Introduction
Electronic devices and components exposed to moisture, humidity, condensation, or direct water contact are susceptible to corrosion, electrical leakage, and reliability failures. Protecting circuits and assemblies with waterproof coatings and encapsulants is critical for applications where electronics will encounter wet environments.
But with the wide variety of waterproofing materials available, how do engineers choose the optimal protective coating? Key factors to consider include the application requirements, coating properties, material compatibility, application methods, and qualification testing.
This article provides a detailed overview of waterproof coating technologies for electronics along with guidance on selection criteria and best practices. With an understanding of the different coating options and their capabilities, designers can make informed decisions on implementing the best waterproofing protection regimes for their products.
Waterproofing Requirements
The first step is identifying the specific waterproofing needs based on the application and operating environment:
- Liquid exposure: Will the electronics encounter direct water submersion, pressurized sprays, splashing, or only incidental moisture?
- Duration: Is water exposure occasional and temporary, or continuous for extended periods?
- Chemical compatibility: Does the coating need to withstand specific fluids like saltwater, acids, or hydrocarbon oils?
- Temperature range: What are the temperature limits for material performance?
- UV and abrasion: Will the coating experience wear, UV radiation, or chemical degradation factors?
- Repairability: Does the application require repairable/reworkable waterproofing?
- Electrical properties: Are dielectric strength, surface insulation, and other electrical properties important?
- Application method: Can the coating be cast, brushed, sprayed, or dip applied?
With an understanding of the environmental stresses and product requirements, the waterproof coating type can be selected.
Coating Properties
There are a wide variety of waterproof electronics coatings that seal components and PCB assemblies from moisture ingress and corrosion. Here are key performance properties to consider when comparing options:
Solvent and Curing
- Solvent-cured: Use evaporation and chemical reaction of solvents to harden coating after application.
- UV-cured: Cure rapidly upon exposure to UV irradiation. Minimal solvents.
- Heat-cured: Apply as liquid or paste, then cure with oven baking process.
- Room temperature cure: Cure at room temperature using catalytic chemical reactions.
Permeability
- Impermeable: Cannot be penetrated by water molecules. Provides highest reliability.
- Conformal: Thin coats follow PCB topology but allow slow moisture diffusion. Reliance on isolation layers.
- Permeable: Absorbs moisture but provides temporary water resistance. Used with venting strategies.
Thermal and Mechanical
- Operating temperature: Upper limit before coating degrades, typically 120°C to 230°C.
- Coefficient of thermal expansion (CTE): Match to substrate to reduce stress and delamination.
- Hardness and mar resistance: Resists abrasion, nicks, cuts. Shore D 65-85 typical range.
- Adhesion: Bond strength to substrate, measured in psi. 500 psi minimum recommended.
- Flexibility: Ability to bend without cracking. Important for flex PCB applications.
Electrical
- Dielectric strength: Voltage withstand rating, reported in V/mil. 500-1500 V/mil target.
- Dielectric constant: Impacts impedance and capacitance for coating thickness. 2.5-4.0 typical.
- Insulation resistance: Resistivity in Ω or TΩ reflecting conduction losses.
- Dissipation factor: Dielectric losses from polar molecules under AC excitation. Lower is better.
Material Types
Some of the main classes of waterproof electronics coatings include:
Acrylics
- Simple, low cost resin coatings
- Fast room temperature cure
- Medium hardness and flexibility
- Moderate temperature rating
- Easy application by dip, spray, or brush
Urethanes
- Tough, abrasion resistant coatings
- Single or two part chemistries
- Temperature, UV, and chemical resistant
- Excellent adhesion and flexibility
- Can be rigid or rubber-like
Silicones
- Inorganic silicone polymers
- Very broad operating temperature range
- Excellent moisture and UV resistance
- Conformal thin film coats with high elasticity
- Moderate dielectric strength
Epoxies
- Strong adhesion and environmental resistance
- Range of flexible to rigid coatings
- Excellent dielectric properties
- Withstand rugged use conditions
- Room temperature or heat cure options
Parylenes
- Unique vapor deposited polymer films
- Extremely conformal thin coats
- Superb dielectric strength and moisture barrier
- Expensive process with high capital costs
This covers some of the major coating material types. Comparing options within each category based on specific needs is key.
Waterproof Coating Methods
There are several approaches to protect electronics from water exposure:
Conformal Coatings
Conformal coatings provide thin layers that follow the contours of components and PCBs. Common coating methods include:
- Spray: Automated selective spray systems or manual aerosol cans deposit controlled coats.
- Brush: Manual application allows selective brushing but can leave voids.
- Dip: Full immersion followed by controlled withdrawal leaves a thin uniform coat.
- Vapor deposition: Parylene coatings are applied as gaseous monomers that polymerize onto surfaces.
Typical conformal coat thickness ranges from 25-75 μm. While offering some water resistance, conformal coats rely on the PCB soldermask layer as the primary water barrier.
Conformal epoxy coating applied over PCB assembly
Potting Compounds
Potting fills the entire electronics enclosure with a thick protective resin encapsulant:
- Casting: Liquid resins poured or injected then cured to harden
- Compression: Potting compounds compressed around components
- Impregnation: Vacuum draws resin into complex assemblies
Typical potting thickness from 2mm up to 25mm. Provides excellent water resistance but limits access for rework.
