Parker Chomerics CHO-FOIL CCH is a copper-foil pressure-sensitive adhesive tape in which the pressure-sensitive layer is a conductive acrylic system rather than a dielectric transfer adhesive. The product uses an embossed copper carrier with a nominal foil thickness of 0.0356 mm and a total tape thickness of 0.1016 mm. The conductive acrylic adhesive contains conductive particulates dispersed through the adhesive matrix, providing a through-plane path between the foil and the bond line after the tape is burnished onto a substrate. The tape is supplied on a release liner in slit rolls; standard widths include 12.7 mm, 25.4 mm, 50.8 mm, and 101.6 mm, with standard roll length of 16.5 m. The continuous-use temperature range is specified as -40 °C to 125 °C, and surface resistivity is specified below 0.005 Ω/square when tested according to ASTM D257.

Typical assembly uses include seam shielding in electronic enclosures, grounding of printed circuit board support rails, cable connector wrap transitions, and repair of discontinuities in shielded housings. Because the foil is solid copper, shielding effectiveness at low frequency is governed primarily by aperture and seam geometry rather than the intrinsic conductivity of the tape. In a flat overlap seam, the tape can provide a continuous conductive cover over the gap; if the gap is wider than approximately 1/20 of the shortest wavelength of interest, aperture leakage will dominate and the tape cannot recover the full shielding floor. For a gap of 3 mm, this corresponds to a transition near 5 GHz; below this frequency, the aperture remains electrically small and the tape’s surface impedance is the dominant variable.

Shielding effectiveness published for CCH is commonly expressed as 75–85 dB over 30 MHz–1 GHz under IEEE 299 test conditions. At 10 GHz, published values are lower and more dependent on seam preparation; continuous metal foil tapes do not behave as perfect conductors at higher frequencies because surface roughness and adhesive thickness introduce impedance discontinuities. The embossed foil surface can also create small air pockets under the adhesive at the edges of the embossed pattern; these pockets are compressed during burnishing but may remain in recessed areas if the substrate is textured. Published data for this specific configuration above 10 GHz is limited; when system-level performance at 10 GHz is required, sample-level shielding effectiveness should be measured using a mode-stirred chamber or fixture per IEEE 299.

What Limits Shielding Effectiveness Below 100 MHz in Seam Applications?

Below 100 MHz, the tape’s primary contribution is to reduce the impedance of the seam by bridging the gap with a continuous conductor. The shielding floor in this regime is generally set by the bond-line resistance between the copper carrier and the enclosure substrate. If the substrate is aluminum or nickel-plated steel, a conductive acrylic adhesive of this class typically develops bond-line resistance below 0.01 Ω across a 25 mm lap after burnishing. However, chromate conversion coatings, zinc-plated surfaces, and conductive paints can introduce higher initial interface resistance. Production-line measurements on shielded enclosures show that bond-line resistance rises when the tape is not burnished with a firm roller; the conductive particles in the adhesive require compression to break through the thin oxide and establish particle-to-foil and particle-to-substrate contact. Without compression, the adhesive may behave as a partially insulating layer at low frequencies, reducing shielding performance by 10–30 dB in the 10–100 MHz range.

Because the tape is copper-based, storage and handling require exclusion of moisture and sulfur compounds. Rolls should be kept in the original polyethylene packaging at 15–30 °C and below 60% RH until use. Once the container is opened, partial rolls can develop edge oxidation if stored for extended periods. Oxidation at the exposed copper edge increases contact resistance and may require the first 100 mm of a slit roll to be discarded after long idle periods. Die-cut parts should not be transferred to cardboard or paper carriers because sulfur-bearing paper fibers can accelerate tarnishing.

When the Application Surface Contains Silicone Mold Release or Plasticizer Residue

Silicone mold release agents and phthalate plasticizers are common on injection-molded enclosure walls and gasket seating surfaces. These low-surface-energy contaminants inhibit adhesive wet-out and can reduce peel adhesion by more than 50% compared with clean stainless steel under ASTM D3330 conditions. Conductive acrylic adhesives are less tolerant of these residues than soft rubber adhesives because the high filler loading reduces the volume fraction of polymer available to absorb the contaminant. Before tape application on molded polycarbonate or ABS, a solvent wipe with 70% isopropyl alcohol or a plasma treatment is normally required. On surfaces that continuously exude plasticizer, adhesive bond strength may decline over time; a tin-plated foil tape or a form-in-place EMI gasket may be a more stable choice. Qualification should include 48 h of adhesion coupons at 60 °C with visual inspection for edge lift and four-wire bond resistance remeasurement.

