the surface of the particle core 81A does not need to have the same distribution of the metal oxide in the film. For example, in the case where one or more metals constituting the particle core 81A may form a plurality of oxides, different kinds of oxides may be unevenly distributed with each other within the oxide film 81B, and in addition, the oxide film 81B may be constituted by a plurality of layers formed of different kinds of oxides.

Next, an insulating film 81C is formed on the oxide film 81B formed on the particle cores 81A. The insulating film 81C is, for example, a film of phosphate glass formed by a mechanochemical method.

[ A-1-1-3 ] 1 st example of Soft magnetic particles ]

27 kinds of samples (samples A1-01 to A1-27) in which the Cr content of the particle cores 81A and the thickness of the oxide film 81B on the surface of the particle cores 81A are different from each other were prepared, and the characteristics were evaluated. The summary of the particles in samples A1-01 to A1-27 is shown in Table 1 below together with their evaluation results. Here, samples a1-03 to a1-08, samples a1-12 to a1-16, and samples a1-20 to a1-24 are examples of the 1 st soft magnetic particles 81 according to one embodiment of the present invention.

Hereinafter, each sample will be described.

< sample A1-01 >

(preparation of particle core)

Amorphous metal fine particles of a (Cr-free) Fe-Si alloy having a zero Cr content were produced as the particle cores 81A by the SWAP method. The content of Fe and Si in the prepared particle core 81A is: 93 wt% of Fe, 3.5 wt% of Si, 3 wt% of B and the balance of C. The average thickness of the oxide film 81B caused by the surface oxidation of the particle cores 81A is 5 nm. The hardness of the prepared particle core 81A was 953 HV.

Here, the contents of Fe and Si were measured by ICP-OES emission spectrometry (spark discharge emission spectrometry). The hardness of the particle core 81A is measured by a nanoindentation method.

(formation of insulating film)

Next, an insulating film 81C made of zinc phosphate as phosphate glass was formed on the surface of the particle core 81A (including the oxide film 81B) obtained above by a mechanochemical method, and the soft magnetic particles on which the insulating film 81C was formed were sample a1-01 of the 1 st soft magnetic particle 81. The thickness of the insulating film 81C formed was 23 nm.

The 1 st soft magnetic particle 81 had an average particle diameter (median diameter) of 25.3 μm.

The average particle diameter was measured using a particle size distribution meter.

The average thickness of the oxide film 81B was measured as follows. The average film thickness (average thickness) of the oxide film 81B is an average value of the thicknesses of the oxide film 81B measured at a plurality of points in the cross section of the particle core 81A in a broad sense, and is a value derived by the procedure described below in a narrow sense. First, 1 particle of the particle core 81A is cut by a Focused Ion Beam (FIB) to produce a sheet. In this case, the cleavage site is arbitrary. Using an electron microscope (TEM) set at a magnification of 10 ten thousand times, 1 particle core 81A was set at 3 arbitrary points at equal intervals along the outer periphery, a cross section of the particle core 81A was taken at 3 fields, and each TEM image was set at 4 arbitrary points at equal intervals, and the thickness of the oxide film 81A was measured. The above measurement is performed on 3 particle cores 81A, and an average value is obtained from the thicknesses of the oxide films measured at all points (3 fields × 4 dots × 3 dots — 36 dots), and this is defined as an "average thickness". The thickness of the insulating film 81C can be measured in the same manner.

(evaluation of thickness unevenness of oxide film and oxygen content)

The difference between the maximum film thickness and the minimum film thickness of the oxide film 81B in the cross section of the particle core 81A (hereinafter referred to as the difference in the thickness of the oxide film) is measured as an index value indicating the unevenness in the thickness of the oxide film 81B. The difference between the maximum film thickness and the minimum film thickness of the oxide film 81B can be measured as follows. First, 1 particle of the particle core 81A is cut by a Focused Ion Beam (FIB) to produce a sheet. In this case, the cleavage site is arbitrary. A cross section of the particle core 81A was observed along the outer periphery of the particle core 81A using an electron microscope (TEM) set at a magnification of 10 ten thousand times, 3 points were set at each of a thin portion and a thick portion, and images were taken in 3 fields. Then, the maximum value and the minimum value were measured for each TEM image, and the maximum value and the minimum value in the 3 fields of view were set as the maximum film thickness and the minimum film thickness, respectively. The evaluation results are shown in table 1.

The oxygen content in the sample a1-01 prepared as described above was evaluated by using the amount of the oxide film 81B formed on the surface of the particle core 81A as a parameter that can be estimated in the post-formation process of the insulating film 81C. The oxygen content was measured and evaluated by the inert gas melting method by weighing 1 g of the soft magnetic particles produced above. The evaluation results are shown in table 1.

(evaluation of bonding Strength of insulating film)

The bonding strength of the insulating film 81C in the soft magnetic particles produced as described above was evaluated as follows using a powder resistance measuring instrument (Hiresta). First, 10g of the powder composed of the soft magnetic particles produced above was weighed and put into a measuring cylinder (a cylinder having an electrical insulator as a side wall and a metal plate connected to ground potential as a bottom plate) provided in a powder resistance measuring instrument. An upper plate made of a metal plate having the same diameter as the inner diameter is brought into contact with the upper surface of the powder put in the cylinder, and a voltage is applied between the bottom plate and the upper plate. A load is applied to the upper plate in a direction toward the base plate, the load is increased, a current flowing between the upper plate and the base plate is observed, and a value of the load (unit MPa) when the current exceeds a predetermined threshold value is measured as an evaluation value indicating a degree of bonding strength of the insulating film 81C. The evaluation results are excellent good x, good 60Mpa or more, good 20Mpa or more and less than 60Mpa, and good x less than 20 Mpa. The evaluation results are shown in table 1.

(preparation of test piece)

In order to evaluate the withstand voltage, specific permeability and saturation magnetic flux density of the molded article made of the soft magnetic particles of sample A1-01 prepared as described above, a test piece was prepared for sample A1-01. The test piece was an annular test piece obtained by compression molding the 1 st soft magnetic particle 81, the 2 nd soft magnetic particle 82, and the epoxy resin, which were sample a 01-01. The 2 nd soft magnetic particles 82 used in the test piece are sample a2-05 described later.

The 2 nd soft magnetic particle 82 used in the test piece was a soft magnetic particle having an average particle diameter of 3 μm, in which an insulating film 82B (described later) having a thickness of 2nm and containing an alkyl group having a long chain portion and having 16 carbon atoms was formed on a particle core 82A (described later) made of crystalline pure iron. The weight ratio of the 1 st soft magnetic particle 81 to the 2 nd soft magnetic particle 82 used in the test piece was 75: 25. in addition, the weight ratio of the total of the 1 st soft magnetic particles 81 and the 2 nd soft magnetic particles 82 to the epoxy resin is 100: 3.1. the test piece was in the form of a ring having an inner diameter of 8mm, an outer diameter of 13mm and a thickness of 5 mm.

(evaluation of withstand Voltage)

The withstand voltage was evaluated by using the test piece corresponding to the sample A1-01 prepared as described above. The withstand voltage was measured using an AC/DC withstand voltage insulation resistance tester. The evaluation results are shown in table 1.

(evaluation of magnetic permeability)

The test piece prepared above for sample A1-01 was used to evaluate the specific permeability. The specific magnetic permeability was measured by a BH analyzer and an impedance material analyzer using a high-frequency signal having a frequency of 1 MHz. The evaluation results are shown in table 1.

(evaluation of saturation magnetic flux Density)

The saturation magnetic flux density was evaluated by using the test piece prepared above for sample A1-01. Inductance change of the test piece when the test piece was superimposed was measured using an LCR meter and a dc power supply, and BH data was inverted to determine a saturation magnetic flux density. The evaluation results are shown in table 1.

< samples A1-02-A1-27 >

Samples A1-02 to A1-27 were prepared in the same manner as the above-mentioned sample A1-01, with the Cr content in the grain cores 81A being changed to values shown in Table 1, and with the Fe and Si contents and oxygen contents being changed, and the bonding strength, withstand voltage, specific permeability and saturation magnetic flux density were evaluated. The average film thickness of the oxide film 81B formed by oxidizing the surface of the particle core 81A becomes thicker as the oxygen content increases in each Cr content.

FIG. 9 is an electron micrograph of the surfaces of the particle cores 81A having an oxygen content of 500, 1200, 1500, 2500, 2600ppm among the particle cores 81A having a zero Cr content (no Cr), i.e., the particle cores 81A of samples A1-01, A1-04, A1-05, A1-06 and A1-07. From the difference in the surface state of the particle core 81A between these samples shown in fig. 9, it can be judged that: the thicker the thickness of the oxide film 81B is, the greater the oxygen content of the particle cores 81A before formation of the insulating film 81C is, and the deeper the irregularities on the surface of the oxide film 81B are. Such an increase in the depth of the irregularities associated with an increase in the film thickness of the oxide film 81B is considered to be due to, for example: the contact state of the particle cores 81A varies, and the surface of the particle cores 81A varies in the dry state, and the degree of difficulty of oxidation of the Fe — Si alloy varies depending on the position.

When the oxygen content is 900ppm or more, the average thickness of the oxide film 81B becomes large, the difference in thickness of the oxide film 81B becomes large, and the bonding strength of the insulating film 81C satisfies the reference value. That is, the oxygen content in the 1 st soft magnetic particles 81 is preferably 900ppm or more from the viewpoint of the bonding strength of the insulating film 81C.

It is considered that the effect of improving the bonding strength of the insulating film 81C accompanying the increase in the film thickness of the oxide film 81B is caused by: as the thickness of the oxide film 81B increases, the irregularities formed on the surface of the oxide film 81B become deeper, and the anchor effect due to the irregularities increases.

Then, from the comparison of the bonding strengths between the samples A1-26 and A1-27, the other samples A1-03 to A1-08, the samples A1-12 to A1-16, and the samples A1-20 to A1-24 in Table 1, it can be seen that: the effect of improving the bonding strength due to such an increase in surface roughness can be obtained when the Cr content is 1.5 wt% or less, and is particularly remarkable when sample a1-01 is Cr-free (content 0).

FIG. 10 is a graph showing the dependence of withstand voltage on oxygen content in samples A1-01 to A1-09 free of Cr. FIG. 11 is a graph showing the dependence of the specific magnetic permeability and the saturation magnetic flux density on the oxygen content in samples A1-01 to A1-09.

FIG. 12 is a graph showing the dependence of the magnetic characteristic coefficient on the oxygen content in samples A1-01 to A1-09.

The withstand voltage shown in fig. 10 increases simultaneously with the increase in the oxygen content. It is considered that this is due to: the increase in the thickness of the oxide film 81B increases together with the bonding strength of the insulating film 81C, and as a result, the insulation resistance of the particle core 81A of the 1 st soft magnetic particle 81 to the surroundings increases, and thus the specific resistance as a test piece (molded body) increases.

The specific permeability shown in fig. 11 decreases with an increase in oxygen content. It is considered that this is an increase in the oxygen content, i.e., an increase in the oxidized portion of the Fe — Si alloy constituting the particle cores 81A, so that the content of the Fe — Si alloy in the particle cores 81A, i.e., the content of the metal portion of the soft magnetic metal, decreases. In addition, it can be seen that: when the oxygen content is 2800ppm or less, the decrease in specific permeability associated with the increase in film thickness of the oxide film 81B can be suppressed to about 15% with respect to the value of the oxygen content of 500 ppm.

The saturation magnetic flux density increases together with the oxygen content. It is considered that the increase in saturation magnetic flux density together with the oxygen content is due to: as described above, the increase in the oxidized portion of the Fe — Si alloy as the soft magnetic metal constituting the particle cores 81A reduces the cross-sectional area of the Fe — Si alloy portion in the particle cores 81A, and as a result, the number of effective magnetic fluxes passing through the Fe — Si alloy portion of the particle cores 81A in the magnetic flux passing through the test piece decreases. In fig. 11, it is considered that the reduction in saturation magnetic flux density among 2500ppm, 2600ppm and 3000ppm in oxygen content is due to the influence of measurement conditions, the workings of the test piece, and the like. In the verification after the evaluation, it was confirmed that: the causes of the abnormality in the measurement values of 2500ppm, 2600ppm and 3000ppm are the heat generation of the test piece and the abnormality in the state of the test piece due to the error in the volume calculation of each test piece and the error in the setting of the measurement conditions.

From the above results, it is understood that the desired oxygen content of the 1 st soft magnetic particle 81 constituting the core 40 is 900ppm to 2800ppm, in order to maintain the withstand voltage at a practical level while securing the bonding strength of the insulating film 81C and to suppress a significant decrease in specific magnetic permeability.

As described above, the soft magnetic powder of the mixed powder used for forming the core 40 includes the 1 st soft magnetic particles 81. The 1 st soft magnetic particle 81 is composed of a particle core 81A including a soft magnetic metal, and an insulating film 81C located on the surface of the particle core 81A. The particle cores 81A have oxide films 81B made of an oxide of the soft magnetic metal between the insulating films 81C. The particle core 81A contains no Cr or 1.5 wt% or less of Cr, and has an oxygen content of 900 to 2800ppm by weight.

According to this configuration, when the mixed powder containing the 1 st soft magnetic particles 81 is compression-molded to form the core 40 as a metallic magnetic body, it is possible to stably realize high withstand voltage while suppressing a decrease in magnetic permeability in the core 40.

In addition, the soft magnetic metal included in the particle core 81A of the 1 st soft magnetic particle 81 may be an iron-based soft magnetic metal containing Fe and Si. With this configuration, the oxide film 81B can be easily formed on the surface of the particle core 81A.

In addition, the iron-based soft magnetic metal may be crystalline. With this configuration, the oxide film 81B can be more easily formed on the surface of the particle core 81A.

The soft magnetic powder constituting the mixed powder may further include, in addition to the 1 st soft magnetic particles 81, 2 nd soft magnetic particles 82 that include a soft magnetic metal and have an average particle diameter smaller than that of the 1 st soft magnetic particles. By using the 2 nd soft magnetic particles 82 described later in addition to the 1 st soft magnetic particles 81, the filling factor of the soft magnetic particles in the core 40 can be increased, and higher magnetic permeability can be obtained.

