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How to Apply PCB Conformal Coating

Applying a conformal coating to a PCB (Printed Circuit Board) is essential in electronics manufacturing to protect the board from moisture, dust, chemicals, and temperature extremes. As manufacturers are under increasing pressure to comply with high IP ratings, some believe new devices—especially portable ones—must have an IP67 classification or above.

Coating printed circuit boards, or PCBs, with a conformal coating, increases their water resistance, even with a lower IP rating. This post will serve as a vital resource for conformal coating on PCBs. Here’s a step-by-step guide on how to apply conformal coating:

What is PCB Conformal Coating?

What is PCB Conformal Coating

You may be familiar with the term “PCB conformal coating” in the context of printed circuit boards. But what exactly is PCB conformal coating? As the name implies, PCB conformal coating involves applying a thin layer of a protective substance to a circuit board.

The board’s components are all protected by the covering. These components include exposed metal, solder joints, exposed traces, and component leads. The conformal coating protects the metal components against fungi, moisture, spray, and other environmental impurities.

Advantages of PCB Conformal Coating

  • Moisture Protection
  • Dust and Debris Protection
  • Chemical Resistance
  • Thermal Resistance
  • Prevention of Corrosion and Oxidation
  • Electrical Insulation
  • Enhanced Reliability and Longevity
  • Vibration and Shock Protection
  • Improved Performance in Harsh Environments
  • Reduced Risk of Short Circuits

Different Types of PCB Conformal Coating

When designing PCBs, engineers often employ two categories of conformal coatings to layer printed circuit boards: classic and modern. This section will delve into the benefits and drawbacks of the highlighted conformal coating types.

1. Acrylic Resin (AR) Coating

Acrylic Resin (AR) Coating

Conformal coatings made of acrylic are frequently utilized because of their superior dielectric strength. They are especially renowned for being simple to remove and offer a passable degree of resistance to moisture and abrasion. They are perfect for applications requiring overhaul or repair because of this attribute.

Pros:

  • Easy to apply by brushing, spraying, or dipping.
  • Quick drying time and relatively simple curing process.
  • Easy to remove and rework with solvents.
  • Cost-effective for many applications.

Cons:

  • Limited chemical resistance, particularly against solvents.
  • Lower protection in high-temperature or high-humidity environments.
  • It is less robust compared to other coatings like epoxy or silicone.

2. Silicone Resin (SR) Coatings

Silicone Resin (SR) Coatings

Silicone coatings are well-known for their exceptional performance and flexibility across a broad temperature range. They are appropriate for environments with high humidity because of their strong resistance to chemicals, dampness, and salt spray.

Pros:

  • Strong chemical resistance, suitable for harsh industrial environments.
  • Excellent protection against moisture and high humidity.
  • Provides good abrasion resistance and durability.

Cons:

  • More difficult to remove than acrylic, requiring specialized solvents.
  • Longer curing time compared to acrylic.
  • Can be affected by UV exposure, potentially leading to degradation over time.

3. Urethane Resin (UR) Conformal Coating

Urethane Resin (UR) Conformal Coating

Urethane coatings are well-recognized for their resilience to chemicals and moisture. They also resist abrasions, making them a reliable option for demanding applications.

Pros:

  • High resistance to extreme temperatures, making it ideal for high-heat applications.
  • Excellent moisture resistance, suitable for marine or high-humidity environments.
  • Flexible coating, protecting against mechanical stress and vibration.

Cons:

  • Difficult to remove and rework.
  • Higher cost compared to acrylic and polyurethane coatings.
  • Can attract dust during curing due to its tacky nature.

4. Epoxy Conformal Coatings

Epoxy Conformal Coatings

Epoxy coatings are renowned for being extremely strong and resistant to dampness. These two-component mixtures form an impermeable, extremely strong layer.

Pros:

  • Extremely strong and durable, offering excellent physical protection.
  • Outstanding resistance to chemicals, moisture, and mechanical damage.
  • High dielectric properties, providing excellent electrical insulation.

Cons:

  • It is challenging to remove or rework once cured.
  • Curing can be complex and require precise conditions (temperature, time).
  • Can become brittle in extreme temperature variations or over time.

