JP2008147580A - Dielectric sheet for reducing electromagnetic noise and electromagnetic noise reducing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dielectric sheet for reducing generation of EMI, and to provide a method for reducing EMI. <P>SOLUTION: The dielectric sheet 20 for reducing electromagnetic noise comprises two dielectric layers having different dielectric constants, i.e., a high dielectric layer 22 having a relatively high dielectric constant and a low dielectric layer 24 having a relatively low dielectric constant, wherein the dielectric constant and the thickness of the high dielectric layer 22 are determined to bring the radiation efficiency of electromagnetic noise entering from the low dielectric layer side and exiting from the high dielectric layer side to the vicinity of a minimal value. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a dielectric sheet for reducing electromagnetic noise and an electromagnetic noise reducing method.

  The printed wiring board is a member on which a circuit of an electronic device is formed. The printed wiring board is formed by bonding a copper foil to the surface of an insulating layer such as glass epoxy or ceramic, and then forming a circuit by etching the copper foil. An electromagnetic wave is generated from the printed wiring board, and this electromagnetic wave is a problem as EMI (Electro Magnetic Interference) that adversely affects the human body and other electronic devices.

An example of an unnecessary radiation absorber for suppressing EMI is shown in a conventional document (Patent Document 1). In this unnecessary radiation absorber, a large number of dielectrics having different dielectric constants are laminated.
JP 10-145074 A

  In the unnecessary radiation absorber described in Patent Document 1, the dielectric constant is only smaller in the outer dielectric layer with respect to the EMI generation source, and optimization for further suppressing EMI is not considered. Therefore, there is room for further improvement in terms of EMI reduction.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide a dielectric sheet and an EMI reduction method that can further reduce the generation of EMI.

  In order to achieve the above-described object, the dielectric sheet for electromagnetic noise reduction according to the present invention is compared with a high dielectric layer having at least two dielectric layers having different dielectric constants and having a relatively high dielectric constant. And a low dielectric layer with a low dielectric constant. The dielectric constant and thickness of the high dielectric layer are electromagnetic waves that are incident from the low dielectric layer side and radiated from the high dielectric layer side. The noise radiation efficiency is determined to be in the vicinity of the minimum value.

  The electromagnetic noise reduction method according to the present invention includes at least two dielectric layers having different dielectric constants, a high dielectric layer having a relatively high dielectric constant and a low dielectric layer having a relatively low dielectric constant. A method for reducing electromagnetic noise using a dielectric sheet comprising an electromagnetic wave that is incident from the low dielectric layer side and radiated from the high dielectric layer side with respect to the dielectric constant and thickness of the high dielectric layer An electromagnetic noise reduction method comprising: preparing a dielectric sheet having a noise radiation efficiency near a minimum value; and attaching the dielectric sheet to an electromagnetic noise generation source.

  ADVANTAGE OF THE INVENTION According to this invention, the dielectric material sheet and electromagnetic noise reduction method which can reduce EMI more can be provided.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a side view of a printed wiring board 10 according to the present embodiment and a dielectric sheet 20 attached thereto. The printed wiring board 10 is a multilayer board formed by laminating a plurality of circuit layers and a plurality of insulating layers. A flexible dielectric sheet 20 is affixed and superimposed on the upper side of the printed wiring board 10.

  The dielectric sheet 20 is a laminate of two dielectrics having different dielectric constants, and includes a high dielectric layer 22 having a relatively high dielectric constant and a low dielectric layer 24 having a relatively low dielectric constant. Consists of. The low dielectric layer 24 is disposed so as to be in close contact with the printed wiring board 10, and the high dielectric layer 22 is disposed on the side far from the printed wiring board 10.

  The high dielectric layer 22 is obtained by uniformly mixing hard dielectric particles, which are ferroelectric materials, in a flexible dielectric polymer. Here, as the dielectric polymer, it is preferable to use a material having a high dielectric constant. For example, polyvinylidene fluoride may be used. Similarly, it is preferable to use a material having a high dielectric constant as the dielectric particles, for example, barium titanate may be used. The dielectric constant of the high dielectric layer 22 as a whole can be adjusted to a desired value by mixing an appropriate amount of dielectric particles into the dielectric polymer while taking into consideration the dielectric constant of the dielectric polymer and dielectric particles. .