In addition, the inductor 1 can be configured by the metallic magnetic body configured by the soft magnetic powder including the 1 st soft magnetic particles 81 according to any one of the above embodiments and the wound wire 31. According to this configuration, a small inductor having a high withstand voltage and high reliability can be realized.

[ A-1-2. 2 nd Soft magnetic particles ]

[ A-1-2-1 ] embodiment of the 2 nd Soft magnetic particle ]

Fig. 13 is a diagram showing the structure of the 2 nd soft magnetic particles 82. The 2 nd soft magnetic particle 82 is composed of a particle core 82A containing a soft magnetic metal, and an insulating film 82B formed on the surface of the particle core 82A. The soft magnetic metal constituting the particle core 82A is, for example, crystalline or amorphous iron (Fe). Specifically, the granular particles of the 2 nd soft magnetic particles 82 are carbonyl iron powder having an onion skin structure, for example, and the Fe content is 95 to 99.8 wt%, preferably 97 to 99.8 wt%. The carbonyl iron powder may contain carbon C, oxygen O, nitrogen N, sulfur S as impurities. The carbonyl iron powder serving as the particle core 82A may have an oxide film of Fe on the surface thereof.

The soft magnetic metal constituting the particle core 82A of the 2 nd soft magnetic particle 82 is not limited to Fe, and may be an iron-based soft magnetic metal containing Fe and other metals, as in the 1 st soft magnetic particle 81.

The insulating film 82B of the 2 nd soft magnetic particle 82 is made of a sol-gel reaction product containing, for example, silica as a component, and contains a hydrocarbon group having a linear portion having 8 or more carbon atoms. The hydrocarbon group having a linear portion having 8 or more carbon atoms is specifically, for example, an alkyl group which is a chain-like saturated hydrocarbon group. The hydrocarbon group having a linear portion having 8 or more carbon atoms may be 1 or more hydrocarbon groups selected from octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl groups. Further, the alkyl group may be any of a primary alkyl group, a secondary alkyl group, or a tertiary alkyl group.

The hydrocarbon group having a long chain portion can be formed, for example, as a product of a sol-gel reaction obtained using a mixture of Tetraethoxysilane (TEOS) and a silane coupling agent having the hydrocarbon group.

By providing the insulating film 82B of the 2 nd soft magnetic particle 82 with an alkyl group having a linear portion having 8 or more carbon atoms, when the core 40 is formed by compression molding the mixed powder including the 1 st soft magnetic particle 81 and the 2 nd soft magnetic particle 82, the filling ratio of these soft magnetic particles in the core 40 can be increased.

The mechanism for increasing the filling ratio of the soft magnetic powder is not limited to a specific theory, and is presumed as follows. As described above, the core 40 is formed by compression molding the mixed powder including the 1 st soft magnetic particles 81, the 2 nd soft magnetic particles 82 and the thermosetting resin (epoxy resin or the like). In this case, for example, when the 2 nd soft magnetic particles 82 (small particles) as one of the soft magnetic particles have hydrocarbon groups having a linear chain part having 8 or more carbon atoms on the surface thereof, hydrogen bonds and/or dipole interactions between the 2 nd soft magnetic particles 82 and the polar groups (epoxy groups, hydrosic acid groups, and the like) of the epoxy resin can be reduced, and thus the fluidity (smoothness) of the 2 nd soft magnetic particles 82 at the time of compression molding can be improved.

As a result, the 2 nd soft magnetic particles 82 having high smoothness can enter the gaps between the 1 st soft magnetic particles 81 (large particles). It is considered that by such a mechanism, the filling ratio of the soft magnetic particles in the core 40 can be increased as compared with the case where the soft magnetic particles do not have long-chain hydrocarbon groups. By increasing the filling ratio of the soft magnetic particles, the density of the soft magnetic powder in the core 40 can be increased, and as a result, the specific permeability of the core 40 can be increased.

Here, the smoothness of the 2 nd soft magnetic particles 82 can be measured by the following procedure using the single-sided shear test apparatus used in JISZ 8835. More specifically, the smoothness can be measured by the following procedure using a single-side shear testing device of a lower unit direct type (powder layer shear force measuring device NS-S500 manufactured by Nano Seeds corporation). The inner diameter of the upper unit (Ring) and the inner diameter of the lower unit (Base) were both set to 15mm, and the gap (minute interval) between the upper unit and the lower unit was set to 0.2 mm. Before the powder is put in, an upper pestle (Lid) is disposed in the upper and lower units, and a zero point is set so that the thickness of the powder layer can be measured using a laser sensor. After a powder sample of 10g of the 2 nd soft magnetic particles 82 was uniformly filled in the Vertical dividing cell and an upper pestle (Lid) was placed quietly, a 150N pressing load was applied by a Vertical servo motor (Vertical servo motor). At the time when the pressing Load of 150N was applied by the vertical servo motor, the position of the Load cell (Load cell) of the vertical servo motor was fixed. The extrusion speed was set to 0.2 mm/sec. The side planing was started 100 seconds after the position of the load cell of the vertical servomotor was fixed. Namely, the start delay of the side planing was set to 100 seconds. After the start of side planing by the operation of a Horizontal servo motor (Horizontal servo motor), the pressure was measured every 0.1 second. The side planing speed was set at 5 μm/sec. The measurement is stopped when the coefficient of variation (CV value) of the measured value is 0.4% or less, while N is continuously measured for each measurement sample at 50 points or more during the period in which the horizontal servo motor is operated. The thickness of the finally compacted powder layer (final powder layer thickness) was measured with a laser sensor.

Then, based on the load (maximum pressing load) applied to the load cell at the time when the position of the load cell of the vertical servo motor is fixed, the load (pressing load at the time of starting the operation of the horizontal servo motor) applied to the load cell at the time of starting the operation of the horizontal servo motor, and the value of the final powder layer thickness, the smoothness can be obtained by using the following equation (for details, see japanese patent application No. 2019 and 224678, for example).

[ number 1]

As described above, by providing the hydrocarbon group having a linear portion having 8 or more carbon atoms on the surface of the 2 nd soft magnetic particle 82, the smoothness of the 2 nd soft magnetic particle 82 is improved, the filling factor of the soft magnetic particles in the core 40 when forming the core 40 is increased, and the magnetic permeability can be increased. Among these, as a result of the improvement in smoothness due to the long-chain hydrocarbon groups being contained on the surfaces of the 2 nd soft magnetic particles 82, the adhesion or bonding between the 2 nd soft magnetic particles 82 and the resin or other soft magnetic particles (the 1 st soft magnetic particles 81 and/or other 2 nd soft magnetic particles 82) therearound may be reduced, and the mechanical strength of the core 40 as a molded article may be reduced.

The present inventors have found that the mechanical strength of the core 40 as a molded article can be improved by controlling the smoothness by reducing the number of long-chain hydrocarbon groups having 8 or more carbon atoms formed on the surface of the particle core 82A of the 2 nd soft magnetic particle 82.

The number of long-chain hydrocarbon groups in the surface of the 2 nd soft magnetic particle 82 can be controlled by, for example, the mixing ratio of tetraethoxysilane and silane coupling agent in the mixing agent for sol-gel reaction used when the insulating film 82B is formed on the surface of the particle core 82A.

The number of long-chain hydrocarbon groups in the surface of the 2 nd soft magnetic particle 82 can be evaluated by the content ratio of silicon Si to carbon C in the insulating film 82B. When the number of the long-chain hydrocarbon groups is not so large as to contain Si and C in the particle core 82A of the 2 nd soft magnetic particle 82, the number can be evaluated by the weight ratio of the amount of Si to C contained in the entire 2 nd soft magnetic particle 82 (Si/C weight ratio). From the viewpoint of suppressing the decrease in mechanical strength of the core 40 and maintaining high magnetic permeability, the Si/C weight ratio in the 2 nd soft magnetic particles 82 is desirably 7.6 to 42.8.

In the present embodiment, the 1 st and 2 nd soft magnetic particles 81 and 82 having different average particle diameters and constituting the mixed powder have long-chain hydrocarbon groups on the surfaces of the particle cores 82A of the 2 nd soft magnetic particles 82 having a small average particle diameter, but the soft magnetic particles having long-chain hydrocarbon groups are not limited to the 2 nd soft magnetic particles 82. For example, instead of the 2 nd soft magnetic particles 82, the insulating film 82B containing the hydrocarbon group having a long chain portion having 8 or more carbon atoms as described above may be formed on the surface of the 1 st soft magnetic particle 81 or the surfaces of the 1 st soft magnetic particle 81 and the 2 nd soft magnetic particle 82. This improves the smoothness of the surfaces of the 1 st soft magnetic particles 81 or the surfaces of the 1 st soft magnetic particles 81 and the 2 nd soft magnetic particles 82, and can realize high magnetic permeability in the core 40 while suppressing a decrease in the mechanical strength of the core 40.

[ A-1-2-2. method for producing 2 nd Soft magnetic particles ]

Next, a method for producing the 2 nd soft magnetic particles 82 according to one embodiment of the present invention will be described. The method described below is merely an example, and the method for producing the 2 nd soft magnetic particles 82 according to the present invention is not limited to the method described below.

(preparation of particle core of Soft magnetic Metal)

First, metal fine particles that are the grain cores 82A of the 2 nd soft magnetic grains 82 are prepared. The details of the average particle diameter of the 2 nd soft magnetic particles 82, the composition of the particle cores 82A, and the like are as described above. It is considered that the average particle diameter of the particle core 82A does not substantially change before and after the surface treatment described later.

(formation of insulating film on surface of particle core)

Next, an insulating film 82B containing a hydrocarbon group having a linear portion having 8 or more carbon atoms is formed on the surface of the particle core 82A. The insulating film 82B can be formed by, for example, a sol-gel reaction of a surface treatment agent containing tetraethoxysilane as an alkoxide and a silane coupling agent. Thus, the insulating film 82B having a hydrocarbon group having a linear portion as a product of the sol-gel reaction can be formed on the particle core 82A.

The alkoxide is not limited to tetraethoxysilane, and any metal alkoxide represented by the chemical formula M- (OR) n may be used. In the formula, the metal species M of the metal alkoxide is preferably 1 or more selected from Li, Na, Mg, Al, Si, K, Ca, Ti, Cu, Sr, Y, Zr, Ba, Ce, Ta and Bi. The alkoxy group OR of the metal alkoxide may be any alkoxy group such as a methoxy group, an ethoxy group and/OR a propoxy group.

Further, a silane coupling agentMay be represented by the formula R' -Si (OR)₃And (4) showing. Wherein R' is a hydrocarbon group having a linear portion of 8 or more carbon atoms, and may be 1 or more hydrocarbon groups selected from the group consisting of octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, and octadecyl. In the formula, OR is alkoxy, preferably methoxy OR ethoxy. For example, when a hydrocarbon group having a linear portion having 16 carbon atoms is formed, hexadecyltrimethoxysilane can be used.

The thus obtained 2 nd soft magnetic particles 82 having smoothness are capable of effectively filling the space between the 1 st soft magnetic particles 81 without excessively constraining the epoxy resin when the core 40 is formed by compression molding together with the 1 st soft magnetic particles 81 and the epoxy resin, and thus, a magnetic body (magnetic core) having a high specific permeability can be realized as the core 40.

Here, the formation of the insulating film 82B on the particle core 82A is desirably performed by the process of the 1 st stage of forming a film of tetraethoxysilane on the surface of the particle core 82A, and the process of the 2 nd stage of forming a film containing a long-chain hydrocarbon group having a linear portion of 8 or more carbon atoms by a sol-gel reaction of tetraethoxysilane and a silane coupling agent on the formed film of tetraethoxysilane. This can suppress the long-chain hydrocarbon group from being buried in the layer of the insulating film 82B, and the long-chain hydrocarbon group can be efficiently arranged on the surface of the insulating film 82B, so that the amount of the silane coupling agent used for forming the insulating film 82B can be reduced.

The surface treatment agent may contain a surfactant in the step 1. By adding a surfactant to the surface treatment agent, the hydrophilic group portion of the surfactant which becomes a micelle forms a hydrogen bond with a silanol group formed by a hydrolysis reaction of tetraethoxysilane. Thus, in the process of the 1 st stage of forming a film of tetraethoxysilane, the micelles are arranged on the surface of the particle core 82A of the soft magnetic metal, and a part where the density of tetraethoxysilane molecules is low and a part where tetraethoxysilane molecules is dense can be present on the surface of the particle core 82A, and therefore, in the process of the 2 nd stage, long-chain hydrocarbon groups having 8 or more carbon atoms can be arranged with a space therebetween. As a result, the long-chain hydrocarbon groups are dispersed and disposed on the surface of the insulating film 82B, and the smoothness can be distributed over the entire surface of the 2 nd soft magnetic particle.

[ A-1-2-3 ] 2 nd example of Soft magnetic particles ]

27 kinds of samples of soft magnetic particles in which the number of carbon atoms in the long chain portion of the hydrocarbon group contained in the insulating film 82B and the weight ratio of Si to C (Si/C weight ratio) in the insulating film 82B were different were prepared as samples a2-01 to a2-27, and the characteristics were evaluated. The outlines of samples A2-01 to A2-27 are shown in Table 2. Here, samples A2-01 to A2-09, samples A2-15 to A2-18, and samples A2-24 to A2-27 are examples of the 2 nd soft magnetic particles 82. In table 2, the silane coupling agents used for producing the insulating films 82B of the samples a2-01 to a2-27 are also shown.

[ Table 2]

Hereinafter, each sample will be described.

< sample A2-01 >

(preparation of particle core)

As the particle cores 82A of the 2 nd soft magnetic particles 82, carbonyl iron powder containing 97 wt% to 99.8 wt% of iron was selected. The particle core 82A has a hardness of 952 HV. This value is substantially equal to the hardness of the grain cores 81A of the 1 st soft magnetic grains 81 used in the above-mentioned samples a1-01 to a 1-08.

(formation of insulating film containing Long-chain hydrocarbon group)

On the particle core 82A prepared as described above, an insulating film 82B containing an alkyl group as a hydrocarbon group having a linear portion having 16 carbon atoms is formed. As a surface treatment agent for forming the insulating film 82B, a mixed solution containing tetraethoxysilane as an alkoxide, hexadecyltrimethoxysilane as a silane coupling agent, and a phosphate ester type anionic surfactant as a surfactant is used.

