Saint-Gobain CHR 2345-1 is a pressure-sensitive tape constructed from a 0.025 mm (1.0 mil) polyimide film backing and a 0.038 mm (1.5 mil) silicone pressure-sensitive adhesive, producing a nominal total thickness of 0.064 mm (2.5 mil). The polyimide backing is non-thermoplastic under electrical insulation Class H temperatures and resists cut-through during coil winding, while the silicone adhesive remains elastomeric after soldering excursions. The tape is applied as interlayer and outer-wrap insulation in dry-type transformers, as finger masking on printed circuit assemblies during wave soldering, and as edge masking in powder coating. Because the backing is thin, each layer adds 0.064 mm to slot build height; this value is used by coil manufacturers to calculate maximum slot fill before varnish impregnation.

On a wave-solder line, the tape is exposed to preheat zones at 120 °C to 150 °C and a solder pot at 260 °C to 280 °C. Published typical property data list a service temperature range of -73 °C to 260 °C (-100 °F to 500 °F). The polyimide backing does not soften at these temperatures, but edge contact with dross can embrittle the film locally. Field observations from wave-solder operations show that tape lift is more often caused by flux wicking under the edge than by thermal decomposition of the backing. This distinction matters because process adjustments to flux deposition and preheat drying are more effective than changing tape thickness when masking failures occur.

How Does Silicone Adhesive Affect High-Temperature Masking Performance?

Silicone pressure-sensitive adhesives maintain peel strength across a wide thermal window but exhibit lower initial room-temperature tack than acrylic systems. Published typical peel adhesion of CHR 2345-1 to stainless steel is 6.9 N/25 mm (25 oz/in) after a 20 min dwell under ASTM D3759/D3759M. In manufacturing terms, the tape adheres to freshly cleaned copper, aluminium, and stainless steel; on low-energy polymers or silicone-contaminated metals, wetting can be incomplete and edge lift may appear within minutes. Unlike acrylic adhesives, silicone releases more readily after heating. This reduces adhesive transfer during soldering mask removal but requires stricter surface preparation because the adhesive cannot tolerate the same residual oil film thickness as a high-tack acrylic.

The surface energy of silicone pressure-sensitive adhesive lies in the range of 22 mN/m to 24 mN/m. Clean copper and stainless steel surfaces after solvent wiping present surface energies above 40 mN/m, providing sufficient spreading pressure. Polypropylene, acetal, and heavily silicone-processed surfaces fall below 30 mN/m or carry low-molecular-weight siloxane films that inhibit wetting. This explains mixed performance on hybrid assemblies where the tape is applied partly to copper and partly to a low-energy polymer carrier. The practical response is not to increase tape thickness but to introduce a final wipe with isopropanol or methyl ethyl ketone and to verify surface contamination with a dyne pen or contact-angle measurement before automatic taping.

The adhesive is a dimethylsiloxane network containing MQ silicate resin; the resin-to-gum ratio controls peel and tack. This chemistry has a low glass transition temperature, typically below -40 °C, which maintains conformability at low temperatures. Crosslink density is adjusted so that the adhesive flows under pressure at room temperature but resists creep at 260 °C for short exposures. This balance is why the tape shows limited adhesive transfer after thermal cycling: cohesive failure is more likely than interfacial failure when the substrate is clean, but peel force can decrease after repeated reheating if the adhesive network undergoes additional crosslinking or embrittlement.

Surface preparation for stator and rotor cores typically consists of single-pass wiping with isopropanol or methyl ethyl ketone. Multi-pass wiping can redistribute oils rather than remove them. On sanded copper bus bar, roughness increases mechanical interlock for the soft silicone adhesive; if roughness exceeds 1.6 µm Ra, air entrapment under the backing creates voids that reduce partial-discharge inception voltage. This boundary is encountered on production lines where the same bus bar surface finish is used for adhesive bonding and electrical termination. The tape should be applied in a single continuous pass without lifting and repositioning; silicone adhesive loses tack after contamination by skin oils or dust even when the underside appears clean.

Electrical Insulation Thresholds at Thermal Class 180

Polyimide film is classified for Class H operation at 180 °C in electrical insulation systems when evaluated under UL 1446 or IEC 60085. The published typical dielectric strength of CHR 2345-1 is 7,500 V at 25 °C; this value applies to the total tape construction and is measured across the thickness. Dielectric strength is not a creepage or clearance rating, and the tape does not replace the dimensional requirements of IEC 60664-1. In dry-type transformer layer insulation, partial discharge testing under IEC 60343 is used to verify that voids from taping do not exceed the discharge inception threshold for the winding voltage class. Single-layer dielectric strength values obtained on flat laboratory fixtures are reduced in production windings by cut edges, wrinkles, and butt splices that create local field enhancement points.

