A laminated planar spiral coil winding

Abstract

The present application relates to the technical field of electromagnetic coil, and particularly relates to a laminated planar spiral coil winding. The laminated planar spiral coil winding is formed by laminating at least two layers of planar spiral coils formed by spiral center fitting lines of each turn of wire. The planar spiral coil occupies the whole winding plane, and the strong magnetic center is offset from the geometric center of the planar spiral coil. The winding has different layers for different phases. The present application aims to provide a competitive choice for the winding of axial flux motor with high copper full rate, low copper loss, large air gap, easy heat dissipation and low production cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electromagnetic coil, in particular to a laminated planar spiral coil winding. BACKGROUND

[0002] The motor winding is the core and physical basis of the motor, which is extremely important to the overall significance of the motor and almost determines all key performances of the motor, and can be understood as the combination of the "heart" and "nervous system" of the motor. The motor winding is the physical carrier and executor of electromagnetic energy conversion, directly shapes the core electromagnetic performance (torque, efficiency, noise, back EMF, inductance, etc.) of the motor, determines the key electrical parameters (voltage, current, power, speed, phase number) of the motor, and greatly affects the power density, efficiency and thermal performance of the motor. The traditional axial flux motor usually adopts multiple concentric cylindrical coils to form the winding, or adopts multiple concentric circles and concentric sectors to form the winding. The common feature of these windings is that the multi-phase windings are uniformly distributed in the same plane, and each winding has different winding copper fullness, copper loss, winding air gap, heat dissipation mode, and magnetic performance, each has advantages and disadvantages. The present application innovatively proposes a laminated planar spiral coil winding, which aims to provide a competitive choice for the winding copper fullness, copper loss reduction, air gap increase, heat dissipation improvement, magnetic performance and economic cost of the axial flux motor. SUMMARY

[0003] The purpose of the present application is to provide a competitive choice for the winding copper fullness, copper loss reduction, air gap increase, heat dissipation improvement, magnetic performance and economic cost of the axial flux motor.

[0004] To achieve the above purpose, the present application provides the following solutions:

[0005] A laminated planar spiral coil winding, characterized in that it comprises:

[0006] 1) composed of at least two layers of planar spiral coil, each layer being a single planar spiral coil covering the entire winding plane;

[0007] 2) the spiral center fitting line formed by each turn of the planar spiral coil is linear;

[0008] 3) the strong magnetic center formed after each layer of the same phase winding is energized is projected on the winding plane and coincides at a point P, and the point P is offset from the geometric center O of the planar spiral coil;

[0009] 4) the strong magnetic centers of each phase winding of different layers are projected on the winding plane and are distributed in a central symmetric manner after each phase winding is energized;

[0010] 5) The internal space of the planar spiral coil is filled with magnetic conductive material to improve the magnetic conductive performance of the coil winding, insulating material to improve the insulation performance, heat dissipation material or hole pipe to improve the heat conduction performance;

[0011] 6) The output ends of each phase winding are short-circuited to form a squirrel cage equivalent rotor of an axial flux asynchronous motor;

[0012] 7) The coil winding manufacturing method includes PCB printing or using round / flat wire and laminated wire winding.

[0013] The offset distance D between the projection point P of the strong magnetic center formed after the planar spiral coil is energized and the geometric center of the planar spiral coil satisfies: 0.2R≤D≤0.67R, where R is the outer diameter of the coil spiral.

[0014] The adjacent interval d of the spiral center formed by each turn of the planar spiral wire is equal or gradiently changed.

[0015] The internal space of the planar spiral coil is filled with amorphous core powder and epoxy resin composite magnetic conductive material or ferrite powder and epoxy resin composite magnetic conductive material, and embedded with micro-channel heat dissipation pipe to improve the magnetic conductive performance, insulation performance and heat conduction performance of the coil winding.

[0016] The center-symmetric through-hole heat dissipation holes are arranged at the internal space of the PCB printed coil winding plane.

[0017] The through-hole is used to realize the interlayer connection of the PCB winding.

[0018] The coil winding adopts N-phase power supply topology, and the phase difference between adjacent phases is 360° / N, where N≥2.

