Conformal Coating Process

Definition, scope, and who this guide is for

The conformal coating process is the application of a protective polymeric film over a printed circuit board assembly (PCBA) to shield electronic components from environmental aggressors. Unlike the structural integrity provided by the PCB substrate itself, this process focuses entirely on surface reliability. It creates a barrier against moisture, dust, salt spray, and chemical contaminants that can cause corrosion or electrical leakage. For procurement leads and engineers, defining this process is not just about selecting a material; it is about defining the exact boundaries of protection versus connectivity.

This guide is designed for decision-makers who need to move beyond the general concept of "weatherproofing" and into the specific engineering requirements that guarantee yield and longevity. It covers the transition from design intent to manufacturing reality, highlighting where specifications often fail and how to prevent costly rework. Whether you are building automotive sensors or industrial controllers, the goal is to establish a repeatable standard that your manufacturing partner can execute without ambiguity.

At APTPCB (APTPCB PCB Factory), we often see that the success of the coating process is determined long before the liquid touches the board. It relies on precise keep-out zone definitions, material compatibility checks, and rigorous cleanliness standards. This playbook provides the technical framework to specify, validate, and audit the conformal coating process effectively.

When to use the conformal coating process (and when a standard approach is better)

Determining the necessity of coating drives the subsequent engineering choices; applying it unnecessarily adds cost and rework complexity, while omitting it in harsh environments guarantees field failure.

Use the conformal coating process when:

High Humidity or Condensation: The device operates in environments where temperature cycling creates dew points (e.g., outdoor IoT, automotive under-hood).
Chemical or Salt Exposure: The PCBA is deployed in marine environments or industrial plants with corrosive gases (sulfur, chlorine).
Tin Whisker Mitigation: You are using lead-free components and need to inhibit the growth of conductive tin whiskers that could cause short circuits.
High-Voltage Arcing: You need to increase the dielectric strength between conductors that are spaced closely together, effectively allowing for tighter designs than air-gap standards permit.
Dust and Debris: The enclosure is vented (IP54 or lower), allowing conductive dust to settle on fine-pitch components.

Stick to standard (uncoated) assembly when:

Controlled Office Environments: The device sits in a climate-controlled server room or office (IP20+) with no risk of condensation.
High-Frequency RF Sensitivity: The dielectric constant of the coating material might detune sensitive RF circuits or antennas if not strictly controlled.
Thermal Dissipation Constraints: While thin coatings have minimal thermal impact, heavy potting or thick coatings can trap heat in high-power power electronics without proper thermal management design.
Frequent Prototyping/Rework: If the design is in early alpha stages and requires constant probing and component swapping, coating makes debugging significantly harder.

Conformal coating process specifications (materials, stackup, tolerances)

Once the decision to coat is finalized, the next step is translating "protection" into quantifiable specifications that a manufacturer can follow.

Material Type Selection:
- Acrylic (AR): Easy to rework, dries quickly, good moisture resistance. Standard for general electronics.
- Silicone (SR): High temperature resistance (200°C+), flexible, good for thermal cycling. Hard to rework.
- Urethane (UR): Excellent chemical and solvent resistance. Very hard to remove/rework.
- Parylene (XY): Vapor deposition process. Ultra-thin, pinhole-free, penetrates everywhere. High cost, specialized batch process.
Coating Thickness Targets:
- Standard liquid coatings (AR, UR, SR): 25µm to 75µm (1-3 mils).
- Parylene: 12µm to 25µm (0.5-1 mil).
- Note: Specifying "as thick as possible" is dangerous; excessive thickness causes cracking during thermal expansion.
Keep-Out Zones (KOZ):
- Clearly define areas not to coat: Connectors, test points, grounding pads, RF antennas, optical sensors, and switches.
- Tolerance for KOZ edges: Typically ±1mm to ±2mm depending on the application method (spray vs. brush vs. dam-and-fill).
Curing Method:
- Heat Cure: Requires oven profiling; ensure components can withstand the dwell time and temperature.
- UV Cure: Fast throughput; requires "shadow curing" mechanisms (secondary moisture cure) for areas under components where UV light cannot reach.
- Moisture Cure: Relies on ambient humidity; slower but low stress.
Viscosity and Solids Content:
- Define the viscosity range to ensure consistent flow under components without excessive capillary action into connectors.
Cleanliness Standards:
- Specify ionic contamination limits (e.g., <1.56 µg/cm² NaCl equivalent) prior to coating. Coating over flux residues traps active chemicals, leading to corrosion under the coating.
Edge Coverage:
- Sharp edges of components and PCB tracks often suffer from "thinning" due to surface tension. Specifying multiple passes or higher viscosity may be needed for critical edge protection.
Fluorescence:
- Require UV tracers in the coating material to facilitate easy visual inspection under black light.
Reworkability Requirements:
- Define if chemical stripping or thermal removal is acceptable for repairs.
Documentation:
- Require a dedicated coating layer in the Gerber files or a detailed mechanical drawing indicating masked areas.

