3.5 Coating & Potting

CoatingResidues left behind in assembly are often invisible, yet they can dictate whether a circuit survives years in the field or fails within months. The decision to apply a protective layer is a core risk management mandate that protects the product from environmental and pottingmechanical aredamage. This chapter details the invisiblecritical shieldstrade-offs between Conformal Coating (thin film, reworkable) and Potting (full encapsulation, maximum protection).

3.5.1 The Protection Mandate: Coating vs. Potting

The choice between the two primary methods of electronics,environmental guardingprotection is a direct trade-off between Reworkability and Weight versus Mechanical and Chemical Resilience.

Feature	Conformal Coating (Thin Film)	Potting (Encapsulation)
Protection Level	Good barrier against moisture, contamination,dust, and mechanicalmild stresschemicals.	Superior: longComplete aftersealing assemblyagainst ismoisture, done.vibration, Whenand matchedharsh correctlychemicals (e.g., fuels).
Mechanical Strength	Minimal support; protects against light abrasion and condensation.	Superior: High resistance to thephysical product’sshock, environment,impact, conformaland coatingsvibration adddampening.
Reworkability	Easy. protectionCan be removed with minimalsolvents, thickness,heat, whileor pottingscraping; compounds create rugged encapsulationideal for theservicing.	Extremely harshest conditions. The challenge lies not in applying material, but in controlling preparation, masking, thickness, cure, and void formation so that protection strengthens reliability instead of hiding latent defects. With disciplined process control, coatings and potting transform from cosmetic layers into proven barriers that extend service life and customer confidence. 3.5.1 What “good” looks like Right chemistryDifficult. forRemoval theoften riskrequires (humidity,mechanical HV,destruction, vibration,risking chemicals).PCB damage.
Clean, dry surfaceWeight/Thickness	that the coating sticks to. Target thickness everywhere that matters,Adds nominimal beads/pools anywhere that doesn’t. Masked keepouts stay clean; labels/readers still scan. Cure verifiedweight (≈ 25 to 200 microns); maintains slim profile.	Adds significant weight and bulk; not justsuitable “timefor elapsed”).space-constrained devices.
NoHeat voidsManagement	Acts as a mild insulator.	Can be formulated to be thermally conductive into potting;dissipate partsheat survive exotherm; connectors fit.efficiently.

Mandate: The application must be based on the end-use environment. If the product faces extreme conditions (e.g., automotive under-hood, industrial vibration, chemical immersion), Potting is mandatory.

---

3.5.2 Conformal coatCoating: chemistriesMaterials (pickand byApplication

environment

Conformal &coating rework)is the preferred method when weight, space, and future reworkability are primary concerns.

A) Coating Types and Properties

Coatings are polymer-based films designed to adhere closely to the component contours.

ChemistryMaterial Type	TypicalKey thicknessCharacteristic	StrengthsReworkability	Watch-outs	ReworkBest easeUse Case
Acrylic (AR)	25–75Excellent µmmoisture resistance, high dielectric strength.	Easiest (removable with common solvents).	FastGeneral dry,consumer easyelectronics, tolow-cost strip, clearprotection.	Solvent/VOC; modest chem resistance	Easy (solvent)
Urethane (UR)	25–125 µm	Solvent/corrosion resistant	Can be isocyanate-based; harder to rework	Medium-hard
Silicone (SR)	50–200High µmflexibility, wide operating temperature range (∆T).	TempRemovable swing(special andsolvents/heat); good for high-vibration champenvironments.	DustAutomotive, magnetaerospace, ifhigh tacky;thermal long cure if RTV	Medium (peel/cut)cycling.
UV-cureEpoxy acrylates(ER)	100–300 µm	Very fasthard, withhighly UV;resistant thickto filmsabrasion and chemicals.	Extremely Difficult (rigid and permanent).	ShadowedHarsh areaschemical needexposure, secondaryabrasion cure	Medium
Paryleneresistance (C/N)	10–25 µm	Pin-hole free, zero edge bead	Capex; mask cost; no spot repair	Hard (ablate/machine)

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)fit-and-forget). 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)

Method	Use when	Notes
Selective spray/robot	Most builds	2–4 passes, 50% overlap, cross-hatch angles; verify fan width & edge sharpness
DipPolyurethane (UR)	HighExcellent volume,toughness simpleand keepoutsmoisture/solvent resistance.	GreatDifficult coverage;(requires maskingspecialized loadsolvents high;or watchthermal wicking into connectors
Flood/brush	Repair/small runsmethods).	ControlledAerospace, byapplications operatorrequiring skill;fuel goodvapor for edges & touch-ups
Parylene	Mission-critical	Vapor deposit; plan mask spend and rework strategyresistance.

