The 3M CN-3490 EMI shielding tape is constructed from a nickel–copper-plated woven ripstop fabric carrier coated with a conductive pressure-sensitive acrylic adhesive. The carrier is a textile rather than a solid metal foil, which determines the product’s mechanical and shielding behavior. Nominal total thickness is 0.12 mm, and the product is supplied as slit rolls with a release liner, commonly in widths from 25 mm to 100 mm depending on converter stock. Published datasheet values commonly report surface resistivity below 0.05 Ω/sq when measured according to ASTM D257, and plane-wave shielding effectiveness from 70 dB to 85 dB over 30 MHz to 1 GHz under ASTM D4935-18. The adhesive is acrylic and electrically conductive in the z-axis, but it is not a structural adhesive and does not replace mechanical fasteners. The tape is intended for enclosure seam shielding, cable shield grounding, and low-profile bonding of conductive fabric segments.

CN-3490 is rated for continuous service from −40 °C to 105 °C. The minimum practical application temperature is approximately 16 °C, below which the acrylic pressure-sensitive adhesive does not wet polycarbonate or powder-coated steel reliably. Storage should be maintained at 18 °C to 27 °C and 40 % to 50 % relative humidity; shelf life is approximately 12 months from manufacture. A qualification lot should be tested for peel adhesion to the actual substrate per ASTM D3330/3330M, because the stainless steel values used in datasheets do not predict adhesion to low-surface-energy plastics. The woven fabric is tear-resistant but can be cut intentionally with a rotary die; unintended nicks in the backing create exposed metal filaments and possible loose conductive particles.

What limits CN-3490 as a magnetic-field barrier below 10 MHz?

At frequencies below approximately 10 MHz, a thin conductive fabric tape cannot provide high magnetic shielding. The shield operates mainly by reflection of the electric field and plane-wave components, while absorption in a thin metal layer is weak. The skin depth in copper at 100 kHz is approximately 206 µm, but the plated metal thickness on a fabric carrier is in the micrometer range. Consequently, magnetic near-fields between 50 kHz and 1 MHz pass around or through the tape with little attenuation. Published CN-3490-specific magnetic-field shielding data is limited; qualification should be performed with loop antennas under CISPR 16-1-1 or MIL-STD-461 RE101 procedures rather than by extrapolating a far-field ASTM D4935 value.

The ASTM D4935-18 coaxial fixture uses a 133 mm annular sample and a 50 Ω measurement path. It does not capture enclosure seam leakage, overlap resistance, or the effect of adhesive voiding along a curved seam. Installed shielding may therefore be 10 dB to 30 dB below coupon-level measurements if the tape is not lapped and wetted continuously. This gap is known from seam attenuation tests; the exact reduction depends on aperture length, tape width, and substrate flatness.

Radio-frequency current on a shielding tape flows primarily on the outermost surface. At 1 GHz, the skin depth of copper is approximately 2.1 µm; a thin nickel–copper plating is therefore sufficient for high-frequency surface conduction. At 100 kHz, however, the skin depth is about 206 µm, which explains why the micrometer-range plating on a fabric carrier cannot support the eddy currents needed for magnetic shielding. The adhesive layer contributes a separate conduction path: z-axis particle loading gives low through-plane resistance, but in-plane resistance through the adhesive is much higher than through the plated fabric. Overlap joints therefore depend on adhesive wet-out and on the direct fiber-to-substrate contact promoted by roll pressure.

On a production line, CN-3490 is applied after the substrate is cleaned with 70 % isopropanol or the solvent system specified in the assembly procedure. A pH-neutral aqueous cleaner may leave alkaline residue that retards adhesion. The tape should be laminated with a pneumatic silicone rubber nip-roller at 3 N/mm² to 5 N/mm², and feed speed should be held between 0.05 m/s and 0.15 m/s to avoid entraining air. For enclosure flanges with a radius of curvature below 20 mm, a hand roller with a narrow contact edge is used after the main laydown. The release liner is removed at a low peel angle to avoid lifting the adhesive from the backing. Surface energy of the substrate should exceed 38 mN/m by ASTM D2578; if not, corona or plasma treatment is required before lamination.