Conformal coating process manufacturing risks (root causes and prevention)

Even with perfect specifications, the physical application involves fluid dynamics and surface chemistry that can introduce defects.

De-wetting (Fisheyes):
- Root Cause: Surface contamination (silicone oil, mold release agents, flux residues) lowering surface energy.
- Detection: Circular voids where the coating pulls away from the board.
- Prevention: Rigorous cleanliness testing pcb protocols and plasma treatment prior to coating.
Capillary Flow (Wicking):
- Root Cause: Low viscosity coating flowing into connectors or switches via capillary action.
- Detection: Intermittent connectivity or insulated contacts after curing.
- Prevention: Use masking gels (peelable masks) or "dam-and-fill" techniques around sensitive components.
Delamination:
- Root Cause: Poor adhesion due to "no-clean" flux residues that are incompatible with the coating solvent.
- Detection: Coating peeling off like a plastic sheet (tape test failure).
- Prevention: Verify chemical compatibility between the specific solder paste flux and the coating material.
Air Bubbles/Voids:
- Root Cause: Trapped air during mixing, spraying at too high pressure, or solvent boiling (curing too hot, too fast).
- Detection: Visible bubbles under magnification.
- Prevention: Vacuum degassing of material; optimized curing profiles (step-cure) to allow solvents to evaporate slowly.
Shadowing:
- Root Cause: Tall components blocking the spray nozzle from coating the area behind them.
- Detection: Uncoated patches visible under UV inspection.
- Prevention: Multi-angle spray heads (tilt and rotate) or manual touch-up after automated spraying.
Cracking:
- Root Cause: Coating applied too thick (CTE mismatch) or cured too rapidly.
- Detection: Micro-cracks visible after thermal cycling tests.
- Prevention: Strict process control on wet film thickness; automated thickness measurement.
Orange Peel Effect:
- Root Cause: Improper solvent evaporation rate or spray pressure issues.
- Detection: Uneven, textured surface finish.
- Prevention: Adjust solvent blend and spray atomization pressure.
Masking Failures:
- Root Cause: Masking tape or boots not applied tightly, allowing leakage.
- Detection: Coating on connector pins.
- Prevention: Use custom rubber boots for high-volume connectors instead of manual taping.

Conformal coating process validation and acceptance (tests and pass criteria)

To catch these risks before shipment, a robust validation plan is required that combines non-destructive routine tests with periodic destructive verification.

