US3512946A - Composite material for shielding electrical and magnetic energy - Google Patents

Description

United States Patent O 3,512,946 COMPOSITE MATERIAL FOR SHIELDING ELEC- TRICAL AND MAGNETIC ENERGY Irving .I. Hutkin, San Diego, Calif., assignor to Lash Manufacturing, Inc., San Diego, Calif., a corporation of California No Drawing. Filed Apr. 17, 1967, Ser. No. 631,187 Int. Cl. 133% /08 US. Cl. 29-195 10 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a composite material for use as shield material and for making printed circuits in which thin metal layers of conductive material are plated on a high temperature resistant material and more particularly to a composite shielding material for shielding magnetic and electrical fields in which thin metal layers of low and high permeability are plated on a thin plastic sheet.

BACKGROUND OF THE INVENTION The electromagnetic shielding materials generally in use are made from alloys of iron and nickel. These alloys usually fall into two categories. One category is the alloys that are designed for every high initial magnetic permeabilities. These are primarily materials having 80 percent nickel and the balance iron, such as the mu metals, permalloy and supermalloys. These alloys may often contain a small amount of copper or molybdenum in the 80 percent nickel plus percent iron alloy. The other general classification of iron-nickel alloys for magnetic shielding is the high permeability alloys for shielding the higher strength fields and include the 50 percent nickel-50 percent iron alloys. In applications where cost is a factor and the particular shielding desired need not have the most eflicient available material, the silicon irons and low carbon steel foils are often used for shielding.

The shielding materials are generally supplied in the form of sheets or strips that are rolled to a to 23 of an inch thickness. The sheets are often in preforms such as cans, boxes, and other shapes designed to fit around components or component assemblies. The same two types of iron-nickel alloys are also used in motors, transformers and certain types of cores for sensitive relays in which the high permeability characteristics of the iron-nickel alloys are required. In such application these materials are used in the shape of tape wound cores or precut laminates. Because of the high cost of rolling these alloys to a very small thickness and the fact that significant annealing cycles must be performed to get optimum magnetic properties, these shielding materials are quite expensive. Their ductility is also limited and, in some cases, heat treatments after gradual use or extended use periods are often required to maintain ductility and the magnetic characteristics.

Where shielding materials are used in their sheet and strip form, better results are usually obtained when several layers of such strips are wound concentrically around the object of the shield. This requires that the strips be flexible and thin. This requirement for thinner materials must then be balanced by the increased cost in preparing the nickel-iron alloys in small thicknesses.

Other types of shielding materials that are available are woven wire mesh, often supplied for cable shielding, and iron particle bearing resin systems that are generally laminated onto a kraft paper Ibacking. The wire mesh type of cable shielding while still providing flexibility, does not provide the same high degree of electro-magnetic shielding because of the voids and open areas around the circumference of the cable. The iron loaded 3,512,946 Patented May 19, 1970 resin systems are also effective; however, the overall permeability of these materials is fairly low. Besides being expensive, these laminates are fairly bulky with common thicknesses of 40 to 60 mils. Other disadvantages of the iron loaded resin systems laminated onto the kraft paperis that they cannot be easily grounded because of their nonsolderability; and they are not useable at temperaturers much above 125 Fahrenheit.

SUMMARY OF THE INVENTION Thus there is a need in the art for a new and improved electromagnetic shielding material that is inexpensive, flexible light weight, high temperature resistant and that is capable of providing improved shielding.

It is therefore an object of this invention to provide a new and improved shielding material.

It is another object of this invention to provide a new and improved electromagnetic shielding material that is flexible, pliant, less expensive, light weight and is high temperature resistant.

It is another object of this invention to provide a new and improved shielding material in which the shielding metal is thin and yet retains its inherent ductility.

It is another object of this invention to provide a new and improved electromagnetic shielding material having combined low and high permeability shielding capability.

It is another object of this invention to provide a new and improved electromagnetic shielding material having a low density shield with a broad frequency range.

It is another object of this invention to provide a new and improved electromagnetic composite shielding material.

My invention accomplishes the foregoing and other objects by providing a material and a method of manufacture by which magnetic shielding alloys and similar metals for magnetic cores and laminates are formed in thin sheets or sections by plating these metals onto both sides of, preferably, a high temperature resistant plastic sheet; and can comprise a plurality of layers.

