Metal packaging lines processing drawn-and-ironed aluminium cans require white basecoats that withstand necking and flanging without coating fracture. The cationic photoinitiator under evaluation, a triarylsulfonium hexafluorophosphate salt supplied by Yangfan New Material, is incorporated at 1.0–3.0 wt% relative to total resin solids in a cycloaliphatic epoxy white basecoat. The formulation is applied by roll coating at a dry film weight of 8–12 g/m². The coated can body is passed under a mercury arc lamp delivering 300–600 mJ/cm² in the UVA range. Surface cure is immediate because cationic ring-opening polymerization is not inhibited by oxygen. Mechanical integrity develops further through dark cure. The coated can is held at 40–60°C for 24–48 h before necking and flanging operations. Moisture content in the pigment concentrate must be controlled below 600 ppm. Water quenches the propagating oxonium ion. Titanium dioxide is pre-dried at 120°C for 4 h before dispersion. Batch-to-batch variation in pigment moisture above 600 ppm is a known cause of intermittent surface tack on can coating lines. The formulation must be free of amines and thiols. These nucleophilic species neutralize the photogenerated acid. Food-contact compliance is verified by extraction testing. The cured coating must meet the chloroform-soluble extractives limit of 0.5 mg/in² under FDA 21 CFR 175.300. Overall migration must not exceed 10 mg/dm² under Commission Regulation (EU) No 10/2011. Where non-listed photolysis products are present, the analytical detection threshold of 10 µg/kg under Swiss Ordinance SR 817.023.21 is applied. The absence of oxygen inhibition allows full cure at the sharp cut edge of the can. However, thick stencil deposits above 20 g/m² are not recommended because the mercury arc lamp cannot generate sufficient acid at the substrate interface. A post-cure thermal step is then required to drive conversion.

What governs acid diffusion in shadow-cured cationic epoxy adhesives?

A persistent process conflict in UV-cured cationic adhesives arises from the need to balance photospeed against acid diffusion distance. When bonding opaque substrates, the adhesive receives UV only at exposed fillet edges. The bondline interior cures by thermal diffusion of photogenerated acid. The triarylsulfonium salt generates a strong Brønsted acid upon irradiation. The counterion controls the acid strength. Hexafluorophosphate salts generate a weaker acid than hexafluoroantimonate salts. The weaker acid reduces moisture sensitivity but slows dark cure. In optical component assembly, a loading of 1.5–2.5 wt% of triarylsulfonium hexafluorophosphate provides a lap shear strength of 12–18 MPa on fused silica after 24 h dark cure at 23°C and 1 h post-bake at 80°C. Lap shear is measured according to ISO 4587. For metal-to-metal bonds, ASTM D1002 is used. Amine-based surface primers must be avoided. Residual amines neutralize the photogenerated acid and create an uncured boundary layer. Aluminium adherends are prepared by phosphoric-acid anodizing or chromic-acid anodizing per ASTM D2651. Adhesive viscosity is typically held at 2,000–8,000 mPa·s at 25°C to prevent starvation of the bondline during fixturing. A thixotropic fumed silica addition of 2–5 wt% prevents slump on vertical surfaces. Silica scatters UV and reduces cure depth. Therefore, the minimum UV dose must be increased by 20–40% when fumed silica is present. The dark cure rate depends on free volume. Below the glass transition temperature, acid diffusion is restricted. Post-bake at 60–100°C increases free volume and accelerates conversion. However, temperatures above 100°C can volatilize low-molecular-weight epoxide diluents and create voids. For microprocessor die-attach applications, the adhesive must survive 1,000 h of 85°C/85% RH damp heat without delamination. Surface insulation resistance is measured after moisture conditioning according to IPC-TM-650. Published data for specific cationic adhesive systems on mixed substrates is limited. Formulators must screen each substrate combination for acid neutralization and moisture inhibition.