[0019] The interlayer connection of the same phase winding sub-coil includes opposite spiral direction series connection, same spiral direction parallel connection, or opposite spiral direction series connection and parallel connection of sub-coils, and the overall winding is preferably connected into a fractional slot winding.

[0020] The coil winding center or internal part includes an axial transmission shaft hole or does not include an axial transmission shaft hole.

[0021] A manufacturing method of any one of the aforementioned winding, characterized by the steps of:

[0022] (a) Laser etching spiral wire grooves on the PCB substrate, with groove width error ≤±5μm;

[0023] (b) Electroplating to fill the grooves to form wires, with depth-to-width ratio ≥1.5:1;

[0024] (c) Vacuum pressure infusion of magnetic conductive composite material, with pressure 0.5-0.8MPa;

[0025] (d) Laminated with 50-100mT alternating magnetic field to orient the amorphous core powder or ferrite particles;

[0026] (e) Filling the interlayer via holes with conductive paste.

[0027] The planar spiral coil is a special planar spiral coil, the minimum spiral center of which is not at the geometric center of the planar coil, the connecting line of the spiral centers formed by each turn of the wire or the fitting line of the spiral centers is linear, the planar spiral coil is divided into a dense wire area and a sparse wire area as a whole, the minimum spiral of the planar spiral coil is adjacent to the dense wire area, and the maximum spiral center of the planar spiral coil is the geometric center of the planar spiral coil. After the planar spiral coil is energized, the magnetic lines are asymmetrically distributed, in the dense wire area, due to the high current density and strong coupling effect, a strong magnetic center is formed in the minimum spiral turn area, and other areas outside the strong magnetic center are magnetic field diffusion areas, which is significantly different from the magnetic field mode of the strong magnetic center formed by the traditional cylindrical solenoid and the planar spiral coil.

[0028] The smaller the spacing between the spiral centers of each turn of the wire of the planar spiral coil, the higher the copper fullness rate of the planar spiral coil, the stronger the magnetic field generated, and the higher the uniformity of the spacing between the spiral centers of adjacent turns, and the stronger the uniformity of the magnetic field.

[0029] The planar spiral coil of the present application is a single-layer planar spiral coil that fills the entire winding plane, after being energized, the magnetic vectors generated by all the wires of the planar spiral coil participate in the magnetic field coupling / stacking of the entire winding, and has the characteristics of low copper loss and high electromagnetic utilization rate.

[0030] The laminated planar spiral coil winding can be wound by traditional wire to form a planar spiral coil, and then laminated and connected.

[0031] The planar spiral coil of the present application is wound by traditional wire, preferably by a multi-layer flat copper strip formed by laminating thin copper sheets with insulating skins.

[0032] The minimum spiral port or the maximum spiral port of two planar spiral coils of the present application with opposite spiral directions are connected in series to form a double-layer planar spiral coil symmetrically up and down, the unconnected port is used as a current inlet and outlet port, and the two coils connected in series can effectively form a magnetic field in the same direction after being energized. Compared with a single-layer planar spiral coil, it has higher magnetic field strength and interlayer space utilization.

[0033] Three above-mentioned double-layer planar spiral coils are centered on the coil center, three-phase 120° stacking is performed, six-layer coil windings are formed with the minimum spiral center of each double-layer planar spiral coil being 120° apart, the coil winding is cylindrical, and the radius is the maximum spiral outer diameter. After each double-layer planar spiral coil is electrified, the same magnetic direction (for example, upward) is generated, three double-layer planar spiral coils are respectively connected with three-phase alternating current, and an axial magnetic field rotating magnetic field is formed.

[0034] Two above-mentioned double-layer planar spiral coils (in series at the minimum spiral port) are symmetrically stacked with the coil center as the center, the maximum spiral ports with opposite spiral directions in the four-layer coil are connected in parallel, one end of the parallel port is used as the current inflow end, the other end is used as the current outflow end, and after electrification, the symmetric two minimum spiral centers are the north and south poles of the four-layer coil winding. After three above-mentioned four-layer coils are three-phase 120° stacked and then respectively connected with three-phase power, an axial magnetic field rotating magnetic field can be formed.