B) Application Methods

ThicknessThe control:method usemust wetensure filmuniform gaugescoverage onand coupons;avoid verifycoating with eddy-current gaugesconnectors or cut-upscontact 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:points.

• LowerSelective solidsCoating / reduce pass thickness(Robotic):; moreMandatory thinfor passeshigh-volume, >high-density onePCBs. thickA flood.robotic nozzle sprays the coating only onto the required areas, minimizing the need for manual masking.
• Spray angleDipping: slightlyEffective offfor high volume and full encapsulation of the boardassembly, edgebut sorequires theextensive fanpre-masking shedsof overall air,connectors notand piles on the rim.contacts.
• Air-knifeVapor passDeposition (Parylene): Applies a highly uniform, ultra-thin film in a vacuum chamber. Non-contact (soft)zero aroundstress), perimeterbut beforeslow gel.
• Keepoutand dams:requires aspecialized tiny mask lip or pallet dam at the rail absorbs meniscus.
• Rotate & drain 10–20 s before bake.equipment.

3.5.3 Potting: Material Selection and Process Control

Potting involves pouring a liquid compound (resin) into an enclosure or shell that completely encases the PCB. This provides maximum mechanical reinforcement.

---

3.5.8

A) Potting &Compound encapsulationSelection

(when

The coatingchoice isn’tof enough)

resin dictates the final mechanical and thermal properties of the potted block.

FamilyCompound	ShoreKey Characteristic	NotesPrimary Advantage	Where it fitsDrawback
Epoxy	D–rigidHigh mechanical strength, rigid, excellent chemical resistance.	Strong,Superior adherent,impact highresistance exotherm;and candefense beagainst brittleharsh solvents.	Structural,Rigid chemicalstructure exposure
Polyurethane	A–D	Softer,can goodcause shock,stress moderatecracking chemduring resistance	Generalextreme electronics,temperature vibrationcycling.
Silicone	A–softHighly flexible, wide ∆T range, rubbery nature.	WideSuperior tempvibration swing,dampening forgivingand CTE,thermal lowcycling modulusprotection.	Sensors,Lower mechanical strength; higher cost.
Polyurethane (PU)	Good balance of flexibility and toughness.	Excellent abrasion resistance and moderate cost.	Limited high-temperature range compared to silicone.

B) Process Control Mandates

• Pre-Bake: PCBs must be perfectly dry before potting to prevent outgassing and void formation.
• Dispensing: Compounds (usually two-part systems) must be mixed under vacuum and dispensed slowly to prevent air entrapment and voiding, which compromises the seal integrity.
• Curing: The cure profile (time/temperature) must be strictly followed to ensure maximum chemical properties are achieved without creating excessive shrinkage stress on the PCB.

Final Checklist: Coating and Potting DFM

Parameter	Mandate	Rationale
Surface Preparation	Cleaning (Aqueous/Solvent) is mandatory before coating or potting.	Residues compromise adhesion, leading to delamination and under-film corrosion.
Material Match	Coating/Potting material must be qualified against the PCB laminate and component plastics for compatibility.	Prevents chemical attack, swelling, or plastic degradation.
Rework Consideration	Product design requires future servicing – Acrylic or Silicone coating is mandatory.	Ensures the product remains serviceable without requiring full board destruction.
Potting Process	Potting compound must be vacuum-mixed and dispensed; curing profile logged.	Prevents void formation, which creates thermal cycling,hotspots delicateand partsmoisture entry paths.
DFM/Keepout	All connectors, grounding points, test points, and unique labels must be masked or remain outside the coating/potting area.	Ensures electrical conductivity and external functionality are maintained.

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

---

By selecting chemistries wisely, keeping surfaces clean and dry, and verifying coverage and cure, protective layers become consistent, predictable, and trustworthy. Done right, they safeguard electronics for years with minimal rework or mystery failures.