The present invention relates to an electromagnetic interference (EMI) shielding material. More particularly, it relates to a corrosion resistant, electroconductive EMI shielding material, such as gaskets, sheets, tapes, etc.. This application is a continuation-in-part of U.S.S.N. 78,699, filed on June 13, 1993 (pending).

BACKGROUND OF THE INVENTION

As the regulation of unwanted electromagnetic energy becomes more prevalent, the use of EMI shielding material has become more commonplace.

A major problem that involves such shielding materials is the potential for corrosion that may occur in the material and/or the adjacent surfaces to which the shielding material is mated. This is believed to be caused by the difference in the electrical potentials between the conductive filler of the shielding material and the adjacent conductive substrates. When moisture, especially in the form of salt water, is present between the shielding material and the adjacent substrate, a galvanic coupling occurs between the two different materials causing pitting and corrosion of the lesser noble material. For example, a silver-filled silicone gasket mated to an aluminum substrate will, in the presence of salt water, cause the rapid and widespread corrosion of the aluminum substrate. The phenomenon is called galvanic coupling.

Not only is this corrosion unsightly, but it lessens the structural and shielding integrity of the interface.

One method for limiting the potential for corrosion is to subject the substrates to a chromate treatment. Chromated surfaces are more resistant to corrosion than non-chromated surfaces. However, the chromate process is expensive and generates hazardous materials, making such a method undesirable.

Another method has been to modify the conductive filler of the EMI shielding material so that corrosion is less likely. For example, one can use a silver coated/aluminum powder in EMI shielding materials to reduce corrosion. See, U.S. Patents 4,434,541, 4,507,359, 4,678,716 and 4,734,140.

Further, the use of corrosion inhibiting agents on the substrates or fillers have also been attempted. However, these agents are generally expensive and have an adverse effect on the other physical properties of the shielding materials.

Another method has been to recess the conductive filler within the matrix of the elastomer so that it is only exposed upon the application of pressure to the elastomer. See, e_g.. U.S. Patent 2,796,457 and Canadian Patent 1,116,650. This product is difficult to produce without exposing some of the conductive material prematurely. Further, the conductive material is exposed upon the application of pressure, so corrosion can still occur. Moreover, the shielding effectiveness of these materials can be reduced due to the lack of conductive material near the surface. Lastly, the material requires a high deflection force in order to cause the filler to reach the surface and render the material conductive. This is contradictory to the desires of the industry for lower and lower deflection forces and reduced compression set.

Lastly, it is a well established belief in the art that to have an effective shielding material, one must have a continuous particle to particle contact between the two outer surfaces of the shielding material. Failure to do so will result in a lack of shielding properties.

The present invention provides a means for reducing corrosion of the shielding materials and/or adjacent substrates at a reasonable cost and in a manner unknown in the prior art.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention is formed of electrically conductive EMI shielding material having at least a portion of the major outer surface adjacent to and preferably covered by a dielectric layer. The dielectric layer is sufficiently thin so as to allow for effective EMI shielding but which is sufficiently thick so as to prevent corrosion due to galvanic coupling and which is resilient and abrasion resistant.

It is an object of the present invention to provide an EMI shielding material comprising an inner layer formed of an elastomeric matrix and one or more electrically conductive fillers in an amount sufficient to provide at least 20 decibels of EMI shielding and a dielectric layer adjacent to and covering at least a portion of the conductive layer, so as to maintain its electrical conductivity, yet be free from corrosion.

It is another object of the present invention to provide an EMI shielding material comprising a first formed of an electrically conductive material, elastomeric material, and a second layer formed on the first layer and being formed of a dielectric material.

A further object of the present invention is to provide an EMI shielding material comprised of a sheet, the sheet being formed of a first layer of electrically conductive material and a second layer formed on the first layer, the second layer being formed of a thin dielectric material and wherein the sheet is capable of providing at least 20 decibels of EMI shielding.

An additional object is to provide a system for providing EMI shielding between two adjacent, electrically conductive substrates comprising an EMI shielding material formed of an electrically conductive material and a resilient elastomer and a dielectric

layer, interposed between the EMI shielding material and at least one adjacent substrate so as to maintain electrical conductivity between the adjacent substrates while preventing galvanic coupling from occurring.

