3.5 Coating & Potting
Chemistries, masking, cure profiles, and void/edge-bead control—so protection adds reliability, not mystery.
Coating and potting protect electronics from moisture, dust, chemicals, and mechanical stress, but only when the right materials and methods are matched to the product’s risks. Conformal coatings—like acrylic, urethane, silicone, or parylene—form thin protective films, while potting compounds fully encapsulate components for maximum durability. Success starts with clean, dry surfaces and careful masking of connectors, test points, and other keepouts. Application must be even, with edges controlled to avoid thick beads or pools, and curing verified rather than assumed. Potting brings extra challenges like avoiding trapped air (voids) and managing heat buildup (exotherm), which require degassing, slow fills, and sometimes staged cures. When properly planned, documented, and inspected, these barriers add years of reliability without creating rework headaches or obscuring critical markings.
3.5.1 What “good” looks like
- Right chemistry for the risk (humidity, HV, vibration, chemicals).
- Clean, dry surface that the coating sticks to.
- Target thickness everywhere that matters, no beads/pools anywhere that doesn’t.
- Masked keepouts stay clean; labels/readers still scan.
- Cure verified (not just “time elapsed”).
- No voids in potting; parts survive exotherm; connectors fit.
3.5.2 Conformal coat chemistries (pick by environment & rework)
Rule: if you will ever need to rework, avoid parylene and thick UR unless the product truly needs them.
3.5.3 Surface prep (adhesion lives here)
- Cleanliness: no-clean is fine only if thin & fully activated (15.1–15.2). Heavy residue → clean or plasma.
- Dryness: bake PCAs 90–110 °C for 30–60 min (material-safe) to purge moisture before coat/pot.
- Activation (optional): plasma or corona improves wetting on solder mask, FR-4, and plastics.
- Adhesion checks: cross-hatch tape test on a coupon; quick dyne pen/contact angle spot check if you have the tools.
3.5.4 Masking strategy (protect what must stay bare)
- Tapes: polyimide + silicone adhesive; remove within the de-mask window to avoid glue transfer.
- Custom boots/caps: silicone boots on headers/switches—fastest at volume; design them during NPI.
- Peelable mask (latex-style): good for odd shapes; verify no ionic residue after peel.
- Selective spray/robot: reduce masking by drawing keepout polygons from CAD; tune edge overlap.
- Do-not-coat list: connectors, test pads, adjustment pots, mating grounds, heat sinks where thermal contact is critical.
3.5.5 Application methods (how to lay it down)
Thickness control: use wet film gauges on coupons; verify with eddy-current gauges or cut-ups during NPI.
3.5.6 Cure profiles & verification
- Solvent-borne (AR/UR): staged flash-off → bake 60–90 °C to drive solvent; watch bubbles.
- UV-cure: ensure UVA intensity meets spec (mW/cm²) and dose (J/cm²); run secondary cure (heat/moisture) for shadows under BTCs.
- RTV silicones: humidity/condensation cure; allow full through-cure before test; avoid closed totes that starve humidity.
- Proof, not hope: durometer/tack test on coupon, UV tracer uniformity, and—if coated for Hi-Z/HV—SIR spot checks.
Log recipe ID, lamp intensity, oven charts, and ambient RH.
3.5.7 Edge-bead control (keep rims from growing)
Edge beads trap solvent and foul bezels. Fight them with:
- Lower solids / reduce pass thickness; more thin passes > one thick flood.
- Spray angle slightly off the board edge so the fan sheds over air, not piles on the rim.
- Air-knife pass (soft) around perimeter before gel.
- Keepout dams: a tiny mask lip or pallet dam at the rail absorbs meniscus.
- Rotate & drain 10–20 s before bake.
3.5.8 Potting & encapsulation (when coating isn’t enough)
Key numbers to mind: viscosity, mix ratio, pot life, exotherm, CTE, thermal conductivity, dielectric strength.
Design for potting: vent paths, bottom-up fill access, avoid deep single pours (>10–15 mm) without staging, protect connectors.
3.5.9 Voids & exotherm control (potting)
- Prep: warm resin to 30–40 °C to lower viscosity; vacuum degas resin (−80 to −95 kPa) 5–10 min; pre-bake PCAs.
- Pour: slow bottom-feed or wall pour; avoid free-fall streams. Stage deep fills with cool-down between.
- Assist: vacuum potting (pull then release) or pressure cure (2–4 bar) to crush micro-bubbles (only if chemistry allows).
- Exotherm: small batches; wider trays; fillers reduce heat; monitor peak temp on first article.
- Proof: cut/scan a coupon pot, or X-ray thick sections if critical.
3.5.10 Inspection & acceptance (simple, visual, measurable)
Conformal coat
- Coverage continuous on required areas; no pools, craters, fisheyes, pinholes.
- Thickness within target window (per chemistry); no edge beads beyond cosmetic spec.
- Mask lines crisp; barcodes/marks readable; connectors/test pads clean.
Potting
- Level fill, no exposed components where protection is required.
- No voids visible on transparent resins; for opaque, acceptance via process proof (degassing log / pressure cure) + destructive sample at NPI.
- No cracks after cure; connectors seat; weight within tolerance.
3.5.11 Rework & repair (be honest about effort)
- Acrylic: solvent strip → clean → recoat.
- Urethane: chemical stripper (timeboxed), or mechanical scrape + local recoat.
- Silicone: slit/peel; residue wipe; recoat.
- UV acrylate: local UV/heat softening + mechanical.
- Parylene: laser/micro-abrasion only; plan test points up front.
- Potting: plan access wells; otherwise, rework often means replace the assembly.
Always restore thickness and re-run adhesion/coverage checks after repair.
3.5.12 Pocket checklists
Before you coat
- Chemistry picked to risk (AR/UR/SR/UV/parylene)
- Boards clean & baked; adhesion check on coupon
- Masking plan: tapes/boots/keepouts loaded from CAD
- Spray program & thickness target set; UV/oven verified
During
- Even passes, cross-hatch; thin films; edges controlled
- Cure proven (UVA dose / bake profile / RTV time)
- Remove masks on time; connectors/test pads clean
Potting
- Resin lot & mix ratio logged; vac degas done
- Bottom-fill; staged pours for thick sections
- Exotherm temp monitored; cure per spec
Inspect & release
- Coverage complete; thickness in band; no beads/defects
- Labels readable; SN scans; photos for first lot
- Records: recipe ID, viscosity/dose, cure proof attached
Bottom line: choose chemistry by risk and rework reality, make the surface clean and dry, and apply thin, even films with edges under control. For potting, beat air and heat with degas, bottom-fill, and staged cures. Prove coverage and cure every time, and coating/potting will quietly extend life instead of adding surprises.