3M 616 Lithographers Tape is a red polyester film tape with a rubber pressure-sensitive adhesive system. The product is supplied in slit roll form for temporary masking, stop-off, and edge sealing in wet chemical processing. It is not a photoresist, not a primary dielectric film, and not a permanent conformal coating. Manufacturer technical data list total tape thickness at 0.061 mm, tensile strength at break at 438 N/100 mm, elongation at break at 100%, adhesion to stainless steel at 24 N/100 mm when tested according to ASTM D3330, and continuous service temperature up to 121°C. The polyester backing supplies chemical resistance and dimensional stability. The rubber adhesive phase supplies wet-out and edge sealing. The process envelope in downstream applications is governed by the interaction of these two layers with the etch chemistry, bath temperature, and mechanical shear at the tape-substrate edge.

At What Point Does a Red Polyester Tape Serve as the Primary Mask in Mixed-Surface PCB Processing?

In printed circuit board fabrication, 3M 616 is not a replacement for dry film photoresist in primary outer-layer imaging. Its functional position is secondary masking: protecting gold-plated edge fingers during solder stripping, isolating bussed panel edges during alkaline etching, and preventing tin-lead overplate on plated through-hole pad surfaces that are designated for other finishes. On a six-layer ENIG surface-finished array, the tape is applied after electrolytic gold plating and before nitric/ferric solder stripping. Strip width is selected to overlap the gold terminal edge by at least 0.5 mm per side. Narrower overlap produces capillary wicking of stripper chemistry under the polyester backing. Roller lamination pressure is maintained below the threshold that causes adhesive ooze, typically 30–40 N on a 60 mm wide applicator roller, although the setting is adjusted for panel thickness and copper feature density. Tape edges are burnished with a PTFE blade to close air channels before immersion.

The solder stripper is a proprietary nitric acid/ferric nitrate formulation held at 35–45°C. Immersion time is 5–15 min depending on solder volume and bath age. Free nitric acid is maintained at 0.8–1.2 N by automatic analysis. After stripping, boards are rinsed in cascading deionized water with resistivity ≥18.2 MΩ·cm and dried with air knives set below 60°C. Tape removal is performed after the panel cools below 40°C. Hot peel above that temperature leaves adhesive residue on gold surfaces. Residual adhesive is removed with the solvent blend specified by the stripper supplier. Peel adhesion after processing is inspected according to IPC-TM-650 2.4.1. A drop below 12 N/100 mm indicates edge attack. Finished boards are inspected to IPC-A-600H Class 3 for foreign matter, undercut, and dielectric spacing. Terminal products include ENIG edge-connector boards used in line cards, automotive engine control unit substrates, and high-density routing panels.

Aerospace chemical milling lines for 2024-T3 and 7075-T6 skins maintain an alkaline etch bath at 130–180 g/L NaOH and 10–15 g/L Na₂S, with bath temperature 70–85°C. Etch rate on 2024-T3 is controlled to 15–30 µm/min through temperature and dissolved-aluminum concentration. The tape is laminated over scribed maskant boundaries to seal the line where a strippable elastomeric maskant meets bare aluminum. Application is performed at 20–25°C on dry substrate. Application below 18°C reduces adhesive wetting and increases edge voids that permit etchant penetration. The polyester backing is cut after lamination with a scalpel-type blade to leave a 0.8 mm overlap onto the liquid maskant. After immersion, the tape edge is inspected at 10× magnification for undercut. Visible air channels require re-masking before the next etch step. After chemical milling, the tape is removed before deoxidizing in concentrated nitric acid. Prolonged contact with concentrated HNO₃ is not listed in the manufacturer’s chemical resistance data for this tape, so qualification per ASTM D543 is required before any such use. The milled pocket is then processed to MIL-DTL-5541 Type I Class 3 conversion coating or to the applicable AMS 2473 chemical milling specification. Tape adhesion before immersion is qualified using ASTM D3359 Method A on a representative aluminum coupon. A minimum classification of 4A is required. Terminal parts include fuselage skin pockets, wing leading-edge skins, and rib webs with step-milled thickness gradients.

Leadframe Selective Plating Stop-Off: Drag-Out Control and Reel-to-Reel Tension Limits

Copper alloy leadframes stamped from C19400 strip are selectively plated with nickel, palladium, and gold in continuous reel-to-reel cells. The tape functions as a stop-off mask on carrying rails, tie bars, and areas where acid gold deposition must not occur. The tape is laminated at 0.2–0.4 MPa nip pressure and slit in-line to match the leadframe panel layout. Line tension is maintained at 8–15 N per 25 mm width. Higher tension produces adhesive yield and tape walk. The polyester backing resists deformation at cell temperatures up to 60°C. The rubber adhesive phase is the limiting layer and must be shielded from direct impingement of high-velocity electrolyte jets. Selective gold electrolyte is a weak acid gold cyanide solution with 8–12 g/L Au and a cobalt or nickel brightener at 0.3–0.8 g/L, pH 3.8–4.5, temperature 45–60°C, and current density 10–50 A/dm² in the impingement zone. The tape prevents gold overplate outside the functional contact area.