Visual Inspection (100%):
- Objective: Verify coverage and keep-out compliance.
- Method: UV (black light) inspection.
- Criteria: Continuous glow over coated areas; no glow on connectors; no bubbles bridging conductors.
Wet Film Thickness (Process Control):
- Objective: Real-time thickness monitoring.
- Method: Wet film comb gauge used on a test coupon or frame rail immediately after spraying.
- Criteria: Reading falls within the wet-equivalent of the specified dry thickness range.
Dry Film Thickness (Sample Basis):
- Objective: Verify final cured thickness.
- Method: Eddy current probe (non-destructive on metal) or micrometer on test coupon.
- Criteria: 25-75µm (or per spec).
Adhesion Testing (Periodic):
- Objective: Ensure coating bonds to the substrate.
- Method: Cross-hatch tape test (ASTM D3359).
- Criteria: Class 4B or 5B (less than 5% removal).
Ionic Contamination Testing:
- Objective: Ensure board was clean before coating.
- Method: ROSE testing (Resistivity of Solvent Extract) or Ion Chromatography.
- Criteria: <1.56 µg/cm² NaCl equivalent (IPC-J-STD-001).
Dielectric Withstanding Voltage (Type Test):
- Objective: Verify electrical insulation.
- Method: High-pot test across coated tracks.
- Criteria: No breakdown or arcing at specified voltage.
Thermal Shock (Reliability Test):
- Objective: Verify resistance to cracking.
- Method: -40°C to +125°C cycling (e.g., 100 cycles).
- Criteria: No visible cracking or delamination.
Solvent Resistance (Material Verification):
- Objective: Confirm full cure.
- Method: Solvent rub test (e.g., MEK rub for certain coatings).
- Criteria: Coating does not soften or dissolve (unless designed to).

Conformal coating process supplier qualification checklist (RFQ, audit, traceability)

Validating the product starts with validating the partner. Use this checklist to audit potential suppliers or APTPCB during the RFQ phase.

1. RFQ Inputs (What you must provide)

Gerber Files: Including a dedicated layer for coating areas and keep-out zones.
Master Drawing: Specifying material type (IPC-CC-830), thickness, and inspection class (Class 2 or 3).
Volume Estimates: Determines if manual spray, dipping, or selective robot is used.
Connector List: Identify which connectors need masking boots vs. tape.
Test Requirements: Specify if 100% UV inspection is required.
Packaging: Requirements for ESD bags and handling after coating (coating must be fully cured before bagging).

2. Capability Proof (What they must demonstrate)

Equipment List: Do they have automated selective coating machines (e.g., Asymtek, PVA) or is it manual spray?
Curing Ovens: Are ovens profiled specifically for the coating material?
Dispensing Accuracy: Can they hit ±1mm edge tolerance for selective coating?
Viscosity Control: Do they have automated viscosity monitoring or manual logs?
UV Inspection Booths: Are inspection stations properly lit with UV sources?
Material Handling: Storage of moisture-sensitive coatings (shelf life management).

3. Quality System & Traceability

IPC Standards: Certification to IPC-A-610 (Acceptability of Electronic Assemblies) and IPC-CC-830.
Batch Traceability: Can they trace a specific board to a specific batch of coating material and curing profile?
Operator Training: Are operators certified for manual masking and touch-up?
Cleanliness Data: Do they perform onsite cleanliness testing (ROSE/IC) before coating?
First Article Inspection (FAI): Do they provide a detailed FAI report with thickness measurements for the first run?

4. Change Control & Delivery

PCN Policy: Will they notify you before changing coating material brands?
Rework Capability: Do they have a defined process for stripping and re-coating defective boards?
Throughput: Does their curing capacity match your volume requirements?

How to choose the right conformal coating process (trade-offs and decision rules)

Different application methods yield different results. Choosing the right one depends on your volume, design complexity, and budget.

If you have high volume (>10k units) and simple geometry:
- Choose: Automated Dip Coating.
- Trade-off: Fast and cheap, but requires extensive masking of connectors. Coating thickness can be uneven (wedge effect).
If you have high mix, medium volume, and complex connectors:
- Choose: Automated Selective Coating.
- Trade-off: Higher setup cost (programming), but eliminates most manual masking. Very consistent thickness and edge definition.
If you have low volume or prototypes:
- Choose: Manual Spray (Aerosol or Spray Gun).
- Trade-off: Low setup cost, but highly operator-dependent. Consistency varies; requires 100% inspection.
If you need extreme protection (IP67/IP68 equivalent) or high voltage:
- Choose: Parylene (Vapor Deposition).
- Trade-off: Most expensive and slow (batch process). Requires specialized masking. Unmatched protection.
If you need to coat under low-standoff components (BGAs):
- Choose: Low-viscosity Selective Spray with Tilt.
- Trade-off: Risk of wicking into nearby vias or connectors. Requires careful selective solder design to ensure flux is removed from under the BGA first.
If you prioritize ease of repair:
- Choose: Acrylics.
- Trade-off: Lower chemical resistance compared to Urethanes or Epoxies.