The specific steps are as follows. As tetraethoxysilane and hexadecyltrimethoxysilane, KBE04 (manufacturer: shin-Etsu chemical Co., Ltd.) and X-88-422 (manufacturer: shin-Etsu chemical Co., Ltd.) were used, respectively. Further, PLYSURF AL (manufacturer: 1, manufactured by Industrial pharmaceutical Co., Ltd.) was used as the phosphate ester type anionic surfactant.

First, the process of the 1 st stage of forming a film of tetraethoxysilane is performed on the surface of the particle core 82A. An aqueous solution of isopropyl alcohol, ammonia water, and plursf AL was added thereto, and the mixture was stirred to prepare a dispersion 1. A predetermined amount of the prepared particle cores 82A was weighed, isopropyl alcohol was added, and ultrasonic vibration was applied to disperse the powder of the particle cores 82A, thereby preparing a dispersion 2. Then, dispersion 1 was added to dispersion 2, and the mixture was stirred with a stirrer to prepare dispersion 3.

Next, tetraethoxysilane was added to isopropyl alcohol and mixed to prepare a surface treatment liquid 1. The surface treatment liquid 1 was added to the dispersion liquid 3 to prepare a reaction liquid 1, and the reaction liquid 1 was stirred by a stirrer to form a film of tetraethoxysilane on the surface of the particle core 82A.

Next, the process of the 2 nd stage in which an alkyl group having a linear portion of 16 carbon atoms is formed on the surface of the film of tetraethoxysilane formed in the process of the 1 st stage described above is performed. First, isopropyl alcohol was mixed with hexadecyl trimethoxysilane and tetraethoxy silane to prepare a surface treatment liquid 2. The surface treatment liquid 2 was added to the reaction liquid 1 produced in the step 1 to prepare a reaction liquid 2, and the reaction liquid 2 was stirred by a stirrer to form an alkyl group having a linear portion having 16 carbon atoms on the surface of the tetraethoxysilane film formed above.

Thereafter, the reaction solution 2 was suction-filtered by a membrane filter, and the particles forming the insulating film 82B were separated. The separated particles were washed with acetone and dried at room temperature in a natural environment. The dried particles were filtered through a metal mesh, and the remaining particles were prepared as sample A2-01 for the 2 nd soft magnetic particles 82.

The average particle diameter (median diameter) of the 2 nd soft magnetic particles 82 produced as described above was 1.7 μm. The average particle diameter was measured using a particle size distribution meter.

(method of confirming Si/C weight ratio in particle core after formation of insulating film)

The Si/C weight ratio in sample a2-01 after the formation of the insulating film 82B was confirmed as follows as a parameter indicating how many hydrocarbon groups having a long chain portion having 16 carbon atoms were present on the surface of the formed insulating film 82B.

First, information on the peak intensity of the element contained in the insulating film 82B is obtained by a method called wide scan spectrometry by irradiating X rays onto the 2 nd soft magnetic particle 82 on which the insulating film 82B is formed using an X-ray photoelectron spectrometer. Next, by a method called narrow scan spectrometry, the element Si and the element C included in the insulating film 82B are focused, the area intensity of the peak intensity is obtained, and the total is normalized to 100% from the relative sensitivity coefficient of the element orbit, and the atm% concentration is calculated. Next, this atm% is multiplied by the atomic weight of each element to obtain the weight ratio of Si to C.

(evaluation of smoothness)

10g of the soft magnetic particles of the sample A2-01 thus prepared were weighed, and the slipperiness was evaluated using a single-side shear test apparatus of the bottom cell direct type (powder layer shear force measuring apparatus NS-S500 manufactured by Nano Seeds corporation). The inner diameter of the upper unit (Ring) and the inner diameter of the lower unit (Base) were both set to 15mm, and the pressing load was set to 150N. The evaluation results are shown in table 3.

(preparation of test piece)

In order to evaluate the molded article constituted of the soft magnetic particles of sample A2-01 thus prepared, a test piece for sample A2-01 was prepared. The test piece was an annular test piece obtained by compression molding the 1 st soft magnetic particle 81, the 2 nd soft magnetic particle 82 as sample a02-01, and an epoxy resin.

The 1 st soft magnetic particle 81 used in the above test piece was the above sample A1-04, which was obtained by forming an oxide film 81B of 5nm and an insulating film 81C of 23nm in a granular material of 25.3 μm average particle size made of an amorphous Fe-Si alloy containing no Cr. The mixing ratio of the 1 st soft magnetic particle 81 to the 2 nd soft magnetic particle 82 is 75: 25. the mixing ratio of the total of the 1 st soft magnetic particles 81 and the 2 nd soft magnetic particles to the epoxy resin is 100 in terms of weight ratio: 3.1. the test piece was in the form of a ring having an inner diameter of 8mm, an outer diameter of 13mm and a thickness of 4 mm.

(evaluation of magnetic permeability)

The test piece corresponding to the sample A02-01 prepared above was used to evaluate the specific magnetic permeability. The specific magnetic permeability was measured by a BH analyzer and an impedance material analyzer using a high-frequency signal having a frequency of 1 MHz. The evaluation result of the specific magnetic permeability is represented by good, and good when the measured value is equal to or greater than the reference value 30, and good when the measured value is less than the reference value 30. The measured values of the specific permeability and the results of the evaluation are shown in Table 3.

(evaluation of radial compressive Strength)

The test piece of the sample A02-01 prepared as described above was used to evaluate the radial compressive strength. The method for measuring the radial compressive strength is to measure the pressure at which the test piece formed into a ring shape is broken by applying pressure in the radial direction. The evaluation results of the radial compressive strength are represented by good values, and the measured value is a reference value of 85N/mm²In the above case, the quality is good, and in the below reference value 85N/mm²In the case of (2), is X. The measured values of the radial compressive strength and the results of the evaluation are shown in Table 3.

(evaluation of withstand Voltage)

The withstand voltage was evaluated by using the test piece corresponding to the sample A2-01 prepared as described above. The withstand voltage was measured using an AC/DC withstand voltage insulation resistance tester. The evaluation results are represented by good quality, good quality when the measured value is 50V/mm or more, and good quality when the measured value is less than 50V/mm. The evaluation results are shown in table 3.

< samples A2-02-A2-27 >

In each of samples a2-02 to a2-27, the silane coupling agents shown in table 2 were used so that the number of carbon atoms in the long chain portion of the hydrocarbon group contained in the insulating film 82B was the number shown in table 2, and the Si/C weight ratio in the insulating film 82B was the value shown in table 2. Except for this, the insulating film 82B was formed in the same manner as in the sample a2-01, and the smoothness was evaluated. In addition, with respect to each of the samples a2-02 to a2-27, the same test pieces as the sample a2-01 were produced using the soft magnetic particles of each sample as the 2 nd soft magnetic particles 82, and the specific permeability, the compressive strength in the radial direction, and the withstand voltage were evaluated by the same procedures as the sample a2-01 using the produced test pieces.

Here, the Si/C weight ratio in the samples A2-01 to A2-27 was adjusted by changing the mixing ratio of tetraethoxysilane and silane coupling agent used in the formation of the insulating film 82B.

The evaluation results of samples A2-01 to A2-09 are shown in Table 3. The evaluation results of the samples A2-10 to A2-18 are shown in Table 4, and the evaluation results of the samples A2-19 to A2-27 are shown in Table 5.

From the measured values of smoothness and the evaluation results of magnetic permeability in table 3 or table 5, it is understood that: since the insulating film 82B formed on the surface of the particle core 82A contains a hydrocarbon group having a long chain part having 8 or more carbon atoms, the smoothness as soft magnetic particles is improved, the molding density of the molded article (test piece) is improved, and the specific magnetic permeability is improved.

There is a trade-off relationship between specific permeability and radial compressive strength and the Si/C weight ratio.

In the insulating film 82B having a linear portion with a carbon number of 8 or more, if the Si/C weight ratio is in the range of 7.6 to 42.8, the compressive strength in the radial direction and the specific permeability in a trade-off relationship with each other can be made 85 or more and 30 or more, and a core having a high specific permeability that can withstand practical mechanical strength can be realized. Particularly, the Si/C weight ratio is more preferably in the range of 9.7 to 13.4 from the viewpoint of reproducibility in production.

As described above, the soft magnetic powder constituting the mixed powder includes the 2 nd soft magnetic particles 82. The 2 nd soft magnetic particle 82 is composed of a particle core 82A containing a soft magnetic metal, and an insulating film 82B located on the surface of the particle core 82A. The insulating film 82B contains Si and a hydrocarbon group having a linear portion having 8 or more carbon atoms, and the weight ratio of Si to C in the insulating film 82B is 7.6 to 42.8.

According to this configuration, when the core 40, which is a metallic magnetic body, is formed by compression molding the mixed powder including the 2 nd soft magnetic particles 82, high mechanical strength and high magnetic permeability can be simultaneously achieved in the core 40. Here, the Si is derived from, for example, a silane coupling agent used for producing the insulating film 82B.

The hydrocarbon group in the insulating film 82B of the 2 nd soft magnetic particle 82 may be an alkyl group. With this configuration, a hydrocarbon group having a linear portion having 8 or more carbon atoms can be easily formed on the surface of the particle core 82A.

The grain core 82A of the 2 nd soft magnetic grain 82 may be formed of carbonyl iron. With this configuration, when the core 40 is formed as a metal magnetic body, a higher magnetic permeability can be achieved in the core 40.

The soft magnetic powder constituting the mixed powder may further include, in addition to the 2 nd soft magnetic particles 82, 1 st soft magnetic particles 81 including a soft magnetic metal and composed of particle cores 81A having a larger average particle diameter than the particle cores 82A of the 2 nd soft magnetic particles. With this configuration, the filling ratio of the soft magnetic particles in the core 40 can be further increased, and a higher magnetic permeability can be obtained.

Further, the inductor 1 can be configured by a metallic magnetic body made of soft magnetic powder including the 2 nd soft magnetic particles 82 according to any of the above embodiments, and the wound wire 31. According to this configuration, a small and highly reliable inductor can be realized.

[ A-2. resin ]

The proportion of the resin is 2.0 to 3.5 wt% based on the total weight of the soft magnetic powder and the resin. The resin may contain at least a bisphenol a type epoxy resin and a rubber-modified epoxy resin, and may further contain a novolac type epoxy resin.

The present inventors found the 1 st resin blending ratio, which is a preferable blending ratio of the bisphenol a type epoxy resin and the rubber modified epoxy resin in the case where the novolac type epoxy resin is not blended in the mixed powder, through the evaluation experiment described later. The 1 st resin compounding ratio is based on the total weight of the resins contained in the mixed powder, and the bisphenol A type epoxy resin accounts for 50-90 wt%, and the rubber modified epoxy resin accounts for 10-50 wt%.

Here, the bisphenol a type epoxy resin is a main component of the resin contained in the mixed powder, and if the resin contained in the mixed powder is only the bisphenol a epoxy resin, the unit body 10 formed is easily embrittled. Therefore, by blending a rubber-modified epoxy resin with the resin contained in the mixed powder, the unit cell 10 formed can have toughness, and the brittleness of the unit cell 10 can be improved. Then, the ratio of the bisphenol a type resin to the rubber-modified epoxy resin in the resin contained in the mixed powder is set in accordance with the 1 st resin compounding ratio, and the unit body 10 in which the coil 30 is sealed is molded by the unit body molding and curing step, whereby the inductor 1 having improved strength of the unit body can be manufactured.

The present inventors found, through the evaluation experiments described later, the 2 nd resin blending ratio, which is a preferable blending ratio of the bisphenol a type epoxy resin, the rubber modified epoxy resin and the novolac type epoxy resin in the case where the novolac type epoxy resin is blended in the mixed powder. The 2 nd resin compounding ratio is based on the total weight of the resins contained in the mixed powder, and the bisphenol A type epoxy resin accounts for 40-80 wt%, the rubber modified epoxy resin accounts for 10-50 wt%, and the novolac type epoxy resin accounts for 1-30 wt%.

Here, the novolac-type epoxy resin fulfills the function of adjusting the viscosity of the mixed powder when forming the unit cells and the glass transition temperature of the unit cells when flowing in the unit cell forming and curing step, and improving the strength of the unit cells when the temperature is high. Therefore, the strength of the unit body can be improved by appropriately blending the novolac type epoxy resin according to the blending ratio of the 2 nd resin 1.

Further, the present inventors have found that inductors having the following specific configurations 1 to 3 can be produced by forming a unit body using a mixed powder obtained by mixing a resin according to the above-mentioned 1 st resin mixing ratio or 2 nd resin mixing ratio.

In the specific configuration 1 …, in the cross section of the cell body 10, the ratio of the area of the voids to the total area of the soft magnetic particles (1 st soft magnetic particle and 2 nd soft magnetic particle) and the resin in the surface region of 1 μm to 100 μm from the surface of the cell body 10 is smaller than the ratio of the area of the voids to the total area of the soft magnetic particles and the resin in the central region of the cell body 10, and the surface region is denser than the central region.

Specific configuration 2 … referring to fig. 1 to 3, the amount of resin in the ridge line portion where the main surfaces 12,14 and the 2 nd side surface 18 meet is smaller than the amount of resin in the ridge line portion where the main surfaces 12,14 and the 1 st side surface 16 meet.

Specific configuration 3 … referring to fig. 1 to 3, the 1 st or 2 nd polished soft magnetic particles are exposed from the core, and the 1 st or 2 nd soft magnetic particles exposed from the core are covered with the element protective film 50. Further, the surface roughness of the 2 nd side surface 18 is larger than the surface roughness of the 1 st side surface 16, and the small distance between the 2 nd side surface 18 and the winding portion 32 of the coil 30 is 1 times larger and 4 times smaller than the diameter of the 1 st soft magnetic particle.

