A Comparison of the Effectiveness of Conformal Coatings for Use with IoT Components

Sensors Insights by Sean Horn

 

The Internet of Things as a Concept and Reality

As a concept, the term Internet of Things (IoT) encompasses smart networks' escalating collaborative power for communicating with each other, connecting electronic processing systems, locally or worldwide. Relying on the Internet's interactive capacities, IoT energizes enhanced smart automation in a growing variety of applications, including those for aerospace/aeronautic communications, embedded systems, industrial equipment, instant-on MCUs, neural networking systems, smart RFID-memory tags, and wearable medical devices.

Wired or wireless, the IoT connects these products and systems to the Internet, each other and their users and, as such, is poised to influence most of the things people do throughout the 21st century. By 2020, Web-connected things will comprise 85% of 29.5 billion Internet global connections. In addition to the somewhat specialized applications mentioned above, IoT systems are already in play for items used on a daily basis, including cameras, personal fitness bands, thermostats, washing machines or weather monitors — virtually anything with an on/off switch and Internet connectivity.

By 2018, sales of IoT-associated products and systems are expected to tally in excess of $100 billion. Internet users' growing expectations of guaranteed IoT functionality as an integral component of most new web-enabled objects and related products are reflected by this forecast of the commodities' dollar-value, representing an increase of 67%, in just three years (from $60 billion, 2015).

Protection of printed circuit boards (PCBs), semiconductors, sensors, and similar electrical components that power the IoT is essential to their ongoing and efficient performance. Maintaining optimal functionality at low levels of power is critical to IoT systems' cost-effective implementation. Equally as important is robust yet flexible adaptability to a wide range of operating conditions. As the IoT expands its uses and influence, microcontrollers, sensors and related assemblies will be increasingly used to drive public infrastructure applications.

For instance, cities will become further connected through the IoT, exemplified by smart electric grids, roads and street lights. In this regard, the trend toward municipal LED lighting helps both cities and citizens, potentially diminishing city energy costs in excess of 30%. Further adding to this trend, evidence suggests these developments:

  • Private industry will experience considerable IoT input, as the industrial Internet develops and implements Internet-driven factory, logistic, and medical systems' applications.
  • Connected homes and passenger cars will become more prevalent, as will wearable systems that connect to the Internet
  • It is vital that chips, sensors and devices powering IoT activities are properly protected. Conformal coatings have proven exceptionally useful protecting these components.
  • Conformal Coatings and Their Uses

Composed from a variety of polymeric materials, conformal coatings primary purpose is safeguarding the function of electronic and mechanical assemblies, circuitry, and related components. By insulating the substrate, they protect PCBs, sensors and associated parts and products from environmental contamination during use, extending their operational life, while improving performance. Without conformal coatings, many of the products we commonly take for granted would not function nearly as well, and would need to be more frequently replaced. In general, the protection conformal coatings afford IoT assemblies and products include: security from:

  • Contamination caused by exposure to extreme temperatures, humidity, and other elements of harsh physical environments.
  • Contact with acids, solvents or, in the case of medical IoT systems, bodily fluids.
  • Conductor electro-migration, corrosion, dendritic growth, or short circuits to electronic assemblies and circuitry.
  • Stress and exposure during operation, insulating the item with a flexible, durable coating to assure ongoing functionality.

To provide appropriate protection, conformal coatings are applied to the substrate surfaces of IoT assemblies in fine layers, typically in the range of a few millimeters at a time until the desired coating-thickness is achieved. Most of the commonly used conformal coatings — acrylic, epoxy, silicone, urethane — are applied by wet techniques, dipping the object into the liquid coating material, or through flow-coating or spraying the object; however, robotic-dispensing and select-coating methods are increasingly used with these materials.

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The major exception is parylene, which is applied through a vapor deposition process. Of the major conformal coating materials, parylene may be best suited for IoT purposes. It specific properties are best-positioned to assure manufacturers provide their IoT devices more comprehensive protection, improving the overall functional and performance quality of their products.