Conformal coating process FAQ (cost, lead time, Design for Manufacturability (DFM) files, materials, testing)

1. How much does the conformal coating process add to the PCBA cost? Typically, coating adds 5% to 15% to the assembly cost. This varies heavily based on the masking labor required. Automated selective coating reduces labor costs for high volumes but has higher NRE (programming) fees.

2. How does conformal coating affect lead time? It usually adds 1 to 3 days to the production schedule. This accounts for cleaning, masking, application, curing (which can take hours), de-masking, and final inspection.

3. What DFM files are required for the conformal coating process? You should provide a Gerber layer (e.g., GKO or a dedicated user layer) that clearly highlights the areas to be coated or the keep-out zones. A PDF drawing with dimensions is also helpful for quality control to verify masking tolerances.

4. Can I use "No-Clean" flux with conformal coating? Yes, but it is risky. "No-clean" residues can interact with the coating, causing delamination or inhibiting cure. We recommend a chemical wash process and cleanliness testing pcb verification even if using no-clean flux to ensure long-term reliability.

5. What is the difference between conformal coating and potting? Conformal coating is a thin film (microns) that follows the shape of the components. Potting (encapsulation) fills the entire enclosure with a thick resin (millimeters). Potting offers superior impact and IP protection but is much heavier and impossible to rework.

6. How do you inspect coating thickness on a populated board? We use wet film gauges during the process. For cured boards, we can use eddy current probes on copper ground planes or measure witness coupons processed alongside the batch. Destructive cross-sectioning is used for dispute resolution.

7. Does conformal coating waterproof the electronics? It makes them water-resistant, not waterproof. It protects against humidity, condensation, and splashes. It does not allow for submersion unless combined with a watertight enclosure (potting is better for submersion).

8. What are the acceptance criteria for bubbles in the coating? According to IPC-A-610, bubbles are acceptable provided they do not bridge two conductive paths (reducing electrical clearance) and do not expose the component surface (voids).

9. How does selective solder design impact coating? If your board uses selective soldering, flux residues are concentrated around through-hole pins. If not cleaned properly, the coating will not adhere to these specific joints. Design the board with enough clearance to allow cleaning nozzles to reach these areas.

PCB Conformal Coating Services – Detailed capabilities of APTPCB’s coating lines, including material options and equipment.
PCBA Testing & Quality – Learn how we validate reliability through environmental stress screening and cleanliness testing.
DFM Guidelines – Best practices for designing your PCB to be manufacturing-ready, including coating layer setup.
Automotive Electronics PCB – Case studies on high-reliability boards where coating is a mandatory requirement.
Final Quality Inspection – How we integrate UV inspection and thickness checks into the final outgoing audit.

Request a quote for conformal coating process (Design for Manufacturability (DFM) review + pricing)

Ready to secure your electronics against the elements? At APTPCB, we combine automated precision with rigorous validation to ensure your coating specifications are met on every unit.

To get an accurate quote and DFM review, please provide:

Gerber Files: With a defined coating layer.
BOM: To check component compatibility.
Assembly Drawings: Highlighting critical keep-out zones and connector types.
Volume: To determine the most cost-effective application method (Spray vs. Dip vs. Robot).
Material Preference: (e.g., Acrylic, Silicone, or "Recommend Best for X Environment").

Click here to Request a Quote & DFM Review

Conclusion (next steps)

The conformal coating process is more than a finishing step; it is a critical engineering control that defines the environmental lifespan of your product. By specifying the right materials, defining precise keep-out zones, and enforcing strict cleanliness standards, you transform a standard PCBA into a ruggedized assembly ready for the field. Use the checklist and validation steps in this guide to align with your manufacturer, ensuring that every board leaving the line is protected exactly as designed.