The mechanism obtained by the above-described specific configurations 1 to 3 will be described. Referring to fig. 6, as described above, the coil 30 is provided on the 1 st plate 70, and the coil 30 is sandwiched between the 1 st plate 70 and the 2 nd plate 72 to be integrated. The 1 st plate 70 and the 2 nd plate 72 are heated, and the 1 st plate 70 and the 2 nd plate 72 are pressed in the overlapping direction to fluidize the mixed powder, thereby obtaining a core in which the coil 30 is embedded.

Fig. 14 is an enlarged image of the peripheral portion Ar of the coil 30 which is pressed in the superimposing direction with the coil 30 sandwiched by the 1 st plate 70 and the 2 nd plate 72 shown in fig. 6. Fig. 15 is a cross-sectional view of the 1 st plate 70. As shown in fig. 14, the lateral regions of the coil 30 may be gaps s1, s2, s 3. Therefore, when the 1 st plate 70 and the 2 nd plate 72 are pressed in the overlapping direction, the mixed powder can be filled in advance from the gap in the vicinity of the outer peripheral portion of the core.

That is, the amount of movement of the mixed powder increases near the outer periphery of the coil 30, and thus the gap is easily embedded, and the packing density is easily increased. In contrast, in the region inside the coil 30, since the amount of movement of the mixed powder is small, the gap is difficult to be embedded, and the filling density is likely to decrease. Therefore, as shown in fig. 15, the filling rate of the mixed powder in the outer peripheral portions s10 to s13 of the coil 30 is higher than that in the inner portion s14 of the coil 30. Therefore, the inductor 1 having the above-described specific configuration 1 can be obtained.

If there are voids in the surface region of the cell body, moisture may enter the cell body from the voids, which may deteriorate the moisture resistance of the inductor. In addition, when forming the external electrodes, the plating solution may enter the cell from the gaps, which may accelerate the deterioration of the cell. Therefore, the density of the surface region of the cell body can be increased by the above-described specific configuration, and the occurrence of such problems can be prevented.

Here, fig. 16 is an explanatory diagram of the LT plane and the WT plane as reference planes of the inductor 1, and fig. 17 is a sectional image of the LT plane and the WT plane of the inductor 1 shown in fig. 16. As described above, before the cell protective film is formed on the cell 10, the 2 nd side surface 18 is ground by the cell grinding process, but the 1 st side surface 16 is not ground. Therefore, in the ridge line portions s22, s23 where the main surface (mounting surface here) 12 of the cross-sectional image of the WT surface meets the 2 nd side surface 18, the metal magnetic powder is ground to be flush with the 2 nd side surface 18, the exposed area of the metal magnetic powder in the ridge line portions s22, s23 increases, and the amount of resin in the ridge line portions s22, s23 is smaller than the amount of resin in the ridge line portions s20, s21 where the main surface 12 meets the 1 st side surface 16 in the cross-sectional image of the LT surface. Thereby, the inductor 1 having the above-described specific configuration 2 can be obtained. According to specific configuration 2, by grinding the vicinity of the ridge line portion where main surface 12 and 2 nd side surface 18 contact each other, it is possible to prevent the decrease in the amount of soft magnetic particles protruding from the cell protective film and the decrease in the insulation property of the inductor.

Fig. 18 is a table comparing the photomicrographs of the surfaces of the LT surface and the WT surface after the processing in the unit body molding and curing step and after the unit body grinding step with the maximum height. The maximum height Sz is used as an index value of the surface roughness. The larger the maximum height Sz is, the larger the surface roughness is.

The LT surface is ground in the cell body grinding step, and at this time, since the 1 st soft magnetic particle or the 2 nd soft magnetic particle is degranulated, the surface roughness is increased. Therefore, the maximum height Sz (50 μm) of the LT surface is larger than the maximum height Sz (43 μm) of the WT surface not ground. By increasing the surface roughness of the LT surface, the adhesiveness between the unit cell protective film and the core in the LT surface can be improved. The surface roughness was determined by scanning the center of the surface of the T-plane and the WT-plane in the L-longitudinal direction using a shape-analysis laser microscope (VK-X250, Keyence corporation) and measuring the maximum height (Sz).

As shown in fig. 19, by setting the narrow interval between the 2 nd side surface 18 and the coil 30 at intervals SG1 and SG2 which are larger than 1 st soft magnetic particle and smaller than 4 th soft magnetic particle, the moisture resistance of the unit body can be secured even when the interval is small, and the inductor 1 having the above-described specific configuration 3 can be obtained.

[ A-2-1. embodiment using a resin having the 1 st resin content ]

The unit body of the inductor was prepared by using the mixed powder containing the resin of the 1 st resin compounding ratio through the above-described granulating step → coil forming step → unit body molding and curing step → unit body grinding step → unit body protective film forming step → unit body protective film removing step → external electrode forming step, and the unit body of each sample was evaluated. The production conditions in the unit forming/curing step were 135 ℃ and 10MPa of pressure.

(evaluation of Unit body Strength)

For each sample, the strength was evaluated from the breaking load at the time of 3-point bending using a 3-point bending test apparatus (3-point bending test apparatus AGS-5kNX manufactured by shimadzu corporation). The breaking load is 30MPa or more, G (pass), and the breaking load is less than 30MPa, NG (fail).

(evaluation of Density)

For each sample, an image of a cross section was subjected to image processing, an image portion of a void was extracted, a ratio of the total area of the image portion of the void to the area of the cross section was calculated, and a void ratio was measured.

The void fraction was measured as follows: the cell was cut at 1/2 in the longitudinal direction, and the cut surface was obtained by taking an image of 4 sites (1 site on each surface) in a region of 1 to 100 μm from the surface of the cell using a Scanning Electron Microscope (SEM) set at a magnification of 1000 times, measuring voids included in the cut surface, and calculating the average value. The cell was cut at 1/2 in the longitudinal direction, and in the cut surface, 4 positions in the central region of the cell were photographed using a Scanning Electron Microscope (SEM) set at a magnification of 1000 times, and voids included in the cut surface were measured, and the average value was calculated to measure the void ratio.

As a result, in addition to the samples b4 to b8, b22, b23, and b27, the average porosity of the 8 dots in the surface region and the center region of the cell body was small, the ratio of voids in the surface region of the cell body was smaller than that in the center region of the cell body, and the center region was denser than the surface region.

(proportion of resin in the powder mixture)

FIG. 20 shows the relationship between the amount of resin in the mixed powder and the density of the cells, and the vertical axis is set to the density (g/cm) of the cells³) The horizontal axis represents the amount of resin (wt%) in the mixed powder. The molding conditions of the unit body are 180 ℃, the pressure of 30MPa and the pressure time of 100 seconds. In FIG. 20, if the amount of resin is less than 2.0%, the density of the unit cell decreases. This is presumably because the fluidity of the mixed powder is lowered, and the filling property of the mixed powder at the time of molding the unit body is deteriorated.

From the evaluation results of the samples of table 6, it can be found that: the preferred compounding ratio of the resin in the mixed powder for an inductor having improved strength of the unit body is 50 to 90% by weight of the bisphenol a type epoxy resin and 10 to 50% by weight of the rubber-modified epoxy resin (the compounding ratio of the above-mentioned 1 st resin).

[ A-2-2. embodiment using resin of the 2 nd resin compounding ratio ]

In the same manner as in the embodiment shown in A-2-1, samples of the unit bodies of the inductors were prepared using the mixed powder containing the resin having the above-described 2 nd resin compounding ratio, and the unit bodies of the respective samples were evaluated. The proportion of the resin in the mixed powder is 2.0 to 3.5 wt% of the mixed powder, as in the embodiment shown in A-2-1.

From the evaluation results of the samples of table 7, it can be found that: the preferred compounding ratio of the resins in the mixed material for the inductor having improved strength of the unit body is 40 to 80 wt% of the bisphenol a-type epoxy resin, 10 to 50 wt% of the rubber-modified epoxy resin, and 1 to 30 wt% of the novolac-type epoxy resin (the above-mentioned 2 nd resin compounding ratio).

[ A-2-3 ] embodiments relating to side clearances ]

In order to evaluate the quality of the moisture resistance between the gaps (side gaps) SG1 and SG2 between the 2 nd side surface 18 and the coil 30 shown in fig. 19 and the inductor 1, samples shown in tables 8 and 9 below were prepared and moisture resistance was evaluated.

(evaluation of moisture resistance)

For each sample, a moisture resistance test was performed using a moisture resistance cell set at a temperature of 85 ℃ and a humidity of 85%, and G (passed) was determined when the weight increase of the element due to water absorption was 2 wt% or less, and NG (failed) was determined when the weight increase was more than 2.

(Specification of Mixed powder)

The ratio of the resin in the mixed powder is 2.0 to 3.5% by weight, and the resin content is the above-mentioned No. 1 resin content. The average particle size of the large soft magnetic particles (1 st soft magnetic particles) in the mixed powder was 21 μm (sample in table 8) or 28 μm (sample in table 9), and the average particle size of the small soft magnetic particles (2 nd soft magnetic particles) was 2 μm.

The evaluation of samples b51 to b60 shown in table 8 will be described below. The samples b54 to b60 are examples of the present invention, and the samples b51 to b53 are comparative examples.

[ Table 8]

Large grain size: average particle diameter 21 μm, pellet: average particle diameter of 2 μm

The samples b51, b52, b53 are comparative examples.

(example A-2-3-1)

< example A-2-3-11 (sample b54) >

A cell body having a small side gap of 25 μm and a large side gap of 85 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-12 (sample b55) >

A cell body having a small side gap of 29 μm and a large side gap of 81 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-13 (sample b56) >

A cell body having a side surface with a small gap of 33 μm and a side surface with a large gap of 77 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-14 (sample b57) >

A cell body having a small side gap of 40 μm and a large side gap of 70 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-15 (sample b58) >

A cell body having a small side gap of 45 μm and a large side gap of 65 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-16 (sample b59) >

A cell body having a small side gap of 50 μm and a large side gap of 60 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-17 (sample b60) >

A cell body having a small side gap of 55 μm and a large side gap of 55 μm was formed. The result of evaluation was … moisture resistance G.

Comparative example A-2-3-1

< comparative example A-2-3-11 (sample b51) >

A cell body having a small side gap of 0 μm and a large side gap of 110 μm was formed. The result of the evaluation … was moisture resistance NG.

< comparative example A-2-3-12 (sample b52) >

A cell body having a small side gap of 10 μm and a large side gap of 100 μm was formed. The result of the evaluation … was moisture resistance NG.

< comparative example A-2-3-13 (sample b53) >

A cell body having a small side gap of 18 μm and a large side gap of 92 μm was formed. The result of the evaluation … was moisture resistance NG.

From the evaluation results in table 8, it was found that a cell body in which the 1 st soft magnetic particle having an average particle size of 21 μm had a smaller side gap in the range of larger than 1 st soft magnetic particle and smaller than 4 was able to obtain good moisture resistance.

The evaluation of samples b61 to b70 shown in table 9 will be described below. The samples b65 to b70 are examples of the present invention, and the samples b61 to b64 are comparative examples.

[ Table 9]

Large grain size: average particle diameter 28 μm, pellet: average particle diameter of 2 μm

The samples b61, b62, b63, b64 are comparative examples.

(example A-2-3-2)

< example A-2-3-21 (sample b65)

A cell body having a small side gap of 29 μm and a large side gap of 81 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-22 (sample b66) >

A cell body having a side surface with a small gap of 33 μm and a side surface with a large gap of 77 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-23 (sample b67) >

A cell body having a small side gap of 40 μm and a large side gap of 70 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-24 (sample b68) >

A cell body having a small side gap of 45 μm and a large side gap of 65 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-25 (sample b69) >

A cell body having a small side gap of 50 μm and a large side gap of 60 μm was formed. The result of evaluation was … moisture resistance G.

< example A-2-3-26 (sample b70) >

A cell body having a small side gap of 55 μm and a large side gap of 55 μm was formed. The result was evaluated as … G.

Comparative example A-2-3-2)

< comparative example A-2-3-21 (sample b61) >

A cell body having a small side gap of 0 μm and a large side gap of 110 μm was formed. The result of the evaluation … was moisture resistance NG.

< comparative example A-2-3-22 (sample b62) >

A cell body having a small side gap of 10 μm and a large side gap of 100 μm was formed. The result of the evaluation … was moisture resistance NG.

< comparative example A-2-3-23 (sample b63) >

A cell body having a small side gap of 18 μm and a large side gap of 92 μm was formed. The result of the evaluation … was moisture resistance NG.

< comparative example A-2-3-24 (sample b64) >

A cell body having a small side gap of 25 μm and a large side gap of 85 μm was formed. The result of the evaluation … was moisture resistance NG.

From the evaluation results in table 9, it was found that a cell body in which the side gap was smaller than the 1 st soft magnetic particle and was in the range of larger than 1 and smaller than 4 was able to obtain good moisture resistance for the 1 st soft magnetic particle having an average particle size of 28 μm.

[ A-2-4. other items of study ]

In the above embodiment, as the resin contained in the mixed powder, a bisphenol a type epoxy resin, a rubber modified epoxy resin, and a novolac type epoxy resin are used. The bisphenol a epoxy resin is a generic concept of epoxy resin, and the rubber modified epoxy resin is a generic concept of rubber or resin having flexibility.

Therefore, examples of a resin that can be studied as a substitute for bisphenol a epoxy resin include bisphenol a, F, and S type phenoxy resins. Examples of the resin or rubber which can be used as a substitute for the rubber-modified epoxy resin include urethane modification, nbr (acrylonitrile Butadiene rubber) rubber modification, CTBN (carboxyl Terminated Butadiene acrylonitrile) rubber modification, and CTBN rubber. Further, as a resin having room for study to replace the novolak type epoxy resin, cresol, dicyclopentadiene, phenol aralkyl, biphenyl, naphthol, xylene, triphenylmethane, tetraphenylethane may be mentioned, if the resin is limited to the novolak type, naphthalene, biphenyl, and triazine may be mentioned, if the resin is not limited to the novolak type.

[ coil B ]

Next, a portion of the coil 30 of the inductor 10 including the core 40 formed of the mixed powder of the soft magnetic particles and the resin described in [ a.

(guide wire)

In the inductor 1, the conductive wire 31 used in the coil 30 may be either a round wire or a flat wire (flat wire in fig. 3), and by using a flat wire as the conductive wire 31, a gap is not generated between the conductive wires 31 at the time of forming the winding portion 32, and the winding is easy.