5. Parylene Conformal Coating

Parylene Conformal Coating

Vapor phase deposition is a unique application method for parylene coatings. It offers remarkable dielectric strength and provides unmatched resistance to solvents, moisture, and high temperatures.

Pros:

  • Vapor-deposited, providing uniform coverage even on complex geometries.
  • Excellent protection against moisture, chemicals, and extreme temperatures.
  • High dielectric strength and excellent electrical insulation.
  • It is lightweight and thin, making it ideal for size-constrained applications.

Cons:

  • It is expensive due to the complex deposition process.
  • Requires specialized application equipment (chemical vapor deposition).
  • Difficult to rework or remove once applied.

6. Thin Film/Nano Coatings

Thin Film Nano Coatings

You apply it in a very thin layer, which provides a basic level of hydrophobicity and protects against brief water exposure.

Pros:

  • Easy to Apply
  • Basic Hydrophobic Protection
  • Quick Drying Time
  • Cost-Effective

Cons:

  • Limited Durability
  • Minimal Chemical Resistance
  • Short-Term Water Protection
  • May Require Frequent Reapplication

How Is a PCB Conformal Coating Applied?

There are various ways to apply conformal coatings to PCBs. Specific techniques are better suited for a given application or level of production than others.

  • Brushing: This is a simple method if you’re working with small batches or making repairs. Use a soft brush and carefully apply an even coat across the PCB.
  • Dipping: In this method, you dip the entire PCB into the coating material, then let it drain. This ensures even coverage, but you might need to do some additional cleaning afterward.
  • Spraying: You can use an aerosol can or a spray gun, especially if you’re handling larger batches or automated processes. Be sure to apply the coating evenly in thin layers.
  • Selective Coating Machines: These automated systems allow you to apply coating only to specific areas of the PCB, making them perfect for high-volume production runs.

How To Determine PCB Conformal Coating Thickness

To determine the thickness of your PCB conformal coating, follow these steps:

1. Use a Wet Film Gauge

Before the coating dries, place a wet film gauge on the coated surface. Read the thickness directly from the gauge. This gives you an estimate of the coating’s thickness while it’s still wet.

2. Use a Micrometer or Calipers

Measure the PCB’s thickness before and after applying the coating using a micrometer or calipers. The difference between the two measurements indicates the coating thickness.

3. Opt for Non-Destructive Methods

  • Ultrasonic Thickness Gauge: This tool sends ultrasonic waves through the coating to measure its thickness without damaging the PCB.
  • Eddy Current Testing: This method measures thickness based on electrical impedance and is suitable for non-metallic coatings.

4. Cross-Sectioning (Destructive Testing)

For more accurate results, you can cut a section of the coated PCB and examine it under a microscope to measure the exact thickness of the coating layers.

5. Check Manufacturer’s Recommendations

Always consult the conformal coating manufacturer’s data sheet for guidance on the optimal coating thickness for your application.

The Curing Mechanism of PCB Conformal Coating

When applying PCB conformal coating, understanding the curing mechanism is essential for achieving optimal protection. Here’s how the curing process works:

  • Evaporation of Solvents 

    • If you’re using a solvent-based conformal coating, the curing process begins with the evaporation of solvents. As the solvent evaporates, the coating thickens and forms a solid layer on the PCB.
  • Chemical Reaction

    • For two-part systems, you’ll mix a hardener with the resin. This initiates a chemical reaction that causes the coating to cure. Follow the manufacturer’s instructions for the correct mixing ratio and application time.
  • Heat Curing

    • If your conformal coating requires heat curing, you must place the coated PCB in an oven at the specified temperature for a designated time. This process helps the coating reach its final properties.
  • UV Curing 

    • If you’re using a UV-curable coating, you will expose the coated PCB to ultraviolet light. UV exposure triggers a chemical reaction that instantaneously cures the coating, forming a hard protective layer.

