For inner layer patterning of rigid multilayer boards, Rongda photosensitive dry film is laminated onto cleaned copper foil at 0.5–0.6 MPa roll pressure, 105–120°C roll temperature, and 1.0–1.5 m/min transport speed. Supplied in roll format with a PET cover sheet and LDPE separator, the film requires conditioning at 20–25°C for 24 h before use. The process replaces liquid photoresist on 18 µm and 35 µm copper-clad FR-4 and high-Tg epoxy laminates. Copper surfaces are prepared by pumice scrub or chemical microetch to a roughness of 0.3–0.5 µm Ra, followed by drying at 70–80°C for 30 s. Lamination is conducted in a cleanroom at 40–60% RH to prevent moisture entrapment at the resist/copper interface. Exposure through a phototool uses a 365 nm collimated UV source at 45–80 mJ/cm² for 20 µm and 25 µm film thicknesses. Developer solution is 1.0 wt% Na₂CO₃ at 28–32°C, sprayed at 1.5–2.0 bar with a breakpoint of 45–55% of the total development time. Development beyond 3× breakpoint increases line width loss and undercuts fine traces. After etch in cupric chloride or alkaline ammoniacal etchant, the remaining resist is stripped in 3–5 wt% NaOH at 45–55°C for 60–120 s. The terminal circuit features are conductors and pads for multilayer stack-ups used in automotive engine control units, industrial motor drives, and network switches. Process acceptance follows IPC-A-600J Class 3 visual and microsection criteria and IPC-6012E Class 3 performance requirements.

Adhesion is evaluated by cross-cut tape test per ASTM D3359-17 with a Class 5B result required before etching. The main process conflict is resolution versus adhesion: higher exposure energy raises cross-linking density at the film surface but can scatter into the resist foot, producing a lip that impairs fine-line etching. At 25 µm film thickness, line/space resolution below 50 µm requires a collimated source and vacuum contact; otherwise scattered light reduces the etch factor and causes open or short conditions after etching. Panel-to-panel variance in lamination temperature of ±5°C is sufficient to change adhesion by a measurable level, so roll temperature uniformity across the working width must be maintained within ±2°C. Contamination from amine-based cleaners or silicone release agents must be excluded because these residues inhibit polymerization at the copper interface and cause resist peeling during etching.

What Prevents Pattern Plating Resist from Lifting in Acid Copper Electrolytes?

Outer layer pattern plating uses the dry film as a sacrificial mask over electroless copper to define conductors and pads prior to electrolytic copper and tin deposition. The panel enters a pre-plating sequence of acid cleaner, microetch, and sulfuric acid dip. The critical failure mode is resist lifting at the leading edge of plating current densities above 2.0 A/dm². The dry film must maintain adhesion to both smooth electroless copper and the roughened epoxy surface during 60–120 min immersion in acid copper sulfate at 20–25°C, containing 200 g/L CuSO₄·5H₂O, 50–70 g/L H₂SO₄, and 30–50 ppm chloride ion. Electrolytic copper is plated at 2–3 A/dm² to a thickness of 20–25 µm in vias and 5–8 µm on surface patterns, followed by tin as an etch resist at 1.0–1.5 A/dm². Underexposure below 50 mJ/cm² for 25 µm film leaves a soft resist foot that swells in the acid bath and allows underplating; overexposure above 90 mJ/cm² increases adhesion but narrows the development window and leaves residual resist in isolated gaps. Development uses the same 1.0 wt% Na₂CO₃ at 28–32°C, but the breakpoint is targeted at 50–60% to retain a thicker resist shoulder against plating solution ingress. Alkaline stripping after tin etch is performed in 3–5 wt% NaOH at 45–55°C. The terminal products are outer-layer circuits for two-layer and multilayer PCBs in power modules, LED lighting boards, and handheld devices. Finishing inspection references IPC-6012E Class 2 and Class 3 conductor spacing and annular ring requirements.