The number of turns of the winding portion 32 may be appropriately determined according to the characteristics realized by the inductor 1.

Further, as the lead 31, a copper wire 36 formed of copper is preferably used.

For example, in the inductor 1 having the dimensions of 2.0 ± 0.2mm in length L, 1.2 ± 0.2mm in width W, and 0.7 ± 0.1mm in thickness T, the winding portion 32 of the coil 30 has a height of 0.4mm, and an outer diameter of 1.17mm and an inner diameter of 0.55mm in the width W direction.

Here, when the lead wire 31 of the coil 30 is a flat wire, the length of the short side of the cross section of the flat wire is, for example, 0.118mm or less. The length of the short side of the cross section of the flat line is preferably 0.052mm or more.

The length of the long side of the cross section of the flat line may be, for example, 0.203mm or less. Further, the length of the long side of the cross section of the flat line is preferably 0.141mm or more.

The aspect ratio (long side/short side) of the cross section of the flat line is, for example, 1.3 to 3.4.

In the inductor 1 in which the thickness T is changed to a size of 0.55 ± 0.1mm (so-called downsizing) among the inductors 1 having the above dimensions, the winding portion 32 of the coil 30 has, for example, an outer diameter of 1.17mm, an inner diameter of 0.48mm, and a height of 0.30mm in the width W direction. The lead wire 31 of the coil 30 constituting the winding portion 32 of the preferred size is, for example, a flat wire having an aspect ratio (long side/short side) of 1.3, a length of, for example, a short side of the cross section of 0.11mm, and a length of a long side of the cross section of 0.14 mm.

(insulating coating Material)

The material of the insulating coating layer 61 forming the insulating coating material 60 is not particularly limited, and examples thereof include a polyurethane resin, a polyester resin, an epoxy resin, and a polyimide amide resin is preferable.

The thickness of the insulating coating layer 61 is preferably 4 μm.

The material for forming the fusion-bonded layer 62 of the insulating cover material 60 is polyamide resin.

The thickness of the fusion-spliced layer 62 is preferably 1 μm to 25 μm, more preferably 2 μm to 25 μm, and still more preferably 2 μm to 4 μm.

By setting the thickness of the fusion-bonded layer 62 to the above value, it is possible to obtain a fusion-bonding force that sufficiently suppresses peeling due to springback of the conductive wire 31 in the outermost periphery of the winding portion 32 while suppressing an increase in size of the winding portion 32 of the coil 30, thereby preventing a shape failure of the coil 30.

As described above, since the welding layer 62 is melted by heating the conductive wire 31 at the time of winding in the coil forming step and the conductive wires 31 of the winding portion 32 are fixed to each other, the material of the welding layer 62 can be selected so that the melting point temperature is, for example, 180 ℃.

Since the melting point temperature is equal to the heating temperature in the reflow soldering step when the completed inductor 1 is mounted on the printed wiring board, the solder layer 62 can be melted in the reflow soldering step. The material of the fusion-bonded layer 62 melted in the reflow welding step is usually solidified by allowing a part thereof to permeate into the inside of the unit body 10, but this permeation range is not problematic because the material of the fusion-bonded layer 62 is viscous and stays in the vicinity of the coil 30.

However, when the one-side gap Sg of the unit cell 10 (the distance between the coil 30 and the 2 nd side surface 18 in the unit cell 10) is 50 μm or less, the material of the fusion-spliced layer 62 melted in the reflow-splicing step may bulge out of the 2 nd side surface 18.

Therefore, when the 2 nd side surface of the unit cell 10 is formed thinner than 50 μm as described above, it is important that the fusion-bonded layer 62 be made of a material having a melting point temperature equal to that described above and having a higher viscosity when melted.

The material of the fusion-bonded layer 62 can be configured, for example, as a material containing a plurality of resins having different molecular weights. Generally, the smaller the molecular weight, the smaller the viscosity of the resin at the time of melting. Therefore, by forming the fusion-bonded layer 62 from a material including a plurality of resins having different molecular weights from each other, the viscosity of the fusion-bonded layer 62 at the time of melting can be adjusted to a desired value by adjusting the weight ratio of the respective resins, and the material can be prevented from bulging out of the unit body 10 in the reflow welding step.

Such a material containing a plurality of resins having different molecular weights can be produced, for example, as follows: the resin composition is prepared by mixing resins having different molecular weights, and polymerizing a part of the resin by adding a catalyst to a resin having a small molecular weight or depolymerizing a part of the resin by adding a catalyst to a resin having a large molecular weight.

In one embodiment, weld layer 62 is, for example, 2 polyamides having different molecular weights.

[ C. magnetic circuit ]

Next, a description will be given of a relationship between the structure of the magnetic powder of the core 40 and the lead wire 31 of the coil 30 in the inductor 10 including the core 40 formed of the mixed powder of the soft magnetic particles and the resin described in [ a. The conductive line 31 uses a flat line.

[ C-1. line-to-line magnetic powder ]

In the inductor 1, since the soft magnetic powder formed of the metal magnetic particles is used as the magnetic material, a favorable dc superposition characteristic can be obtained as compared with the case of using a magnetic material such as an iron unit.

Here, fig. 21(a) is an image showing the lower winding portion 32L of the coil 30 together with the surrounding material, and fig. 21(B) is an image showing the upper winding portion 32L of the coil 30 together with the surrounding material. In fig. 21(a) and (B), the vertical direction of the paper corresponds to the thickness direction of the unit body 10, and the horizontal direction of the paper corresponds to the radial direction of the winding portion 32. In the figure, reference symbol CW denotes the roll width of the winding portion 32.

In this configuration, as shown in fig. 21(a) and (B), a part of the 2 nd soft magnetic particle 82 as a small particle enters between the winding portions 32L. A part of the 2 nd soft magnetic particle 82 is located near the outer peripheral portion of the winding portion 32, and a region near the outer peripheral portion is denoted by reference numeral 10S. The 2 nd soft magnetic particles 82 in the region indicated by reference numeral 10S form a magnetic path in the vicinity of the winding portion 32L along the flow of the magnetic flux, and thus local saturation of the magnetic flux density can be suppressed. In this configuration, the 2 nd soft magnetic particles 82 also enter the vicinity of the inner peripheral portion of the winding portion 32 (not shown). However, as will be described later, a part of the 2 nd soft magnetic particle 82 may enter between the winding portions 32L, and the position of the entry is not limited to the vicinity of the outer periphery and the vicinity of the inner periphery of the winding portion 32. The structure in which the 2 nd soft magnetic particles 82 enter between the winding portions 32L is referred to as a line-to-line magnetic powder structure.

The structure of the magnetic powder between lines will be explained.

As shown in fig. 21(a) and (B), the soft magnetic powder includes the 1 st soft magnetic particle 81 of large particles and the 2 nd soft magnetic particle 82 of small particles.

The gaps between the winding portions 32L are small, and the 2 nd soft magnetic particles 82 of small particles can enter and the 1 st soft magnetic particles 81 of large particles cannot enter. The 2 nd soft magnetic particle 82 is formed to have an average diameter smaller than the thickness of the fusion-bonded layer 62 of the winding portion 32L. Therefore, the 2 nd soft magnetic particles 82 easily enter the vicinity of the weld layer 62.

In this configuration, in the unit body molding and curing step of compression molding the coil 30 in the mixed powder including the 1 st and 2 nd soft magnetic particles 81 and 82, the pressure P during compression molding is adjusted to a value higher than before so that the 2 nd soft magnetic particles 82 positively enter between the winding portions 32L. Since heating is performed during the compression molding, the fusion-bonded layer 62 in the insulating coating material 60 on the surface of the winding portion 32L is melted, and the 2 nd soft magnetic particles 82 easily enter the melted fusion-bonded layer 62.

More specifically, as shown in fig. 22, when the pressure P is applied from above, the pressure P is applied from below and from the left and right sides by the law of reaction or the like at the winding portion 32 of the coil 30 and the periphery thereof, simultaneously with the application of the pressure P from above. This applies pressure to the 2 nd soft magnetic particles 82 from the outer peripheral side of the coil 30 to the respective winding portions 32L, and the 2 nd soft magnetic particles 82 are easily filled between the winding portions 32L.

The condition of the pressure P at this time may include not only the value of the pressure P but also various parameters related to pressurization such as the time for applying the pressure P. By appropriately setting this condition, the 2 nd soft magnetic particle 82 can be easily filled between the winding portions 32L. In this case, the 2 nd soft magnetic particles 82 can be easily filled between the winding portions 32L by adjusting the heating conditions, the distance between the winding portion 32 and the peripheral wall (the inner surface of the forming die 74 and the punch 76), and the like.

As shown in fig. 21(a) and (B), by allowing the 2 nd soft magnetic particles 82 to enter between the winding portions 32L, local saturation of the magnetic flux density in the vicinity of the winding portions 32 can be suppressed, and the dc superposition characteristics can be improved.

Next, the length LS of the 2 nd soft magnetic particle 82 between the winding portions 32L will be described.

This length LS corresponds to the length of the coil 30 constituting the winding portion 32L in which the 2 nd soft magnetic particle 82 is in contact.

The wire width of the winding portion 32L of the coil 30 is 95 μm, the thickness of the winding portion 32L of the coil 30 is 180 μm, the thickness of the fusion-bonded layer 62 between the winding portions 32L of the coil 30 is 6 μm, the average particle diameter of the 1 st soft magnetic particle 81 is 10 μm or more, the average particle diameter of the 2 nd soft magnetic particle 82 is 5 μm or less, and the pressurizing force P is 300kg/cm²The length LS of the 2 nd soft magnetic particles 8 between the winding portions 32L was varied, and the dc superimposed rated current Isat was calculated and examined.

The dc superimposed rated current Isat is a current value when the inductance is reduced by a certain ratio with respect to the initial characteristics of the non-superimposed current, and is a measure of the maximum current that can flow when the inductance is not magnetically saturated. The current value at which the initial inductance value is reduced by about 30% is defined as the dc superimposed rated current Isat. The results of the study are shown in Table 10.

[ Table 10]

	Length LS of 2 nd soft magnetic particles between the winding portions	Isat
			Comparative example CK-1	(non-magnetic powder)	100
Comparative example CK-2	The length of the wires connected with each other is 5%	100.07
			Example C1-1	The length of the wires connected to each other is 10%	100.14
Example C1-2	The length of the wires connected with each other is 50%	100.7
			Comparative example CK-3	55% of the length of the conductors connected to each other	100.77

In comparative example CK-1, the length LS of the 2 nd soft magnetic particle 82 between the winding portions 32L is zero in the cross section (for example, WT cross section) of the coil 30 constituting the winding portions 32L, that is, the 2 nd soft magnetic particle 82 does not exist between the winding portions 32L. In comparative example CK-2, the length LS is 5% of the length of the coil cross section where the winding portions 32L are in contact with each other, in other words, 5% of the length of the coil cross section where the lead wires 31 of the coil wire are in contact with each other. The ratio of the length LS of the 2 nd soft magnetic particles 82 between the winding portions 32L to the length in which the winding portions 32L are in contact with each other is set at the center in the L direction of the unit 10 when the WT cross-section is viewed, and is (the length LS of the 2 nd soft magnetic particles 82 between all the winding portions 32L)/(the length in which all the winding portions 32L are in contact with each other).

Example C1-1 shows a case where the length LS is 10% of the length of the coil cross section where the wires 31 are connected to each other. Example C1-2 shows a case where the length LS is 50% of the length of the coil cross section where the wires 31 are connected to each other. Comparative example C1-3 shows a case where the length LS is 55% of the length of the coil cross section where the leads 31 are connected to each other. The dc superimposed rated current Isat in the case of the comparative example CK-1 is defined as 100.

As a result of the study by the inventors, if the length LS is 10% or more of the length of the coil cross section over which the leads 31 contact each other, the dc superimposition rated current Isat is larger than that in the case where the length LS is zero, and therefore the length LS is preferably 10% or more of the length over which the leads 31 contact each other. However, if the length LS is greater than 55% of the length of the coil cross section where the leads 31 meet each other, cracks are likely to occur in the fusion-spliced layer 62 where the wound portions 32L meet each other, and the wound portions 32L are likely to peel off from each other.

In view of these things, it can be judged that: from the viewpoint of suppressing magnetic saturation and improving the dc superimposition characteristic, the length LS is preferably 10% or more of the length over which the lead wires 31 are in contact with each other in the coil cross section, and from the viewpoint of suppressing separation of the winding portions 32L from each other, the length LS is preferably 10% to 50% of the length over which the lead wires 31 are in contact with each other in the coil cross section. Therefore, the length LS is preferably set to be in the range of 10% to 50% of the length of the coil cross section where the leads 31 are in contact with each other.

Fig. 23 is a characteristic graph showing a simulation result of the line-to-line magnetic powder structure.

In fig. 23, the horizontal axis represents the current value and the vertical axis represents the inductance value (L value). In fig. 23, comparative example CK-4 shows a case where no interline magnetic powder structure is provided, that is, the 2 nd soft magnetic particles 82 are not present on any of the surface between the upper and lower stages of the winding portion 32 on which the upper and lower 2 stages are wound, and between the winding portions 32L on the upper stage and the winding portions 32L on the lower stage.

Example C1-3 shows a case where the 2 nd soft magnetic particles 82 are present on the entire circumference of each of the surface between the upper and lower stages of the winding portion 32, the space between the upper winding portion 32L, and the space between the lower winding portion 32L.

Example C1-4 shows a case where the 2 nd soft magnetic particles 82 are present only in the outermost periphery between the upper and lower segments of the winding portion 32, the upper half between the upper segment of the winding portion 32L, and the lower half between the lower segment of the winding portion 32L.

Example C1-5 shows a case where the 2 nd soft magnetic particles 82 are present in the upper half between the upper-stage winding portions 32L and the lower half between the lower-stage winding portions 32L.

Example C1-6 shows a case where the 2 nd soft magnetic particles 82 were present around the entire circumference between the upper winding portion 32L and the lower winding portion 32L.