The material for such electromagnetic shielding should be highly conductive to prevent the passage of electric fields and highly permeable to prevent the passage of magnetic fields. At radio frequencies, the magnetic field produces eddy currents in the conducting shield that largely prevents the flux from penetrating through the shield. In direct current fields, the high permeability shield short-circuits the flux lines. At high frequencies the high permeability, require less skin depth than non-magnetic materials. However, the effectiveness of a magnetic shield is normally directly proportional to the thickness of the shield, since the reluctance that the shield offers to magnetic fiux is inversely proportional to its thickness.

The degree of shielding achieved by a given total thickness of material can be increased, as in my invention, by dividing the thickness of magnetic shielding material into two separate sheets of joined, aligned, parallel shields. This creates a highly effective shielding material in a compact space that is made possible by using a layer of medium-permeability material covered by an outer layer of high-permeability material. The medium permeability laminate acts as a buffer, and sufficiently diverts the field to permit the lower-reluctance, high-permeability material to produce the required attenuation.

By using a high temperature plastic sheet, such as a polyimide sheet, as the substrate in my invention; my magnetic composite shielding material can be used in applications almost identical with that accomplished by the more expensive all-metal magnetic foils. For example, the composite shielding material in my invention can be soldered with an ordinary soldering iron and it can be used in applications where it will be exposed to temperatures as high as 600 Fahrenheit. Applications for the magnetic alloy coated plastic films of my invention are the same as those for rolled and annealed high permeability metal foils. These applications include magnetic shielding, magnetic cores (laminated or tape wound) and motor or transformer laminations.

With the deposition of a conductive metal coating such as, for example, copper, nickel or cobalt by electroless plating techniques on one or both sides of a plastic film; it is then possible to further deposit over these initial layers other magnetic alloy layers, by electroless or electroplating processes, that have thicknesses and properties better suited for the intended shielding application. For example, magnetic shielding alloys of iron and nickel can be deposited by electroplating onto the initial metallic layers on each side of the plastic film. The thickness or composition of these magnetic shielding overplates can be and are made different on each side of the sheet so that the composite material provides more efficient shielding than the thickness of two layers of the alloy. Thus each side of the plastic film has individual layers or several layers deposited one on the other with each layer having different magnetic and/or conductive properties.

As an example of this, consider a configuration consisting of a plastic film on which the initial conductive metal coating has been deposited by an electroless plating technique. Over this conductive metal coating, alternate layers of copper and nickel-iron alloy are deposited in a laminar configuration. Thus, the conductivity of the copper and the high permeability of the nickel-iron alloy would be effectively merged into a single sheet to provide a shield that, within its given thickness, combines the better magnetic and conductive properties of each metal. Another useful multilayer configuration of this invention would consist of layers of nickel with alternate layers of the nickel-iron alloy to provide electromagnetic shielding effectiveness exclusively within the range of the magnetic interference frequencies. In both configurations, several thin layers (i.e., each approximately 200g in. thick) could be deposited on one side of a coated plastic film. Alternatively, more numerous but thinner layers of each metal could be applied without building up the overall thickness of the composite. The optimum combination of layers depends upon the application being considered and the specific shielding requirements. The tradeoff will depend upon the advantages of building up in sequence, additional layers of metal over a single sheet of plastic as opposed to the advantages of using multiple layers of the plastic metal composite.

The only limitation to this increasing of the effectiveness of my invention against fields of different strengths and frequencies is the decrease in flexibility and pliantness and the increase in weight with each succeeding layer.

Other advantages of this construction are also apparent. The composite magnetic shielding material in my invention has a low weight because of the low density of the plastic sheet substrate and the small total thickness 'of the conductive and magnetic layers on the sheet. Be-

cause the cross section of the composite is mostly in the plastic sheet, the composite shield material has the ductility of the plastic in considering formability of the shield. This plastic pliability is not limited by metallic work hardening as ordinary metal shields are, since no metal working is necessary. The complete elasticity of the plastic substrate permits the material to be reused and reformed numerous times. While the coatings may crack locally under the repeated bending, the Shielding ability is not seriously impaired and the structural integrity is maintained. The composite shield can be cut to desired shapes with such simple tools as paper shears, scissors or razor blades. The presence of the plastic film provides thermal insulation to the composite shield and electrical separation to the metal layers on each side to permit their acting as individual shielding layers.