Resist lifting in acid copper is often incorrectly attributed to exposure energy alone; field failures on horizontal plating lines show that developer depletion is a more frequent root cause. As Na₂CO₃ solution loading increases, development rate falls, and operators extend development time, which hydrolyzes the resist surface and reduces wetting resistance to plating bath penetration. Bath pH must be maintained at 10.5–10.8 and carbonate concentration titrated at least once per shift. The dried film thickness after development, measured by non-contact profilometry, should not decrease by more than 2 µm from the nominal thickness. Incompatibility with amine-based pre-cleaners and with sulfuric acid/hydrogen peroxide microetch baths above 35°C must be controlled, because excessive surface roughening or organic residue undermines the resist anchor and produces localized lifting at high current density edges.

Via Tent Integrity Under Hot Roll Lamination and Alkaline Development

The via tenting process applies dry film over drilled, desmeared, and electroless copper surfaces to form a continuous mask over through-holes that must survive alkaline development, acid etching, and stripping. The dry film is laminated at 0.5–0.6 MPa and 105–120°C, but for via fields the transport speed is reduced to 0.8–1.2 m/min to allow the film to soften and drape into hole openings without puncturing. Hole diameter is the controlling variable: as diameter increases, film thickness must increase and exposure energy must be raised to maintain a residual tent cap after development. Film breakage over vias is a known failure mode when the unexposed film is too thin or when lamination creates trapped air that expands in the developer. Exposure through a phototool is performed with 365 nm collimated UV; the exposure dose for tent hold is typically 10–20 mJ/cm² higher than for trace imaging at the same thickness. Development in 1.0 wt% Na₂CO₃ at 30°C removes unexposed film from the via interior only if the hole is not fully collapsed by the tent. Developer breakpoint is held at 45–55%; longer development thins the tent cap and reduces its resistance to etch spray pressure.

After etching, tented vias are inspected by backlight and microsection for cap thinning, pinholes, and incomplete tent formation. The process is limited to holes below 0.60 mm for conventional hot roll lamination; larger holes require vacuum lamination or plugging. Terminal boards using tented vias include high-density interconnect modules and radio-frequency circuit boards where via plugging is not permitted. Acceptance follows IPC-A-600J Class 3 and IPC-6012E Class 3 void and material criteria.

In selective electrolytic nickel and gold plating for edge connectors, the Rongda photosensitive dry film is used as a plating mask over a partially processed PCB surface. The film is imaged to open only the connector finger area while the remaining circuitry is protected. The masked panel passes through alkaline degreasing, microetch, acid dip, and then enters a gold plating line with a nickel underlayer. The dry film must withstand 60–70°C nickel and gold electrolytes at pH 4.5–5.5 for 10–20 min without lifting, blistering, or creeping. Typical gold thickness for edge connectors is 0.75–1.25 µm over 2.5–5.0 µm nickel, plated at 0.5–1.0 A/dm². The exposed resist surface is cross-linked enough to resist cyanide-based or sulfite-based gold solutions, but film adhesion at the nickel/gold interface is the primary control point. Pre-plating bake at 90–100°C for 10–20 min after development improves gold bath resistance but can reduce stripping speed later. Stripping after plating is performed in 3–5 wt% NaOH at 50–55°C with ultrasonic agitation, but extended gold bath exposure can harden the resist surface and increase strip time by 30–50%. The terminal products are printed circuit boards with separable edge contacts for industrial computers, telecommunications backplanes, and instrumentation modules. Gold coating quality is assessed by ASTM B488-18 thickness and adhesion test methods, while the remaining panel must meet IPC-6012E Class 3 surface finish and mask removal criteria.

The main process incompatibility is with cyanide-containing cleaning baths that are too aggressive for the resist; cyanide baths used before gold plating must be kept below 50°C and limited to 5 min to avoid swelling. Humidity above 60% RH during storage or lamination causes moisture entrapment that produces blisters when the panel enters hot gold baths. Published data for this specific Rongda configuration is limited, so qualification on production-scale horizontal gold lines is required for each lamination batch.