A further object is to provide an EMI shielded enclosure formed of two adjacent electrically conductive substrates and a gap there between, an electrically conductive gasket placed adjacent the gap and having electrically continuity with the two adjoining substrates, the improvement comprising a relatively thin dielectric layer interposed between the gasket and at least one of the two adjacent substrates so as to prevent galvanic co pling from occuring.

IN THE DRAWINGS:

Figure 1 shows a cross-sectional perspective view of one embodiment of the present invention.

Figure 2 shows another embodiment of the present invention in cross-sectional view.

Figure 3 shows a cross-sectional perspective view of one embodiment of the present invention.

Figure 4 shows a cross-sectional view of a third embodiment of the present invention.

Figure 5 shows a cross-sectional view of the embodiment of Figure 3 in use.

Figure 6 shows a cross-sectional perspective view of a further embodiment of the present invention.

Figure 7 shows a cross sectional view of another preferred embodiment of the present invention.

Figure 8 shows a graphical representation of the shielding effective of the present invention and the prior art.

Figure 9 shows a cross sectional view of a testing jig for use in testing corrosion resistance.

DETAILED DESCRIPTION

The present invention is an electrically conductive EMI shielding material which has at least a portion of an outer major surface adjacent to and preferably covered by a dielectric layer. This dielectric layer allows for continued EMI shielding capabilities while reducing or preferably eliminating the potential for galvanic coupling between the shielding material and the adjacent substrate to which it mates.

Figure 1 shows a cross-sectional view of a preferred embodiment of the present invention, in which the dielectric layer is incorporated as part of the shielding material. The shielding material 1 is in the form of a gasket. The gasket has an inner highly conductive layer 2, surrounded by a thin protective dielectric outer layer 3.

The material 1 may be in the form of a gasket, tape, sheet, cloth, etc. If, in the form of a gasket, it may be solid or hollow; foamed or unfoamed. It may be conductive throughout or it may have a non-conductive innermost layer surrounded by a conductive layer. The material may have a circular, square, rectangular, P-shaped, D-shaped, L-shaped, Z-shaped or any other cross section that is generally used as EMI shielding materials.

The inner layer is highly conductive, generally formed of one or more elastomeric materials such as silicone or fluorosilicone rubber, EPDM rubber, polyurethane, neoprene, thermoplastic rubbers, such as Santoprene® polymers from Monsanto, various styrene containing rubbers, such as styrene butadiene rubbers, butyl rubber and other well-known elastomers. The

elastomer contains a conductive filler such as a noble metal _=_g.- gold or silver; a non-noble metal e_a. copper, iron, aluminum, cobalt or nickel; a noble metal-coated noble metal, e_g. silver-plated gold; a noble metal coated non-noble metal, eσ. silver-coated copper, silver-coated nickel, silver-coated iron, silver coated aluminum; a noble metal coated non-metallic filler, eg. silver-coated graphite, silver-coated glass microspheres, silver-coated plastic particles; or a conductive non-metallic filler, e_g_. carbon, or graphite. Such fillers are well-known and commercially available and described in U.S. Patents 3,140,342; 4,434,541 and 4,678,716 all of which are incorporated in their entirety herein.

The amount of conductive filler in the elastomer depends upon the conductivity of the filler, the level of conductivity desired or required and the ability of the elastomer to retain the filler while remaining resilient and elastic.

It is preferred for the present invention that the inner layer be highly conductive. By highly conductive it is meant that the inner layer by itself (without the outer protective layer) be capable of providing at least 20 decibels preferably at least 40 decibles of EMI shielding at frequencies from about 10 MHZ to 40 GHZ.

To achieve such levels of conductivity, typically the level of filler in the elastomer should be from about 20 to about 90 volume percent, such as are taught in 3,140,342, 4,434,541 and/or 4,678,716. The levels cited in these incorporated patents have proven to be useful in the present invention.

Alternatively, the conductive inner layer may be formed of a metal layer over a nonconductive core. Such

a layer may be a metal foil, mesh or fabric such as aluminum, copper, tin, MONEL or FERREX metal, or a noble metal-coated layer such as silver-plated copper or tin, or a metal coated fabric or mesh, such as a silver-plated nylon rip stop fabric or a silver coated plastic yarn. The inner conductive layer should be sufficient to provide the desired conductivity and should be at least 0.0001 inch thick, more preferably 0.010 inch thick. Moreover, it should not be too thick so as to render the gasket inflexible or nonresilient.