Table 11 shows the simulation results of the initial inductance (initial L value) and the DC-superimposed rated current Isat for comparative example CK-4 and examples C1-3 to C1-6. The conditions such as the line width and thickness of the coil 30 and the thickness of the fusion-bonded layer 62 are the same as those in table 10.

[ Table 11]

As shown in FIG. 23, in examples C1-3 to C1-6, a larger inductance value was obtained in a wide current range of 0A to 10A as compared with comparative example CK-4, and it was confirmed that magnetic saturation was suppressed and the DC superposition characteristics were improved. As shown in Table 11, it was found that the examples C1-3 to C1-6 were larger than the comparative example CK-4 in terms of the rated current Isat superimposed on the direct current.

In addition, examples C1-3 and C1-6 gave almost the same characteristics (inductance value and DC-superimposed rated current Isat), and a large inductance value was obtained as compared with the other examples C1-4 and C1-5. The examples C1-3 and C1-6 are the same in that the 2 nd soft magnetic particles 82 are present between the upper winding portions 32L and between the lower winding portions 32L, and it is presumed that this point is advantageous in suppressing magnetic saturation and improving the dc bias characteristics.

In this way, the core 40 in which the coil 30 is embedded includes the 1 st soft magnetic particle 81 having large particles and the 2 nd soft magnetic particle 82 having small particles, and a part of the 2 nd soft magnetic particle 82 enters between the winding portions 32L to form a magnetic path in the vicinity of the winding portion 32L, so that even when magnetic particles that can obtain good dc superposition characteristics are used, local saturation of the magnetic flux density can be suppressed.

Further, by setting the length LS of the 2 nd soft magnetic particles 82 between the winding portions 32L to a length that is 10% or more of the length of the coil cross section at which the conductive wires 31 are in contact with each other, local saturation of the magnetic flux density, that is, magnetic saturation can be further suppressed as compared with the case where the length is less than 10%.

Further, by setting the length LS of the 2 nd soft magnetic particles 82 between the winding portions 32L to a length of 50% or less of the length of the contact between the conductive wires 31 in the cross section of the coil, it is easy to avoid the occurrence of cracks in the fusion-bonded layer 62 where the winding portions 32L are joined to each other and the separation of the winding portions 32L from each other.

Further, since the adjacent winding portions 32L are joined to each other by the welding agent formed by the welding layer 62, the 2 nd soft magnetic particle 82 has a diameter smaller than the thickness of the welding layer 62, and a part of the 2 nd soft magnetic particle 82 enters the welding layer 62 to form a magnetic path between the winding portions 32L, the adjacent winding portions 32L are joined to each other to effectively form a magnetic path between the winding portions 32L, and magnetic saturation is easily effectively suppressed.

Further, when the unit body 10 is formed by compression molding a material formed of the material of the core 40 (the 1 st and 2 nd soft magnetic particles 81,82, etc.) and the coil 30, since at least the pressure P at the time of compression molding is adjusted so that a part of the 2 nd soft magnetic particles 82 enters between the winding portions 32L, a magnetic path can be easily provided between the winding portions 32L.

The method of allowing a part of the 2 nd soft magnetic particles 82 to enter between the winding portions 32L is not limited to the conditions of the pressure P and the heating conditions. For example, the adjustment of the diameters of the 1 st and 2 nd soft magnetic particles 81 and 82, the adjustment of the smoothness of the surface layer between the particles 81 and 82, the selection of the resin, and the like may be appropriately combined to facilitate the penetration of the 2 nd soft magnetic particles 82 between the winding portions 32L.

In the case where the coil 30 has the winding portions 32 wound in a plurality of upper and lower stages (including 2 or more stages) in a state where the lead wire 31 is connected, the 2 nd soft magnetic particles 82 are preferably present between the winding portions 32L in the uppermost stage and/or the lowermost stage. This facilitates the provision of a magnetic circuit effective in suppressing magnetic saturation.

The material of the core 40 may be appropriately increased or decreased and the shape of the coil 30 or the like may be appropriately changed within a range in which a part of the 2 nd soft magnetic particles 82 can enter between the winding portions 32L. The 2 nd soft magnetic particles 82 may be inserted between the winding portions 32L by another method, not limited to the case where the 2 nd soft magnetic particles 82 are inserted between the winding portions 32L in the unit body molding and curing step.

The winding method of the coil 30 is not limited to the α -winding, and may be, for example, edgewise (edgewise) winding. In edgewise winding or the like, a magnetic path is formed between the adjacent winding portions 32L joined by the welding layer 62 by the 2 nd soft magnetic particles 82, and magnetic saturation is easily and effectively suppressed.

[ C-2. magnetic gap ]

In the inductor 1, a magnetic gap may be provided near the winding portion 32 in order to further suppress saturation of magnetic flux density near the winding portion 32 of the coil 30.

Fig. 24 is an image in the case where an air gap 40K is provided as a magnetic gap in the vicinity of the winding portion 32.

The air gap 40K extends in the direction in which the winding portions 32L are arranged, and is substantially perpendicular to the magnetic flux. The air gap 40K may be formed during the unit body molding and curing process. Specifically, as shown in fig. 22, by applying pressures P to the upper, lower, left, and right sides of the coil 30, the 1 st and 2 nd soft magnetic particles 81 and 82, etc., which are core materials, are compressed around the winding portion 32 of the coil 30. Thereafter, by rapidly retracting the punch 76 or removing the unit cell 10 and the like from the die before the core material is completely solidified, the air gap 40K shown in fig. 24 is formed around the winding portion 32 by the rebound (repulsive force) of the 1 st and 2 nd soft magnetic particles 81,82 and the like.

The springback includes springback of at least one of the 1 st, 2 nd, and 1 st and 2 nd soft magnetic particles 81, 82. By appropriately utilizing these rebounds, the air gap 40K functioning as a magnetic gap can be formed. Note that the springback may include springback between the coil 30 and the core 40 (1 st and 2 nd soft magnetic particles 81 and 82).

That is, the air gap 40K extending in the direction of the arrangement of the winding portions 32L is formed around the winding portion 32L by appropriately adjusting various conditions during the compression molding, such as the pressure P during the compression molding, the pressing speed, the pressing time, the retracting speed of the punch 76, and the timing of taking out the unit body 10. This facilitates the provision of the air gap 40K that functions as a magnetic gap, and the saturation of the magnetic flux density in the vicinity of the winding portion 32 of the coil 30 can be suppressed by the air gap 40K, thereby improving the dc superimposition characteristic.

Next, the position, length, and width of the air gap 40K will be explained.

The wire width of the winding portion 32L of the coil 30 is 95 μm, the thickness of the winding portion 32L of the coil 30 is 180 μm, the thickness of the fusion-bonded layer 62 of the winding portion 32L of the coil 30 is 4 μm, the average particle diameter of the 1 st soft magnetic particle 81 is 10 μm or more, the average particle diameter of the 2 nd soft magnetic particle 82 is 5 μm or less, and the pressure P is 300kg/cm²The position, length and width of the air gap 40K were varied, and the dc superimposed rated current Isat was calculated and studied. The dc superimposed rated current Isat is defined as a current value at which the initial inductance value is reduced by about 30%.

The results of the investigation of the position of the air gap 40K are shown in table 12. The dc superimposed rated current Isat in table 12 indicates a case where the value of the dc superimposed rated current Isat in the case where the air gap 40K is at the position from the winding portion 32 to 11 μm is taken as 100.

[ Table 12]

Position of the air gap	Isat
			0 μm from the winding part	67
1 μm from the winding part	70
		2 μm from the winding part	73
3 mu from the winding partm	76
		4 μm from the winding part	79
5 μm from the winding part	82
		6 μm from the winding part	85
7 μm from the winding part	88
		8 μm from the winding part	91
9 μm from the winding part	94
		10 μm (one time of the average particle diameter of the 1 st soft magnetic particle) from the winding part	97
11 μm from the winding part	100
		15 μm from the winding part	113
20 μm (twice the average particle size of the 1 st soft magnetic particle)	130
		30 μm (three times the average particle size of the 1 st soft magnetic particle) from the winding part	130
40 μm from the winding part (four times the average particle size of the 1 st soft magnetic particle)	113
		50 μm (corresponding to five times the average particle diameter of the 1 st soft magnetic particle)	100

As shown in table 12, in the range of the air gap 40K from the winding portion 32 to 30 μm, the value of the direct-current superimposed rated current Isat is larger as the distance from the winding portion 32 is larger, and if it is larger than 50 μm, the value of Isat is lower than 100. In the studies by the inventors, the air gap 40K is an effective range for suppressing magnetic saturation within a range from the winding portion 32 to 50 μm, in other words, within a distance of 5 times the average particle diameter of the 1 st soft magnetic particles 81. More preferably, the air gap 40K is in the range of 20 to 30 μm from the winding portion 32.

Table 13 shows the results of the investigation of the length KL (see fig. 24) of the air gap 40K. The dc superimposed rated current Isat in table 13 indicates a case where the value of the dc superimposed rated current Isat when the length KL of the air gap 40K is 10% of the winding width CW (fig. 24) of the winding portion 32 is taken as 100.

[ Table 13]

Position of the air gap	Isat
		The roll width of the wound portion was 10%	100
The roll width of the wound portion was 20%	102
		The roll width of the wound portion was 30%	103
The roll width of the wound portion was 40%	105
		The roll width of the wound portion was 50%	107
The roll width of the wound portion was 60%	109
		The roll width of the wound portion was 70%	111
The roll width of the wound portion was 80%	113
		The roll width of the wound portion was 90%	114
The roll width of the wound portion was 10%	116
		The roll width of the wound portion was 110%	118

As shown in table 13, the length KL of the air gap 40K is up to 110% ま of the winding width CW of the winding portion 32, and the value of the dc superimposed rated current Isat increases as the air gap 40K is longer. In the studies of the inventors, it is effective to suppress magnetic saturation that the length KL of the air gap 40K is equal to or greater than the width of the single winding portion 32L (equal to or greater than 33% of the winding width CW).

On the other hand, if the length KL of the air gap 40K is substantially greater than the roll width CW, more specifically, if it is greater than 110% of the roll width CW, there is a risk of the inductance value being lowered.

Therefore, the inventors judged that: the length KL of the air gap 40K is preferably equal to or greater than the width of the single winding portion 32L (corresponding to 33% or greater of the winding width CW of the winding portion 32) to equal to or less than 110% of the winding width CW of the winding portion 32, and more preferably equal to or greater than 1.5 times the width of the single winding portion 32L (equal to or greater than 50% of the winding width CW of the winding portion 32) to equal to or less than (equal to or less than 100%) the winding width CW of the winding portion 32.

Table 14 shows the results of the investigation of the width KW of the air gap 40K (see fig. 24). The width KW corresponds to the length of the air gap 40K orthogonal to the arrangement direction of the winding portions 32L. The dc superimposed rated current Isat in table 14 indicates a case where the width KW of the air gap 40K is 1 μm smaller than the diameter of the minimum particle (defined as no air gap.) is 100.

[ Table 14]

Width of air gap	Isat
		Less than smallest particle	100
1μm	103
		2μm	107
3μm	l11
		4μm	114
5μm	118
		6μm	122
7μm	126
		8μm	131
9μm	135
		10μm	140
11μm	140

As shown in table 14, the value of the dc superimposed rated current Isat was the same as that of the dc superimposed rated current Isat when the width KW of the air gap 40K was up to 10 μm and the width KW was 10 μm and 11 μm as the width KW was larger. Further, if the width KW of the air gap 40K is larger than 11 μm, the width KW is larger than the average particle diameter of the 1 st soft magnetic particles 81, the bonding strength of the soft magnetic particles of the resin bonded to the 1 st soft magnetic particles 81 becomes weak, and cracks are likely to occur along the unit 10 in the extending direction of the air gap 40K.

Therefore, the width KW of the air gap 40K is preferably equal to or larger than the average particle diameter of the 2 nd soft magnetic particles 82 (5 μm) to 11 μm, and more preferably a value close to 10 μm within a range in which cracks in the cell body 10 can be suppressed.

Fig. 25 is a characteristic graph showing simulation results corresponding to the presence or absence of the air gap 40K. In fig. 25, the abscissa indicates the current value, the ordinate indicates the inductance value (L value), the characteristic curve K1 indicates the case where the air gap 40K is absent, and the characteristic curve K2 indicates the case where the air gap 40K extends in the upper, lower, inner, and outer peripheries of the wound portion 32, and in both the longitudinal and short-side directions of the wound portion 32.

Table 15 shows the simulation results of the initial inductance value (initial L value) and the dc superimposed rated current Isat corresponding to the characteristic curves K1 and K2, respectively. The conditions of the line width and thickness of the coil 30, the thickness of the fusion-bonded layer 62, and the like are the same as those in tables 10 and 11.

[ Table 15]

As shown in fig. 25 and table 15, it was confirmed from the simulation results that the magnetic saturation can be suppressed in the case where the air gap 40K is present as compared with the case where it is not present, and the effect of suppressing the magnetic saturation can be obtained particularly in the range of the current value 0A to 6A.

In this way, the unit body 10 has the air gap 40K extending in the direction in which the winding portions 32L are arranged, on the outer periphery of the winding portion 32 of the coil 30, within a distance of 5 times the average particle diameter from the winding portion 32 to the 1 st soft magnetic particle 81, and therefore, even when the magnetic particles obtained by good dc superposition characteristics are used, magnetic saturation can be suppressed.

The air gap 40K has a length from the width of the single winding portion 32L to the winding width CW of the winding portion 32 in the direction in which the winding portions 32L are arranged, and has a width from the average particle size of the 2 nd soft magnetic particles 82 to 10 μm in the radial direction of the winding portion 32L, and therefore, magnetic saturation can be effectively suppressed.

Further, since the air gap 40K is formed by utilizing the spring back when the unit body 10 is formed by compression molding of the material including the core 40 and the material of the coil 30, the air gap 40K can be easily provided.

The material of the core 40 may be appropriately increased or decreased within a range in which the air gap 40K can be formed, or the shape of the coil 30 or the like may be appropriately changed. The air gap 40K is not limited to the case where the air gap 40K is formed in the unit body molding and curing step, and the air gap 40K may be formed by another method.