[0035] In the application process of using both sides of the winding as the working surface, the transmission hole can be arranged at the center position of the winding, and the magnetic poles of the stator and the rotor on both sides of the winding are connected through the transmission pipe, but this may need to adjust the wire at the center position of the winding, which will have a certain impact on the magnetic field, especially for the winding with small diameter and small number of turns. However, the joint effect of the two working surfaces is greater.

[0036] The inner space of the planar spiral coil in the application includes the single-layer planar coil inner wire spacing gap, the layer spacing gap, and the gap through the entire coil winding (including the wire spacing and layer spacing gaps). The gap through the entire coil winding is usually in a central symmetric state due to the small number of multi-layer stacking factors. The space of the motor winding is valuable, and in the process of producing the winding by using the traditional wire, the heat dissipation pipe / hole can be arranged in the gap through the entire coil winding, then the amorphous iron core particles or ferrite particles are configured into a filler together with the epoxy resin glue, the wire spacing gap and the layer spacing gap in the winding are filled, and the vacuum die casting method is used for integrated die casting to achieve the purpose of improving the overall magnetic, insulation and heat dissipation of the coil winding. The utilization of the above-mentioned heat dissipation, insulation and magnetic materials to fill the gap in the winding has a synergistic optimization effect on the performance of the winding, and significantly improves the overall performance of the winding.

[0037] In the production cost of the PCB winding, the blind buried hole is an important part of the PCB production cost, especially for high multi-layer boards. The stacked planar spiral coil winding of the application printed on the PCB can realize multi-layer coil layer connection by using all through holes, which has a significant economic cost advantage. However, the use of blind buried holes in the high multi-layer application of the coil winding of the application can save more space for the winding and significantly improve the winding efficiency.

[0038] The laminated planar spiral coil winding of the application adopts N-phase power supply topology, and the phase difference between adjacent phases is 360° / N, wherein N≥2. When N=2, the phase difference is 90°, which is consistent with the mode of increasing the capacitance phase modulation of the traditional two-phase motor. When N>2, the phase difference between adjacent phases is 360° / N.

[0039] The interlayer connection of the sub-coil layers of the in-phase winding of the application includes series connection of opposite spiral directions, parallel connection of the same spiral direction, or parallel connection of sub-coils in series connection of opposite spiral directions. The overall winding is preferably connected into a fractional-slot winding.

[0040] The laminated planar spiral coil winding of the application can be applied to an axial flux motor system, a magnetic suspension system, a sensor system, a magnetic bearing system, an eddy current magnetic coupling system, a magnetic stirring system, and a wireless charging system.

[0041] Compared with the prior art, the application has the following advantages and technical effects:

[0042] 1. The coil winding of the application is stacked by planar coil layers, which significantly improves the copper fullness rate of the winding.

[0043] 2. All conductors of the coil winding of the application participate in the fitting of the magnetic flux of each phase of the winding, which significantly improves the magnetic flux utilization rate, reduces the copper loss, and improves the winding efficiency.

[0044] 3. The coil winding of the application is designed to be eccentric, and the strong magnetic field in the dense area of the conductors significantly improves the air gap of the winding.

[0045] 4. In the application of the winding to high multi-layer PCB winding, the number of PCB winding drilling holes is significantly reduced, and the production cost is lower.

[0046] 5. In the traditional winding motor winding, the internal space of the laminated planar spiral coil winding can be fully utilized to add insulating materials, magnetic conductive materials, and micro-porous heat dissipation pipes, and the addition of the three can produce a synergistic optimization effect on the winding, which significantly improves the overall performance of the winding.

[0047] 6. The winding coil structure of the application is simple and easy to mass-produce, and the winding can be modularly expanded. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 It is a single-layer planar spiral coil

[0049] Figure 2 It is a double-layer planar spiral coil

[0050] Figure 3 It is a double-layer planar spiral coil with a center transmission shaft hole

[0051] Figure 4 It is a six-layer three-phase coil winding top view

[0052] Figure 5 Top view of a twelve-layer PCB coil winding

[0053] Figure 6 A plan view of the magnetic field simulation of a twelve-layer PCB coil winding.

[0054] Figure 7 Simulated cross-sectional view of the magnetic field of a twelve-layer PCB coil winding

[0055] Figure 8 A planar spiral coil wound with eight layers of thin copper wire.