The inner layer may be formed in any manner generally known in the art. For example, it may be molded, extruded, laminated or otherwise formed into the desired shape.

The dielectric layer is a relatively thin, protective, nonconductive material. It may be in the form of a coating, film, extrusion, deposition or any other such layer which is capable of being bonded to the conductive layer (if so desired) without adversely affecting the conductivity of the conductive layer and yet provide the corrosion resistance desired.

The selected material for the dielectric layer should be compatible with and be capable of mating directly to the conductive layer. Examples of suitable materials include but are not limited to silicones, fluorosilicones, polyurethanes, polytetrafluoroethylenes, polyesters, nylons, acrylics, acrylates, epoxies, polyolefins, polyolefin copolymers, such as ethylene vinyl acetate copolymers, ethylene propylene copolymers, etc. and blends thereof.

By relatively thin, it is meant that the layer is thin enough so as to allow for EMI shielding properties to continue, but is thick enough to ensure adequate

coverage of the inner layer so as to prevent corrosion of the conductive filler or the adjacent substrates to which the shielding material is mated. Preferably, the outer layer is less than 5 mils. (.005 inch, .1274mm) in thickness, more preferably it is less than 2.0 mils. (.002 inch, .0508 mm) most preferably, less than 1.0 mil (.001 inch, .0254 mm) in thickness.

More importantly, the outer layer must be substantially uniform and constant over the selected surface of the inner layer so as to ensure that the conductive layer filler is not exposed, even upon compression.

In some embodiments, the entire conductive layer is covered by the dielectric outer layer. In other embodiments, only a portion of the conductive layer is covered by the outer layer. Such an embodiment is shown in Figure 2 wherein only a portion of the conductive layer 4 which is exposed or potentially exposed to the outside elements or corrosive environments is covered by the dielectric outer layer 3.

Various manners for applying the outer layer to the outer surface of the inner layer are well-known. For example, the outer layer can be extruded over or coextruded with the inner layer to form an integral layered structure. Another method is to coat a preformed inner layer with a liquid coating. Such liquid coating include solutions, dispersions, latices and molten forms. The coating may be sprayed, rolled, electrostatically or vapor deposited onto the inner layer.

If desired, a separately formed film layer as the outer layer may be heat-bonded or laminated to the inner layer. Likewise, the outer film layer may be formed into the desired shape and an inner layer can then be formed

into and bonded to the film layer. For example, one can form the outer layer into a circular cross-section and then proceed to inject the conductive layer into the interior of the film layer and allow it to bond to the . outer layer. Alternatively, the outer layer could be a heat shrinkable tube and be placed over and heat shrunk onto the inner layer. Heat shrinkable plastic tubes are well known and include for example, various polyolefins such as polyethylene and polypropylene.

Figure 3 shows another preferred embodiment of the present invention. It relates to a gasket 11 having an innermost core 12 of non-conductive elastomer, an intermediate layer 13 of highly conductive elastomer and a dielectric outer layer 14.

Figure 4 illustrates another embodiment of the present invention. The EMI shielding material 21 is in the form of a surface conductive sheet or tape. A first surface 22 of the tape is designed to be attached to a conductive surface (not shown) . The opposite surface 23 of the sheet is either exposed to the elements or isdetachably mated to an adjacent conductive substrate. It therefore has an inner conductive layer 24 and an outer dielectric layer 25. Preferably the dielectric layer 25 extends beyond the sides 26 A, B of the tape and under a portion of the first surface 22 of the tape to ensure corrosion resistance. Alternatively, a caulk or sealant could be used to seal the edges of such a tape once it is placed into position.

Figure 5 shows the conductive tape 30 of Figure 4 as used in place to bridge a gap 31 between two adjacent conductive surfaces 32. 33. A layer of conductive adhesive 34, such as a pressure sensitive acrylic adhesive is shown. If desired, one could use a non-conductive adhesive so long as it did not fully

extend along the surface 22 of the tape 21 or one could use a mechanical means such as rivets, clamps, brackets, etc. Without a bridging conductive layer, the gap 31 would permit the transmission of electromagnetic energy through the gap. Likewise, the gap 31 would radiate electromagnetic energy, much like an antenna. Such tapes are particularly useful in the assembly of modular shielding enclosures, such as portable test rooms, computer rooms, mobile communications trailers, etc.