After plating, the strip enters a three-stage counterflow rinse at 45°C to remove cyanide drag-out from the tape edge. Final rinse conductivity must not exceed 10 µS/cm above feedwater. Tape removal occurs before trim-and-form. Terminal products are QFP and QFN leadframes and automotive connector contact carriers. Gold plating thickness is verified according to ASTM B488. Adhesion after peel is checked using IPC-TM-650 2.4.1. Nickel underlayers are controlled to ISO 4527 for phosphorus content and internal stress. If the tape edge retains plating bath residue after normal rinsing, the counterflow cascade is adjusted rather than raising rinse temperature above 60°C, which shortens adhesive holding time.

Sulfuric acid anodizing lines running at 15–20 wt% H₂SO₄ and 18–22°C use the tape to mask threaded holes, counterbores, and faying surfaces on 6061-T6 machined components. The tape is applied after vapor degreasing and alkaline cleaning but before racking. Edges are burnished with a PTFE tool to close air channels. The applied tape must withstand voltage ramp to 18 V and current density 1.2–1.8 A/dm² for 30–45 min. The polyester backing is hydrophobic. Wetting agents in the anodizing bath do not plasticize the adhesive. Edge wicking is controlled by the burnish step and by bath agitation below 2 L/min per 100 L bath volume. After anodizing, the tape is peeled before hot sealing in DI water at 96–100°C. Early removal prevents adhesive degradation from prolonged exposure near the upper service temperature. Parts are then sealed for 20–30 min and inspected per MIL-A-8625F Type II Class 2. Masked areas remain uncoated and are subsequently conversion coated or left as machined. Tape performance is qualified on test coupons by immersion in the production anodizing bath and by adhesion evaluation per ASTM D3359 Method A. Classification below 4A after 45 min immersion disqualifies the tape lot. Terminal components include avionic enclosure covers, pneumatic valve bodies, and optical mounting plates.

If Solder Stripping Bath Temperatures Exceed 45°C, Which Failure Mode Appears First?

In mixed-metallurgy PCB arrays where gold edge connectors must remain solder-free, the tape experiences a competitive failure sequence as solder stripper temperature rises above 45°C. The polyester backing retains dimensional stability. The rubber adhesive begins to plasticize and swell at the tape-copper interface. The first observable defect is edge lift along the gold-to-copper transition, not dissolution of the polyester film. The mechanism is diffusion of nitric acid and ferric nitrate complexes into the adhesive edge. High free acid increases the rate. At bath temperatures above 50°C, edge wicking typically appears within 5–10 min on unburnished tape lines. Properly burnished edges show visual wicking only after 15–20 min. Published data for wicking depth as a function of free acid for this specific tape configuration are limited. Production lines therefore use edge-wicking coupons to set the upper temperature boundary for each stripper formulation. The process window is bounded by adhesive plastication and not by the 121°C backing thermal rating alone.

Solder stripping baths are maintained with automatic free-acid analysis. Free nitric acid is held within the stripper supplier’s control band, commonly 0.8–1.2 N for a ferric nitrate based stripper at 35–45°C. Immersion time is 5–15 min. Tape removal after stripping is performed at substrate temperature below 40°C. Terminal products are mixed-metallurgy boards with ENIG edge connectors and HASL-free pads. Qualification includes ASTM D3330 peel adhesion before and after a production-representative immersion coupon, and inspection per IPC-A-600H. If edge wicking exceeds 0.2 mm per side, the tape application method is corrected rather than reducing bath temperature below the level needed for complete tin-lead removal.

Temporary Masking of Fiducials and Alignment Holes in Ferric Chloride Etching of AISI 304

Photochemical machining of AISI 304 half-etch features uses ferric chloride solution at 42–45°Bé and 45–50°C to etch both sides of a cold-rolled stainless steel sheet. The tape is applied as a secondary mask over fiducial holes, stress-relief slots, and alignment features that have been protected by photoresist but require an additional barrier during immersion. The polyester backing resists degradation in ferric chloride at these operating temperatures. The rubber adhesive maintains interfacial contact at etch chamber spray pressures of 0.5–1.0 bar. The tape is not used on fine-line features below 200 µm because adhesive cold flow can close small clearances. After etching, the tape is peeled prior to stripping the photoresist in alkaline solution. Residual adhesive is removed in a final degreasing stage with 99.9% isopropyl alcohol or a proprietary hydrocarbon cleaner, followed by passivation per ASTM A967.

Dimensional inspection of half-etch pockets uses an optical coordinate measuring machine with ±5 µm accuracy. Finished parts include encoder disks, fine-aperture screens, and fuel cell bipolar plates. The primary process limit is mechanical shear on the tape edge during high-pressure spray. A drop in peel adhesion from baseline to below 15 N/100 mm after 20 min exposure requires adjustment of spray nozzle spacing or a reduction in chamber turbulence.