Exposure to moisture is one of the major reliability risks for printed circuit boards (PCBs) used in humid environments or applications involving water contact. Moisture ingression into a PCB can lead to short circuits, corrosion, electrical leakage, dendritic growth, and other failure mechanisms. Therefore, implementing moisture protection solutions for PCBs is critical to ensure robust field performance and long operating life. This article provides an overview of various techniques used for waterproofing PCBs against moisture ingress and condensation issues.

Need for Waterproofing PCBs

PCBs consist of conductive copper traces separated by insulating polymeric substrates. But when exposed to moisture, the insulation resistance degrades over time and may eventually cause short circuit faults. Some ways moisture can damage PCBs are:

• Electrochemical migration of conductive metal dendrites between adjacent copper traces
• Corrosion of metallic traces and component leads
• Swelling and delamination of polymeric substrates
• Promotion of electrochemical reactions leading to leakage currents
• Lowering of surface resistivity
• Increase in dielectric constant of insulating materials

Moisture damage risks are elevated for PCBs used in applications like:

• High humidity environments
• Outdoor electronics exposed to rain or water splashes
• Automotive under-the-hood electronics
• PCBs in proximity to cooling water or plumbing
• Appliances and hydronic systems prone to leaks or spills
• Marine and nautical electronic equipment
• Aerospace electronics exposed to condensation issues

Implementing suitable waterproofing is vital in these scenarios to enhance the safety, durability and reliability over product lifespan.

Waterproofing Methods for PCBs

Various waterproofing techniques are adopted during PCB design, manufacture, and assembly to guard against moisture related failures.

Conformal Coatings

Applying a thin polymeric conformal coating over the assembled PCB creates a protective barrier against moisture and other contaminants. Acrylic, polyurethane and silicone conformal coatings are popular choices. Selective coating of critical areas helps reduce costs. The coating electrically insulates the PCB from any surface wetting or condensed moisture. Conformal coatings with higher hydrophobicity provide better moisture resistance.

Potting Compounds

Encapsulating the PCB assembly in a polymer potting compound creates robust mechanical protection as well as moisture resistance. Epoxy, polyurethane and silicone gels or elastomers are common potting materials. The compound is poured or dispensed to immerse the PCB within the protective barrier. Potting offers protection against direct water immersion but rework is difficult. Selective potting of critical components is an option.

Laminated Barriers

Protective plastic films like polyvinylidene chloride (PVDC) laminated over the PCB surface act as moisture barrier layers, especially for flexible circuits. The film prevents moisture penetration through the substrate while allowing circuit functionality. The barrier films are attached using adhesive lamination. Adequate ventilation is required to avoid condensation under the film.

Board Coatings

Moisture-resistant coatings are applied to the PCB substrate during fabrication. These include:

• Humiseal coatings: Urethane or acrylic coatings deposited directly over substrates
• GelMA coatings: UV-cured methacrylated gelatin coatings reduce water swelling
• Hydrophobic nanocoatings: Depositing silicon dioxide nanoparticles create a hydrophobic layer over board surface

Such coatings provide in-built moisture resistance integrated into the PCB structure.

Sealing with Resins

Exposed areas of PCB substrates can be sealed using ultraviolet (UV) or thermal curable resins. Elastic sealing resins like silicon, polyurethane, or epoxy effectively cover pores within the board material, preventing moisture wicking. Resins cure into solid barriers. But they add weight and may require significant preparation.

Via Protection

Vias acting as pathways for moisture ingress are sealed using liquid EPoxy or METhacrylate (epoMeT) resins. The resin is applied over via sidewalls through micro-dispensing and cured. Via filling also enhances insulation resistance and prevents chemical leakage through inner layer vias.

Solder Mask Over Bare Copper (SMOBC)

In SMOBC PCBs, solder mask coverage is extended over bare copper traces in addition to the traditional pad masking. This provides environmental protection for traces against moisture and other contaminants. The solder mask over traces must be removed during rework.

choosing Moisture-Resistant Materials

PCB substrates should exhibit high resistance to moisture absorption and good dimensional stability across humidity changes. Choosing materials like polyimide, PTFE, and some ceramic-filled laminates which are not hygroscopic minimizes moisture uptake.