Printed circuit board solder mask formulations based on epoxidized novolac resins and triarylsulfonium salts are processed as photoimageable negative resists. The liquid resist is applied by screen printing or curtain coating to a dry film thickness of 15–35 µm. The panel is pre-baked at 80°C for 20–30 min to remove solvent. UV exposure is performed through a phototool at 365 nm with a dose of 200–500 mJ/cm². Unsensitized triarylsulfonium salts absorb primarily below 320 nm. Therefore, 0.2–0.5 wt% of 2-isopropylthioxanthone is added as a photosensitizer. The photosensitizer extends absorption into the 365–405 nm region. After exposure, a post-exposure bake at 90–110°C for 15–30 min drives acid diffusion into unexposed regions. Development is carried out in 1% sodium carbonate solution at 30°C for 45–90 s. The unexposed resist is removed to form solder dams. Final thermal cure is performed at 150°C for 60 min. The cured solder mask must meet IPC-SM-840E class T/H requirements. Adhesion is tested by tape test per ASTM D3359 with a rating of 4B or better. Pencil hardness is measured per ASTM D3363 at 6H or harder. The mask must withstand solder float at 288°C for 10 s without delamination or blistering. Moisture absorption above 2.0% in the cured film increases ionic conductivity. High-humidity storage of printed boards before conformal coating should be limited to 72 h at 30°C/60% RH. The cationic photoinitiator loading is typically 3–5 wt% of resin solids. Excessive photoinitiator loading increases residual acid and reduces surface insulation resistance. The formulation must avoid amine hardeners and imidazole accelerators because they quench the photogenerated acid and retard cure. The solder mask ink is dispersed on a three-roll mill with a gap setting of 5–15 µm to achieve a fineness of grind below 5 µm. Incomplete dispersion above 5 µm causes pinholes and reduces edge definition after development.

Photoacid generation kinetics in i-line epoxy photoresist systems for MEMS

Chemically amplified epoxy resists used in MEMS and microfluidics derive their lithographic contrast from the generation of a strong Brønsted acid by the cationic photoinitiator. The triarylsulfonium salt is dissolved at 5–10 wt% of total solids in an epoxidized novolac resin. After spin coating to a thickness of 10–500 µm, the film is soft-baked at 65°C for 5–10 min to remove residual solvent. Exposure is performed at 365 nm with a dose of 150–400 mJ/cm² using a contact aligner or mask aligner. The post-exposure bake is a two-step ramp: 65°C for 2 min followed by 95°C for 5–10 min. During the bake, the photogenerated acid diffuses through the glassy film and catalyzes ring-opening of the epoxide groups. Typical lateral acid diffusion lengths are 2–8 µm under these bake conditions. The post-exposure bake temperature window is narrow. A variation of ±5°C changes acid diffusion length by approximately 20%. Development is carried out in propylene glycol methyl ether acetate. Development time ranges from 5–15 min depending on film thickness. Aspect ratios greater than 10:1 are achievable. Stress cracking becomes probable above 15:1 due to crosslink shrinkage and coefficient of thermal expansion mismatch with the silicon substrate. Tensile properties of cured films are measured by ASTM D638. Published tensile strength for SU-8-type photoresists typically falls between 50–70 MPa. Adhesion on silicon wafers after piranha cleaning is assessed by ASTM D3359 cross-hatch method. A rating of 5B is required. The cationic photoinitiator must be free of chloride and other anionic contaminants. Chloride residues above 5 ppm can corrode aluminium bond pads in MEMS devices. Storage of the formulated resist must be at 5–25°C under amber glass or opaque containers. Ambient moisture above 45% RH can accelerate hydrolysis of the epoxide groups and reduce shelf life.