Adhesion can also be degraded by anodized aluminum with thick oxide layers. The conductive acrylic adhesive does not dissolve the oxide; it requires mechanical burnishing to deform the foil and penetrate the thin oxide layer. For hard anodized surfaces, surface roughness above 1.6 µm Ra is necessary to obtain reproducible bond resistance. If the enclosure surface is masked before anodizing, a bare aluminum contact pad under the tape footprint improves electrical continuity but reduces corrosion protection.

Die-Cut Edge Quality, Liner Release, and Roll-Fed Processing Boundaries

The embossed copper foil and conductive acrylic construction can be die-cut into complex shapes on flat-bed or rotary lines. Kiss-cutting on a polyethylene terephthalate liner with a controlled liner release of 25–50 g/25 mm is typical. If liner release is too high, the tape can delaminate from the liner during high-speed unwind and pick-and-place; if too low, the die-cut part may carry the liner or tear during removal. The copper foil is thin but does not stretch significantly; rotary die-cutting with excessive penetration can fracture the foil and create burrs along the edge. These burrs can produce shorting paths or metal slivers in an enclosure. Slitting of master rolls should be performed with carbide rotary knives at controlled tension. Production logs show edge burr formation is minimized when the ratio of knife overlap to foil thickness is held within 0.5–1.0; published data for this specific configuration is limited, and line trials should be run on the final slit width.

Application by roll lamination uses a silicone rubber roller with durometer 60–70 Shore A. A single pass at 0.2–0.5 m/s with firm hand pressure is normally sufficient to wet the adhesive. On textured surfaces, a second pass perpendicular to the first is recommended to fill depressions. The tape should not be stretched during application because reducing the copper thickness locally increases sheet resistance and can open microcracks in the foil at the outside of a bend radius smaller than 5 mm.

Grounding Continuity Depends on Compression Through the Filled Adhesive Layer

The conductive acrylic adhesive in CCH is not a zero-ohm bond line. Its filler network is established only after compression. The through-plane resistance of the adhesive is sensitive to the application pressure because the conductive particles must be brought into contact. Hand burnishing with a plastic applicator on a flat surface generally provides sufficient compression; however, when the tape is assembled into a compression joint under a gasket or clamping rail, the joint pressure may dominate the final bond resistance. Bench measurements using a four-wire milliohm meter show that resistance between the foil face and a nickel-plated substrate can be reduced by an order of magnitude when applied pressure is increased from light finger pressure to 0.3 MPa roller compression. In a joint compressed after assembly, creep of the adhesive can alter bond resistance over thermal cycling, particularly if the tape is exposed to 85 °C and 85% RH. For that reason, grounding paths that rely on the adhesive alone should be verified after environmental test per IEC 60068-2-78 or the relevant product qualification sequence.

From a compliance standpoint, the product is generally declared by the manufacturer to meet Directive 2011/65/EU (RoHS) and Regulation 1907/2006/EC (REACH). For aerospace or defense platforms, the end user should verify whether the specific product configuration is qualified to the platform-specific specification; commercial datasheet compliance does not substitute for lot-level testing under MIL-STD-810 or RTCA/DO-160 if the tape is used in an electromagnetic environmental effects control plan.

Large die-cut CCH parts are often applied to enclosure flanges with a roller fixture that controls bond-line thickness and excludes trapped air. In vertical flanges, the tape may be specified with a narrower width to avoid overhang beyond the mating surface; overhang creates a copper sliver that can lift during handling and contact adjacent circuit nodes. When the flange width is less than 6 mm, the tape is typically slit to 4 mm and applied under magnification to maintain alignment. After assembly, the seam is checked with a milliohm probe between the enclosure halves; the acceptance threshold is often <0.05 Ω across the seam, though this value is assembly-specific and must be derived from the system’s grounding budget.