GB2062358A - Radio frequency electromagnetic radiation shield - Google Patents

Abstract

A multi-layer radio frequency electromagnetic radiation shield structure, primarily for lamination to a housing of plastics material 10, comprises alternating layers of relatively high electrical conductivity metals, e.g. Al, Cu, B, D, F and low conductivity metals, e.g. stainless steel, Al-Cu alloy, A, C, E to combine the effects of reflection and absorption and thereby maximize the attenuation of the radiation Eo. Alternatively a similar structure of layers of materials with differing magnetic permeabilities may be used for the same purpose. High permeability films may consist of an alloy of Fe, Ni, Mo and Mn. Low permeability films may be made of Al. <IMAGE>

Description

SPECIFICATION Radio frequency electromagnetic radiation shield The present invention relates to shielding structures for shielding against electromagnetic interference or radio frequency interference. The invention more particularly relates to a composite layered coating which may be applied, for example, to plastics substrates to provide an effective shield for RFI and EMI.

With the growing replacement of metal housings by plastics housings in TV, audio equipment, medical instruments, process controls, computers, microprocessors and other sources of electromagnetic or radio frequency radiation, the problem of interference created by the components within the housing, or the effect of radiation created outside the housing on the components within the housing becomes important. Since plastics are materials which are essentially transparent to such radiation, no natural shielding exists as it does with metal housings.

RFI or EMI shielding on plastics housings has typically been accomplished by several methods providing attenuation or a decrease in magnitude in transmission of interference from one point to another. For example, spraying the plastics material with an electrically conductive paint has been utilized in certain applications. A high conductivity material, such as silver or copper, has to be added to the paint to provide attenuation.

The painting technique is subject to adhesion problems and to non-uniform metal fill in the coating on the plastics material.

Plastics material which includes electrically conductive materials as a filler has also been used to effect the shielding of radio frequency energy.

However, frequently the addition of these conductive materials compromises the strength and other factors leading to the decision to use plastics materials for the housing.

Vacuum deposition techniques are also used to obtain a thin film of conductive material on the plastics material and these generally provide acceptable attenuation values depending on the material deposited and the frequencies desired to be shielded. It is mainly relative to this type of technique that the invention is directed.

Attenuation, as used herein, is intended to mean a reduction of radiation or signal strength as the radiation or signal passes through an obstruction, in this case a shielding structure such as a plastics housing with a vacuum deposited film upon it. Attenuation is given in decibels (dB) from the equation: Attenuation (dB) = +20 log A/Ao = 20 log Ao/A where, Ao is the original signal amplitude and, A is the remaining signal amplitude after passing through the obstruction.

Attenuation of the signal generally occurs by reflection of the signal; or by absorption of energy.

A brief description of both of these phenomena will help to explain this invention.

Reflection: As a signal travels through one medium and encounters an interface with another medium, the signal is either reflected or transmitted, or more commonly is partially reflected and partially transmitted.

By defining a predetermined signal with energy Eo which approaches an interface of media, and defining ER as the reflected energy and ET as the transmitted energy, a reflection coefficient R and transmission coefficient T can be defined as follows: R = ER/EO T= ET/EO Note that ER + ET = EO and therefore R + Ti.

Some general statements concerning Rand Tcan be made: 1. If the signal in question is in a medium of relatively low conductivity and encounters a medium of higher conductivity, then most of the energy of the signal is reflected, or: RLH TLH 2. If the signal travels from a highly conductive medium to a medium of low relative conductivity then most of the energy of the signal is transmitted, or: THOL R H < L 3. The above relationships are especially pronounced for interfaces where the media have ratios of resistivity in the order 10:1; and the highly conductive material has a resistivity < 5 y ohm cm.

A special consideration when reflecting a signal from a thin film is the opacity of the film. To get the maximum reflection, one must have a certain minimum film thickness; this may be termed the point of opacity which is easily determined from experiment. For example, with reference to aluminum, the minimum thickness for maximum reflection is approximately 3000 angstroms. With reference to nickel, the point of opacity is near 10000 angstroms.

Absorption: As a signal traverses a medium, some of its energy is dissipated in the medium in the form of heat. The energy of a signal as a function of the distance it travels through a medium is given by: E(x)= EOe 2X/? where, EO is the original energy E(x) is the remaining energy, x is the distance travelled in the medium and a is the "skin depth" or the depth at which the remaining energy is 1/e2 the original energy. (corresponds to ""-9 dB attentuatio~n). For a good conductor a is proportional to 1/ where # = magnetic permeability and cr = conductivity.

For a signal having a frequency of 100 Mhz, travelling through copper, which is a high conductivity, low permeability (y = 1) material, the skin depth is 71,000 angstroms. To provide -36dB attenuation, by absorption only, 71,000 x 4 angstroms =284 KA of copper, or a little over 0.001 to of copper would be needed.