The high dielectric layer 22 is further mixed with particles of a conductive material. The conductive particles are for improving dielectric loss in the high dielectric layer 22 and reducing electromagnetic noise generated from the printed circuit board. Various materials can be used as the conductive material. For example, carbon may be used. Since carbon has a conductivity of 0.061 × 10 ⁶ / mΩ, the effect of reducing electromagnetic noise is high, and the permeability is 20 H / m to 100 H / m for a magnetic field change of 1 MHz. Since it hardly affects the electric field in the dielectric layer 22, it is particularly suitable for reducing the radiation efficiency of electromagnetic noise in this embodiment.

Considering carbon as a reference, the conductive material described above has a magnetic permeability of less than 100 H / m, more preferably a magnetic permeability of less than 60 H / m, with respect to a magnetic field change of 1 MHz. It is preferable to use a material having a magnetic permeability smaller than 20 H / m. In addition to the magnetic permeability condition, it is preferable to use a material having a conductivity higher than 0.061 × 10 ⁶ / mΩ as the conductive material.

  The low dielectric layer 24 is a layer made of a low dielectric material having a lower dielectric constant than that of the high dielectric layer 22. As the low dielectric layer 24, various materials can be used as long as the material has a relatively low dielectric constant, such as polyethylene, polystyrene, nylon, polytetrafluoroethylene, and the like. The material used for the low dielectric layer 24 is a dielectric having a relative dielectric constant of generally less than 10, preferably less than 8, more preferably less than 5. The smaller the relative dielectric constant of the low dielectric layer, the better the effect of suppressing the generation of EMI, which is preferable.

  In the dielectric sheet 20 of the present embodiment, by laminating two dielectrics having different dielectric constants, the electromagnetic wave generated from the printed wiring board 10 is refracted and incident from the low dielectric layer 24 side to enter the high dielectric layer. The radiation of electromagnetic noise radiated from the 22 side is suppressed. Hereinafter, the principle of refracting electromagnetic waves generated from the printed wiring board 10 will be described with reference to FIG. In this embodiment, the dielectric sheet 20 is formed by laminating two dielectrics. However, if the electromagnetic wave generated from the printed wiring board 10 is refracted, the dielectric sheet 20 is formed by laminating three or more dielectrics. It may be.

FIG. 2 is a conceptual diagram showing a bonding state of a substrate in which a dielectric I having a relative dielectric constant ε1 and a dielectric II having a relative dielectric constant ε2 are in contact. In this bonded state, the magnitude of the electric flux density of the dielectric I is D1 and the direction is θ1 with respect to the normal of the boundary, and the magnitude of the electric flux density of the dielectric II is D2 and the direction is the normal of the boundary. Assuming that there is no electric charge at the boundary surface when the angle θ2 is, the normal component D1n of the electric flux density of the dielectric I and the normal component D2n of the electric flux density of the dielectric II are between ,
D1n = D2n (1)
Thus, the normal component of the electric flux density is continuous.

On the other hand, the tangential component is continuous for the electric field, and between the tangential component E1t of the electric field of the dielectric I and the tangential component E2t of the electric field of the dielectric II,
E1t = E2t (2)
The relationship holds.

For this reason, the following relationship is established among D1, D2, and E2.
D2 = D1 {cos ² θ1 + (ε2 / ε1) sin ² θ2} ^1/2 (3)
E2 = D1 {cos ² θ1 + (ε2 / ε1) sin ² θ2} ^1/2 / ε2 (4)
θ2 = tan ⁻¹ {(ε2 / ε1) tan θ1} (5)

  Therefore, assuming that dielectric I is a high dielectric, dielectric II is a low dielectric, and ε1 is sufficiently larger than ε2 (ε1 >> ε2), the following relationship holds, and the electric field radiated from the substrate is It is considered that it can be confined in the high dielectric layer I.

D2 = D1 cos θ1 (6)
E2 = D1 cos θ1 / ε2 (7)
θ1 ≒ 90 ° (8)
θ2 ≒ 0 ° (9)

  Since the electric field and the magnetic field interact, it is possible to prevent electromagnetic noise from being emitted to the outside of the dielectric sheet 20 (EMI) as a result by not releasing the electric field to the outside.