Contact resistance in a lapped seam is controlled by the conductive adhesive and the physical fiber-to-fiber contact at the tape surface. A single-layer flat sample may show low surface resistivity, but a two-layer overlap on a painted or anodized substrate can show higher resistance because the adhesive cannot displace thick oxide layers. For aluminum substrates, chromate conversion coating or an appropriate conductive primer is usually required. For steel with zinc plating, the surface must be cleaned with a non-alkaline solvent and dried before lamination. The acrylic adhesive develops strength over time; measurements made immediately after lamination may be below the 24 h value. Peel and contact resistance should be re-measured after a dwell period of 24 h at 23 °C and 50 % RH.

When foil-backed shielding tapes crack and CN-3490 remains viable

Copper foil tape may be selected when enclosure seams are flat and the objective is the highest possible plane-wave attenuation. However, foil work-hardens when it is bent around radii below approximately 3 mm or when it is applied over repeatedly flexing cable shields. Those conditions produce microcracks that increase DC resistance and degrade shielding after thermal cycling. CN-3490 distributes bending strain through its woven carrier and is therefore less prone to conductor fracture. The outer nickel surface also maintains lower contact resistance in indoor environments than bare copper foil that has not been passivated. In contrast, unsupported conductive transfer tape can be thinner and die-cut into narrower features, but it tears during liner removal and does not bridge gaps. CN-3490 bridges a 0.5 mm to 1.0 mm seam gap under light pressure; bridged areas should not be used across moving joints because the adhesive may creep and the backing may flap.

Another distinction involves corrosion compatibility. Copper foil tape can develop cuprous and cupric oxide films at elevated humidity; the nickel–copper plating on CN-3490 is more stable in indoor electronics. However, if CN-3490 is placed in direct contact with magnesium, zinc-plated steel, or bare aluminum in a marine environment, galvanic corrosion can occur. Qualification in such conditions should follow IEC 60068-2-52 or ASTM B117, with contact resistance measured before and after exposure. Published CN-3490-specific salt-spray performance is limited, so the product should not be approved for outdoor sealed enclosures without environmental testing.

The tape does not provide compressible environmental sealing. A fabric-over-foam gasket can accommodate dynamic gaps between 0.5 mm and 2.0 mm and maintain contact force after vibration; CN-3490 is a permanent adhesive bond. Rework is difficult and may leave conductive residue on the housing. If the joint is opened repeatedly, spring-finger contacts or conductive elastomers are more suitable. CN-3490 is therefore selected for permanent seams or for applications where low installed height is more important than repeated openability.

In high-volume conversion, the roll must be slit with minimal edge damage. During kiss-cutting on a flatbed plotter, liner penetration of 30 µm to 50 µm is common; deeper penetration cuts the woven backing and produces loose metal-coated fibers. Loose fibers can move into PCBA clearances and create short-circuit paths. The slitting room should use static-dissipative work surfaces and filtered air extraction. If the tape is wound under high tension, the roll telescopes during storage and loses edge alignment. These are known conversion issues for conductive fabric tapes and are not unique to CN-3490.

Comparative selection attributes and standards referenced during qualification

The following methods are used during incoming inspection or substitution evaluations. The table is a compliance matrix, not a performance claim.

The acrylic adhesive is sensitive to ketone-based solvents, toluene, and chlorinated cleaners. These chemicals can soften the adhesive and reduce peel strength before any visual degradation occurs. Avoid applying the tape over surfaces coated with amine-containing curing agents because amine residues can inhibit the adhesive. The maximum service temperature is 105 °C, so CN-3490 is not rated for reflow soldering or powder-coat oven cycles. When the tape must survive a powder-coat oven at 180 °C, a different shielding material with a silicone adhesive or a mechanically fastened gasket is required.

A representative production case is grounding a metallized fabric cable shield to a connector backshell. The tape is cut to length and wrapped around the assembly with a 10 mm overlap. The surface must be free of mold-release agents; if mold release is present, the first step is a 70 % isopropanol wipe followed by a 30 s flash. After wrapping, a 2 bar silicone roller compresses the tape, and a stainless-steel band clamp provides mechanical strain relief. The adhesive by itself will not maintain a stable bond under repeated cable flexure, so the clamp or ferrule is required. First-article qualification should include contact resistance measured through the overlapped joint after 500 flex cycles and shielding effectiveness measured with the connector fully assembled. Published data for this specific configuration is limited, so production approval should rely on measured values from the actual harness.