Protecting Sensors in Harsh Environments with Parylene

Many different sensors require protection from corrosion, dirt, and internal and external attack. Coatings provide a viable, reliable solution for complete electronic system and sensor protection.

artKuva

Illustration of Parylene vs Liquid Coatings.

Sensors exist in every size and configuration, with specific capabilities for specific needs, from the smallest CMOS to giant load cells – and thousands of types in between. All sensors, however, tend to have two things in common. First, sensors act as watchdogs, receiving some type of data and subsequently transmitting signals that are designed to keep systems running reliably.

Alan Hardy

ElectronicsMarket Manager,

Specialty Coating Systems,

ahardy@scscoatings.com

Second, virtually all sensors require protection from corrosion, dirt, and internal and external attacks. Protective measures often include conformal coatings, special paints, corrosion preventative compounds (CPCs), and/or a physical housing/case designed to protect the sensitive electronics.

While these options are often effective for a period of time or in mild conditions, many protection options degrade in hostile environments, reducing protection and triggering expensive maintenance downtime. In addition, many conformal coatings are not able to protect highly intricate sensor systems due to their uneven thickness or voids within the coating. This is a particularly detrimental issue for electrical signal conditioning integrity needed for remote monitoring, automation and wireless transmission systems.

The Parylene Solution

Parylene conformal coatings provide a viable, reliable solution for complete electronic system and sensor protection. Parylene is a unique series of polymeric organic coating materials, which are polycrystalline and linear in nature and possess useful dielectric and barrier properties per unit thickness. They are chemically inert, ultra-thin, pinhole-free and truly conform to components due to their molecular-level polymerization – basically growing on the deposition surface one molecule at a time.

Parylene coatings are applied using a vapour deposition process. The parts to be coated are placed in the deposition chamber and the powdered raw material, known as dimer, is placed in the vaporizer at the opposite end of the deposition system.

The dimer is heated, causing it to sublimate to a vapour, then heated again to break it into a monomeric vapour. This vapour is then transferred into the ambient temperature chamber where it spontaneously polymerizes onto the substrates to be coated, forming the thin Parylene film.

The Parylene process is carried out in a closed system under a controlled vacuum, with the deposition chamber remaining at room temperature throughout the process. No solvents, catalysts or plasticizers are used in the coating process. Because there is no liquid phase in this deposition process, there are no subsequent meniscus, pooling or bridging effects as often seen in the application of liquid coatings, thus dielectric properties are never compromised.

The molecular growth of Parylene coatings also ensures not only an even, conformal coating at the thickness specified by the manufacturer, but because Parylene is formed from a gas, it also penetrates into every crevice, regardless of how seemingly inaccessible. This ensures complete encapsulation of the substrate without blocking small openings.

Parylene coatings are extremely lightweight, offering excellent pinhole-free barrier properties without adding dimension or significant mass to delicate components. Parylene is typically applied in a thickness ranging from 500 angstroms to 75 microns.

A 25-micron coating, for example, will have a dielectric capability of 7,000 volts. Simply, no other coating material can be applied as thin as Parylene and still provide the same level of protection, making Parylene an ideal option for components used in an extremely wide range of sensor system configurations.

The family of Parylene conformal coatings (Parylenes N, C and D) has been trusted as a reliable protection solution for a wide range of applications for over 40 years.

Parylene coatings provide excellent barriers to moisture, chemicals and biological agents. They are also RoHS compliant and have been proven effective to mitigate metallic whisker growth in lead-free solder applications.

Parylene HT®, which more recently became commercially available, offers additional protective capabilities for end users. Parylene HT was developed by replacing the alpha hydrogen atom of the Parylene N dimer with fluorine.

The resulting Parylene film possesses increased dielectric capabilities and superior thermal and UV stability, making it ideal for electronics that need reliable, long-life performance in harsh environments, including oil and gas drilling operations, surface mining, forest products and chemical plants.

Parylene N is a primary dielectric, exhibiting a very low dissipation factor, high dielectric strength, and a low dielectric constant invariant with frequency.

Parylene C has a useful combination of electrical and physical properties, plus a very low permeability to moisture and corrosive gases.

Parylene D has properties similar to Parylene C with the added ability to withstand higher temperatures (up to 100°C short-term, 120°C continuous).

Parylene HT is useful in high temperature applications (short-term exposures up to 450°C, continuous up to 350°C) and those in which UV stability is required. Parylene HT also has an extremely low dielectric constant and dissipation factor, enabling it to provide small, tight packages with dielectric insulation via a thin coating, and its extremely small molecular structure allows the coating to ingress deeper through open areas on the top or bottom of any package, regardless of the size or complexity of integrated devices.

Superior Adhesion to All Substrates

Many electronic device designers, including those providing components to machine automation, mining, oil & gas, and renewable energy sectors, are moving away from using standard multiple-layer FR4 board configurations to using more flex materials for interconnects.