Figure 6 shows another preferred embodiment of the present invention. In this embodiment, the shielding material 41 is shown in the form of a gasket.

It is formed of a non-conductive core 42. Instead of a highly conductive elastomer layer, a metal layer is used as the conductive layer 43.

The outer layer 44 can be formed of any of the preferred materials discussed above. While this embodiment may not have as high a shielding effectiveness as some of the other embodiments, it is nevertheless very useful, especially for commercial applications where shielding requirements are typically less restrictive.

Figure 7 shows another embodiment of the present invention wherein the dielectric layer 45 is formed on either of the adjoining substrates 46 (A) and (B) (as shown) rather than on the conductive layer 47 of the shielding element 48 or the layer 45 is interposed between one or both of the adjoining substrates 46 and the conductive layer 47. In either of these alternative embodiments, the dielectric layer 45 is not bonded to nor forms part of the shielding element 48 per se.

The conductive layer 47, and the dielectric layer 45 may be formed of the same materials discussed above

with other embodiments. The dielectric layer 45, if bonded to an adjacent substrate, may be attached by an adhesive, especially an electrically conductive adhesive, deposited as a spray or coating or retained by a mechanical means such as a rivet, clip or frame.

The following is an example of one embodiment of the present invention and is meant only to elucidate the teachings of the present invention and not to be a limitation thereon.

EXAMPLES

EXAMPLE I

In order to test for shielding effectiveness, two test samples were formulated as follows:

1.) A conductive gasket Sample 1, formed in the design of Figure 2 of a DACRON fabric reinforced gasket containing a silver plated aluminum filled silicone, known as CHO-SEAL® 1285 gasket, available from Chomerics.

2.) A material similar to that of Sample 1, designated as Sample 2 was prepared, having a .5 mil thick layer of MYLAR® film bonded to its upper surface.

The samples were tested for EMI shielding capabilities using MIL-G-83528 procedures, at the following frequencies: 100 MHz, 200 MHz, 400 MHz, 600 MHz, 800 MHz, 1000 MHz, 2000 MHz, 4000 MHz, 6000 MHz, 8000 MHz and 10000 MHz.

The samples were each mounted to a one inch wide MIL-G-83528 brass flange bonded to the perimeter of a 24 by 24 inch opening in a wall of an EMI shielded test chamber. A .375 inch (9.525 mm) thick aluminum mounting plate (26x26 inches) (test panel) was placed over each

sample and attached to the chamber wall by 8 bolts (2 per side) to deflect the sample gasket by 50%. A transmitting antenna, signal generator and amplifier were placed outside of the chamber in line within and one (1) meter from the opening covered by the sample. A test panel receiving antenna and spectrum analyzer were placed in the shielded chamber, in line with and one (1) meter from the test panel.

Shielding effectiveness was measured by taking power readings recorded when no test panel was in place and subtracting from it the power readings recorded when a test panel was in place.

Open Reference - Closed Reference - Shielding Effectiveness (in decibels (dB)).

Figure 8 shows the shielding effectiveness measured for the samples tested over the frequency range as well as the results taken with an air gap. (simultating any non conductive gasket). The gap was maintained at the same distance between the wall and the mounting plate as with a gasket in place by a series of plastic shims.

As can be seen, Sample 2 had lower shielding capability than Sample 1, yet it provided consistently better and acceptable shielding effectiveness than the air gap over the tested frequency range.

The performance of the dielectrically coated sample was quite significant in view of the air gap test results. The air gap represents any non-conductive gasket. One skilled in the art would have anticipated obtaining results similar to those of the air gap inview of the dielectric coating rather than the higher, satisfactory results obtained with the dielectric containing gasket.

EXAMPLE I I

A test program was developed to determine the effectiveness of the electrically insulating material of the present invention as a barrier in the prevention of corrosion between an EMI shielding gasket (i.e. conductive elastomer) and a 6061-T6 aluminum substrate coated with a Mil-C-5541 Class 3 conversion coating.

Corrosion occurs if the necessary requirements of a galvanic couple are satisfied; two dissimilar metals, an electrolyte, and an electric current pathway, for instance, a silver-plated elastomer mated against an aluminum substrate, in the presence of salt fog. If one. of these variables are eliminated, the corrosion will be minimal. The weight loss of the less noble metal (aluminum coupon), will measure the amount of corrosion.