[0056] The diagram is labeled as follows: 1001, geometric center point O of the coil winding; 1002, center point P of the smallest spiral turn of the coil; 1003, end of the smallest spiral turn of the coil; 1004, end of the largest spiral turn of the coil; 1005, coil radius R; 1006, center of each spiral turn of the coil; 1007, connecting / fitting line of the center of each spiral turn of the coil; 1008, counterclockwise spiral coil; 1009, clockwise spiral coil; 1010, spacing gap between lines of the double-layer planar coil; 1011, wire for adjusting the drive shaft hole; 1012, center drive shaft hole of the double-layer planar coil; 1013, PCB coil winding frame; 1014, interlayer connection through hole in the PCB coil winding; 1015, symmetrical heat dissipation through hole in the PCB coil winding; 1016, wire formed by eight layers of thin copper sheets stacked together. Detailed Implementation

[0057] The present invention will now be described in further detail with reference to the accompanying drawings and specific preferred embodiments.

[0058] It should be understood that the specific values ​​of the number of planar spiral coil layers, the number of conductor layers, and the number of winding phases in the description of the embodiments of the present invention are only for illustrating the technical solution and should not be construed as limiting the present invention or the scope of protection of the present invention.

[0059] Example 1: Six-layer three-phase coil winding

[0060] The single-layer planar helical coil described in this invention is as follows: Figure 1 1001 is the geometric center point O of the coil winding, 1002 is the center point P of the smallest spiral turn of the coil, and point P is also the strong magnetic center point after the coil is energized. The line segment OP is the offset distance D between the strong magnetic center and the geometric center after the spiral coil is energized. 1003 is the end of the smallest spiral turn of the coil, 1004 is the end of the largest spiral turn of the coil, 1005 is the coil radius R, 1006 is the spiral center of each turn of the coil, and the interval between two adjacent spiral centers of turns is d; 1007 is the line connecting the spiral centers of each turn of the coil.

[0061] The above Figure 1The coils are mirror-symmetrically arranged and vertically stacked along the OP line, with the smallest spiral turn end 1003 vertically connected to form a double-layer planar spiral coil with opposite spiral directions. Figure 2 In this coil, 1008 is a counter-clockwise spiral coil, and 1009 is a clockwise spiral coil. The ends with the largest spiral turns 1004 are located on both sides, serving as the current inlet and outlet ends, respectively. Figure 2 A double-layer planar helical coil will generate a magnetic vector in the same direction. Figure 1 and Figure 2 The left side (from point P to point O) is the sparse area of ​​the conductors, and the right side is the dense area of ​​the conductors. 1010 shows the gap between the double-layer planar coil lines, which can be used to fill magnetic materials, insulating materials, heat dissipation pipes, and holes. Since the double-layer coils are stacked vertically, the upper and lower double-layer conductors overlap in the sparse area when viewed from above.

[0062] The above 3 Figure 2 Double-layer planar helical coils are vertically stacked, and the strong magnetic center point P of three double-layer planar helical coils is adjusted to form a six-layer coil with the geometric center O of the winding as the center, OP as the radius, and an angle difference of 120°. This coil winding is called... Figure 4 The diagram shows a six-layer three-phase coil winding. Figure 4 The six-layer three-phase coil winding shown has its smallest spiral turn ends 1003 internally vertically connected, while its largest spiral turn ends 1004 are exposed. These six ends can be connected in a delta or star configuration to form a six-layer three-phase coil winding as needed.

[0063] Example 2: Double-layer planar spiral coil with a drive shaft hole

[0064] like Figure 3 The double-layer planar helical coil shown is a planar helical coil in which the center of each turn of the coil is arranged in an arc shape. It is mirror-symmetrically arranged and vertically stacked along the OP line, with the smallest helical turn ends 1003 vertically connected, forming a double-layer planar helical coil with opposite helical directions. The red turn helical center 1006 corresponds to a small red circle, and the corresponding light blue-green turn helical center 1006 corresponds to a small light blue-green circle. The OP line is the fitting line 1007 for the helical centers of each turn of the two coils. A drive shaft hole 1012 is provided in the region at the center point O of the double-layer planar helical coil by adjusting the wire position 1011. Both magnetic flux directions of the axial flux winding can serve as working surfaces, and the central drive shaft hole can transmit the torque generated by the two working surfaces uniformly, which has significant practical implications.