These configurations have been historically challenging for protective coatings, often requiring some form of a protective case or shield. Parylene coatings eliminate the need for alternative protection. New adhesion technologies, AdPro Plus® and AdPro Poly®, ensure that Parylene coatings protect even the most intricate flex circuit designs.

Now, rather than building a protective assembly and sealing it around a device, Parylene provides protection for not only flex-circuit configurations, but also many other device or substrate materials, including stainless steel, cobalt-chromium, copper, gold, iridium, nitinol, platinum, solder, tin, titanium, tungsten, aluminium, nickel, chromium, brass, and polycarbonates. AdPro Poly enables Parylene to protect surfaces made from polyimide, epoxy, acrylic and EPDM.

Application Examples in Oil & Gas and Mining

Integral to wireline tools (examples Logging While Drilling (LWD) and Measurement While Drilling (MWD) applications), sensors, which are located above the drill bit and on the drill string itself, log critical data upon which exploration companies and manufacturers rely.

There are also many types of electronics, circuit boards and sensors used above surface for controlling and monitoring ancillary equipment and processes, including the monitoring of drilling fluids and mud, blowout prevention, and the monitoring of power supplies, engines and generators, to name a few. For subsea applications, the reliable operation of sensor and sensor systems used in power and vision systems and remote-operated vehicles (ROVs) is highly critical.

Surface mining typically uses self-driving trucks, which, as indicated by their name, operate with no human intervention. These trucks use numerous sensors to keep them on course and running 24/7. Sensors are also integrated into surface mining processing systems to both power and monitor the crushing system, conveyors, motors and gearboxes. All of these sensors are critical to ensure the overall operation runs properly.

Additionally, sensors are found in underground mine applications, from the tunnelling machinery to hard rock, industrial minerals and longwall systems, to prevent equipment failures and to enable the machinery to communicate remotely. Like in surface mining, underground mining utilizes sensors to monitor and power the crushing equipment, conveyor systems, gearboxes and motors.

Today’s mining operations are linked to industrial plants where monitoring is done within remote control rooms. Sensors are placed in pipes, on valves, in tanks and on the packaging line. Smart sensors and electronics allow an operator to simply touch a screen to control and/or make adjustments to systems that are located in the plant itself or in the field, which can be over 2,000 miles away from the plant.

Heavy Manufacturing Applications

Today’s industrial factories like refineries, chemical plants, steel, aluminium, copper, forest products and paper processing plants use automation on a large scale, which means they rely on wireless sensing systems to operate the facilities and provide protection from system failures.

SCS_Fig_1

Photo of Parylene HT coated (top) and non-coated (bottom) boards after testing in salt-fog environment.

 

In forest products, for example, sensors keep the entire production flow moving – from cutting the raw material to manufacturing end products. Sensors on harvesting equipment begin the process by selecting trees for optimum lumber size. In the processing plant, sensors ensure the wood is cut to the proper thickness. Even remote-guided vehicles are used in the warehouse to move lumber, all operated and monitored through sensor technology.

Taking paper from pulp to final product, sensors measure the web, control motion, and ensure the end product is the right thickness and the proper chemical make-up. The heavy use of water, chemicals and heat in the paper drying process creates a harsh environment, which can adversely affect the life of sensors needed to keep the facility running with minimal downtime. Parylene is virtually the only conformal coating that can protect sensors without adding significant weight or interfering with the perfect integrity of the sensors.

Wind Turbines and Power Generation

Wind turbines can be located on land or in the water, in some cases miles offshore. To generate electric power in these particularly harsh environments, machinery must be fully functional and be able to reliably and accurately measure critical operating parameters.

Exterior and interior (within the nacelle) sensor technologies are designed to measure temperatures, pressures and other critical aspects of wind farming. Further, sensor technology can be employed to support the design stage for enhanced plant performance, in particular, for industrial gas turbines.

For sensors and electronics to successfully monitor and capture data, the technologies must survive exposure to temperature extremes, humidity, salt, dust and other environmental conditions. The properties of Parylene coatings offer protection to the smallest, most advanced turbine technologies so they may operate reliably, avoiding costly and unexpected turbine downtime.

SCS_Fig_2

Ultra-thin Parylene conformal coatings are applied as a vapor at room temperature.

Parylene coatings provide reliable temperature measurements in the most challenging environments in standard power and distribution plants. The measurement data gives quantitative assessments of component degradation and operating conditions, which enables the accurate prediction of remaining service life and contributes toward the adoption of new, more productive monitoring strategies.

For renewable energy, Parylene protects photovoltaics (PV) and associated system instrumentation, allowing complete systems to meet the required goal of 25 years of continual operation in extremely harsh environments. For sensors and electronics used in applications where protection is needed to ensure a long and productive life, Parylene conformal coatings are a proven, viable solution.
In the plant or out in the field, heavy-duty applications can take advantage of Parylene’s moisture, chemical and dielectric barrier properties, and its thermal and UV stability, to increase sensor life and greatly reduce the opportunity for costly system downtime.

Applications | 20.11.2015

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