It is within the teachings of this invention that the composite materials prepared as directed herein could consist entirely of nonferreous metal coatings over the plastic film. For example, after an initial conductive coating of the electroless copper has been deposited on the plastic film, the composite could be built up entirely with copper that is electroplated over this initial layer. This type of copper coating would have a significant shielding utility in applications where frequencies of 20 kc. or more are to be encountered. Other applications for the copper coated plastic films include printed circuits, memory planes and capacitor manufacture. These utilize only a simple copper coated plastic film and achieve significant property improvements and other advantages over copper clad plastics prepared conventionally by laminating a sheet of copper to one of plastic with the use of an intermediate layer of adhesive. In my invention, no separate adhesive layer is required and adhesion achieved between the copper and plastic film is greatly improved. When such material is used for the manufacture of printed circuits or the fabrication of memory planes, a pattern of etchant resistant materials is generally deposited over areas where it is desired to retain the copper coating. All other areas not similarily protected with the etch resists are removed chemically thereby providing the alternate paths of conductivity and insulation. The techniques for preparing such resistant patterns are well known to those skilled in the art. The use of copper clad plastic films prepared in accordance with this invention, however, possess significant improvement over conventional laminates because small copper thicknesses can be deposited in this process which permits precise etching of very fine patterns. Also, the lack of an intermediate adhesive during the pattern etching step provides freedom from such problem areas as undercutting and bleeding through the adhesive. Finally, the elimination of the adhesive interface between the copper and plastic eliminates the problem of swimming which describes the movement of the copper layer with respect to the plastic when the two are heated above the melting point of the adhesive layer. This condition may occur, for example, during the soldering operation. It should also be noted that in the manufacture of printed circuits by the process described herein, nickel is sometimes preferred as the coating metal instead of copper, particularly when it is intended that wire connections to the circuit be made by welding rather than by soldering. 'For this purpose, only nickel would be electroplated over the initial conductive coating, to the desired thickness for the printed circuit design.

While the composite shielding material of this invention is prepared by using known available plastic sheets as the substrate material, it is uniquely preferable that the sheet be of a high temperature plastic. The following detailed description is concerned with a polyimide sheet since its use provides the composite with a high temperature resistance. The only processing step that might vary it a film other than polyimide is used would be the etch procedure prior to the electroless film deposition. Prescribed etching procedures for other plastic films are known to those skilled in the art and are also available as packaged electroless processes (including etchants) from several companies. An example of a particular polyimide that can be used is Kapton, that is made by Du Pont.

The polyimide sheet is resistant to degradation by most acids and is quite susceptible to attack by strong alkaline solutions, particularly at temperatures above Farhenheit. To achieve good adhesion by the metallic coatings, it is first necessary to etch the film surface. To preserve the strength and ductility of thin plastic films, this attack should be limited to the outermost portions of the film surface. There has been found to be a narrow range of etch solution concentrations, temperatures and times, within which good etching is accomplished. Both potassium hydroxide (KOH) and sodium hydroxide '(NaOH) with water can be used as the etching reagent.

If KOH is used, then the recommended concentration should be between 20 and 30 weight percent, with 25 percent preferred. The temperature of this solution should range between 60 C. and 80 C. with 70 C. preferred. Depending upon the agitation of the solution, the etch time could range from 10 seconds to 120 seconds. A period of 45 seconds has been found most suitable under many of the above variations. This etch treatment produces a gelatinous reaction layer that adheres tothe film surface and must be thoroughly rinsed off before proceeding. The sodium hydroxide reagent does not produce the same thick reaction layer and is generally to be preferred to KOH. The etch cycle recommended for NaOH should be 25 to 30 percent with 27 /2 w/o as the preferred concentration. If the etching of the polyimide is attempted at lower temperatures, lower solution concentrations or using shorter immersion times, then underetching results. This shows up as a subsequent lack of desired film adhesion or uneven surface roughness caused by the formation of loose powdery deposits. Overetching can result from excess solution temperature, solution concentration or overlong immersion time. This shows up as a loss of film strength, brittleness of the film and uneven surface roughness.

