Polyimide-backed pressure-sensitive tapes are specified for printed circuit board masking when the tape must survive solder wave contact and still remove without adhesive transfer. Saint-Gobain CHR 2345-5 combines a thin polyimide film backing with a silicone pressure-sensitive adhesive. The backing is a poly(4,4'-oxydiphenylene pyromellitimide) film, a condensation polyimide with no defined melting point before decomposition; the adhesive is a crosslinked polydimethylsiloxane/MQ silicate pressure-sensitive adhesive system. In manufacturer’s published roll-form data, the backing gauge is cited as 0.5 mil (0.0127 mm), with the silicone adhesive layer bringing total tape thickness to approximately 1.5 mil (0.038 mm). The tape is amber, supplied in log rolls or slit rolls, and used for masking, insulating, splicing, and high-temperature process protection. The suffix distinguishes the thinner backing variant from other members of the CHR 2345 polyimide series, which are also available in heavier gauges for applications requiring higher dielectric margin and tensile strength.

In wave soldering, the tape is exposed to flux preheat zones at 100–120°C and a solder pot maintained between 245°C and 260°C for Sn-Pb and many lead-free alloys. For SAC305 lead-free wave soldering, solder pot setpoints near 260°C are common, with nitrogen inerting in some chambers increasing convective heat transfer to the tape surface. Published maximum intermittent temperature for polyimide/silicone tape is generally 260°C, and continuous service is limited to 180°C to reduce oxidative hardening of the silicone. During solder contact, the silicone adhesive remains below degradation for short dwell times; however, sustained exposure above 260°C causes polyimide embrittlement and adhesive charring. Masking failure in dual-wave selective soldering is commonly associated with flux undercutting at the tape edge, not adhesive melting, when pre-clean adhesion is sufficient. On ENIG and OSP surface finishes, the tape edge must resist flux spray and spray-jet impingement; field evaluations on production wave solder equipment are required to establish maximum cycle counts for a specific board design.

What Limits the Silicone Adhesive’s Working Window in Wave Soldering?

Adhesive performance is bounded by low-temperature glass transition, high-temperature oxidation, and residue formation. Silicone pressure-sensitive adhesives retain tack below -73°C, which is lower than acrylic systems; at elevated temperatures, depolymerization begins near 260°C. Peel adhesion is typically measured by ASTM D3330/D3330M on stainless steel, with published CHR 2345 series values around 20 oz/in (5.5 N/25 mm). After one pass through a wave solder line, peel force can drop by 20–40% depending on dwell time, preheat profile, and surface roughness. Silicone adhesives also exhibit low surface energy, which permits wetting on polyimide but limits shear holding on smooth metals; cohesive failure within the adhesive is more common than interfacial failure at tape removal above 150°C. In high-volume printed circuit board assembly, edge lifting is most often traced to insufficient application pressure or oily fingerprint contamination rather than adhesive temperature failure. For critical lead-free soldering, the time above 240°C should be kept below the tape’s recommended thermal exposure limit, because the combination of air and flux vapor accelerates silicone oxidation at the exposed cut edge.

In selective soldering systems with no-clean VOC-free flux, spray flux viscosity is often in the 10–50 cP range. Low-viscosity flux penetrates the tape-board interface more rapidly than rosin-based fluxes. To limit undercutting, the tape edge can be burnished with a rubber roller; process engineers should qualify tape adhesion on board surface finishes including ENIG, OSP, and HASL. On OSP surfaces, adhesion is lower than on bare copper or HASL because the organic surface preservative reduces surface energy. Tape removal after soldering should be conducted below 40°C; peel at elevated temperatures increases adhesive cohesive failure and residue transfer. In high-volume cell assembly, automatic tape removal heads that peel at shallow angles between 90° and 180° reduce backing tearing and edge adhesive transfer.

Mechanical handling requires attention to unwind tension and slitting. A 0.5 mil backing has lower tear initiation resistance than 1 mil polyimide, so automatic die-cutting and lamination equipment must be tuned to lower tension. Roll cores are commonly 76 mm (3 in) internal diameter; telescoping and core collapse occur when thin-gauge rolls are wound at excessive tension or stored vertically. For production slitting, razor-blade slitters with fresh blades at an included angle of 30° reduce edge burrs and micro-tears. Compression-type rotary creasing is preferred over steel-rule scoring because the latter can initiate tears in thin polyimide. High-speed mask placement machines may require a liner or differential unwind to prevent adhesive transfer at die-cut edges. Die-cutting anvils should maintain total indicated runout below 0.01 mm to avoid cutting through the liner or partially into the backing.

Electrical Insulation and Dielectric Failure Modes in Thin-Gauge Backings

Electrical insulation requirements for PCB masking demand a backing with repeatable dielectric strength and low pinhole density. For thin polyimide films, dielectric strength is thickness-dependent; a 0.5 mil backing typically exhibits short-time AC breakdown strength in the 3.0–4.0 kV range under ASTM D149 using a 0.5 in diameter electrode. Volume resistivity of polyimide is typically on the order of 10¹⁵–10¹⁷ Ω·cm under ASTM D257. The silicone adhesive contributes negligible electrical insulation relative to the backing, so dielectric integrity is controlled by film gauge uniformity, pinhole density, and slitting-induced edge defects. In selective soldering masks, edge burrs and micro-tears from dull razor slitting reduce local dielectric strength more than bulk film variation. When dielectric strength testing is required on finished tape, the specimen must be free of creases and edge defects because breakdown initiates at the thinnest point of the backing.

