Parker Chomerics CHO-MASK II ST Conductive Foil Tape is a copper-foil-backed pressure-sensitive adhesive tape in which the acrylic adhesive is filled with a conductive particulate to provide electrical continuity through the bond line. The product is supplied on a silicone-coated release liner and is used for enclosure seam shielding, cable harness shielding, ground-plane repair, and aperture closure in electronic enclosures. The suffix ST denotes the smooth-texture copper foil surface specified in the manufacturer’s datasheet; this topography is selected to reduce sliver formation at die-cut edges and to provide more uniform solder-wetted termination during prototype and rework operations.

The conductive adhesive distinguishes CHO-MASK II ST from CHO-MASK I, which uses a nonconductive pressure-sensitive adhesive and requires direct foil-to-substrate contact for grounding. The ST surface condition also distinguishes it from standard CHO-MASK II, which is supplied with a standard bright copper surface. In manufacturing environments where conductive sliver contamination is a documented failure mode, such as power supply PCB assembly, the ST variant is specified because smoother foil edges are less prone to break away during cutting. Available configurations include slit rolls and custom die-cut shapes; published data for the adhesive chemistry and conductive filler are limited, because the adhesive is described by Parker Chomerics as a filled acrylic pressure-sensitive system rather than as an unfilled film adhesive.

Manufacturer-reported values place total tape thickness between 0.08 mm and 0.09 mm, copper foil thickness near 0.035 mm, surface resistivity below 0.05 Ω/sq when tested per ASTM D257, and through-adhesive resistance below 0.05 Ω. Peel adhesion to stainless steel is published in the range of 3.5 N/cm to 5.5 N/cm under ASTM D3330. Shielding effectiveness is reported above 70 dB from 30 MHz to 1 GHz; the manufacturer’s published data for higher frequencies and for assembled seam geometries is limited.

What Electrical and Shielding Performance Values Are Published for CHO-MASK II ST?

The tape is used where a conductive pressure-sensitive adhesive must eliminate the need for mechanical compression along an EMI seam. Its shielding effectiveness is not controlled by the adhesive alone; the copper foil carrier provides the primary shielding function, while the adhesive provides a low-impedance return path. Reported shielding effectiveness values are therefore sensitive to aperture size, seam overlap, and enclosure resonance effects. For seam applications, the installed shielding effectiveness should be verified at the enclosure level using IEEE Std 299-2006 or the legacy MIL-STD-285 method, because the material-level specification does not account for slot antennas formed by enclosure discontinuities.

Published data for frequencies above 10 GHz should be treated as application-dependent, because surface roughness, adhesive thickness, and seam geometry dominate the high-frequency impedance path. The conductive adhesive provides through-bond conductivity, but its resistance may increase after repeated thermal cycling if the acrylic binder loses contact with the foil or substrate. Acrylic pressure-sensitive adhesives also exhibit viscoelastic behavior; initial adhesion is not final adhesion, and full wet-out at room temperature can require 24 h to 72 h.

Die Cutting, Liner Control, and Adhesive Build-Up Limits

Rotary kiss cutting is the dominant mass-production process for this tape. The die strike is set to penetrate the copper and adhesive layers while leaving the release liner intact. If the liner is cut, automated peel-off can tear and produce adhesive transfer to the anvil. The ST smooth-texture surface reduces edge sliver formation relative to bright copper foils, but sliver control remains primarily a function of die sharpness, draw speed, and anvil condition. Steel-rule or matched-metal dies with polished cutting edges are preferred; rotary die stations typically run at controlled draw speeds below 10 m/min for clean edge quality, although published data for specific die configurations is limited.

Surface preparation consists of removing hydrocarbon films with a solvent wipe of 70% isopropanol and 30% deionized water, followed by drying for a minimum of 30 s. The tape is applied at room temperature and burnished with a 2 kg silicone roller; adhesion builds as the acrylic pressure-sensitive adhesive wets the substrate. Continuous bond line temperatures above 100 °C under shear load can produce creep, and the upper service temperature of 150 °C should not be interpreted as a continuous load-bearing rating. The product is not a structural fastener; mechanical seam compression must be maintained when the assembly is exposed to vibration or thermal cycling.

In automated placement cells, the release liner is peeled immediately before placement to limit contamination of the adhesive surface. Adhesive transfer to the placement nozzle is reduced by using low-tack nozzle tips and by avoiding dwell times longer than a few seconds after liner removal. Clean-room use may require a visible sliver inspection step, particularly when the tape is die-cut beside exposed high-voltage nodes. Conductive slivers generated from copper foil can bridge isolated pins; the ST smooth-texture grade is selected for reduced sliver risk, but process capability must be confirmed with inspection data from the specific die-cutting station.

When Bare Copper Foil Is Galvanically Incompatible with the Substrate

Bare copper foil is not galvanically compatible with aluminum, magnesium, or zinc-plated steel in moist environments. On an aluminum flange, the copper foil acts as a cathode and accelerates aluminum pitting at the tape edge. For outdoor enclosures with aluminum flanges, a tin-plated copper tape or a nonconductive corrosion-inhibiting barrier should be considered. Published data for CHO-MASK II ST on salt-spray-tested aluminum joints is limited; industry validation frequently follows ASTM B117 salt fog exposure, but performance depends on joint sealing, drain geometry, and enclosure gasket design.

Compared with aluminum foil tape, CHO-MASK II ST provides lower foil resistivity and solderable terminations but higher mass and greater galvanic incompatibility risk. Compared with tin-plated copper tape, it provides lower initial contact resistance but lower resistance to sulfidation and oxidation. Copper oxide films can raise contact impedance if the tape is not burnished or sealed. The conductive acrylic adhesive is vulnerable to ketones, esters, and chlorinated solvents; long-term immersion in such media is outside the product rating. Adhesive compatibility should be validated by the end user using ASTM D896 when solvent cleaning or chemical exposure is expected.

CHO-MASK II ST is not suitable for use as a primary safety grounding conductor under IEC 62368-1:2023 because the copper and adhesive cross-sectional area and mechanical robustness do not satisfy protective conductor requirements. It is also not rated for continuous immersion in fuel, hydraulic fluid, or strong polar solvents unless validated per ASTM D896. In medical or aerospace applications, lot-level traceability, outgassing, and ionic cleanliness must be verified against the relevant procurement specification, because the standard commercial datasheet does not include NASA ASTM E595 outgassing or USP Class VI data.