Parylene Coating Blog by Diamond-MT

All About Conformal Coatings

Posted by Sean Horn on Fri, Jun 23, 2017 @ 08:00 AM

Conformal coatings insulate printed circuit boards (PCBs) and similar electronics; their protection increases devices’ tolerance to harsh environments.  The result is undisturbed function through a range of frequently harsh operating environments and performance conditions.  Conformal coatings provide these services for aerospace/defense, automotive, consumer, and medical devices.  They are adaptable for LED uses, as well as MEMS/nanotechnology, and other uses.

There are five basic types of conformal coating.  Four are comprised of materials applied to substrates by liquid (wet) methods; these are acrylic resin (AR), epoxy resin (ER), silicone resin (SR), and urethane resin (UR).  Primary liquid application processes are:

  • brushing the wet coating onto substrate surfaces,
  • dipping, immersion of assemblies into canisters of liquid coating material, and
  • spraying the coating onto substrates.

Liquid methods are generally inexpensive and somewhat easy to use, accounting for most conformal coating assignments.conformal coating process.jpg

          Parylene (XY) is the fifth major type of conformal coating.  It uses a chemical vapor deposition (CVD) process for substrate application.  Unlike liquid coatings, which cover surface areas only, gaseous parylene CVD penetrates deep within substrate areas, creating a truly conformal, uniform and pinhole-free film.  Providing generally superior conformal coverage, parylene is a costly, time-consuming process compared to liquid coatings.

Types of Conformal Coating Compared                                       

          Selecting a conformal coating requires careful analysis.  Each coating material offers a unique blend of benefits and drawbacks that must be considered in relation to a given application.  Beginning with liquid coatings, comparative descriptions of conformal coating materials offer the following information:

Acrylic:  Used primarily for moisture protection on PCBs, acrylic has traditionally been the least expensive and easiest conformal coating to apply, by liquid brush, dip, spray methods.  A very popular conformal coating choice, AR does not shrink during cure, while producing reliable fungus/humidity resistance.  Other AR-advantages include: 

      • Easy coating removal for rework/repair. 
      • Good dielectric barrier properties
      • Long pot life
      • Low glass transition (Tg) temperatures.

These benefits are largely counterbalanced by such drawbacks as: 

  • Limited solvent resistance
  • Limited abrasive/stress-relieving properties.
  • Cost-advantage/effectiveness has declined relative to competing coatings.

These properties indicate AR is less useful for applications requiring higher operating temperatures, prolonged exposure to moderate/strong solvents, and exceptional, longer-term coating strength.

Epoxy:  With reliable abrasive, chemical and humidity resistance, epoxy films are applied similarly to acrylic and other liquid coatings.  ER offers users these benefits:

  • Exceptional surface hardness and service durability.
  • Good dielectric and moisture barrier properties.
  • High glass transition (Tg) temperature in comparison to most coatings.

Drawbacks include:

  • Epoxy’s hardness and durability make it difficult to remove or rework if damaged.
  • Coatings shrink during polymerization.
  • Thermal extremes during operation significantly lower epoxy’s stress resistance.
  • Some lack UV trace

Silicone:   Very versatile, silicone conformal coating can be customized according to a product's precise requirements; surface treatments range from elastomeric, stress-relieving coverings to those far more abrasion-resistant and durable.  SR is particularly suited for automotive applications, because of its endurance in extreme, high-temperature environments.  Its operating temperature range isf -55°C - +200°C.   Other benefits include:

  • Excellent moisture barrier properties.
  • High humidity/UV/corrosion resistance.
  • High dielectric strength.
  • Low surface energy, which promotes enhanced wetting.
  • Low dissipation factor.
  • Good adhesion to most PCB materials.
  • Rapid curing.
  • Thicker film applicability generates thermal protection and vibration dampening
  • Easy application and rework.
  • Low toxicity.

Pertinent drawbacks of silicone conformal films include:

  • Poor abrasion/wear resistance.
  • Poor solvent resistance, compared with other conformal coatings
  • Achieving optimal coating thickness requires more time and attention than other liquid materials.

Urethane:  UR demonstrates exceptional chemical resistance, consistent dielectric properties, and low moisture permeability.  Available in single- or dual-component, UV-curable, or water-based versions, UR is often the optimal conformal coating choice for PCB protection and applications that demand exposure to chemical solvents.  Urethane conformal coating:

  • Provides dependable abrasion/humidity/solvent resistance.
  • Protects substrates from moisture and contaminants
  • Offers durability/hardness, and wear resistance.
  • Good temperature flexibility at low temperatures
  • Mitigates tin whisker growth
  • Facilitates miniaturization, insulating signal-traces situated in close proximity.

UR drawbacks include

  • Slow cure.
  • Removal/rework a challenge, because of its solvent-resistant nature.
  • Can't withstand high-vibration/high-heat environments.
  • Limited bond-strength.
  • Coating break-down when used over larger areas.
  • Limited reparability.

Parylene:  CVD-derived parylene conformal films provide the thinnest effective coating available, with excellent substrate coverage.  Film consistency is exceptionally uniform and pinhole-free, preventing leakage. Parylene's exceptional dielectric properties recommend their use for a wide array of electrical components.  Benefits of parylene:  

  • Adhesion to a wide range of substrate geometries.
  • Can be applied and effective at sub-micron thicknesses
  • Chemically inert. 
  • Conformability and flexibility.
  • Excellent dielectric/moisture barrier properties.
  • High optical clarity.
  • Tin whisker mitigation.
  • Penetrates extremely small spaces and crevices.

Parylene’s major drawbacks are: 

  • More expensive, attributed to higher raw materials’, labor, and lot-volume costs.
  • Limited throughput; batch process.
  • Sensitive to contamination. 
  • Requires an abrasion-based method for removal.

Optimal conformal coating use and protection is reliant upon selection of the conformal coating type appropriate the performance needs and environment of the project.  Thus, carefully balancing benefits and drawbacks of the types of conformal film under consideration with the coating assignment is necessary.  The professionals at Diamond MT are always on-hand to help you make the correct choice for your conformal coating projects.

To learn more about the conformal coating process, download our whitepaper now:

Conformal Coating Process Whitepaper

Tags: conformal coating, conformal coatings