[ grinding of Unit body ]

Next, surface grinding of the unit body 10 of the inductor 10 including the core 40 formed of the mixed powder of the soft magnetic particles and the resin described in [ a.

As described above, the unit body 10 of the inductor 1 is a molded body formed by embedding the coil 30 and compression-molding the mixed powder, and includes the coil 30 and the core 40.

The unit body grinding step shown in fig. 5 is a step of grinding the width W to a predetermined width by applying abrasive grains to the 2 nd side surface 18 (fig. 1) of the unit body 10 after compression molding, as described above. By this grinding, the unit cell 10 is reduced to a predetermined size, and the occupancy of the coil 30 in the unit cell 10 is increased. Further, by adopting a method of reducing the size of the unit cell 10 by grinding and processing the unit cell to a predetermined size, it is possible to reduce the size variation of the unit cell 10 as compared with a case where the size of the unit cell 10 is controlled to a predetermined size by adjusting the size of the cavity of the molding die. After the grinding, in order to chamfer the corner generated by the grinding of the 2 nd side surface 18, barrel polishing, for example, may be performed.

(grinding device)

Fig. 26 is a diagram schematically showing an example of a grinding apparatus 101 for grinding a unit body.

The grinding apparatus 101 includes a housing tool 102 for housing a unit body 10 (workpiece) to be ground, and an upper grinding wheel 103 and a lower grinding wheel 104 for sandwiching the unit body 10 housed in the housing tool 102; the housing tool 102 is housed such that the 2 nd side surface 18 as a ground surface of the unit body 10 faces upward and downward.

In the unit body grinding, the grinding apparatus 101 presses the upper and lower 2 nd side surfaces 18 with a predetermined load of the upper and lower grindstones 103 and 104, respectively, and moves the upper and lower grindstones 103 and 104 relative to the upper and lower 2 nd side surfaces 18, whereby the upper and lower 2 nd side surfaces 18 are simultaneously ground by the abrasive grains 105 of the upper and lower grindstones 103 and 104 (so-called double-side grinding).

(size of grinding wheel)

The size of the abrasive grains 105 is proportional to the grinding rate, and the larger the abrasive grains 105 are, the more the soft magnetic powder particles are threshed in the grinding surface, and the larger the surface roughness is, which can be confirmed by the experiment of the inventors.

Specifically, if a compact of soft magnetic powder is ground, a considerable amount of particles of soft magnetic powder are threshed by using abrasive grains 105, and irregularities due to particle defects are generated on the ground surface. In the soft magnetic powder including large particles and small particles, the small particles are more easily threshed than the large particles, and the larger the abrasive grains 105 are, the more the large particles are threshed, and a large number of large irregularities are generated on the grinding surface, whereby the surface roughness of the grinding surface becomes large.

As for the surface roughness, there is no correlation between the surface roughness and the load, which can be confirmed by the inventors' experiments.

In the present example, the arithmetic mean height was used for the evaluation of the surface roughness. Specifically, a plurality of (for example, 3 to 4 points) measurement regions of a predetermined size (about 200 μm × 290 μm in the present embodiment) are set on the surface to be measured, the maximum height in each measurement region is measured with a laser microscope, and the arithmetic average height is obtained from the average of these maximum heights. The laser microscope used was a model VK-X250 manufactured by Keyence.

(grinding speed)

It can be confirmed by the experiment of the inventors that the larger the grinding speed (the moving speed of the upper and lower grinding stones 103 and 104), the lower the surface roughness of the ground surface resulting from the cutting of the particles of the soft magnetic powder, and that the grinding speed is in a proportional relationship with the above-described grinding rate.

(grinding speed)

A target value of the grinding rate can be appropriately set, and the size of abrasive grains 105 and the grinding speed necessary for realizing the target value can be determined. The size and grinding speed of these abrasive grains 105 are each related to the surface roughness of the grinding surface as described above. In this embodiment, the size and grinding speed of the abrasive grains 105 are set so that the surface roughness after grinding is larger than that before grinding, and further, the roughness of the 2 nd side surface 18 after grinding is larger than that of the top surface 14 and the mounting surface 12 which are the outer surfaces to be ground.

The surface roughness Sa is increased by grinding, thereby improving the bonding strength of the cell protective film 50 covering the 2 nd side surface 18 of the cell 10. The unit cell 10 is covered with the unit cell protective film 50 on the surface thereof except the external electrode 20, and the unit cell protective film 50 can improve the moisture resistance, rust resistance, and electrical insulation of the unit cell 10.

(grinding time)

The grinding time is defined as the time from the grinding start time Ts to the grinding end time Te, and may be determined based on the difference between the width W of the unit cell 10 before grinding the unit cell and a predetermined width, which is a target value of the width W, and the grinding rate.

Then, at the time of grinding the unit body, a control device (not shown) controls the grinding operation of the grinding device 101 based on the load curve and the grinding time, and performs grinding until the width W of the unit body 10 after compression molding becomes a predetermined width.

(side gap)

The side gap Sg is defined as a thickness from the coil 30 inside the unit body 10 to the 2 nd side 18 in the inductor 1, as shown in fig. 27. When the unit cell 10 is covered with the unit cell protective film 50, the side gap Sg is the thickness of the unit cell protective film 50 removed.

In this example, the side gap Sg is larger than the thickness corresponding to the average particle diameter of the large particles of 1 soft magnetic powder and smaller than the thickness corresponding to the average particle diameter of the large particles of 4 soft magnetic powders in the unit cell 10 ground to the predetermined width W. In other words, in the present embodiment, both or one of the predetermined width and the width WLc (fig. 27) of the winding portion 32 of the coil 30 is adjusted in advance so that the side gap Sg of the unit cell 10 ground to the predetermined width has the thickness.

In the unit cell 10 after grinding, the thickness of the side surface gap Sg is made larger than the thickness corresponding to the average particle diameter of 1 large particle of the soft magnetic powder, so that even if the 2 nd side surface 18 is threshed by grinding, at least 1 or more large particles remain between the 2 nd side surface 18 and the coil 30, and the coil 30 is prevented from being exposed.

In the unit body 10 after grinding, the thickness of the side gap Sg is limited to a range smaller than the thickness corresponding to the average particle diameter of the 4 large particles of the soft magnetic powder, so that the unit body 10 can be prevented from being enlarged, the occupancy of the coil 30 can be maintained at a sufficiently high value, and the reduction in inductance can be prevented.

Tables 16 and 17 show the maximum value and minimum value of the side gap Sg in the inductor 1, and the inductance value and moisture resistance measurement results of the inductor 1.

In table 16, inductors 1 in which the average particle diameters of large particles and small particles of the soft magnetic powder were 21 μm and 2 μm, respectively, were measured, and in table 17, inductors 1 in which the average particle diameters of large particles and small particles of the soft magnetic powder were 28 μm and 2 μm, respectively, were measured.

In the soft magnetic powder, a magnetic powder is used which is composed of a chromium-free Fe — Si-based amorphous alloy powder, the particles of which correspond to large particles, and crystalline pure iron, which corresponds to small particles. The surface of the large particles is laminated with SiO layer and Fe₂SiO₄The oxide film of the layer covers the surfaces of the small particles with the oxide film of Fe, and each particle has electrical insulation properties due to the oxide film.

Then, the mixed powder of the soft magnetic powder and the epoxy resin is compression molded, thereby molding the unit body 10 of the inductor 1.

Further, by using the sample A1-04 of the 1 st soft magnetic particle as the large particle and using any one of the samples A2-02-08 of the 2 nd soft magnetic particle as the small particle, a resin, Fe or an oxide of Fe, a phosphate glass, and SiO were present on the surface of the cell body₂And an alkyl group having a chain length of 16 carbon atoms.

The inductance value was measured by an LCR meter, and the inductor 1 was exposed to moisture resistance in an environment of 85 ℃ and 85% humidity, and based on the test results, the case where the moisture resistance was not satisfied based on the predetermined product quality standard was regarded as "NG".

[ Table 16]

[ Table 17]

As shown in tables 16 and 17, if the minimum value of the side gap Sg is larger than the average particle diameter of 1 large particle, sufficient moisture resistance can be obtained. It is also understood that the inductance value decreases as the maximum value of the side gap Sg increases.

It is also found that in the range where the minimum value of the side gap Sg is larger than the average particle size of 1 large particle and the maximum value of the side gap Sg is smaller than the average particle size of 4 large particles, when the ratio of the minimum value to the maximum value of the side gap Sg is 1 to 1, the inductor 1 having excellent moisture resistance and inductance value can be obtained.

As described above, the inductor 1 of the present embodiment is the inductor 1 in which the pair of external electrodes 20 is provided on the unit body 10 formed of the plate-shaped molded body in which the coil 30 is embedded, the unit body 10 is molded of the mixed powder of the soft magnetic powder and the resin including the small particles of the large particles having different average particle diameters, and the side gap Sg, which is the thickness from the 2 nd side surface 18 located in the radial direction of the coil 30 to the coil 30, is larger than the thickness corresponding to 1 large particle and smaller than the thickness corresponding to 4 large particles.

By making the thickness of the side gap Sg of the unit cell 10 larger than the thickness corresponding to the average particle diameter of at least 1 large particle of the soft magnetic powder, at least 1 or more large particles are present between the 2 nd side surface 18 and the coil 30, and the coil 30 is prevented from being exposed.

Further, by limiting the thickness of the side gap Sg of the unit cell 10 to a range smaller than the thickness corresponding to the average particle diameter of the 4 large particles of the soft magnetic powder, it is possible to prevent the unit cell 10 from being enlarged, maintain the occupancy of the coil 30 at a sufficiently high value, and prevent the inductance from being reduced. This makes it possible to realize the inductor 1 which is small in size and can obtain practical direct-current resistance and saturation magnetic flux density.

In the inductor 1 of the present embodiment, the 2 nd side surface 18 of the unit body 10 is covered with the unit body protective film 50, and the surface roughness is larger than that of at least other 1 or more surfaces (the mounting surface 12 and the top surface 14).

This can improve the bonding strength between the 2 nd side surface 18 and the cell protective film 50.

In the inductor 1 of the present embodiment, the surface of the unit cell 10 is covered with the unit cell protective film 50 except for the external electrode 20.

This improves the moisture resistance, rust resistance, and electrical insulation of the unit cell 10, and a high-quality inductor 1 can be obtained.

[ E. protective film for cell body ]

Next, the cell body protective film 50 formed on the surface of the cell body 10 in the inductor 10 including the core 40 formed of the mixed powder of the soft magnetic particles and the resin described in [ a.

As described above, the unit body 10 of the inductor 1 is a molded body formed by embedding the coil 30 and compression-molding the mixed powder, and includes the coil 30 and the core 40.

The cell body protective film 50 is a layer that covers the entire surface of the cell body 10 except at the external electrode 20 and improves the electrical insulation and the rust resistance of the cell body 10. Even if the soft magnetic powder is exfoliated on the ground surface (the 2 nd side surface 18) in the cell grinding step, the cell protective film 50 covers the ground surface, thereby preventing the deterioration of the electrical insulation property, the corrosion resistance, and the rust resistance.

(Unit body protective film Forming Process protective film Forming apparatus)

As described with reference to fig. 5, in the cell protective film forming step, a protective film material containing a thermosetting resin is applied to the entire surface of the cell 10 by an appropriate method such as spraying (spray) or dipping (Dip).

Fig. 28 is a diagram schematically showing an example of a protective film forming apparatus 201 for forming a cell protective film.

The protective film forming apparatus 201 is an apparatus for spraying a protective film material on the surfaces of the plurality of unit cells 10 (workpieces 208) to form a film. As shown in the drawing, the protective film forming apparatus 201 is rotatably provided in the apparatus main body 202, and includes a drum 203 into which the plurality of unit bodies 10 (workpieces 208) are put, a heater 204 for supplying heat, a duct 205 serving as an exhaust passage of the drum 203, and a spray nozzle 206 disposed in the drum 203.

In the protective film forming apparatus 201 for forming the unit protective film, the rollers 203 to which the plurality of units 10 are put are preheated by the heater 204 to a temperature (for example, 30 to 70 ℃) at which the protective film material is not thermally cured (preheating step).

Next, in the protective film forming apparatus 201, the unit 10 is agitated by rotating the drum 203 (so-called drum rotation), the protective film material is sprayed to the unit 10 from the spray nozzle 206, and the unit 10 is coated with the protective film 50 on the surface of the unit 10 by blowing hot air 207 from an air nozzle (not shown) (coating step). Next, the cell protective film 50 of the cell 10 is appropriately dried by stirring the cell 10 and blowing hot air 207 onto the cell 10 (drying step). After the cell protective film 50 is dried, the cell 10 is taken out from the drum 203 (work taking-out step).

In the drying step, when the drying is insufficient, pores (pores) or swelling occurs in the cell protective film 50, and the adhesion between the cell protective film 50 and the cell 10 is also deteriorated. When the drying is excessive, the cell protective film 50 becomes a so-called discontinuous film, and the adhesion between the cell protective film 50 and the cell 10 is also deteriorated. Therefore, the drying is preferably performed to an appropriate extent that the cell protective film 50 becomes a so-called continuous film and the adhesion between the cell protective film 50 and the cell 10 is maintained to be good.

(protective film Material)

As the protective film material, a mixed solution of a resin component as a base material of the cell protective film 50, a solvent component for diluting the resin component, and a filler component as an additive can be used.

(resin component)

The resin component is based on an epoxy resin, and preferably a resin to which one or both of a phenoxy resin and a novolac resin are added is used. The phenoxy resin is added to improve the toughness of the cell protective film 50. By adding the novolac resin, the heat resistance of the cell protective film 50 can be improved.

The resin of the resin component preferably contains a pigment.

By using a resin containing a pigment, in the cell protective film removing step and the external electrode forming step shown in fig. 5, the surface of the cell 10 is irradiated with laser light to remove the cell protective film 50, thereby improving the workability in forming the external electrode 20. Carbon black is preferably used as the pigment.

(solvent component)

As the solvent component, a solvent that can be sprayed with the resin component in a mist form in the coating step and can obtain an appropriate drying property in the drying step can be used, and for example, a solvent containing Methyl Ethyl Ketone (MEK) or the like used as a diluent of a paste-like resin is preferably used.