Polycarbonate headlamp lenses receive a UV-cured cationic epoxy primer to promote adhesion of subsequent organosilicon hardcoat layers. The primer is applied by flow coating or spray coating at a wet film thickness of 5–15 µm. The coated lens is passed under a gallium-doped mercury lamp with a UVA dose of 250–500 mJ/cm². The cationic photoinitiator loading is 1.0–2.0 wt% of primer solids. Cationic cure develops an interpenetrating interface with the polycarbonate surface through acid-catalyzed transesterification. This interface contributes to adhesion retention. Adhesion is assessed by tape test per ASTM D3359 before and after 1,000 h xenon arc exposure per ASTM G155 cycle 1. Retention of 5B adhesion is expected. The primer must also withstand 85°C/85% RH damp heat for 1,000 h without blistering or loss of adhesion. Residual free acid from incomplete cure can attack the polycarbonate substrate. A post-UV thermal bake at 120°C for 20 min is required to consume residual acid and prevent microcrazing under thermal cycling between -40°C and 80°C. Published data for specific cationic primer systems on polycarbonate is limited. Formulators must screen photoinitiator loading against long-term weathering data. The primer must be free of aromatic amines and hindered amine light stabilizers. These additives neutralize the photogenerated acid and retard interfacial cure. Moisture in the primer must be kept below 500 ppm because water competes with the polycarbonate for the acid catalyst.

When UV-LED output is confined to 365–405 nm, cationic cure depth requires a sensitizer

When UV-LED arrays are required for line-speed retrofits, the absorption envelope of the cationic photoinitiator determines whether adequate acid is generated. Triarylsulfonium salts absorb primarily below 300 nm. Therefore, a 365 nm or 395 nm UV-LED source generates negligible acid unless a photosensitizer is added. 2-Isopropylthioxanthone is used at 0.1–0.5 wt%. The sensitizer absorbs at 380–400 nm and transfers energy to the photoinitiator. Anthracene derivatives are sometimes used at 0.05–0.3 wt% for lower yellowing. Sensitizer addition introduces yellowing and can reduce dark cure storage stability. In LED-cured cationic adhesives for electronic encapsulation, the formulation viscosity is held at 500–3,000 mPa·s at 25°C. A typical LED cure condition is 2–8 s at 1,000–4,000 mW/cm² peak irradiance. The adhesive receives a top-side LED exposure. Shadow regions cure by dark cure at 25°C for 24 h. Adhesion on copper lead frames is tested by die shear per MIL-STD-883 method 2019.5. Failure mode must be cohesive within the adhesive rather than adhesive at the interface. Hexafluoroantimonate counterions increase dark cure speed but increase moisture sensitivity. Encapsulation adhesives with SbF₆⁻ counterions are stored under nitrogen and used within 8 h of dispensing to prevent surface tack from ambient humidity. The photoinitiator loading for LED-cured systems is typically higher than for mercury-lamp systems. A loading of 2.0–4.0 wt% is used to compensate for lower photon absorption at longer wavelengths. Excessive loading can cause residual acid corrosion on copper traces. Therefore, a post-cure bake at 80–100°C for 30 min is recommended to consume free acid.

In vat photopolymerization of cycloaliphatic epoxide resins, the use of a cationic photoinitiator instead of a free-radical photoinitiator reduces linear shrinkage from 5–10% to 0.5–2%. A typical DLP resin for prototyping contains 50–70 wt% cycloaliphatic epoxide, 10–30 wt% oxetane diluent, 1–3 wt% triarylsulfonium photoinitiator, and 0.1–0.5 wt% photosensitizer for 405 nm DLP projection. The viscosity at 25°C is maintained at 300–1,500 mPa·s to allow recoating between layers. Layer thickness is set at 25–100 µm. Exposure time per layer is 2–10 s at an irradiance of 10–20 mW/cm² measured at the vat surface. Because cationic polymerization continues after the light is off, the cured layer develops additional conversion during the dark interval. This dark cure can cause overgrowth at layer interfaces if the inter-layer delay exceeds 30 s. Tensile properties are measured per ASTM D638. Flexural strength is measured per ASTM D790. Shore D hardness is measured per ASTM D2240. Published tensile modulus for cationic epoxy DLP resins typically lies between 2.5–3.5 GPa. Elongation at break is usually below 5%. This brittleness limits use in functional load-bearing parts. The resin must be stored in amber containers at 5–25°C and protected from moisture above 45% RH. Water quenches the propagating oxonium ion and reduces green strength. The photoacid catalyst remains active after printing. Printed parts must be post-cured at 60–100°C for 1–2 h to complete conversion and reduce residual acid. Unreacted photoinitiator can leach from the part surface if the post-cure is omitted.