Several different products were compared in a salt fog test. The first group of samples was formed of a flat sheet, silver plated copper filled silicone (CHO-SEAL 1215) . The second group was formed of a more corrosion resistance silver plated aluminum filled conductive material (CHO-SEAL 1298) .

Samples from each group was prepared and tested with (a) no dielectric layer, (b) with a dielectric layer that covered most but not all of the exposed surface of the conductive material and (c) by a dielectric layer that covered all of the exposed surface of the conductive layer.

Each sample was placed in a test assembly 50 as shown in Figure 9.

A top block 51 is connected to a bottom block 52 by a bolt and nut assembly 53. The conductive material 54 is placed between the blocks such that the conductive material to be tested 54 is adjacent the top block 51.

An aluminum disk 55 is below the conductive material and a nonconductive rubber 56 gasket is located below the aluminum disc 55. The assembly 50 was then slightly compressed by the bolt and nut assembly 53 and then subjected to a salt fog test. The embodiment in Figure 8 shows the example where the dielectric layer 57 completely covers the exposed edges of the conductive material 58.

After 168 hours exposure to an ASTM B117 salt fog test, the samples that has the dielectric extend over the exposed surface of the elastomer, resulted in minimal corrosion of the aluminum. However, the dielectric that did not completely cover the elastomer had no effect in minimizing corrosion. In both, the dielectric did not have an impact on the volume resistivities of the elastomers, but the dielectric had a positive impact on the dimensional stability of the elastomers. (See Table 1).

Since the large diameter dielectric extended beyond the termination of the elastomer, it prevented galvanic coupling from occuring to the aluminum coupon. The weight loss of the coupon using the dielectric layer was greatly minimized, compared to the weight loss in examples in which the dielectric was not used or only partially used.

The small diameter dielectric did not surround the exposed elastomer, and therefore allowed an area where galvanic coupling could occur to the aluminum coupon. Thus, the weight loss of the coupon was similar to that of the sample without the use of the dielectric.

While not wanting to be bound by any particular theory as to why the present invention provides shielding effectiveness, when according to the conventional wisdom in the art, it should not, as it

fails to provide a complete electrical path way across the gap. Applicant offers the following explanation: A factor typically associated with shielding effectiveness is the reduction of the interface resistance between the two mating flanges. In conventional gasket systems this is achieved by implementing a gasket with low volume resistivity and interface resistance via a highly filled, surface to surface metal contact pathway. The intent is to eliminate a "voltage" drop across the interface by lowering the resistance of the seam.

The conventional technique obtains this by the high conductivity across the interface, the present invention obtains this in a capacitive sense. With the presence of the dielectric between the interface, (which*- eliminates the direct electrical conductivity) the interface surfaces separated by the dielectric layer depicts an ideal capacitor (two metal plates separated by a thin dielectric). From an electrical standpoint, a capacitor's ability to suppress RF energy increases as one increases the frequency. For example, a 0.1 micro Faraday capacitor is widely used on printed circuit boards to shunt unwanted high frequency signals to ground. Different capacitance values will be effective at difference frequencies.

With regard to the present invention, this capacitance effect shunts RF energy to the ground associated with the mating flanges and eliminates the ability of the energy to radiate through the flange. As in an ideal capacitor, the ability of this capacitor to work at given frequencies is directly proportional to the capacitance value. For example, thicknesses of other than 1/2 mil used in the Example I will provide more or less signal attenuation and at different frequencies. This can be altered to create different

responses via the use of dielectric of different thickness and/or conductive layers of different conductivity.

While the present invention has been described in relationship to its preferred embodiments, other embodiments can achieve the same result. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents as fall within the true spirit and scope of this invention.

Weight Loss of Al (og)

Dielectric Material Large Small ithout

Diameter Diameter Dielectric

GROUP. 1.9 299.4 270 GROUP II .6 1.3 2.2

Volume Resistivity (mohm-cm)

Dielectric Material

Larσe Diameter Snail Plameter,

Before After Before After Spec.

GROUP I 1.49 2.22 1.60 2.18 4

GROUP II 5.72 5.96 5.87 6.28 12

Dimensional Stability (%swell)

Dielectric Material atg-t — _> to All Without Diameter Diameter Dielectric

GROUP I -1.14 4.75 5 GROUP II 3.29 4.75 5