Hermetic Sealing

For highly moisture-sensitive PCBs, hermetic sealing of the entire assembly within a metal or ceramic package provides the highest level of waterproofing. The enclosure is sealed either using seam welding or adhesive bonding techniques. A desiccant pack absorbs trace moisture inside the package. Hermetic sealing prevents external moisture ingress but increases costs.

Hydrophobic Coatings on Components

Applying hydrophobic conformal coatings over moisture-sensitive components like MOSFETs and chip capacitors safeguards them from internal moisture absorption and surface leakage. Nano-coatings using molecules with hydrophobic functional groups are also being developed.

Process Considerations for Waterproofing

Some best practices during PCB fabrication, assembly and inspection enable effective moisture protection:

• Ensure complete copper coverage by solder mask after wet etching
• Seal exposed laminate edges using edge coats or potting
• Minimize pores in copper and substrate layers
• Optimize lamination temperature, pressure and dwell time
• Implement automated inspection of coatings and seals
• Adhere to IPC specifications for coating materials and thickness
• Verify moisture resistance using leak testing, insulation resistance and high voltage testing
• Assess performance under temperature-humidity cycling and environmental stress testing

Conformal Coatings for PCB Waterproofing

Conformal coatings deserve special mention as the most widely adopted PCB waterproofing technique. Some key considerations when using conformal coatings are:

Coating Selection

Popular options are acrylic, polyurethane and silicone-based conformal coatings. Silicones provide the best hydrophobicity and moisture resistance. Acrylics are moderately hydrophobic. Cost, flexibility, thermal stability and ease of application are other selection criteria.

Coating Method

Application techniques include:

• Selective coating using automated dispensers
• Dipping in coating baths
• Manual spray coating
• Brushing using foam swabs
• Inkjet printing deposition of coatings

Coating Thickness

Typically 25-75 microns coating thickness is adequate. Thicker coatings above 150 microns are harder to cure and may flake off. Insufficient coating thickness leaves pinholes. 50-60 microns offers a good balance.

Inspection

Coatings require automated optical inspection for detecting pores, pinholes, misses and thickness verification to ensure waterproofing reliability.

Strippability

Strippable coatings facilitate repairs. Solvent-removable silicones and acrylics allow minor touchup without recoating the entire board.

Cost Optimization

Selective coating over critical components reduces costs compared to overall conformal coating. Coating materials are also available in aerosol spray cans for low quantity usage.

Conclusion

Waterproofing of PCBs is critical for products used in humid operating environments or involving liquid spill risks and condensation. Techniques like conformal coatings, potting compounds, laminated barriers, board coatings, sealing resins and via filling protect PCBs from moisture ingress right from the board level. Hermetic enclosures provide full system-level waterproofing. A combination of surface coatings, production process controls and inspection steps can help ensure robust moisture protection for enhanced field reliability over product life. However, the waterproofing methods must be chosen carefully to balance performance, design impact, reworkability and cost tradeoffs.

Frequently Asked Questions

Q1. Why is moisture protection necessary for PCBs?

Moisture can damage PCBs through short circuits, electrical leakage, copper corrosion, dendritic growth and substrate swelling. Waterproofing enhances safety, durability and reliability in humid conditions.

Q2. When should PCB waterproofing solutions be used?

Waterproofing should be considered for PCBs used in humid environments, applications prone to spills or direct water contact, outdoor electronics, marine systems, under-the-hood automotive electronics, etc.

Q3. What are some common techniques to waterproof PCBs?

Common waterproofing techniques include conformal coatings, potting compounds, laminated moisture barriers, board surface coatings, sealing resins, solder mask over bare copper (SMOBC) , and hermetic enclosures.

Q4. How are conformal coatings used for PCB waterproofing?

Conformal coatings like acrylic, polyurethane and silicone can be selectively applied over the PCB surface through spraying, dipping or inkjet printing to create a hydrophobic protective layer against moisture.

Q5. How can PCB manufacturers improve moisture resistance?

Using moisture-resistant base materials, sealing exposed edges, minimizing pores in copper and substrates, optimizing lamination and applying hydrophobic coatings on board surface improve moisture resistance.