Comparing the properties of the other conformal coatings is instructive:

  • Acrylic: Applied by wet techniques — via brush, dip, or spray methods — acrylic coatings dry and retain their shape during curing process. While acrylics exhibit persistently reliable fungus and humidity resistance, they can more readily break-down at higher temperatures than competing coatings; in addition, their abrasion and solvent-resistant capacities are limited. Their IOT-effectiveness is restricted because of the coating thickness required to provide dependable effectiveness; acrylic's former cost advantage compared to other coatings has declined in recent years.
  • Epoxy: Epoxy coatings resemble acrylics because they are applied by similar wet techniques, and exhibit good humidity resistance. However, they significantly differ from acrylics in that they demonstrate a high degree of both abrasion and chemical resistance. Because film shrinkage across the substrate during the polymerization process is common, the effectiveness of the extremely durable coatings generated by epoxy is largely neutralized. Moreover, exposure to thermal extremes significantly lowers their stress resistance, largely eliminating them from many IoT uses.
  • Parylene: Offering the thinnest effective coating application available in comparison to other conformal coatings, parylene provides excellent substrate coverage. Unlike its competitors, parylene does not rely on wet techniques for application. Rather its unique chemical vapor deposition (CVD) process, wherein the gaseous parylene penetrates deep into the substrate surface, results in optimal levels of protection for most IoT products. Exceptionally resilient, consistent and uniform, parylene coated-surfaces withstand extremes in temperature and physical stress, while pinhole-free coverage prevents leakage. Electrical assemblies and components benefit from parylene's exceptional dielectric properties. However, reliance on the vacuum-generated CVD application techniques increases their manufacturing costs to levels exceeding competitive conformal coatings.
  • Silicone: Another wet technique conformal coating and very versatile, silicone can be customized according to a product's precise requirements, with surfaces ranging from elastomeric, stress-relieving coverings to those far more abrasion-resistant and durable. With a useful operating range of -55°C - +200°C, silicone conformal coatings can withstand extreme differences in temperature. Other positive qualities include good PCB-adhesion, low dissipation factor, and high levels of resistance to heat, humidity, moisture and ultraviolet light. Their toxicity is low and they are easily repairable. However, the need to apply silicone in generally thicker layers than other conformal coatings restricts its use for many IoT and MEMS' applications.
  • Urethane: With good chemical resistance and low moisture permeability, urethane conformal coatings also display exceptional dielectric properties and reliable chemical temperature flexibility at low temperatures. Despite providing a tough, difficult to penetrate coating surface with dependable resistance to solvents, the bond-strength of urethane coatings is limited; urethane coatings covering larger areas have a tendency to flake and peel. Its superb abrasion resistance is largely negated by this condition; other negative aspects of urethane coatings include limited temperature resistance and repair-ability; coatings lose their consistency at high temperatures and are difficult to restore.

Each conformal coating has its own unique properties which dictate its particular range of product uses, and, for instance, the required coating-thickness necessary to assure reliable performance. These conditions vary according to product and purpose, and will affect the specific coating's designated IoT uses.

For instance, a highly specialized coating thickness is mandated for each conformal material used in space flight and exploration systems by the current regulation, NASA Standard (NASA-STD) 8739.1a. Adjusted for the specific properties and performance qualities of each coating type, these thickness levels ensure dependable, robust performance of IoT assemblies, circuits and systems throughout the extreme performance conditions typical of space flight.

Government assessments of coating types confirm thinner layers of parylene will provide equal or superior protection, compared to other conformal coatings (measured in millimeters (inches)):

  • Parylene — 0.013 – 0.051 (0.0005 to 0.002).
  • Acrylic, urethane, epoxy — 0.025 – 0.127 (0.001 to 0.005).
  • Silicone — 0.051 – 0.203 (0.002 to 0.008).

Each of the major types of conformal coatings offers particular advantages for a range of uses. However, parylene displays superior coating-thickness and temperature advantages for most conformal coating projects, particularly for those requiring reliable protection where thinner coating layers are needed to assure component operation.