(Filler component)

As the filler component, a filler that reduces the gloss of the cell protective film 50 and improves the film quality of the cell protective film 50 can be used, and can be dispersed in a solvent.

The gloss of the cell protective film 50 is reduced, and therefore, when the inductor 1 of the camera is used for appearance inspection, erroneous determination due to color fading can be prevented. The filler is preferably Silica (SiO)₂) And (3) pulverizing.

In addition, as for the filler component, in order to prevent clogging of the spray nozzle 206 of the spray protective film material and to reduce damage to the surface of the unit cell 10 caused by the drum rotation of the drum 203, it is preferable that the particle size of the filler is as small as possible, and in the case of using silica powder as the filler, it is preferable to use nano silica.

(Nano-silica)

The inventors found through experiments that in the case of using nano silica as a filler, there is a correlation between the drying rate in the drying step performed at the end of the coating step and the content of nano silica.

Fig. 29 is a graph showing the results of an experiment on the content of nano silica and the drying rate.

In this experiment, a sample of a protective film material was prepared using an epoxy resin as a resin component, MEK as a solvent component, and nanosilica as a filler component, and using this sample, a cell protective film 50 was formed on the cell 10, and the external electrode 20 was further formed to constitute the inductor 1. Then, in the drying step of the cell protective film 50, the relationship between the dry standing time for standing and the solid content of the cell protective film 50 was examined.

Samples of the protective film material were prepared with 4 types of nanosilica at 0 (no), 50phr, 100prh, and 200 phr. In each sample, nanosilica composed of silica particles having an average particle diameter of 45nm was used as the filler.

Regarding the average particle diameter of the silica particles, in the inductor 1, 4 corners of the top surface 14 were connected and intersected at intersection points of the respective opposite corners, the unit 10 was cut in parallel with the 2 nd side surface 18, and the silica particles were observed by imaging a cross section of the unit protective film 50 at a point equally dividing the length L4 of the unit 10 on the upper and lower main surfaces of the unit 10 by a Transmission Electron Microscope (TEM) at 30 ten thousand times. As the transmission electron microscope, measurement was performed using a field emission type transmission electron microscope (FE-TEM). As the field emission type transmission electron microscope, a multi-function electron microscope (model: JEM-F200) manufactured by Japan Electron Ltd was used, which was equipped with a System (model: NORAN System 7 manufactured by Thermo Fischer Scientific Inc.) of an energy dispersive X-ray analyzer (EDX).

As shown in fig. 29, when the content of the nano silica is large, the solid content is high even in a short dry-standing time. That is, by increasing the content of the nanosilica in the protective film material, the drying rate can be increased and the process time of the drying process can be shortened.

As a result of observing the cell protective film 50 after drying in this experiment, it was found that: when the dried solid content is 80% or less, "sticking" described later occurs, and when the solid content is about 90%, a cell protective film 50 having a good film quality can be obtained, but cracks still occur on the surface of the cell protective film 50.

Therefore, in the drying step, it is preferable to dry the resin in a range of 80% to 90% of the solid content.

(adhesion)

The term "sticking" refers to a phenomenon in which the cell protective films 50 of the cells 10 are bonded to each other when a plurality of cells are put into the drum 203 and spray-coated in the coating step, and is a factor of reducing the quality of the cell protective films 50. The inventors have found through experiments that when nanosilica is used as a filler, the "sticking" in the coating step can be suppressed by changing the particle diameter of the silica particles of the nanosilica.

Fig. 30 is a graph showing the experimental results of the average particle diameter of silica particles of nano silica and the adhesion generation rate.

In this experiment, 2 samples 1 and 2 were prepared as the protective film material.

Sample 1 is a protective film material using an epoxy resin as a resin component, PGM as a solvent component, and nano-silica as a filler component, and sample 2 is a protective film material using an epoxy resin as a resin component, MEK as a solvent component, and nano-silica as a filler component. The nanosilica content of samples 1, 2 was 200 phr.

Then, after coating the cell protective film 50 on the cell 10 using each of the samples 1 and 2 in the coating step, the number of the cells 10 in a joined state is counted when the cells 10 are taken out from the drum 203 in the work taking-out step, and the sticking occurrence rate is determined.

As shown in fig. 30, it is understood that the smaller the average particle size of the silica particles, the lower the sticking occurrence rate.

Comparing sample 2 using MEK as a solvent component with sample 1 using PGM as a solvent component, it is understood that the adhesion generation rate of sample 2 is lower when the average particle size of the silica particles is the same.

In addition, it is seen from sample 2 that if the average particle diameter of the silica particles is 45nm or less, the sticking occurrence rate is significantly reduced.

In both samples 1 and 2, if the average particle size of the silica particles is as small as about 12nm, the sticking occurrence rate can be suppressed to almost zero.

However, in both samples 1 and 2, the occurrence of cracks shown in fig. 31 was observed on the surface of the cell protective film 50 after formation when the average particle diameter of the silica particles was 12 nm. By setting the average particle diameter of the silica particles to 15nm, the occurrence of cracks on the surface of the cell protective film 50 can be suppressed. Therefore, the average particle diameter of the silica particles is preferably larger than 12nm, and more preferably 15nm or more, which can more reliably suppress the generation of cracks.

In addition, it can be observed that: if the average particle diameter of the silica particles is as large as more than 75nm, precipitation of the filler is significantly generated in the protective film material, and if the average particle diameter of the silica particles is 75nm, precipitation of the filler in the protective film material can be prevented. If a filler precipitated protective film material is used in the coating film, a uniform cell protective film 50 cannot be obtained even if adhesion does not occur. Therefore, the average particle diameter of the silica particles is preferably 75nm or less.

When the cell protective film 50 is formed using a protective film material containing silica particles (silica powder) having an average particle diameter of 15nm to 75nm only in an amount (150phr to 250phr) such that a sufficient drying rate can be obtained, the weight ratio of the silica particles to the resin in the formed cell protective film 50 is approximately 150% to 250%.

In other words, when the cell body protective film 50 having the above weight ratio is formed, the cell body protective film 50 is formed at a high drying speed without causing "sticking", and it can be said that a high-quality cell body protective film 50 can be obtained.

(jump plating)

Fig. 32 is a view showing the result of measuring the number of "plating jumps" by changing the thickness of the cell body protective film 50.

"plating jump" means that plating is formed on the surface of the unit cell 10 where the unit cell protective film 50 is not coated, but at an undesired location. For example, when large particles of soft magnetic powder are threshed by grinding the unit body, and large irregularities are generated on the surface of the unit body 10, the protective film material does not sufficiently enter the irregularities, which causes "coating jump".

In this measurement, the presence or absence of the "plating jump" is checked at predetermined measurement intervals for all the edges of the unit cell 10 where the top surface 14 and the mounting surface 12 intersect with each other and the 1 st side surface 16 and the 2 nd side surface 18 intersect with each other, and the number of places where the "plating jump" occurs is counted.

As can be seen from the same figure, if the thickness of the cell body protective film 50 becomes small, a large number of "plating jumps" are generated, if the thickness is 5 μm or more, the number of "plating jumps" is significantly reduced, and if it is 10 μm or more, the "plating jumps" are hardly generated.

Therefore, the thickness of the cell protective film 50 is preferably 10 μm or more, and if it is such a thickness, occurrence of "plating jump" can be suppressed, and the entire surface of the cell 10 can be reliably protected by the cell protective film 50.

However, when the unit cell 10 of the inductor 1 has a predetermined size, the thickness of the unit cell protective film 50 increases, and the size of the unit cell 10 in the portion other than the unit cell protective film 50 is reduced in the portion, and the coil 30 is also reduced, so that the performance of the inductor 1 is degraded.

In the external electrode forming step, the inductor 1 forms the external electrodes 20 by removing the cell protective film 50. Therefore, if the cell body protective film 50 is thicker than the external electrodes 20, the external electrodes 20 do not protrude to the surface of the cell body protective film 50, and the contact of the external electrodes 20 with the substrate is deteriorated.

Therefore, the thickness of the cell protective film 50 is preferably at least equal to or less than the thickness of the external electrode 20.

In order to obtain a high-performance inductor 1, the thickness of the cell protective film 50 is more preferably 30 μm or less in the range of the thickness of the external electrode 20 or less.

The thickness of the external electrode 20 was measured as follows. That is, in the inductor 1, 4 corners of the top surface 14 are connected and intersect at intersection points of respective opposite corners, the unit body 10 is cut parallel to the 2 nd side surface 18, the film thickness of a point at which the longitudinal direction 4 of the external electrode 20 formed on the mounting surface 12 of the unit body 10 is equally divided is measured at a magnification of 1000 times using a microscope, and the average of the respective values is obtained as the 1 st measured value. Then, the 1 st measurement values were obtained for 10 inductors 1, and the average of the 1 st measurement values was defined as the thickness of the external electrode 20. As the microscope, model VHX-7000 manufactured by Keyence corporation was used.

(threshing strategy)

When the surface of the unit cell 10, which is a molded body, is ground, a considerable amount of particles of the soft magnetic powder are threshed during grinding, as described above. In this example, soft magnetic powder including large particles having a large average particle size and small particles having a small average particle size was used, and the large particles were degranulated to form deep irregularities on the grinding surface (the 2 nd side surface 18 in this example).

Table 18 is a graph showing the thickness of the cell protective film 50, the depth of the irregularities due to the degranulation, and the results of the rust resistance test.

As the protective film material in this experiment, the same sample as in the experiment shown in fig. 30 was used. As the soft magnetic powder used for molding the unit body 10, a soft magnetic powder having large particles with an average particle size of 21 to 28 μm is used.

The thickness of the cell protective film 50 is measured as follows. That is, in the inductor 1, 4 corners of the top surface 14 are connected and intersect at intersection points of the respective opposite corners, the unit 10 is cut parallel to the 2 nd side surface 18, and the film thickness of the unit protection film 50 is measured at points of the 2 nd side surface 18 above and below the unit 10, at which the length L4 of the unit 10 is equally divided, at 1000 times magnification using a microscope, and the average of the respective values is determined as the 2 nd measurement value. Then, the 1 st measurement value is obtained for 10 inductors 2, and the average of the 2 nd measurement values is defined as a measurement value of thickness (average thickness). As the microscope, model VHX-7000 manufactured by Keyence corporation was used.

The rust resistance is based on a predetermined product quality standard, and "NG" is used when the rust resistance is not satisfied, and "G" is used when the rust resistance is satisfied.

[ Table 18]

Average thickness [ mu ] m]	Depth of unevenness [ μm]	Average thickness/depth of relief	Rust resistance
					4	39	0.10	NG
11	38	0.29	NG
				16	40	0.40	G
21	42	0.50	G
				27	41	0.66	G
31	43	0.72	G
				36	38	0.95	G

As shown in table 18, it can be seen that: regarding the depth of the irregularities due to the degranulation, if the thickness of the cell protective film 50 is small, the rust resistance is poor, the quality of the cell protective film 50 is insufficient, and if the ratio of the thickness of the cell protective film 50 to the depth of the irregularities is 0.4 or more, sufficient rust resistance can be obtained. Since the depth of the irregularities corresponds approximately to the average particle size of the macroparticles, if the thickness of the unit cell protective film 50 is 0.4 times or more the average particle size of the macroparticles, a unit cell protective film 50 having sufficient quality can be obtained even if degranulation occurs on the surface.

As described above, the inductor 1 of the present embodiment is an inductor 1 having a unit body 10 including soft magnetic powder and resin, a coil 30 embedded in the unit body 10, an external electrode 20 provided on the unit body 10, and a unit body protective film 50 on the surface of the unit body 10; the unit protective film 50 has a thickness of 10 [ mu ] m or more, and contains silica particles and a resin, wherein the silica particles have an average particle diameter of 15 to 75nm, and the weight ratio of the silica particles to the resin is 150 to 250%.

By setting the thickness of the cell protective film 50 to 10 μm or more, the occurrence of "plating jump" can be suppressed, and the entire surface of the cell 10 can be reliably protected by the cell protective film 50.

By including silica particles in the cell protective film 50, the gloss can be reduced, and erroneous determination in the appearance inspection using an optical technique can be prevented.

In addition, in the unit cell protective film 50, the average particle diameter of the silica particles is 15 to 75nm, and the weight ratio of the silica particles to the resin is 150 to 250%, so that the high-quality unit cell protective film 50 which is not deteriorated by adhesion during the formation of the unit cell protective film can be obtained.

In this embodiment, the thickness of the cell protective film 50 is equal to or less than the thickness of the external electrode 20.

Thus, the external electrode 20 can be brought into good contact with the circuit of the substrate without impairing the thickness of the cell protective film 50.

In this example, the cell protective film 50 contains carbon black.

This can improve the workability in removing the cell protective film 50 by laser light in order to form the external electrode 20.

In this embodiment, the cell body protective film 50 contains a phenoxy resin.

This can improve the toughness of the unit cell 10.

In this embodiment, the cell body protective film 50 contains a novolac resin.

This can improve the heat resistance of the unit cell 10.

In this embodiment, the thickness of the unit cell protective film 50 is 0.4 times or more the average particle diameter of the large particles.

Thus, even if the surface is threshed, the cell protective film 50 having sufficient quality can be obtained.

In this example, titanium oxide, zirconium oxide, and aluminum oxide can be used as filler components.

Claims (5)

1. A soft magnetic powder comprising soft magnetic particles composed of a1 st particle core comprising a soft magnetic metal and an insulating film on the surface of the 1 st particle core,

the insulating film contains Si and a hydrocarbon group having a linear portion having 8 or more carbon atoms,

the weight ratio of Si to C in the insulating film is 7.6-42.8.

2. The soft magnetic powder according to claim 1, wherein the hydrocarbon group is an alkyl group.

3. Soft magnetic powder according to claim 1 or 2, wherein the 1 st particle core is composed of carbonyl iron.

4. The soft magnetic powder according to any one of claims 1 to 3, further comprising: soft magnetic particles comprising a soft magnetic metal and composed of a2 nd particle core having an average particle size larger than that of the 1 st particle core.

5. An inductor, having:

a metallic magnetic body comprising the soft magnetic powder according to any one of claims 1 to 4, and a wound wire.

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