The second step in the procedure to prepare the plastic based tape is to effect the deposition of an electrically conductive coating on the plastics surface. After the etch treatment prescribed for the sheet, any of several known electroless plating baths may be used to deposit the initial metallic layer. Many proprietary packaged plating systems are commercially available and many such solutions are described in the literature. One skilled in the art, therefore, may select from metals such as Co, Ni, Cu as the base metallic layer since any of these metals can be suitably deposited on the plastic. Although the specific metal and process for its deposition in this step do not form an important part of this invention, the one found most suitable from the standpoint of adhesion and repeatability is electroless nickel and electroless copper. The film, after etching, is first sensitized by immersion into a solution containing 5 volume percent titanium chloride (or a solution of stannous chloride) and this is followed by a second immersion into a solution of 0.2 gm./liter chloroplatinic acid (H PtCl -H O) or 0.1 gm./liter gold chloride (HAuCl -H O). Good adhesion and uniformity have been obtained from several electroless solution compositions that follow the immersion sensitization steps, such as the following:

Electroless nickel bath Constituents: Grams/ liter Nickel chloride, NiCl 6H O '30 Sodium hypophosphite, NaH PO H O 10 Sodium citrate, Na C H O 5 /zH O Sodium acetate 5 Add NaOH to adjust pH to 5.

Temperature, 90 C.

Electroless copper bath Constituents Rochelle salt-170 g./l. Sodium hydroxideg./l. Copper sulfate, CuSO 5H 035 g./l. Sodium carbonate30 g./l. Versene-T (sodium salt of ethylenediaminetetraacetic acid)20 (ml./l.) Formaldehyde37% by weight To later carry sufficient plating current, a copper flash from a low acid content copper electroplating solution is put down over the electroless metal coating. This can be supplemented by building up the thickness of this layer by electroplating additional copper ontothe flash at conventional current densities. This copper layer acts as a shield to prevent the passage of electric fields and its high electrical conductivity enhances the flow of eddy currents 6 which prevent penetration of electromagnetic fluxes. It should be noted that other platable soft metals may be substituted in this step for the copper, such as nickel, zinc, tin, iron, cadmium, lead or alloys of two or more of the metals just listed.

At frequencies below approximately 14,000 cycles per second, it is known that non-magnetic materials, such as the soft copper layer adjacent to the plastic film surface, provides little shielding effectiveness. If shielding of frequencies below 14 kc. is required, then the ferromagnetic layer or layers must be adequate to provide the desired attenuation with a practical number of wrapped layers. To make a composite having several layers, the composite, for example, consists of several thin alternating layers of copper and iron-nickel alloys with each layer having a thickness of approximately or i -mil. Alternatively, for example, the layers can be nickel interspersed with those of the iron-nickel alloy and/or copper. The controlling factor of each layers thickness is the plating rate of the particular material or alloy being deposited and the length of time that it is in the bath. The manufacture of these multilayer configurations on a continuous basis has the plastic sheet carrying the conductive coating alternately immersed in a plating tank containing the plating solution for the one initial metal layer followed by a rinse water tank and a subsequent tank containing the second metal plating solution. Should a ternary metal structure be desired, such as layers of copper, nickel and nickel-iron alloy, it is necessary to follow the second plating tank with the third metal but to insert between these, another rinse tank. The build up of successive layers in this sequence is accomplished by the addition of plating tanks in the processing line or by reprocessing the film after initial pass as many times as required to build up the desired number of layers.

It has been found by experimentation that an iron plus nickel alloy suitable for shielding purposes can be plated onto the copper coated plastic from the following solutions:

Grams/ liter water Niso 50-25 F6(NH (S0 60-150 NH Cl 25-35 H20, 1 liter.

Another method for depositing an alloy of nickel plus iron for shielding purposes is described in the patent issued to Burns and Warner, No. 1,837,355.

To build up layers of copper, it has been found suitable to use a conventional acid copper bath of the following composition.

Gms./liter CuSO -5H O 200 n so 75 Where it is desired to deposit layers of nickel interspersed with the copper and nickel plus iron alloys, it has been found that a conventional Watts nickel bath performs suitably. A bath composition as follows was used in our experiments.

