Parker Chomerics CHO-FOIL CAD is a copper-foil pressure-sensitive adhesive tape specified for electromagnetic interference shielding, grounding, and electrostatic charge drain in electronic enclosures, cable assemblies, and board-level shield covers. The laminate comprises a rolled copper carrier with a nominal thickness of 0.035 mm (0.0014 in) and a conductive acrylic adhesive layer with a nominal thickness of 0.025 mm (0.001 in), yielding a total thickness of 0.060 mm (0.0024 in). The adhesive is supplied with a differential release liner to support die-cutting and automated placement. Standard roll goods are available in widths from 6.35 mm (0.25 in) to 305 mm (12 in) and a standard roll length of 32.9 m (36 yd). The copper carrier surface resistivity is below 0.005 Ω/sq, and the conductive acrylic adhesive volume resistivity is below 0.05 Ω·cm when measured under ASTM D257. The product is supplied under a RoHS 2015/863 compliance declaration and a REACH SVHC statement.

Installed shielding effectiveness is determined by seam geometry and adhesive interface impedance, not solely by the copper carrier. A continuous 0.035 mm copper sheet at 1 GHz has a skin depth of approximately 2.1 µm, so the carrier represents about 16.7 skin depths and provides an absorption-loss contribution near 145 dB before apertures or joints are introduced. In a practical tape-applied enclosure seam, the measured effectiveness from 30 MHz to 1 GHz is generally 70 dB to 85 dB per IEEE 299. The reduction from the intrinsic material limit arises from unbonded edges, adhesive contact impedance, and substrate discontinuities. At lower frequencies, the copper carrier is electrically thin; at 1 MHz, the skin depth in copper is approximately 66 µm, so a 0.035 mm foil is about 0.53 skin depth and absorption loss is modest. This frequency-dependent behavior must be considered in low-frequency magnetic-field shielding applications.

Why does adhesive conductivity control seam impedance below 1 GHz?

At frequencies below roughly 100 MHz, seam impedance is strongly influenced by the through-thickness resistance of the adhesive layer and the interfacial resistance between copper and substrate. The conductive acrylic pressure-sensitive adhesive contains particulate conductive filler to reduce volume resistivity below 0.05 Ω·cm. Although this value is several orders of magnitude higher than the copper carrier conductivity of 58 MS/m—equivalent to a resistivity near 1.72 µΩ·cm—it is sufficient for shield continuity when the bond line is thin and uniformly compressed. Peel adhesion to stainless steel is typically not less than 2.2 N/cm (20 oz/in) under ASTM D3330. A poorly wetted bond line containing silicone mold-release residues can raise interfacial resistance by orders of magnitude, causing low-frequency shielding degradation. Production qualification should therefore include cross-tape resistance testing, not only peel adhesion.

Skin depth thresholds and the limits of foil shielding at low frequency

Skin-depth thresholds define the low-frequency boundary for copper-foil tape shielding. At 100 kHz, copper skin depth is approximately 206 µm; at 10 MHz, 21 µm; at 100 MHz, 6.6 µm; at 1 GHz, 2.1 µm. The 0.035 mm carrier exceeds one skin depth only above approximately 1.8 MHz. Below that frequency, the tape behaves as a thin conductive sheet with limited absorption; shielding depends on reflection loss and low contact impedance. For low-frequency magnetic fields, high-permeability steel or mu-metal is preferred, and published data for this specific configuration is limited. Above 100 MHz, the copper is multiple skin depths thick, and seam aperture control dominates. Tape-applied seams should be designed so that any unbonded gap is less than λ/20 at the highest operating frequency; at 1 GHz, this corresponds to a maximum gap near 15 mm, but practical seam performance requires much smaller discontinuities because of near-field coupling.