  In the dielectric sheet 20 of this embodiment, by adjusting the dielectric constant and thickness of the high dielectric layer 22, the radiation efficiency of electromagnetic noise (the low dielectric layer of the dielectric sheet 20 generated from the printed wiring board 10) Among electromagnetic noises incident from the 24th side, the ratio of electromagnetic noise radiated from the high dielectric layer 22 side through the dielectric sheet 20) is set to a minimum value vicinity. Note that “near the minimum value of electromagnetic noise radiation efficiency” means that not only the electromagnetic noise radiation efficiency is at a minimum value, but also almost the same effect as when electromagnetic noise radiation efficiency is at a minimum value. It means that the range of the neighborhood is included. Hereinafter, minimization of the radiation efficiency of electromagnetic noise will be described with reference to FIGS.

  FIG. 3 is a graph showing the radiation efficiency of electromagnetic noise with respect to the relative dielectric constant ε of the high dielectric layer 22. FIG. 4 is a graph showing the radiation efficiency of electromagnetic noise with respect to the thickness t of the high dielectric layer 22. In the graph of FIG. 3, the thickness t of the high dielectric layer 22 is set to 25 μm, 50 μm, 75 μm, and 100 μm. In the graph of FIG. 4, the relative dielectric constant of the high dielectric layer 22 is set to 3, 5, 10, 20, 30, 35, 40, 45, 50, 60, 70, 80, 100.

  As shown in FIG. 3, the condition (relative permittivity ε of the high dielectric layer 22) where the radiation efficiency of electromagnetic noise is near the minimum value varies depending on the thickness t of the high dielectric layer 22. Further, as shown in FIG. 4, the condition (the thickness t of the high dielectric layer 22) where the radiation efficiency of electromagnetic noise is near the minimum value varies depending on the relative dielectric constant ε of the high dielectric layer 22. That is, if the relative dielectric constant ε and the dielectric thickness t are large, it cannot be said that the EMI reduction effect is large. Depending on the dielectric thickness t, the EMI reduction effect may be large even if the relative dielectric constant ε is small.

The relationship between the radiation efficiency F of electromagnetic noise, the relative dielectric constant ε, and the dielectric thickness t can be expressed by the following formula (10).
F Ｆ F (t, ε) (10)
This equation (10) means that the radiation efficiency F of electromagnetic noise is determined by the relative permittivity ε and the dielectric thickness t, and in order to make the radiation efficiency F of electromagnetic noise near the minimum value. This means that the relative permittivity ε and the dielectric thickness t need to be optimum solutions.

  It can be understood from the relationship between FIGS. 3 and 4 that the conditions (relative permittivity ε of the high dielectric layer 22 and the thickness t of the high dielectric layer 22) that the electromagnetic noise radiation efficiency is in the vicinity of the minimum value are electromagnetic noise. This means that it cannot be determined without actually measuring and analyzing the radiation efficiency. In the present embodiment, by actually measuring and analyzing the radiation efficiency of electromagnetic noise, the conditions (relative permittivity ε of high dielectric layer 22, high dielectric layer 22) Thickness t). Further, the blending ratio of the dielectric polymer and the dielectric particles is determined so that the relative dielectric constant ε of the high dielectric layer 22 becomes the determined relative dielectric constant ε.

FIG. 5 is a graph showing the dielectric constant ε of the high dielectric layer 22 with respect to the filling rate x of dielectric particles. In this graph, as an example, the relative dielectric constant ε1 of the dielectric polymer is 3, and the relative dielectric constant ε2 of the dielectric particles is 300, 1000, 3000. As shown in FIG. 5, the dielectric constant ε of the high dielectric layer 22 decreases as the dielectric particle filling factor x decreases, and increases as the dielectric particle packing factor x increases. The relationship between the dielectric constant ε of the high dielectric layer 22 and the filling rate x of the dielectric particles can be expressed by the following formula (11).
ε = ε (x) (11)

  As described above, the dielectric loss rate of electromagnetic noise increases as the dielectric particle filling factor x increases and the dielectric constant of the high dielectric layer 22 increases, and the EMI reduction effect of the dielectric sheet 20 increases. Can be larger. However, if the filling rate x of the dielectric particles becomes too large, the dielectric particles are chained in the high dielectric layer 22 and the entire high dielectric layer 22 is hardened. Therefore, there is a limit to the increase in the filling rate x of the dielectric particles, and the filling rate x of the dielectric particles is about 0.74 at the maximum in FIG.