Gms./liter NiSO 300 NiCl 45 H BO 38 7 A sample of the composite shielding material of this invention was tested on a 60 cycle hysteresis loop tester, and as a shielding composite around a pickup coil within a fixed field at various frequencies. The results were as follows:

Foil thickness.Copper coated polyimide sheet 460 micro inches thick with 125 micro inches FeNi layer: BR, 10 mv.; BS, 14 mv.; Hr (scale units), 2.2 units.

When evaluated for shielding thickness the sample performed as follows:

Frequency, kc.: Attenuation, db

It may be observed that the shielding effectiveness of the copper layer is more significant at the higher frequency than at the lower one. It is also evident that with only 125 microinches of ferromagnetic alloy, there is substantial attenuation at the lower frequencies.

Having described my invention, I now claim:

1. A composite material for use as shielding material comprising,

a flexible plastic sheet of high temperature resistant plastic,

a first conductive metal layer being adhered to at least one side of said sheet by electroless deposition,

a second conductive layer being adhered to said first metal layer by electroplating to a thickness greater than said first conductive layer,

said side of said plastic sheet for receiving said first metal layer being etched to provide adhesion for said first metal layer,

said first metal layer having a magnetic permeability different from the magnetic permeability of said second layer,

and an additional layer of electromagnetic shielding metal being adhered to said second metal layer.

2. A composite material for use as shield material as claimed in claim 1 in which,

said first layer and said successive layers of shielding metal being. deposited by separate successive platmgs;

3. A composite material for use as shielding material as claimed in claim 2 in which,

adjoining ones of said several layers of shielding metal having different. magnetic permeabilities.

4. A composite material for use as shielding material as claimed in claim 3 in which,

said several separate layers have selectively different thicknesses.

5. A composite material for use as shielding material as claimed in claim 1 in which,

the other side of said plastic sheet is etched,

a third conductive metal layer being adhered to said other side of said plastic sheet by electroless deposition,

a fourth conductive metal layer being adhered to said third conductive metal layer by electroplating to a thickness greater than said third conductive metal layer,

said third conductive metal layer has a magnetic permeability different from the magnetic permeability of said fourth layer,

and an additional layer of electromagnetic shielding metal being adhered to said fourth metal layer.

6. A composite material for use as a shielding material comprising,

a flexible plastic sheet of high temperature resistant plastic,

a first conductive metal layer being adhered to at least one side of said plastic sheet by electroless deposition,

a second conductive layer being adhered to said first metal layer by electroplating to a thickness greater than said first conductive metal layer,

said side of said plastic sheet for receiving said first metal layer being etched to provide adhesion for said first metal layer,

and said second metal layer having a higher magnetic permeability than the magnetic permeability of said first layer.

7. A composite shielding material for shielding magnetic and electrical energy comprising,

a flexible thin sheet of high temperature resistant plastic,

each side of said sheet is etched,

conductive metal layers are deposited by an electroless process on said etched surfaces,

and high magnetic permeability metal layers are deposited on said conductive metal layers forming a high temperature resistant and flexible composite magnetic and electrical energy shielding material.

8. A composite shielding material for shielding magnetic and electrical energy as claimed in claim 7 in which,

said conductive metal layers have high magnetic permeabilities.

9. A composite shielding material for shielding magnetic and electrical energy as claimed in claim 8 in which,

adjacent ones of said conductive metal layers and said high magnetic permeability metal layers on each side said sheet have selective different magnetic permeabilities.

10. A composite shielding material for shielding magnetic and electrical energy as claimed in claim 8 in which,

said conductive metal layers have low magnetic permeabilities.

References Cited UNITED STATES PATENTS 2,354,556 7/1944 Stahl 17435 X 2,443,756 6/1948 Williams et al. 29-1966 X 2,877,286 3/1959 Vance et al 174-35 3,006,819 10/1961 Wilson et al. 20415 3,231,663 1/1966 Schwartz 17435 3,268,353 8/1966 Melillo 29196.6 X 3,272,911 9/1966 Rollins et al. 29-196.3 X 3,328,195 6/1967 May 29194 X 3,360,349 12/1967 Adomines 29-495 3,375,091 3/1968 Feldtkeller 29195 X L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R.