The skin depth for the same signal traversing a highly permeable material (,u105) will be in the order of 1 KA, so that much of its energy is absorbed while traversing only a short distance.

A similar relationship exists between skin depth and the co-efficient of conductivity relative to reflectivity and transmission.

The invention described herein takes advantage of both reflection and absorption phenomena by causing radio frequency electromagnetic radiation, i.e. a signal, to be "trapped" in a low conductivity material by having higher conductivity layers on either side of the low conductivity material. As the.

signal encounters the low-high interface, the major portion of its energy is reflected back into the low conductivity material; thus, it continues to traverse this medium, continually dissipating its energy. Additionally, a first layer of lower conductivity material will enhance the total reflected portion of the signal by providing a series of reflective interfaces.

To take advantage of this phenomenon, according to this invention, a composite multilayer radio frequency electromagnetic radiation shield structure comprises a plurality of layers of electrically conductive material comprising alternating layers of metal having relatively high skin depth for given frequencies and relatively low skin depth for the same frequencies.

Some examples of structures in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is an enlarged cross-sectional view one example of the structure showing typical disposal paths of waves of Rf electromagnetic energy through the structure; Figure 2 is an enlarged cross-sectional view of another practical example of the structure; Figure 3 is an enlarged cross-sectional view of another example of the structure; and Figure 4 is an enlarged cross-sectional view of another example of the structure which embodies materials of difference magnetic permeability as the alternating layers of electrically conductive material.

Referring first to Figure 1, the invention will be described relative to the various layers on a substrate or housing 10. The housing 10 is shown to be of a plastics material and it is provided with a multilayer composite shielding structure. This composite shielding structure is referred to as 12.

The shielding structure 12 comprises thin conductive layers A, B, C, D, E and F having relative conductivity noted on the drawing. It will be apparent that a relatively high coefficient of resistivity (p) denotes a relatively low conductivity material. On this example, A is the layer exposed to the air and F is the layer which is contacting the substrate.

As a representative wave of energy E0 encounters the top layer A, which is of relatively, low conductivity, the major portion of the wave energy is reflected as R1, since the conductivity of layer A is still much greater than that of air. This reflection R1 is no longer of consequence to the system. As the transmitted portion T, of the wave encounters the next interface AB, again the major portion of its energy is reflected as R2, since the interface AB is that of a low conductivity material to a high conductivity material. The remaining portion of transmitted portion T1 is transmitted through layer B as T2. However, as the reflected wave R2 exits through the air interface, the major portion of the wave is transmitted therethrough as T3 and leaves the system. The reflected portion R3 is additive to transmitted portion T,.

As T2 now encounters the high to low conductivity interface BC, the major portion of this wave is transmitted as T4. A small portion is reflected as R4, and this portion exits the system through the above two high to low interfaces.

Minor reflections, r, at these interfaces are additive toT2andT1.

As T4 encounters the low to high interface CD, the major part of this transmission is reflected as R5. As shown in Fig. 1, R5 now continues to see low to high interfaces CB and CD as it is "trapped" in the layer of C of low conductivity material with only minor transmissions, t, at the interfaces.

These minor transmissions are additive to either R4 or to which is the smaller portion of T4 and which is transmitted through interface CD. As R5 travels within this low conductivity layer, its energy is dissipated until it is no longer significant.

Remaining transmission T5 then encounters another high to low interface DE where a major portion of it is transmitted to T6, with the remaining portion reflected as R6 is additive to R5.

The transmission Te then encounters a low to high interface EF and as in the discussion relative to interfaces CD and AB, the major portion of this transmission is reflected as R7 and trapped in layer E.

The final transmitted signal is T7 plus some minor transmissions as R7 reflects from the low to high interface. The total reflected signal is thus R plus T3 as well as some minor transmissions reflected back and thus it is apparent that a very small portion of signal E0 is able to pass through structure 12.

The above examples only deal with layers of different electrical conductivity, however, it should be understood that a similar structure can be made using materials of different magnetic permeabilities or different combinations of conductivity and permeability. Since the coefficient of permeability and the coefficient of conductivity are related, i.e., high to low permeability interfaces have the same effect as high to low conductivity interfaces by virtue of the relationship between skin depth and the coefficient of conductivity and skin depth to the permeability mentioned above.

Additionally, it should be understood that while an external film substrate is shown, similar effects will be obtained with an internal coating or shielding structure since the nonconductive substrate (plastic housing) is essentially transparent to RF or EM signals.

Turning now to Figure 2, a typical example of the shielding structure of the invention is shown utilizing alternating high and low conductivity layers. A series of layers of aluminum, aluminumcopper alloy, copper and 302 stainless steel are formed over a plastic substrate. It will be apparent that this structure is substantially identical to that of Fig. 1 with the relative coefficient of resistivities of the layers being 2.7, 100, 1.7, and 70 respectively. All of the examples in Figs. 1-4 refer to a coefficient of resistivity (p) in u ohm-cm terms.

