CROSS REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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BACKGROUND OF THE INVENTION

The invention relates to yarns and multi-fibers comprised of metallized stainless steel or low carbon (mild) steel monofilaments suitable for use as RFI/EMI shielding materials especially for military and aerospace purposes.

As the complexity of electronic systems in aerospace applications has grown, designers of the cables that interconnect system components have been obliged to meet increasingly stringent requirements for RFI/EMI protection and, because aircraft performance and operating cost are directly related to weight, demand for lighter weight cable constructions. For cable assemblies with internal shielding, RFI/EMI protection takes the form of a braided mesh that is applied over the core in the cable manufacturing process (FIG. 1). For cable assemblies without internal shielding, RFI/EMI protection takes the form of a separately produced braided wire mesh sleeve (tubing) that is fitted over a cable or cable bundle (FIG. 2) and grounded by clamping the sleeve ends to the terminating connectors. As such, the sleeve also provides the cable assembly with protection against the aggressive mechanical, thermal, and chemical conditions characteristic of the military/aerospace application environment.

Braided RFI/EMI shielding functions by converting incident electromagnetic energy (which may originate within as well as exterior to the cable) into a current that is carried to ground by the shield; in this regard, the shield acts as both a “transducer” and a “conductor”. Shielding effectiveness (SE), a measure of the degree to which incident EMI energy is attenuated by the braided shield, is the resultant of several factors: the resistivity of the shield material; the number of shield layers; and the “optical coverage” or degree to which the braided shield covers the surface compared to an idealized solid cylindrical shield. RFI/EMI energy levels are typically low but braided shielding incorporated into aircraft grounding schemes must also provide enough conductive capacity to withstand lightning strike or electrical fault energies.

Conventional braided shielding is fabricated into flexible tubing from #34 or #36 AWG solid copper wires (0.160 mm or 0.125 mm, 6.3 or 5.0 mils diameter respectively) that are plated with nickel, tin, or silver to provide corrosion protection for the underlying copper. Due to its excellent conductivity, adequate strength, and low cost, plated copper wire has been the “material of choice” in braided tubing designs for many years. However, in a growing number of military/aerospace applications, copper wire-based materials cannot satisfy the more stringent shielding requirements and weight restrictions that are specified in new cable designs. As RFI/EMI frequency increases, its correspondingly shorter wavelengths can leak through the gaps or “windows” in the mesh where the relatively thick ribbons of stiff copper wire intersect. When this occurs, Electromagnetic Compliance (EMC) may necessitate the use of more than one layer of shielding with its attendant weight and stiffness penalties. Moreover, when RFI/EMI induces a current in the shield, the “skin effect” phenomenon confines the current flow to the circumferential surface of the copper wire, which has the effect of rendering the wire core “dead weight”. Braided wire mesh fabricated from smaller diameter wires would presumably improve shielding effectiveness as well as reduce cable weight, but copper wires finer than #36 AWG are too prone to breakage in the braiding process.

DISCUSSION OF RELATED ART

One solution to these problems is proposed in U.S. Pat. No. 5,103,067 issued to Aldissi on Jan. 19, 1993 and related U.S. Pat. Nos. 5,180,884 and 5,473,113 wherein the performance and weight-reduction advantages of metal-coated polymeric fibers for internal cable shielding is described. In this approach, yarns of micron-diameter aramid monofilaments having high strength-to-weight ratio and flexibility are metallized with one or more thin layers of conductive material, then braided or knitted into a tight mesh. The high tensile strength of the aramid monofilaments provides mechanical strength in a small diameter form factor that contributes to improved shielding performance by reducing the size of the gaps in the mesh. The flexibility that the polymeric yarns impart to this braided construction also provides greater conformity to the surface of the underlying insulation, thereby expanding the frequency range over which the shielding is effective. Finally, even with its conductive coating, metallized aramid yarn weighs much less than its conventional copper wire counterpart.

Despite these attributes, the metallized aramid yarn noted above does not completely address the needs of cable designers. The chief drawback is its cost—more than 20 times that of conventional plated wire—which effectively limits its use to selected aerospace applications such as satellites and military aircraft where the performance penalty related to excess weight is very high. The high cost of metallized aramid yarn is attributable to its necessary reliance on an electroless deposition process, which is not only much slower but involves much more expensive chemistry than electrolytic methods, to form the first conductive layer on the polymeric monofilaments. Furthermore, although its performance is superior to conventional materials, metallized aramid yarn is not optimal with respect to either coverage or conductivity, two factors that affect shielding effectiveness. Smaller diameter aramid monofilaments would permit braided shields to be made with smaller air gaps, but constructions of metallized aramid yarn are limited to 15 micron diameter monofilaments due to the fact that smaller diameter filaments are embrittled by the amount of plated metal needed to provide sufficiently low resistance for shielding applications, roughly 1 ohm/ft. Consequently, from a conductivity standpoint, yarns comprised of metallized aramid monofilaments have much higher resistance per unit weight properties, in the range of 18 ohms/mg, compared to about 13 ohms/mg for #36 wire or 5 ohms/mg for #34 wire. Finally, the properties of the polymeric monofilaments themselves limit the temperature and chemical exposure that metallized polymeric yarns can reliably withstand.

