Conformal coating is a critical process in the manufacturing and protection of printed circuit boards (PCBs). This guide explores everything you need to know about conformal coating, including its purpose, materials, application techniques, and best practices to ensure optimal performance and longevity of your PCBs.

What is Conformal Coating for PCBs?

Conformal coating is a thin, protective layer of polymer applied to a PCB to safeguard the board and its components from environmental damage and corrosion. This film seamlessly conforms to the contours of the PCB, covering solder joints, component leads, exposed traces, and other vulnerable areas, ensuring comprehensive protection.

What is Conformal Coating Made Of?

Conformal coatings are typically composed of polymeric resins, sometimes diluted with solvents or water to improve application and flow. The choice of resin depends on the required level of protection, environmental conditions, application method, and ease of repair.

Types of Conformal Coatings:

1. Acrylic Resin (AR):

• Economical and easy to apply/repair.
• Good moisture and abrasion resistance.
• Easily removed with solvents.
• Less effective against solvent vapors (e.g., jet fuel).

2. Silicone Resin (SR):

• Excellent protection across a wide temperature range.
• High flexibility and vibration resistance.
• Ideal for high-humidity environments.
• Challenging to remove, requiring specialized solvents.

3. Urethane Resin (UR):

• Excellent moisture and chemical resistance.
• High abrasion and solvent resistance.
• Difficult to remove, often used in aerospace applications.

Why is Conformal Coating Necessary?

Conformal coating extends the operational lifespan of PCBs by protecting them from environmental hazards such as moisture, salt spray, chemicals, and extreme temperatures. It also enables higher voltage gradients and reduced track spacing, helping designers meet industry standards.

Top 13 Reasons to Use Conformal Coating:

1. Enhanced reliability.
2. Corrosion inhibition.
3. Resistance to fluids and humidity.
4. Temperature resistance.
5. High abrasion and chemical resistance.
6. Arc prevention.
7. Coverage of sharp edges.
8. Ease of application.
9. Specialized formulas for uniform films.
10. Extended product lifespan.
11. Breathable protection.
12. Improved insulation.
13. Minimal weight impact.

How Do You Apply Conformal Coating?

The application process depends on production throughput, board design, and quality requirements. Here are the most common methods:

Application Methods:

1. Manual Spraying:
   • Suitable for low-volume production.
   • Requires masking and is operator-dependent.
2. Automated Spraying:
   • Uses conveyor systems for consistent results.
   • Ideal for medium to high-volume production.
3. Selective Coating:
   • Robotic systems apply coating to specific areas.
   • Eliminates the need for masking in high-volume production.

PCB Cleanliness Prior to Coating

Pre-cleaning PCBs is essential to ensure proper adhesion and avoid defects. Skipping this step can compromise reliability. Common cleaning methods include:

• Ion chromatography.
• Temperature/humidity/bias testing.

Common Coating Defects from Poor Cleaning:

• Dewetting (Fisheyes): Caused by oil, wax, or silicone residues.
• Cracks and Ripples: Result from improper coating mixtures or thermal shock.
• Orange Peel: Uneven texture due to improper drying or application.
• Bridges or Webs: Thick coatings trapping bubbles.
• Dendrite Growth: Moisture absorption leading to surface contamination.

How Thick Should Conformal Coating Be Applied?

Conformal coatings are typically applied between 1 to 5 mils (25 to 127 microns). Thickness can be measured using:

1. Wet Film Thickness Gauge: For quick, on-the-spot measurements.
2. Micrometer: For hard coatings that don’t deform under pressure.
3. Eddy Current Probes: For non-destructive, accurate measurements (requires a metal backplane).
4. Ultrasonic Thickness Gauge: For non-destructive testing without a metal backplane.

How Long Does It Take for Conformal Coating to Dry?

Drying times vary based on the resin type, curing method, and coating thickness:

• Evaporative Cure: Handling time within an hour; full cure in days.
• Moisture Cure: Reacts with ambient moisture; full cure in days.
• Heat Cure: Accelerates polymerization; cure time varies with temperature.
• UV Cure: Near-instant curing in exposed areas; shadowed areas may take days.

How Do You Remove Conformal Coating?

Coating removal is sometimes necessary for rework or repairs. Common methods include:

1. Solvent Removal: Effective for acrylic, silicone, and urethane coatings.
2. Peeling: Suitable for certain silicone and flexible coatings.
3. Thermal/Burn-Through: Using a soldering iron to burn through the coating.
4. Microblasting: For precise removal of tough coatings like Parylene.
5. Grinding/Scraping: A last resort for hard coatings like epoxy.

Step-by-Step Design Guide for Conformal Coating

To ensure successful coating, follow these design considerations:

1. Group coated components together with 2.5mm clearance.
2. Provide 2.03mm spacing around components for coating prep.
3. Avoid placing parts near larger devices that block access.
4. Group connectors for easier dip coating.
5. Tent via holes to prevent coating flow.
6. Use sealed SMT connectors to avoid contamination.
7. Coat only necessary areas.
8. Avoid using coating as underfill.
9. Leave PCB edges uncoated or use handling strips.
10. Account for robotic coating limitations.
11. Clear mounting holes and grounding areas with 2.5mm spacing.

Common Conformal Coating Defects: Identification and Prevention

Common Defects:

1. Dewetting: Caused by surface contamination.
2. Delamination: Due to insufficient tack time or contamination.
3. Air Bubbles: From improper mixing or application.
4. Fisheyes: Resulting from oil or water contamination.
5. Orange Peel: From uneven atomization or rapid evaporation.
6. Cracking/Crazing: Due to excessive thickness or high curing temperatures.

Prevention Tips:

• Ensure thorough cleaning before coating.
• Follow manufacturer guidelines for thickness and curing.
• Maintain application equipment properly.
• Control environmental conditions during application.

Conclusion

Conformal coating is an essential step in protecting PCBs from environmental hazards and ensuring their long-term reliability. By understanding the materials, application methods, and best practices outlined in this guide, you can optimize your coating process and achieve superior results. Whether you’re working on consumer electronics or mission-critical aerospace systems, conformal coating is a key factor in maintaining the performance and durability of your PCBs.

By following this comprehensive guide, you’ll be well-equipped to implement conformal coating effectively, ensuring your PCBs are protected and perform reliably in even the most demanding environments.