  FIG. 6 is a graph showing the radiation efficiency of electromagnetic noise with respect to the frequency of electromagnetic noise. In this graph, the radiation efficiency of electromagnetic noise is measured by setting the thickness t of the high dielectric layer 22 to 50 μm, 100 μm, and 1000 μm, and the dielectric loss rate tan δ to 0.01 and 0.1. The dielectric loss is a phenomenon in which electromagnetic noise is attenuated and lost in the high dielectric layer 22, and the dielectric loss rate increases as the electromagnetic noise attenuation rate in the high dielectric layer 22 increases. The smaller the attenuation rate of electromagnetic noise in the dielectric layer 22, the smaller it becomes.

  As shown in FIG. 6, the radiation efficiency of electromagnetic noise tends to decrease as the frequency of electromagnetic noise decreases and increase as the frequency of electromagnetic noise increases. The radiation efficiency of electromagnetic noise tends to increase over the entire frequency range when the dielectric loss rate tan δ is small, and decreases over the entire frequency range when the dielectric loss rate tan δ is large. Specifically, the radiation efficiency of electromagnetic noise is reduced by about 10 dB when the dielectric loss tan δ is increased 10 times.

The relationship between the radiation efficiency F of electromagnetic noise and the dielectric loss rate tan δ can be expressed by the following formula (12).
F ｔ tanδ * a (12)
Here, a is a constant.

On the other hand, the dielectric loss rate tan δ is smaller as the mixing amount z of conductive particles is smaller, and is larger as the mixing amount z of conductive particles is larger. The relationship between the dielectric loss rate tan δ and the mixed amount z of conductive particles can be expressed by the following formula (13).
tan δ = tan δ (z) (13)

  It can be understood from the above relationship that the radiation efficiency of electromagnetic noise can be reduced as the conductive loss contained in the high dielectric layer 22 is increased and the dielectric loss rate of electromagnetic noise is increased. It is. Based on such consideration, the EMI reduction effect of the dielectric sheet 20 can be further increased by increasing the mixing amount z of the conductive particles contained in the high dielectric layer 22. However, if the mixing amount z of the conductive particles becomes too large, the conductive particles are chained in the high dielectric layer 22, and the entire high dielectric layer 22 becomes a conductor. Therefore, there is a limit to the increase in the mixing amount z of conductive particles.

From the above formulas (10) to (13), the following formula (14) is established.
F = tan δ * a + F (t, ε)
= Tan δ (z) * a + F (t, ε (x)) (14)

  In the dielectric sheet 20 of the present embodiment, the dielectric constant and thickness of the high dielectric layer 22 are determined so that the radiation efficiency of electromagnetic noise is in the vicinity of the minimum value in consideration of the relationship of the above formula (14). Has been. That is, in the dielectric sheet 20 of the present embodiment, the filling rate x of the dielectric particles and the dielectric thickness t of the high dielectric layer 22 are determined so that the radiation efficiency of electromagnetic noise is in the vicinity of the minimum value. Further, the amount z of the conductive particles is increased so as to reduce the radiation efficiency of electromagnetic noise as much as possible, and the dielectric loss in the dielectric sheet 20 is increased.

  Next, a method for optimizing the blending ratio of materials constituting the dielectric sheet 20 from the viewpoint of material cost and weight will be described.

The volume V [m ³ ] (= area S * thickness t) of the high dielectric layer 22, the unit price of dielectric particles is A [yen / kg], the specific gravity is H [kg / m ³ ], and the dielectric polymer The unit price of B is Y [kg / kg], the specific gravity is I [kg / m ³ ], and the unit price of the conductive particles is C [Yen / kg]. Further, in the high dielectric layer 22, the blending ratio of the dielectric particles is x [m ³ ], the blending ratio of the dielectric polymer is (1-x) [m ³ ], and the weight of the conductive particles is z [kg].

