by ADL Technology ADL Technology

Conformal coatings are a protective coating that covers the components of a Printed Circuit Board (PCB), or PCB Assembly (PCBA).   While it is simple to say that you want to protect a board, here are some of the key engineering considerations for conformal coatings

  1. Environmental Requirements: moisture, heat, abrasion, mold, mechanical stress
  2. Chemical corrosion protection
  3. Electrical properties, such as dielectric constant, insulation resistance, dissipation factor, and dielectric breakdown voltage
  4. Outgassing Level requirements
  5. Moisture absorption or vapor transmission requirements

Engineering requirements will usually indicate the coating type because different coating types have specific properties.  The primary types of coating types, properties and benefits are listed below:

Coating Type Properties and Benefits
Parylene Water resistant, thermal endurance, chemically inert, bio-compatible, low outgassing, dry lubricity, adhesion to other coatings
Acrylic Ease of rework, simple to dry, good moisture resistance, high fluorescence, and viscosity easy to adjust
Silicone wide range of temperature stability, flexibility, high dielectric strength, good moisture resistance, low surface energy after wetting
Polyurethane Good dielectric and moisture properties, solvent, and abrasion resistant, less reversion potential
Epoxy Water/humidity resistant, generally not permeable, high level of abrasion and chemical resistant.

ADL has selective conformal coating machines as one of their process options.

Applying Conformal Coatings

Coatings can be applied manually or automatically using a robotic assembly.  The choice of application method will depend on the viscosity of the material, number of units to be protected, and available equipment.  At ADL we offer both manual and automatic conformal coating application.

  1. Manual or automatic. The PCB or PCBA is dipped into the desired coating. While this is effective for larger production runs, maintaining consistency is more problematic, especially in terms of coating thickness, material slump, and purity.
  2. Manual only. production staff paint the coating on with a brush. Excellent for small runs or prototypes, this method is not suited for larger production runs.
  3. Manual or automatic. Overspray requires production staff to mask components that should not be coated.
  4. Needle.  Manual or automatic, this method uses a needle to dispense beads of coating material as needed. Masking is not needed and it tends to be precise and inexpensive.

Curing Methods

Since coatings are in a liquid form when applied, curing the coating properly is vital.   There are 4 ways to cure the conformal coatings added to PCBs:

  • Moisture. Boards are exposed to an controlled humidity level environment for a specific amount of time.
  • Heat curing. Uses convection and/or UV curing depending on the chemistry of the coating material.
  • UV cure. Coated boards are cured by exposing them to short periods of UV light. While this eliminates the need for ovens, the rapid change in temperature can cause components to break off.
  • Reactive/Solvent Curing. Conformal coating elements cure by chemical means, either due to an additive as the coating is applied or mixed.

ADL’s manufacturing facilities offers all four types of curing but we mostly use moisture and reactive curing.

Curing Considerations

Manual application of conformal coating

Making sure that your projects PCB’s are properly protected is a vital part of making sure your final product reflects the standards you company is known for. As you consider your next PCB project, do you want assistance answering the following coating questions?

  • What are environmental conditions does the board need to be protected against? Moisture? Chemicals? Erosion?
  • What temperature range will the electrical device function in?
  • Are there any specific physical, electrical, and chemical requirements for the coating material?
  • Are there any electrical, chemical, and mechanical compatibility considerations with the parts and substances to be coated
  • Is reworking or fixing the part once the coating is applied a consideration
  • Is the price of the coating or its application method a consideration?
  • Is the curing type or speed a consideration?