When Polyimide Substrates Enter Roll-to-Roll Lamination, Process Temperatures Shift

Flexible printed circuit fabrication with polyimide laminates shifts lamination conditions away from rigid FR-4 parameters. The rolled copper foil is 12–18 µm thick and the polyimide core is 12.5–25 µm thick, so the dry film is laminated at 90–105°C, 0.3–0.5 MPa, and 1.0–2.0 m/min in roll-to-roll equipment. Lower temperature is required because polyimide expands and contracts more than FR-4 during thermal cycling; excessive lamination temperature above 110°C distorts the panel and shifts the phototool alignment. Tension control is critical: web tension that is too high stretches the polyimide and changes hole-to-hole distances, while tension that is too low causes wrinkles and trapped air under the dry film. Exposure uses 365 nm UV at 40–70 mJ/cm² for 20 µm film. The developer is 1.0 wt% Na₂CO₃ at 28–30°C; because flex circuits have smaller feature sizes, breakpoint is held at 40–50% to minimize swelling. Etching of the flexible copper is performed with cupric chloride at 45–50°C, spray pressure 1.0–1.5 bar, followed by NaOH stripping. The terminal products are single- and double-sided flexible circuits for cameras, medical ultrasound probes, and OLED display connectors. Qualification follows IPC-6013D Class 3 flex board requirements and IPC-TM-650 dimensional stability methods.

A production-scale failure mode in roll-to-roll lamination is resist edge chipping along the slit edges of the web. This occurs when the slitting burr on the copper foil is greater than 2 µm or when the lamination roll durometer is outside 60–70 Shore A; both conditions create localized pressure spikes that crack the dry film before exposure. Edge chipping is detected by automated optical inspection after development. The incompatibility with silicone release agents is more severe on flex because the smoother polyimide-copper interface offers less mechanical anchoring than FR-4 copper. Pre-cleaning must therefore include plasma treatment or chemical adhesion promotion rather than mechanical scrubbing, which can tear thin polyimide cores.

Chemical milling of thin metal sheet with the Rongda photosensitive dry film requires a different adhesion balance because the metal surface is smoother than PCB copper and the etchant is more aggressive. The substrate is 0.05–0.30 mm thick austenitic stainless steel, copper alloy, or molybdenum sheet. Surfaces are degreased and micro-etched to 0.15–0.30 µm Ra, and the dry film is laminated at 110–125°C, 0.5–0.6 MPa, 0.8–1.2 m/min. Exposure uses 365 nm collimated UV at 50–100 mJ/cm² through a phototool. Development in 1.0 wt% Na₂CO₃ at 30°C opens fine apertures. The etch resist must withstand ferric chloride at 40–50°C, 42–45°Bé, spray pressure 1.5–2.5 bar, for 10–30 min without edge lifting. For stainless steel, the etchant is ferric chloride; for copper alloys, cupric chloride or alkaline ammoniacal etchant is used. Etch factor is controlled by nozzle oscillation and pressure. The dry film residue is stripped in 3–5 wt% NaOH at 50–60°C for 2–5 min. The terminal products are SMT stencils, encoder discs, precision screens, EMI shielding grids, and battery current collector foils. Feature tolerance is typically ±0.025 mm for apertures above 0.10 mm, but published data for this specific Rongda configuration is limited and must be confirmed by first article inspection against ISO 9001 controlled work instructions and customer drawings.

Adhesion loss on stainless steel is accelerated by chloride pitting at the resist edge if the developer is not fully rinsed. Deionized water rinse at 20–25°C for 30–60 s is required after development; residual carbonate ions react with ferric chloride and weaken the resist/metal interface. The dry film is incompatible with solvent-based pre-cleaners that leave oily films, and with nitric acid-containing etchants above 35°C, which attack the resist and cause premature stripping. The lamination environment must be below 60% RH; otherwise micro-condensation on the metal sheet causes random pinhole formation after development.