When the Polyimide Gauge Falls Below 1 Mil

Choosing the -5 variant over a 1 mil backing alters handling and electrical margin. A 0.5 mil backing reduces total tape stiffness, improves conformance around sharp gold-finger pads, and lowers build-up at mask edges. However, tear initiation in a 0.5 mil polyimide occurs at lower tear strength than in a 1 mil backing, and tensile strength decreases proportionally with thickness. Typical published polyimide film tensile strength is 25 lb/in for 1 mil gauge; a 0.5 mil backing may be specified near 12–15 lb/in. Elongation at break is commonly reported near 55% for polyimide film, but this value is gauge-sensitive and should be taken from the applicable lot test certificate. This lower tensile strength requires lower unwind tension during automatic die-cutting and laminating. Slit-edge quality and core alignment are more critical because thin-gauge polyimide is prone to telescoping on 76 mm plastic or paper cores if wound above the manufacturer’s recommended tension. For high-speed mask placement, the thinner backing also reduces spring-back after die-cutting, but it increases the probability of necking during roll-to-roll lamination unless tension control is maintained within ±5%.

Adhesive coverage and coat-weight uniformity become more important with a lower backing gauge. Silicone adhesive coat weight in the CHR 2345 series is typically applied at 1.0 mil (0.0254 mm) dry film thickness. A backing-to-adhesive thickness ratio of 1:2 means that variations in adhesive thickness have a proportionally larger influence on total caliper and unwind behavior. Roll-to-roll converters using nip-roller lamination should maintain nip pressure below levels that cause adhesive bleed at the edges of thin polyimide. In converting trials on center-surface winders, lay-on pressure below 1.5 N per linear inch has been used to avoid edge roll deformation.

Outgassing limits for vacuum and sealed electronics are relevant when a silicone adhesive is specified. Silicone pressure-sensitive adhesives can release low molecular weight siloxanes under vacuum. Qualification for space or optical applications typically requires ASTM E595 total mass loss (TML) below 1.0% and collected volatile condensable material (CVCM) below 0.1%. Published data for CHR 2345-5 under these conditions is limited; therefore, a lot-specific outgassing test must be requested before use in sealed relays, optical cavities, or launch hardware. The polyimide backing itself is considered low-outgassing, but the silicone adhesive dominates the measured response.

Chemical Compatibility Boundaries Are Defined by Solvent Swelling and Adhesive Residue

Silicone pressure-sensitive adhesives are selected for thermal stability, but they are susceptible to swelling in non-polar solvents and can leave siloxane residue after extended high-temperature exposure. For critical conformal coating processes, polyimide/silicone tapes should be evaluated for silicone transfer onto gold contacts. Acrylic adhesive polyimide tapes are often preferred when ionic contamination or silicone transfer cannot be tolerated; however, acrylic adhesives are routinely limited to 150°C continuous service and exhibit lower peel retention after 200°C exposure. In contrast, rubber-based adhesives are unsuitable for solder masking because they soften below 120°C and leave heavy residue. Compared with glass cloth silicone tapes, CHR 2345-5 provides a uniform thin polyimide dielectric and lower caliper, but it offers less conformability and lower puncture resistance. Compared with FEP or PTFE film tapes, polyimide has higher dielectric strength per unit thickness and better dimensional stability above 200°C; however, it is more susceptible to alkaline hydrolysis and is not used where concentrated hot alkali is present.

Compatibility with conformal coatings and potting compounds should be validated on the exact board finish. Silicone adhesive may act as a barrier to certain solvent-based acrylic and urethane coatings, but it can interfere with coating adhesion if residue transfers to solder mask surfaces. The backing itself is resistant to most organic solvents and hydrocarbon oils, but it is attacked by strong alkalies and some hot concentrated acids. In masking for plasma cleaning or aggressive aqueous alkaline cleaners, the user must confirm chemical exposure limits with the manufacturer’s chemical resistance data.

Common usage extends beyond wave soldering to powder coating, where the tape must withstand cure schedules at 180–200°C for 15–30 min; transformer and motor winding insulation; coil wrap; and temporary protection of thermocouple leads in ovens. The thin polyimide backing allows the tape to be placed between tightly spaced conductors, while the silicone adhesive offers removability after thermal cycling. In powder coating, silicone adhesive is more heat-tolerant than rubber or acrylic; however, edge bleed under powder cure can occur if the tape is not applied with sufficient pressure. Published data for this specific configuration is limited, so powder coat cure time and board topography must be qualified on the production line.

Comparison with polyester-backed silicone tapes is instructive. Polyester tape is lower in cost but has a continuous service limit near 130°C, lower dielectric strength per unit thickness, and higher elongation. In wave solder environments above 230°C, polyester backing shrinks and distorts, while polyimide retains dimensional stability. Glass cloth silicone tapes provide thicker dielectric and greater mechanical toughness but are more difficult to die-cut cleanly and may produce loose fiber contamination in clean rooms. Thus, CHR 2345-5 is used when a thin, high-dielectric, high-temperature masking tape is required; published data for this specific configuration is limited, so lot-based verification is necessary for critical aerospace or military soldering processes.

Storage conditions and incoming inspection require controlled ambient conditions. Rolls should be kept in original polyethylene packaging at 10–30°C and 30–60% RH. Exposure to direct sunlight and ozone should be avoided because both accelerate silicone adhesive crosslinking. At relative humidity above 60%, dimensional stability of the polyimide backing can be influenced by absorbed moisture, and incoming rolls should be allowed to equilibrate before precision slitting or die-cutting. The product’s pressure-sensitive adhesive has a finite open time after liner removal; high-speed placement must occur before contamination of the adhesive surface. For log rolls used in high-volume wave solder masking, automatic tape applicators should maintain a consistent dwell time from adhesive exposure to placement. Published data for the -5 variant’s exact shelf life and roll dimensions is limited; the user should confirm maximum roll diameter, core ID, and splice-free length from the manufacturer’s production specification before ordering converting equipment.