Figure 1

Of available conformal coatings, parylene withstands specialized and often harsh environments, retaining maximum functionality at the most dependable levels of performance. While competing coating types are all less costly than parylene and easier to apply, none of them currently exhibit parylene's versatility of IoT uses — including proliferating MEMS/nano applications — throughout the technological evolution of the IoT future.

Figure 2

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Microelectromechanical (MEMS) and Nano Technology for the IoT

Microelectromechanical systems (MEMS) and nano technologies have operating components no larger in size than 100 micrometers, and can be smaller than one nanometer. Nano is smaller than micro; for instance, a sheet of paper is 100,000 nanometers thick (100 micrometers). Advances in modified semiconductor fabrication technologies has stimulated the development of MEMS and nano devices and structures suitable for an extensive range of IoT purposes. In many cases, integrated micro- or nano-electronics energize complex electromechanical systems, integrating their functions onto a single micro- or nano-chip.

Better-designed, more efficient micro/nano controllers and sensors are critical components of IoT systems and communications. Continued evolution and elaboration should represent one of the most significant technological breakthroughs of the 21st century.

The increasingly small scale of a wide range of IoT components encompassing MEMS/nano uses requires often specialized protection to assure expected functioning. Having to frequently operate under circumstances combining high functional expectations and harsh performance environments, MEMS/nano IoT applications rely on microcontrollers, whose functions include managing user-interface displays, sensor-interface and processing, system security, and both wired and wireless connectivity.

IoT systems may have to contend with exposure to abrasive liquids, temperature extremes, and related conditions that can compel system degradation or malfunction. Another problem confronting micronized/nano components is stiction -- friction that interferes with stationary surfaces being set in motion during operation.

Exceptionally thin films of substrate covering material are necessary to assure the expected levels of performance for IoT MEMS/nano assemblies. Coating layers may need to be as thin as five nanometers to effectively minimize stiction, while providing insulation, appropriate dielectric function, surface flexibility and otherwise securing components' functions. Coatings exceeding 100 micrometers interfere with components' performance, rendering them ineffective for MEMS/nano purposes. Most conformal coatings — acrylics, silicones, urethanes, potting compounds — cannot be effectively applied in layers sufficiently fine to assure appropriate performance of these micro/nano components.

Conventional liquid-phased, monolayer coating processes, such as spraying and dipping, have proven less useful for MEMS/nano purposes. Vapor deposition processes are more effective for MEMS/nano because they completely eliminate the wet deposition methods necessary for such coating materials as epoxy, silicone, or urethane. Thus, conformal substances using wet methods — acrylic, epoxy, silicon, urethane — have fewer MEMS/nano applications.

In contrast, parylene's higher CVD process temperatures yield a better quality coating surface, with fewer defects, effective at thicknesses as slight as 0.1 microns, making them exceptionally adaptable for IoT uses requiring MEMS/nano components. In addition to uniform, pinhole-free surface-coverings for an exceptional diversity of substrate materials, further beneficial qualities of CVD-deposited parylene for IoT include:

  • dependable barrier protection against acids, bodily fluids, chemicals, salts, solvents, water vapor, and other caustic solutions,
  • thermal stability between -200°C to +125°C,
  • electrical insulation,
  • high tension strain, and
  • adaptability to an exceptional range of complex coating requirements, encompassing exposed internal surfaces and substrates with crevices, points, or sharp edges.
  • Parylene's CVD processes produce precise MEMS/nano-coating thicknesses for IoT assemblies, where even an additional micrometer of coating can generate component dysfunction or failure.

Conclusion

IoT already represents an exceptional range of product and process applications, many using MEMS/nano components for at least part of their function. These require a level of specialized protection not currently available from wet application conformal coatings, whose substrate coverings are frequently too thick and uneven for reliable operation or use.

In contrast, IoT microcontrollers, semiconductors, sensors and similar assembles benefit from ultra-thin, durable and flexible parylene protection. Its ability to completely cover all surfaces, without affecting product-function demonstrates parylene's value future as an integral component of IoT systems.

About the Author

Sean Horn is the Vice President of Diamond MT, a conformal coating services provider for OEMs in the electronics/MEMS, life sciences, industrial, automotive, aerospace and defense markets. He can be reached at [email protected].

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