Roll stock is supplied in slit widths according to customer specification; slitting tolerances are commonly held to ±0.08 mm on widths below 25 mm and ±0.13 mm on wider tapes. Liner removal should not be performed in solvent-vapour zones because liner peel at unwind speeds above 0.5 m/s can generate triboelectric charge. Static dissipative work surfaces and ionizers with discharge times below 2 s are common controls in coil taping cells. Charge generation is a process characteristic of thin polymer liners and does not indicate a material defect.

When Solvent Immersion and Vacuum Outgassing Exclude Polyester or Rubber Systems

Silicone adhesives swell and lose peel resistance in continuous contact with toluene, xylene, methyl ethyl ketone, and chlorinated solvents. Polyimide backing is attacked by strong alkalis, concentrated mineral acids at elevated temperature, and hot hydrazine. These exposures are uncommon in electrical assembly but appear in chemical milling and solvent degreasing masking. In vacuum applications, polyimide film offers lower total mass loss than polyester backings, although silicone adhesives can release low-molecular-weight siloxanes. When outgassing is critical, testing per ASTM E595 and requesting product-specific total mass loss and collected volatile condensable material values is required; publicly available data for this specific configuration is limited.

In powder coating masking, the tape is applied before spray coating and enters a cure oven at 190 °C to 205 °C for 10 min to 30 min. The silicone adhesive is selected because it releases from the substrate after cure without leaving carbonized residue, while the polyimide backing prevents bleed-through of coating powders at edges. Adhesive transfer can occur if the tape is removed while the substrate is above 100 °C or if the coating film builds to more than 0.15 mm along the tape edge. In the latter case, scoring along the edge with a blade before removal reduces peeling of the cured powder. These are operating limits derived from coating-shop practice rather than intrinsic material specifications.

Conformal coating cells also employ the tape to protect connectors and test points. If the board has a low-surface-energy solder mask, adhesion may be reduced; burnishing with a stiff rubber roller improves contact but cannot overcome contamination from no-clean flux residues. Cleaning the board with isopropanol prior to tape application is required when no-clean flux residue is present, especially under areas where the tape spans exposed contacts. Tape removal after conformal coat cure should be at an angle of 180° and at a speed below 0.1 m/s to avoid tearing the coating fillet; if the coating is fully cured, a scalpel score along the tape edge is common practice. Residue transfer from silicone adhesive is sometimes detected by a water break-free test; if adhesive transfer occurs, an isopropanol wipe may not remove siloxane films, and a second cleaning with a suitable silicone remover or plasma treatment may be required before coating.

Comparative Product Families: Silicone Polyimide, Acrylic Polyimide, and Glass Fabric Tapes

CHR 2345-1 differs from acrylic polyimide tapes in that the silicone adhesive maintains serviceability to 260 °C; acrylic systems commonly lose peel strength above 150 °C and can leave gummy residues after high-temperature cycling. Acrylic versions provide higher initial peel on stainless steel at room temperature and better wetting on low-energy films, which makes them suitable for lower-temperature holding applications where initial tack is the main requirement. Compared with polyester tapes, CHR 2345-1 has a wider thermal window and higher cut-through resistance; polyester constructions are commonly limited to 130 °C continuous service and soften in wave-solder preheat. Compared with glass cloth tapes, CHR 2345-1 provides a lower build height and a smooth backing that does not absorb insulating varnish as readily, but it offers lower abrasion resistance and less tear strength during heavy motor winding. Selection therefore depends on whether the dominant process variable is thermal exposure, mechanical abuse, or adhesive residue tolerance.

Reducing Edge Lifting in Automated Taping Operations

On spindle and linear taping heads, edge lifting is frequently caused by insufficient unwind tension rather than adhesive failure. The release liner must be peeled at a constant angle; if the liner path reverses across a guide roll, adhesive can transfer to the liner and create intermittent low-tack zones. In transformer production, taping heads running at 0.15 m/s to 0.75 m/s apply the tape over magnet wire bundles with varying conformability. At the upper end of this range, the thin polyimide backing can fold at splices or over sharp corners unless the guide roller radii are above 12 mm. Operators should verify that the adhesive has wet out on the substrate by inspecting for air channels under the backing after application; these channels appear as silvery streaks in the amber polyimide and reduce dielectric strength locally.

Storage at 21 °C and 50% RH in original packaging is standard for this class of silicone pressure-sensitive tape. Exposure to direct sunlight, ozone, or high-pressure mercury lamps can age the silicone adhesive and increase gel content, reducing tack. Typical shelf life for silicone pressure-sensitive tapes of this class is 12 months from date of manufacture when stored below 27 °C and 60% RH; product-specific shelf-life documentation should be confirmed with the supplier. Rolls should not be stored adjacent to solvent-soaked rags or in the same cabinet as silicone sealants because low-molecular-weight siloxane vapours can migrate into the adhesive and alter release properties.