Parker Chomerics CHO-MASK II HT Conductive Foil Tape is specified as a seam-shielding and grounding material where a conformable metallic foil carrier must be combined with a conductive pressure-sensitive adhesive that survives elevated-temperature assembly or service. The product is supplied in roll and slit-roll formats on a release liner and is subsequently converted into hand-cut strips, die-cut pads, or machine-applied seam closures. The model designation separates the HT adhesive system from the standard CHO-MASK II grade; the metal carrier itself is not the limiting element under most enclosure conditions. Rather, the adhesive layer controls the upper service temperature, the surface preparation requirements, and the long-term seam impedance. Because the tape is used to close apertures in shielding enclosures, grounding paths, and cable transitions, the selection process typically begins with the adhesive temperature requirement rather than with the carrier thickness.

What limits the upper service temperature of the adhesive system?

For the HT grade, the manufacturer’s published upper continuous service temperature is commonly listed at 180 °C, with short-term excursions above that value requiring mechanical fastening. The standard grade is generally rated lower, often near 150 °C, because the adhesive chemistry differs. These values are referenced to the pressure-sensitive adhesive and not to the metal foil. The carrier in typical configurations is a rolled copper foil specified by basis weight, such as 1 oz/ft² or 2 oz/ft², and the actual thickness follows from the weight designation and rolling tolerance. Peel strength after heat aging is characterised by ASTM D3330/D3330M. In production acceptance testing, peel specimens are aged in a forced-air oven and tested at a 180° peel angle; a drop after 168 h at the rated temperature indicates adhesive embrittlement or loss of tackifier. That data set is more directly useful than thermogravimetric analysis alone because it captures the effect of crosslink density on joint strength.

High-temperature acrylic conductive adhesives are formulated with fillers that provide z-axis electrical conduction while retaining sufficient cohesive strength. The filled system has a different viscoelastic response than an unfilled acrylic. At ambient application temperatures, the adhesive wets out under roller pressure; at elevated service temperatures, the same adhesive can crosslink or lose tackifier and shift toward elastic behaviour. For this reason, the tape is not specified for continuous service above the supplier’s upper limit unless the seam is mechanically clamped along its full length.

In shielding applications, the tape is applied to the perimeter of removable covers, enclosure seams, cable terminations, and board-level shield edges. The performance objective is not bulk metal resistivity alone but the seam transfer impedance. Shielding effectiveness is commonly evaluated by IEEE 299 or by the legacy MIL-STD-285 method in a shielded room. The measurement compares insertion loss with and without the taped seam. Backing surface resistance may be measured with ASTM D257, but the seam impedance is dominated by the adhesive interface and substrate contact. The conductive adhesive contains filler particles that establish a z-axis path through the bond line. That path depends on bond-line thickness and on wet-out achieved during application. A hand roller or pneumatic lamination roller is used to force the adhesive into surface asperities. Without positive pressure, air entrapment and asperity bridging create local sites of high contact resistance, particularly on machined aluminium or zinc-plated steel.

For low-frequency magnetic-field shielding, a copper foil tape provides only limited absorption. The material is primarily an electric-field shield and a conductive seam bridge. If magnetic-field shielding below 100 kHz is required, a high-permeability carrier such as MuMetal or a steel-backed product may be specified separately. A conductive adhesive seam with low DC resistance does not automatically imply high shielding effectiveness at low frequencies; the shield’s thickness and permeability are limiting.

Surface Preparation, Application Pressure, and Contact Resistance

Bond performance is inseparable from substrate preparation. The substrate is cleaned with a solvent wipe compatible with the surface metal; isopropanol or a 50:50 isopropanol/water mixture is common for aluminium and steel. Silicone-containing cleaners are avoided because they can deposit a low-surface-energy film that reduces wet-out. Heavy oxidation on aluminium is removed with a nonwoven abrasive pad to expose clean metal. The time between surface preparation and tape application is controlled to avoid re-oxidation. Application temperature is usually maintained above 16 °C to prevent stiffening of the adhesive and below the dew-point margin to avoid condensation at the bond line.