[0065] Example 3: Twelve-layer PCB coil winding

[0066] Using wire avoidance method Figure 2The double-layer planar spiral coil is provided with a center transmission hole, and then six double-layer planar spiral coils are vertically stacked, the strong magnetic center points P of the six double-layer planar spiral coils are adjusted, and twelve layers of coils are formed with the winding geometric center O as the center and OP as the radius, and the angles are 60° apart. The coil winding is a twelve-layer PCB coil winding as shown in the figure. Figure 5 The interlayer connection of the winding follows the same phase winding sub-coil layer interlayer series connection in the opposite spiral direction, and the same phase winding sub-coil layer interlayer parallel connection in the same spiral direction. The interphase connection of the winding adopts a triangular connection. Figure 5 1013 is a PCB winding frame, and 1014 is an interlayer connection through hole in the PCB coil winding. The total number of connection through holes of the PCB coil winding of the application is 17, which is much smaller than the number of connection holes required by the centralized PCB winding and the distributed PCB winding, and these windings need to be provided with blind buried holes to meet the connection function of the winding. 1015 is a symmetrical heat dissipation through hole in the PCB coil winding. The diameter of the heat dissipation through hole 1015 is determined according to the actual space. The same setting in this embodiment can be applied to 18-layer, 24-layer, 30-layer and 36-layer PCB coil windings.

[0067] Example 4: Twelve-layer PCB coil winding without heat dissipation through hole

[0068] The difference between this embodiment and example 3 is that the twelve-layer PCB coil winding of this embodiment does not have a heat dissipation through hole 1015, and the strong magnetic center point P is offset from the geometric center point O by a distance D = 0.6R. Figure 6 The magnetic field simulation plan view of the twelve-layer PCB coil winding is shown in the figure, Figure 7 The magnetic field simulation profile view of the twelve-layer PCB coil winding is shown in the figure. Figure 7 The magnetic vector high points on both sides of the coil winding center point O are the edges of the transmission shaft through hole, and the magnetic field distortion caused by adjusting the wire position 1011. Figure 7 At the same time, it also shows that the strong magnetic center point P of the planar spiral coil after being energized is located in the smallest spiral region.

[0069] Example 5: Planar spiral coil wound with thin copper sheet

[0070] The thin copper sheet with an insulating skin is stacked into a strip as a wire, and then wound according to the design requirements of the coil to form a planar spiral coil. As shown in the figure, Figure 8 The planar spiral coil wound with eight layers of thin copper sheet is shown in the figure, and the enlarged view on the right shows that each turn of the coil is a wire 1016 formed by stacking eight layers of thin copper sheet. The stacked planar spiral coil winding made of the planar spiral coil of this embodiment can significantly reduce the influence of current skin effect.

[0071] Example 6: Planar spiral coil wound with flat copper wire

[0072] The flat copper wire with insulation is used as the conductor, and then is wound to form a planar spiral coil according to the coil design requirements. The planar spiral coil made by the planar coil structure of the embodiment has a large current characteristic.

[0073] Example 7: Squirrel cage equivalent rotor of axial flux asynchronous motor

[0074] The output ends of the laminated multi-layer multi-phase coil winding made by the planar coil structure of the embodiment are short-circuited, and a squirrel cage equivalent rotor of an axial flux asynchronous motor can be formed. The equivalent rotor is essentially a current generated by guiding the magnetic flux with a specific winding method, and the current flows along a specific path to generate a magnetic field again, and a rotating magnetic field is formed according to the winding phase characteristics. The winding generated by the embodiments 1, 3 and 6 can form a squirrel cage equivalent rotor after the output ends are short-circuited. In actual design, the performance of the overall squirrel cage equivalent rotor is improved by using magnetic conductive materials, insulating materials and heat dissipation pipes or hole filling technology, focusing on the current carrying capacity. In practical application, the squirrel cage equivalent rotor winding can not only be used as the rotor of the axial flux asynchronous motor, but also can replace the eddy current plate to form a more concentrated and controllable eddy current magnetic field.