Electronic device encapsulated with thick potting material
Sealed Housing
For the most critical applications, electronics can be entirely sealed within a watertight enclosure:
- Plastic or metallic housings: O-ring or gasket sealed, often with added potting material
- Hermetic housings: Use welded or soldered metal enclosure with added desiccant to maintain <1% internal humidity
This provides protection for continual or extreme submersion but with higher cost. Regular seal inspection and maintenance may be required. Accessing internals becomes difficult.
Electronics assembly sealed within a hermetic metal enclosure
Selection Criteria
Choosing an optimal waterproofing approach requires tradeoffs across many factors:
Exposure Level
- For temporary moisture or rain exposure, conformal coating provides adequate protection.
- Applications with prolonged submersion will require potting material or a fully sealed housing.
Mechanical Properties
- Conformal coats maintain flexibility for dynamic components and flex PCBs.
- Brittle potting compounds require fixed rigid support.
Thermal Properties
- Conformal coats allow convection and air cooling of electronics.
- Thick potting compounds act as a thermal insulator requiring other cooling methods.
Electrical Performance
- Conformal coats have limited dielectric strength and higher capacitance.
- Potting better withstands voltage exposures but may require insulation displacement.
Repairability
- Conformal coats can be selectively reapplied after rework.
- Encapsulants prevent access to components without full removal.
Weight
- Thin conformal coats minimize weight impact.
- Dense potting resins increase weight, which may be prohibitive.
Cost
- Adding conformal coat has lower material cost but process impact.
- Potting resins and sealed housings have higher material expense.
With an understanding of these considerations against application requirements, the optimal coating method can be selected.
Qualification Testing
Verifying a chosen waterproof coating will meet product reliability requirements under real-world conditions is critical. Typical qualification tests include:
Coating Cure Validation
- Measure shore hardness to confirm full cure
- Exposure to excess working time to check for tackiness or soft spots
- Thermal shock or rapid temperature ramping
- Sample cross-sectioning to validate uniform cure
Adhesion Testing
- ASTM D3359 crosshatch tape tests
- Initial adhesion and after thermal cycling
- Pull-off adhesion tests before and after liquid exposure
Electrical Properties
- Dielectric breakdown voltage
- Insulation resistance
- Combing resistance under bias voltage
- Capacitance changes from coating
Fluid Exposure
- Salt spray fog per ASTM B117 standard
- Thermal shock and moisture cycling
- Water submersion and chemical compatibility
- Flammability testing after exposure
The specific test conditions and performance requirements will depend on the product specifications and reliability models. But following a thorough qualification protocol matched to application conditions ensures the coating will function as needed.
Application Best Practices
Properly applying waterproof electronics coatings is vital for achieving protection. Key guidelines include:
Surface Preparation
- Ensure PCBs are clean and dry before coating
- Remove contaminants like rosin residue
- Lightly abrade glossy soldermask
- Apply adhesion promoter if needed
Process Controls
- Maintain ambient conditions like temperature and humidity
- Use clean tools dedicated for coating
- Have proper PPE and ventilation
- Carefully measure mixing ratios for multi-part coats
Coverage Assurance
- Mask connectors or test points to avoid coating
- Ensure full coverage on all components and PCB edges
- Apply multiple coats for thick buildup if needed
Curing
- Gradually ramp oven temperature for heat cure coatings
- For UV cures, match wavelength to coating reaction
- Validate full cure with testing before proceeding
Coating Repair
- Spot repair thin coats rather than full removal
- Have contingency plan for thicker encapsulant removal
- Plan accessibility provisions into design
Adhering to structured processes ensures the chosen coating consistently provides reliable waterproofing protection.
Summary
Protecting electronic products from water exposure threats requires selecting the optimal protective coating for the specific application conditions and requirements. Key takeaways include:
- Carefully consider liquid exposure levels, coatings properties, material compatibility, reparability needs, and other criteria.
- Determine if thin conformal coating, thick potting encapsulant, or sealed housing provides the right level of protection.
- Match candidate coatings to physical, electrical, and environmental reliability requirements.
- Qualify top coating options with standards-based testing tailored to end use stresses.
- Follow robust process controls and best practices during coating application.
By understanding the wide range of waterproofing options and how to select the best fit, engineers can implement effective water protection regimes for electronics operating in wet environments.
FQA
What are some typical applications requiring waterproof coatings on electronics?
Typical applications are outdoor equipment, marine electronics, automotive systems, appliances, IoT sensors, medical devices, industrial controls in washdown environments, and consumer electronics where liquid resistance is valued.
When is it preferable to use a thick potting encapsulant versus a thin conformal coating?
Potting makes sense for full submersion applications where conformal coats would allow moisture diffusion over time. Use thin coats when flexibility, repairability, weight impact, or thermal dissipation are critical.
What electrical factors should be considered when selecting a waterproof coating?
Key electrical factors are dielectric breakdown voltage rating, insulation resistance, dielectric constant, dissipation factor, and surface resistivity. Match coating properties to PCB layout clearances and any high voltage exposures.
How is a waterproof coating qualified for use in a particular product?
Typical qualification steps are evaluating coating cure consistency, verifying adhesion strength before and after reliability testing, measuring electrical properties, and exposing samples to simulated end use environments like salt fog or liquid submersion.
What are some best practice guidelines for applying waterproof electronics coatings reliably?
Best practices include proper surface preparation, process controls on ambient conditions and mixing, full coverage verification, controlled ramps for heat cures, validating full cure prior to further assembly, and having repair contingencies planned for.