Surface energy is another process variable. The release liner and adhesive are controlled by the tape manufacturer, but the panel surface may be altered by cutting oils, rust inhibitors, or passivation treatments. A dyne test fluid of 38 mN/m or higher is used on production lines as a quick check before tape application; for aluminium with a thin conversion coating, the tape may wet out even when the surface fails a water-break test. However, the water-break test is not quantitative and is used only as a go/no-go indicator. In high-humidity assembly areas above 60% RH, condensation can form on cool metal panels and create an interfacial layer that prevents the conductive adhesive from contacting the substrate. Local dehumidification or panel pre-warming by 3 °C to 5 °C above dew point is often used.

Peel adhesion is characterised by ASTM D3330/D3330M or, for electrical tapes, ASTM D1000. Conductive adhesives are filled systems, so peel and tack are generally lower than an unfilled pressure-sensitive adhesive of the same base chemistry. Contact resistance is evaluated using a four-wire Kelvin measurement across a lapped joint. Acceptance limits are set by the end user rather than by the tape manufacturer because joint resistance depends on substrate type, pressure, and clamping geometry. Field data from high-volume enclosure lines indicate that contact-resistance variation is more often caused by irregular tape tension during manual application than by variation in the tape roll itself.

Contact-resistance measurements are sensitive to fixture geometry. A single clamp may press the tape against the panel at the measurement location and produce a misleadingly low reading; the seam resistance should be evaluated with a four-wire resistor network over the entire joint length, with current injection at one end and voltage sense points at defined intervals. For a seam length of 300 mm, resistance values are often expressed in milliohms per joint rather than per unit length because the current path is not uniform. The tape does not replace the need for multiple mechanical fasteners on shield lids if the lid is load-bearing.

During thermal cycling, differential expansion between the copper carrier and the enclosure substrate imposes shear on the adhesive bond line. The acrylic adhesive dissipates some of that strain through viscoelastic flow, but repeated high-temperature dwells can shift the adhesive toward elastic response. This raises the probability of microvoid coalescence at the bond line and a detectable increase in joint resistance. Thermal cycling tests are often performed according to IEC 60068-2-14 between -40 °C and 85 °C, with the upper soak raised to 125 °C or to the actual bake-process peak for HT-grade validation. Four-wire resistance measurements before and after cycling, rather than visual inspection, are required to detect adhesive relaxation because the foil surface may appear intact while the z-axis path degrades.

When galvanic couples form between copper-foil tape and aluminium or magnesium substrates

The copper carrier is cathodic relative to aluminium and magnesium in most electrolyte environments. When the tape edge forms a galvanic couple with an anodic enclosure base, the aluminium or magnesium can corrode preferentially, and the corrosion product is non-conductive. This failure mode is not unique to CHO-MASK II HT, but it is a boundary condition that must be reviewed before specifying a copper-foil tape on lightweight enclosures. Salt-spray exposure per ASTM B117 for 96 h is a common screening test; edge sealant or a corrosion-resistant topcoat is applied where condensing humidity or outdoor exposure is expected. In indoor telecom cabinets with controlled humidity, the risk is reduced, but a conformal coating over the tape edge may still be required for designs submitted to IEC 60068-2-52 cyclic salt-mist testing. Published data for this specific configuration is limited, so qualification on the actual substrate stack is recommended.

Shelf life and storage are controlled by the pressure-sensitive adhesive; rolls are stored in the original packaging below 32 °C and below 60% RH to limit moisture uptake and release-liner dimensional change. Out-time on the production bench is minimised to prevent surface contamination from finger oils or airborne particulates.