[0075] Example 8: Manufacturing the PCB coil winding of the embodiment by laser etching

[0076] 1) Line slot etching:

[0077] On the basis of mechanical positioning and template calibration, a fiber nanosecond laser machine is used to etch the planar spiral coil structure of the embodiment on the PCB substrate.

[0078] 2) Electroplating slot filling

[0079] a. Chemical copper plating is used to chemically deposit copper on the etched line slot;

[0080] b. Air stirring and moving cathode plating method is used;

[0081] c. Chemical mechanical polishing is used to remove the surface overflow copper layer.

[0082] 3) Vacuum negative pressure perfusion of magnetic conductive material, stepwise temperature rise and solidification;

[0083] 4) Alternating magnetic field directional arrangement of magnetic conductive material

[0084] Pre-pressing (80℃, 30min) → placing in a rotatable variable magnetic field → gel period (100℃, 60min, changing the magnetic pole every 10min) → removing the magnetic field → full curing (130℃, 120min)

[0085] 5) Silver-coated copper paste is used to fill the interlayer through hole.

[0086] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the same; although the present application has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that the technical solutions recorded in the foregoing examples can still be modified, or some of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A layered planar helical coil winding, characterized in that, include: 1) It consists of at least two layers of planar spiral coils stacked together, each layer being a single planar spiral coil covering the entire winding; 2) The fitting line of the spiral center formed by each turn of the conductor in the planar spiral coil is linear; 3) The strong magnetic center formed by the spiral center windings of each layer after being energized coincides with a point P in the plane of the coil winding, and point P is offset from the geometric center O of the planar spiral coil. 4) The windings are in different layers for each phase, and after each phase winding is energized, the strong magnetic center is projected onto the plane of the coil winding in a centrally symmetrical distribution; 5) The internal gaps of the planar helical coil are filled with magnetic materials to improve the magnetic permeability of the coil winding, insulating materials to improve the insulation performance, and heat dissipation materials or perforations to improve the thermal conductivity. 6) Short-circuit the output terminals of each phase winding to form the squirrel-cage equivalent rotor of the axial flux asynchronous motor; 7) Coil winding manufacturing methods include using PCB printing or using round / flat wires or laminated wires for winding.

2. The coil winding according to claim 1, characterized in that... The offset distance D between the projection point P of the strong magnetic center formed by the planar helical coil after it is energized and the geometric center of the planar helical coil satisfies: 0.2R≤D≤0.67R, where R is the outer diameter of the coil helix.

3. The coil winding according to claim 1, characterized in that... In a planar helix, the spacing d between adjacent spiral centers formed by each turn of the conductor is equal or varies with gradient.

4. The coil winding according to claim 1, characterized in that... The internal gaps of the planar spiral coil are filled with a composite magnetic material of amorphous iron core powder and epoxy resin or a composite magnetic material of ferrite powder and epoxy resin, and a microchannel heat dissipation tube is embedded to improve the magnetic permeability, insulation performance and thermal conductivity of the coil winding.

5. The coil winding according to claim 1, characterized in that... The PCB printed coil winding plane has centrally symmetrical through-holes for heat dissipation.

6. The coil winding according to claim 1, characterized in that... The coil windings printed on the PCB are connected between PCB winding layers using through holes.

7. The coil winding according to claim 1, characterized in that... The coil winding adopts an N-phase power supply topology with an adjacent phase difference of 360° / N, where N≥2.

8. The coil winding according to claim 1, characterized in that... Interlayer connections of in-phase winding sub-coils include series connection with opposite helical directions, parallel connection with the same helical direction, or parallel connection of series sub-coils with opposite helical directions, and the overall winding is connected as a fractional-slot winding.

9. The coil winding according to claim 1, characterized in that... The coil winding center or interior may or may not have an axial drive shaft hole.

10. A method for manufacturing a winding according to any one of claims 1 to 9, characterized in that... step: (a) Laser etching of spiral grooves on PCB substrate, with groove width error ≤ ±5μm; (b) Electroplating filler formed wires with a depth-to-width ratio ≥ 1.5:1; (c) Vacuum pressure infusion of magnetically conductive composite material, pressure 0.5-0.8 MPa; (d) Applying an alternating magnetic field with an intensity of 50-100mT during lamination to oriented amorphous iron core powder or ferrite particles; (e) Use conductive paste to fill the interlayer vias.

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