Accordingly, it would be desirable to employ a simpler, lower cost metallization process to provide a shielding material consisting of a conductive multi-filament yarn having a small form factor (diameter), high conductivity to weight aspect, excellent flexibility, and the ability to withstand a wide range of thermal and chemical exposure. One approach to such an objective is found in U.S. Pat. No. 7,291,391, issued to Watson et al. on Nov. 6, 2007, wherein the use of an electrolytic metallization process to reduce the resistivity of yarns comprised of micron-diameter stainless steel monofilaments is proposed for use as an electrical resistance heating element for heated car seats.

However, the metallized stainless steel yarn proposed in Watson et al. is not suitable for use in cable shielding applications. Its principal deficiency is that the metallized coating is not uniformly applied around the circumference of each monofilament in the yarn bundle. Indeed, as illustrated in the accompanying figures, most of the monofilaments in the interior of the yarn bundle are not metallized at all. Although details about the metallizing process are not presented in the patent, those skilled in the art will recognize that this result is consistent with the use of conventional wire plating methods wherein single strands of wire are pulled under high tension through long plating baths. When such a process is used to plate a multi-filament yarn bundle, particularly one that is tightly twisted, the axial tension in the yarn compresses the bundle and prevents the plating solution from properly wetting the inner filaments. From a functional standpoint, therefore, the un-plated filaments would represent “dead weight” in an airborne application. Moreover, by concentrating the plating activity at the periphery of the yarn bundle, a high tension process makes it very likely that most if not all of the outer filaments will plate together, thus stiffening the yarn bundle and making it behave more like a solid strand of wire in a braided matrix. Finally, since it would be difficult to reliably remove chemicals trapped within the tightly bound yarn bundle, the use of a high tension plating method presents the risk that long-term corrosion sites will be incorporated into the material.

Military/aerospace cable applications also impose rigorous environmental tests (temperature cycling, shock/vibration, etc.) that stress the adhesion of plated metal coatings to their substrate materials (e.g., silver, or tin coatings plated on copper wire). Although it is well-known that stainless steel has an oxidized or “passivated” surface that must be chemically altered or “activated” in order to achieve good adhesion to plated coatings, Watson et al does not prescribe this pre-conditioning step or, for that matter, even a rudimentary cleaning process to remove any residual lubricants or dirt from the wire drawing process itself.

These deficiencies are overcome in the metallized yarn described in U.S. Pat. No. 7,923,390 (390) issued on Apr. 12, 2011 and U.S. Pat. No. 10,314,215 (215) issued on Jun. 4, 2019 to the applicant herein, Burke et al, wherein a low tension/low twist electrolytic plating process provides complete and substantially uniform coatings on each of the stainless steel monofilaments. In present practice the metallized yarn described in these two patents has evolved into a 14 filament construction, each filament comprised of a 35 μm stainless steel core plated with a two-part (strike plus acid) copper layer nominally 4.8 μm thick. The copper-plated core is then over-plated with one or more layers of other metals (e.g. silver, nickel, tin) in various thicknesses to address a variety of application requirements (corrosion resistance, solderability, etc.). As illustrated in these patents, all plating processes are conducted sequentially in a continuous roll-to-roll production line.

However, this single step “plating only” method of producing lightweight RFI/EMI shielding materials is vulnerable to random microscopic plating anomalies that can become sites for corrosion. The most insidious of these anomalies results in adhesion failure between the first metal layer and the stainless steel core of a monofilament, an “early-stage” defect that is impossible to detect in production or even visualize without the aid of a scanning electron microscope (SEM). A SEM photomicrograph of such a defect is shown inFIG. 3. The probable cause is intermittent failure of the surface conditioning chemistry to fully penetrate the yarn bundle and completely “activate” the surfaces of the 316L stainless steel filaments, thereby leaving passivated chromium oxide sites that inhibit robust copper adhesion. Process adjustments have minimized the incidence of these defects but not achieved the goal of eliminating them entirely. Plating anomalies can also occur in the “later stage” plating processes where metals known for corrosion resistance such as nickel, silver, or tin are coated over the copper layers. These anomalies can take the form of inadequate adhesion, leading to poor abrasion resistance and generation of microscopic particles classified as FOD (Foreign Objects and Debris) in the subsequent braiding process. Micropores in insufficiently plated outer layers can also compromise the corrosion resistance of the metallized yarn in marine environments by providing salt water access to the underlying copper layers. “Later stage” defects also include broken filaments or plated-together filaments that expose underlying copper when they are pulled apart in the braiding process.