The cost Cost of the dielectric sheet 20 can be calculated by the following formula (15).
Cost = A * V * x * H + B * V * (1-x) * I + C * z
= A * (S * t) * x * H + B * (S * t) * (1-x) * I + C * z
... (15)

The weight Weight of the dielectric sheet 20 can be calculated by the following equation (16).
Weight = V * x * H + V * (1-x) * I + z
= (S * t) * x * H + (S * t) * (1-x) * I + z
... (16)

  When the material constituting the dielectric sheet 20 has already been determined, the parameters [A, B, C, H, I] are parameters that cannot be adjusted at the time of design. Therefore, the cost and weight of the dielectric sheet 20 can be optimized by adjusting the parameters [t, S, x, z] at the time of design. That is, by using the above formulas (15) and (16), the blending ratio of each material is obtained so that the cost of the dielectric sheet 20 becomes the minimum value, or the weight of the dielectric sheet 20 becomes the minimum value. Thus, what is necessary is just to obtain | require the compounding ratio of each material. However, it is necessary to determine the blending ratio of the dielectric particles within a range where the dielectric sheet 20 is not hardened. Moreover, it is necessary to determine the blending ratio of the conductive particles within a range where the dielectric sheet 20 does not become a conductor.

  In the above-described embodiment, the dielectric sheet 20 is affixed to the printed wiring board 10, but the dielectric sheet 20 may be affixed to other noise sources and used.

It is a side view which shows the dielectric material sheet affixed on the printed wiring board. It is a conceptual diagram which shows the joint surface of two dielectric material layers from which a dielectric constant differs. It is a graph which shows the radiation efficiency of the electromagnetic noise with respect to the dielectric constant (epsilon) of a high dielectric material layer. It is a graph which shows the radiation efficiency of the electromagnetic noise with respect to the thickness t of a high dielectric material layer. It is a graph which shows the dielectric constant (epsilon) of the high dielectric material layer with respect to the filling factor x of a dielectric material particle. It is a graph which shows the radiation efficiency of the electromagnetic noise with respect to the frequency of electromagnetic noise.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Printed wiring board, 20 ... Dielectric sheet, 22 ... High dielectric layer, 24 ... Low dielectric layer

Claims (6)

At least two dielectric layers having different dielectric constants, including a high dielectric layer having a relatively high dielectric constant and a low dielectric layer having a relatively low dielectric constant;
The dielectric constant and thickness of the high dielectric layer are determined so that the radiation efficiency of electromagnetic noise incident from the low dielectric layer side and radiated from the high dielectric layer side is in the vicinity of the minimum value. A dielectric sheet for reducing electromagnetic noise.   The high dielectric layer is formed by blending dielectric particles and a dielectric polymer, and the dielectric constant of the high dielectric layer is adjusted by adjusting the blending ratio of the dielectric particles and the dielectric polymer. The dielectric sheet for electromagnetic noise reduction according to claim 1, wherein the dielectric sheet matches the determined dielectric constant.   The high dielectric layer is formed by blending a conductive material, and has the predetermined dielectric loss rate by adjusting a blend ratio of the conductive material. Item 2. A dielectric sheet for reducing electromagnetic noise according to Item 1. 2. The dielectric for reducing electromagnetic noise according to claim 1, wherein the conductive material is a material having a permeability smaller than 100 H / m and a conductivity larger than 0.061 × 10 ⁶ / mΩ. Body sheet.   The dielectric sheet for reducing electromagnetic noise according to claim 4, wherein the conductive material is carbon. A dielectric sheet comprising at least two dielectric layers having different dielectric constants and including a high dielectric layer having a relatively high dielectric constant and a low dielectric layer having a relatively low dielectric constant was used. An electromagnetic noise reduction method,
A dielectric sheet is prepared in which the permittivity and thickness of the high dielectric layer are incident from the low dielectric layer side and the radiation efficiency of electromagnetic noise radiated from the high dielectric layer side is set to a minimum value. Steps,
Affixing the dielectric sheet to an electromagnetic noise source;
An electromagnetic noise reduction method including:

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