Die-cutting of CHO-FOIL CAD uses the differential release liner. The heavier liner remains through the cutting station and is removed after die-cutting; the lighter liner remains on the adhesive until final placement. Rotary die presses or matched-metal flat-bed presses are used, and tooling clearances should be qualified for the total 0.060 mm laminate. Dimensional tolerances for parts up to 100 mm are commonly ±0.25 mm; for parts from 100 mm to 300 mm, tolerances of ±0.50 mm are typical, depending on press registration. Production speeds between 10 m/min and 30 m/min require periodic cleaning of kiss-cut blades because conductive acrylic adhesive can transfer to tool steel. Tape rolls should be conditioned at 20 °C–25 °C for 4 h before die-cutting to reduce liner moisture and adhesive viscosity variation. Substrate surface energy should be above 38 dyn/cm by ASTM D2578 wetting inks; isopropyl alcohol or methyl ethyl ketone wiping is required for metallic surfaces with machining oils or fingerprint residue.

Typical applications include grounding of display panels to chassis frames, EMI sealing of connector cutouts in sheet-metal enclosures, and shielding of flat-flex cable terminations. In each case, the tape is specified as a non-structural, electrically conductive bridge; mechanical retention is provided by adjacent fasteners or enclosure compression. The choice of width is based on seam length and required overlap area. A minimum overlap of 3 mm to 5 mm on both sides of a seam is common for achieving low junction impedance. For long seams, multiple parallel strips are preferred over a single narrow strip because adhesive compliance and substrate flatness affect contact uniformity along the full length.

Compared with copper foil tapes having non-conductive acrylic adhesives, CHO-FOIL CAD provides a continuous conductive path through the adhesive rather than relying solely on metallic fasteners at discrete points. This reduces seam impedance in long joints but does not eliminate the need for a primary ground termination. Compared with aluminium-foil tapes, the copper carrier offers higher conductivity—58 MS/m versus 35 MS/m for aluminium—but introduces a larger galvanic potential when bonded directly to aluminium or magnesium. The copper-to-aluminium potential difference can range from 0.35 V to 0.50 V in humid electrolyte, exceeding the 0.25 V differential commonly used under MIL-STD-889 for severe environments. Compared with conductive fabric tapes, the foil is a continuous metal plane with lower sheet resistance but poor drape and flex endurance. Repeated bending beyond the annealed copper yield point of approximately 70 MPa creates creases that can fracture the carrier and increase resistance.

Automated placement of die-cut CAD parts on cast aluminium enclosures requires attention to galvanic isolation at the exposed edge. A polyimide isolation layer or zinc-nickel underplating is used in some production lines to separate the copper tape from the aluminium housing. In a humid environment, water ingress at the exposed edge forms an electrochemical cell in which the copper cathode accelerates aluminium oxidation. This failure mode is not observed with nickel-plated aluminium or chromate-sealed surfaces, provided the chromate layer is not damaged. For prototype assemblies, a clear acrylic conformal coating applied to the tape edge after placement reduces moisture wicking but does not substitute for a compliant surface finish.

Operational boundaries must be observed in production. The acrylic adhesive is not a structural bond and is not a solderable termination. Continuous service above 150 °C accelerates oxidative degradation of the acrylic polymer; soldering temperatures near 260 °C destroy the adhesive and liner. The tape should not be used for power-return paths. The adhesive resistivity is roughly 4 to 5 orders of magnitude higher than the copper carrier, so high-current ground paths require mechanical fasteners or solder joints. Copper is susceptible to oxidation in acidic or sulfur-bearing environments; if edge corrosion is a concern, exposed foil edges should be overcoated. In salt-fog testing per ASTM B117, direct copper-to-aluminium couples are not recommended unless sealed; copper-to-zinc-plated steel is less active but still requires compatibility verification. Storage in original packaging at 10 °C to 32 °C and below 60 % relative humidity preserves die-cutting registration and adhesive tack. For qualification, current lot certificates and manufacturer test reports should be consulted because public data for specific configuration is limited.