The use of copper-clad (as opposed to copper-plated) stainless steel filaments has been proposed to eliminate “early stage” defects. In this method, a copper clad stainless steel “source wire” is formed by wrapping copper foil of a pre-determined thickness around a relatively large diameter stainless steel wire and welding the seam prior to feeding the composite into an initial wire drawing facility. The reduced source wire is then transferred to another facility (usually a separate business entity) that specializes in drawing of fine and ultra-fine wire. The copper clad filaments produced in this operation are then transferred to another facility (likewise usually a separate business entity) where they are assembled into yarn bundles for the plating process. Although this three-step (“draw-draw-plate”) method eliminates the “early stage” defects mentioned above, it does not address “later stage” plating process defects such as micropores and broken or plated-together filaments.

The method proposed in the allowed U.S. patent application Ser. No. 16/995,406 overcomes these deficiencies by changing the foregoing sequence to one in which the plating process is inserted at an intermediate point prior to, rather than after, the final drawing process. As a result, the plating of various protective layers over the clad copper stainless steel (alternatively low carbon steel) source wire is carried out while the wire is still in a relatively large diameter, single-wire format, not a yarn bundle of delicate, micron-diameter filaments. The plated wire is then transferred to the drawing facility where it is reduced to ultra-fine filament form. Although somewhat more expensive, this three-step “draw-plate-draw” sequence provides several improvements over the foregoing “plate-only” or “draw-plate” methods: by removing ultra-fine filaments from the plating process entirely, it eliminates “later stage” plating defects; its use of wire drawing enhances plated layer adhesion and integrity; and, unexpectedly, it proves to be more efficient in terms of plating capacity utilization.

However, the process proposed in application Ser. No. 16/995,406 does not completely address the evolving demands placed on RFI/EMI shielding materials. Having enjoyed the benefit of substantial (up to 80%) weight savings that the ‘390/’215 materials provide, some aerospace engineers are now looking for more conductive materials to provide greater lightning strike protection even if it means adding back weight in the form of thicker copper. Conversely, satellite designers are looking for even lighter weight solutions for their microwave applications. As a derivative of utility wire production, copper clad “source wire” is not well-suited to these requirements because industry standards specify the thickness of the copper cladding as a percentage (10%) of the radius of the composite wire. Thus, applicant herein was motivated to devise a cost-effective metallization process that retains the quality improvements of the Ser. No. 16/995,406 application but provides greater flexibility to meet growing demand for customized shielding designs.

BRIEF SUMMARY OF THE INVENTION

The invention provides an RFI/EMI shielding material in the form of a yarn or multi-fiber comprised of a plurality of micron diameter stainless steel monofilaments which have been separately rendered more conductive in the following two-step “plate-draw” process. A stainless steel “source wire” with a diameter larger than 100 μm is fed into a continuous metallization sequence in which the wire surface is first conditioned by cleaning and etching, then electroplated with various metals or metal alloys to prescribed thicknesses. In this sequence the first electroplated layer, which is completely and substantially uniformly disposed over the stainless steel core, is deposited from an alkaline copper solution or, alternatively a Woods nickel solution, both of which promote adhesion to the underlying wire surface. A second electroplated layer, likewise completely and substantially uniformly disposed over the first layer, is next deposited from an acid copper solution. Alternatively, this second layer can be formed from aqueous solutions of other metals or metal alloys known for their conductivity, such as silver or brass. To the extent that these alternative metals or metal alloys also provide good resistance to corrosion and/or compatibility with solder, this layer would be the last in the sequence. In a similar manner, when the second layer is copper, a third layer of metal or metal alloy such as nickel may be electroplated over the second layer to protect the underlying copper from corrosive agents. When the third layer in this multilayer construction consists of nickel, a fourth layer of silver or tin/tin alloy may be added to enhance solderability or serve as a lubricant in the drawing process. The plated composite wire is then transferred to a drawing facility where the diameter of the wire is reduced to an ultra-fine filament size in the range of about 40-80 μm. The filaments are then twisted or bunched together to form a yarn suitable for braiding or weaving into various RFI/EMI shielding articles.

Alternatively, the conductive monofilaments of this invention can be fabricated from low carbon (<0.25%) steel wire that is likewise plated and drawn to the same ultra-fine diameters. Although suitable primarily for applications with reduced tensile strength requirements, these lower cost alternative shielding materials provide comparable improvements in resistance/weight properties as well as improved shielding efficiency at low frequency.

Thus, this two-step process enables the cost-effective production of a wide range of customized RFI/EMI shielding materials that incorporate the advantages of the prior “three-step” Ser. No. 16/995,406 application without its limitations. This more flexible method is further distinguished by the fact that it facilitates the production of certain lightweight configurations that cannot be made by the prior method.