Slit-edge debris is a documented failure mode on high-volume tape conversion lines

Slitting and die-cutting of metal-foil tapes introduce process risks that are not present with polymer-backed tapes. The foil does not deform like a film; it shears and can produce conductive slivers at the cut edge. On rotary shear slitters, blade clearance, blade sharpness, and tension control determine whether the edge is clean or burred. If burrs are generated, conductive debris can separate from the roll and bridge adjacent circuit nodes. Production lines therefore use vacuum extraction at the slitting head and inspect the slit edges at intervals. Die-cut parts are typically produced by kiss cutting to the release liner without severing the liner, so automated pick-and-place equipment can handle the parts. The kiss depth is set as a percentage of the liner caliper; if the kiss is too deep, the liner is nicked and the part can fold or mis-feed. If the kiss is too shallow, the tape does not release cleanly and pulls adhesive stringers from the part edge. These conversion parameters are tool-specific and are qualified on the actual roll width and liner thickness.

In automated lamination, the tape is applied with controlled tension to prevent stretching of the metal foil. Excessive tension reduces width and can curl the tape after liner removal. Lamination equipment with servomotor-driven unwind and closed-loop tension control is preferred for parts above 500 mm in length. Published data for specific slitting parameters is limited because the required blade gap and kiss depth vary with liner caliper and adhesive coat weight.

Die-cut parts are commonly supplied on rolls with exposed adhesive or as individual parts on a continuous liner. Automated placement equipment may require a liner peel force within a specific range. The release liner should be removed at a low peel angle to avoid curling the foil edge. If liner release is heavy, the operator may use a higher peel angle, which can kink the metal foil and introduce edge cracks. Production experience indicates that kinked edges create localised high-resistance paths and should be removed rather than burnished.

Why specify the HT variant instead of standard CHO-MASK II or non-conductive copper tape?

The standard CHO-MASK II grade is used for general shielded seams, while the HT variant is selected when the assembly experiences higher continuous or peak temperatures. The adhesive, not the copper carrier, is the distinction. A generic copper tape with a non-conductive adhesive does not provide through-adhesive conductivity; the adhesive acts as a dielectric spacer unless the tape is perforated or the foil edge contacts the substrate directly. In a shielded seam, relying on edge contact alone produces a narrow, mechanically unstable conductive path. The conductive adhesive in CHO-MASK II HT provides a distributed z-axis path that lowers seam impedance. Because the adhesive is filled, peel force per unit width is typically lower than an unfilled adhesive of equivalent coating weight; users should compare ASTM D3330/D3330M values, not total foil thickness, when selecting a replacement. CHO-MASK II HT is also distinct from conductive fabric tapes: the metal foil provides continuous plane coverage but has lower tear resistance and higher bending stiffness, so it is not the preferred material for repeated flex-cycle cabling.

The copper carrier accepts soldered terminations at the tape edge. Soldering is performed under conditions that avoid adhesive char; solder joint acceptability is commonly assessed according to IPC J-STD-001, and dwell time is limited to the minimum required for solder wetting. Heat shielding is used to limit adhesive damage adjacent to the joint. This is a manufacturing advantage over aluminium-foil shielding tapes, which are not reliably solderable with standard fluxes. The tape backing itself is not a wrapper for high-current conductors; it is a grounding and shielding aid, and current-carrying capacity should be derated according to the copper foil cross-section rather than the adhesive.

The tape is not intended for continuous immersion in aromatic solvents, ketones, or strong acids; these can attack the acrylic adhesive and reduce peel strength. Chemical compatibility is evaluated by ASTM D896 for adhesive bonds exposed to solvents. The product is also not intended for repeated flex-cycle cable joints because the metal foil work-hardens and may crack over time. For dynamic flex applications, a conductive fabric or elastomer-based product is more suitable. The product is not a replacement for conductive elastomer gaskets in closure applications with repeated open/close cycles and compressive set requirements. Conductive elastomers provide spring recovery; foil tape is permanent or semi-permanent. If the joint is opened for service, the tape is typically replaced rather than reused.

Regulatory and standards matrix for CHO-MASK II HT applications

The product is incorporated into electrical and electronic equipment and is reviewed against the material restrictions relevant to the end market. The table below lists the test methods and standards most often used for qualification; it is not a manufacturer declaration of certification for every roll format.

Compliance documentation should be requested for the specific roll width and liner format because adhesive coat weight and carrier treatments can vary by conversion batch. Users should not transfer the material declaration from a different Parker Chomerics tape without verifying the adhesive grade and backing.