Figure 3 shows yet another example of the shielding structure of the invention wherein a plurality of layers of aluminum are interspersed between alternating layers of stainless steel with aluminum having a coefficient of resistivity of 2.7 and stainless steel of approximately 70.

Figure 4 shows an alternative example of the invention wherein a plurality of layers of material which is of a high magnetic permeability are interspersed between layers of a very conductive material, namely aluminum, having a normal coefficient of permeability. (y ~ 1). The high permeability alloy X may be 15.7% Fe, 79% Ni, 5% Mo, 9.3% Mn with y "" 100,000.

Obviously, the materials utilized in such a multilayer shielding structure are a matter of choice. The structure shown in Fig. 2, where each layer has a thickness of approximately 3-5 thousand angstroms produced45 dB attenuation over the range from 65-265 Mhz.

This is compared to attenuation factors in the order of -20 dB for copper paint over the same frequency ranges and -35 dB for a single thin layer of aluminum or a single layer of silver paint.

A further aspect of the invention is the thickness of the films or layers themselves. It is preferred that the layers be very thin film as by vapor deposition and preferably ion deposition as will be discussed later herein. It has further been found that the optimum thickness for each film is that of opacity. A film which is thicker than its point of opacity has been found to be no more effective in attenuation by reflection over a given range of frequencies than that which is right at its point of opacity. Thus a preferred thickness of each layer of the film is the point of opacity for the material in that layer, as previously discussed.

Obviously, all of the films contemplated by the invention are very thin as typified by a vacuum deposition technique.

The use ofan ion deposition technique as discussed in U.S. Patent 4,039,41 5 is particularly advantageous in practicing this invention since the technique described therein is capable of plating a thin film of any conductive material on a substrate of plastic. Furthermore, the ion deposition technique is capable of providing a uniformity of thickness and coating even if the substrate includes deeply recessed areas that should be coated with a shielding. Since the ion deposition is not a straight line technique, it is particularly effective in producing the layers desired by this invention.

It should also be apparent that while the shielding structure described herein is particularly effective on plastic on nonconductive substrates, it will also be effective as an RF or EM shield for conductive substrates.

While any variety of metals can be utilized and come within the scope of this invention, particularly effective combinations of layers would have a ratio of conductivity in the order of 20:1.

This will provide the multiple reflections and absorption rates desired to enhance and maximize the attenuation.

It should be further noted that any sequence of placement of the various layers can be utilized and still come within the scope of the invention as long as there is in some way a plurality of layers with high conductivity layers sandwiched between low conductivity layers, or interspersed layers of different permeability. The example shown describes a low conductivity layer of, for example, 302 stainless steel material at the top surface.

This is to provide a certain amount of corrosion resistance to the overall structure. However, for various functional and aesthetic purposes, any series of materials can be used.

Claims (14)

1. A composite multi-layer radio frequency electromagnetic radiation shield structure comprising a plurality of layers of electrically conductive material comprising alternating layers of metal having relatively high skin depth for given frequencies and relatively low skin depth for the same frequencies.

2. A structure according to Claim 1, comprising at least three layers of electrically conductive material including alternating layers of metal of relatively high electrical conductivity and relatively low electrical conductivity.

3. A structure according to Claim 1 or Claim 2, in which each layer is a thin film having a thickness substantially at the point of opacity for the material of which the layer is made.

4. A structure according to Claim 3, in which each layer is formed by vacuum deposition.

5. A structure according to Claim 3, in which each layer is formed by ion deposition.

6. A structure according to Claim 2 or any one of Claims 3 to 5 when dependent on Claim 2, in which adjacent layers have a conductivity ratio of at least 20:1 at the interface of the layers.

7. A structure according to Claim 1 , including alternating layers formed by thin films of material having high magnetic permeability and low magnetic permeability.

8. A structure according to Claim 7, in which the high permeability films are of an alloy consisting of 15.7% Fe, 79% Ni, 5% Mo, 0.3% Mn except for impurities and the low permeability films are of aluminium.

9. An electrically non-conductive substrate having a structure according to any one of Claims 1 to 8 laminated thereto.

10. An electrically conductive substrate having a structure according to any one of Claims 1 to 8 laminated thereto.

11. A housing of plastics material forming a substrate in accordance with Claim 9, the structure being laminated on the exterior thereof.

12. A housing of plastics material forming a substrate in accordance with Claim 9, the structure being laminated on the interior thereof.

1 3. A structure according to Claim 2, comprising a plastics substrate having applied thereto a first thin film of aluminium, a second thin film of aluminium-copper alloy, a third thin film of copper and a fourth thin film of a stainless steel material.

14. A substrate having a structure according to Claim 1 laminated thereto substantially as described with reference to any one of the Figures of the accompanying drawings.