PCB Conformal Coating Removing Process

1. Solvent Removal
You can dissolve conformal coating with solvents, but choose a solvent that won’t affect any components or parts on the PCB. Acrylic coatings are among the easiest and fastest to remove with solvents. For coatings like urethane and silicone, you may need to allow more soak time and use additional brushing for complete removal.
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2. Peeling
One advantage of some conformal coatings is that you can easily peel them off the board. This is typically possible with flexible, conformal coatings and certain silicone coatings.
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3. Microblasting
Microblasting involves using a concentrated mix of compressed air and soft abrasive materials to remove the conformal coating. This method is effective for small areas and commonly removes epoxy and parylene coatings.
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4. Thermal/Burn Through
This common method involves using a soldering iron to remove the conformal coating. It generally works well for most types of conformal coatings.
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5. Grinding or Scraping
For harder conformal coatings like epoxy, parylene, and polyurethane, you might need to use grinding or scraping methods. However, this process can be challenging and may lead to serious damage to the PCB if not done carefully.
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PCB Conformal Coating Standards

PCB Conformal Coating Standards

You apply conformal coatings according to specific industry standards that cover PCB acceptance requirements, the acceptability of electronic assemblies, and qualification and performance specifications for rigid PCBs. Your conformal coatings comply with the following standards.

1. IPC-CC-830

Establishes requirements for conformal coating materials, including their testing for thermal, moisture, and insulation properties.

2. IPC-A-610

Defines the acceptability criteria for conformal coating application, covering aspects like coverage, thickness, and defects.

3. IPC-HDBK-830

Guides on selecting appropriate conformal coatings and best practices for their application.

4. MIL-I-46058C

Specifies requirements for conformal coatings used in military applications, including performance under harsh conditions.

5. UL 746E

It covers safety requirements for conformal coatings used in electronics, particularly those related to flammability and electrical insulation.

6. IEC 61086

The international standard for conformal coating materials defines their properties, testing methods, and performance criteria.

Common PCB Conformal Coating Defects

  1. Adhesion Failure
  2. Moisture Ingress
  3. Chemical Resistance Failure
  4. Mechanical Stress Damage
  5. Thermal Degradation
  6. Contamination and Dirt Accumulation
  7. Cracking or Peeling
  8. Ultraviolet (UV) Degradation
  9. Incomplete Coating Coverage
  10. Electrical Failure due to Coating Breakdown

Common PCB Conformal Coating Testing

PCB conformal coating is a thin film applied to protect circuit boards from environmental damage. To ensure the quality and reliability of the coating, several tests are typically performed:

Thickness Measurement:
Purpose: To determine if the coating thickness meets design requirements. Too thin may compromise protection, while too thick may affect circuit performance.
Method: Common measurement methods include dry film thickness gauges and coating thickness gauges.
Example: Using a dry film thickness gauge to measure the thickness of the coating in different areas to ensure uniform thickness.
Adhesion Testing:
Purpose: To evaluate the adhesion strength between the coating and the substrate to prevent coating delamination.
Method: Common test methods include the cross-cut test and tensile test.
Example: Using the cross-cut test to create a grid on the coating surface, and then repeatedly applying and removing the tape to observe if the coating peels off.
Humidity Resistance Testing:
Purpose:To evaluate the performance of the coating in high humidity environments to prevent moisture penetration and circuit shorting.
Method:Place the coated samples in a high humidity environment and periodically check the appearance and electrical performance.
Example: Place the samples in a 85°C/85%RH constant temperature and humidity chamber for 24 hours and then inspect.
UV Stability Testing:
Purpose: To evaluate the stability of the coating under ultraviolet radiation to prevent coating aging, discoloration, or cracking.
Method: Expose the samples to ultraviolet light and periodically check the appearance and performance changes.
Example: Use a xenon arc weatherometer to simulate outdoor environments for accelerated aging tests.
Chemical Resistance Testing:
Purpose: To evaluate the corrosion resistance of the coating to various chemicals to prevent chemicals from corroding the circuit.
Method: Immerse the samples in different chemical solutions and observe if the coating dissolves, discolors, or produces bubbles.
Example Immerse the samples in salt spray, acid, or alkaline solutions to observe corrosion.

Conclusion

Consider applying a conformal coating to your PCB to shield it from harsh environments like moisture and extremely high or low temperatures. This layer increases dependability, strengthens electrical characteristics, and protects against contaminants.

Applying PCB conformal coatings such as acrylic, silicone, urethane, epoxy, parylene, and thin film is our company’s specialty at fscircuits. We follow industry guidelines. If you want to learn more about our services, please approach us.

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