Conformal coatings encapsulate circuit boards and their electronic components in order to prevent the ingress of moisture, fun
Use of Conformal Coatings, and Particularly Silicone, for Electronic Products in Chemically Harsh and High Humidity Environments such as Oil, Gas and Petrochemical Facilities
Eileen R. Hess, P.E., GarrettCom, Inc. Industrially hardened products such as industrial Ethernet switches are extremely robust, meeting and exceeding IPC/IEC/MIL standards for survivability in utility and heavy industrial environments. However, the typical hardened switch does not address the effects of H2S gas and other corrosive agents found in oil, gas, and petrochemical facilities. Even very low concentrations of corrosive agents, such as H2S gas, hydrocarbons, chlorobenzene and chemical by-products produced as a result of the refining process, which are found in certain industrial environments such as in the oil, gas and petrochemical production process, can impact electronic components. For example “Silver Whiskers” can form on the contacts of electronic surface-mount electronic components when exposed to even low levels of hydrogen sulfide (H2S). Heat, chemicals, and moisture can further accelerate degradation, potentially creating shorts or open circuits that can cause electronic components to malfunction or fail. The application of a Harsh Environment Conformal Coating can address the issue of contamination, including H2S, for hardened Ethernet switch components including circuit boards and critical components. Conformal coatings are thin layers of synthetic resins or organic polymers applied to printed circuit boards and electronic components for protection against environmental, mechanical, electrical, and chemical problems including contaminants such as dust, dirt, fungus, moisture, chemicals, thermo-mechanical stress, mechanical shock, and vibration. They are "conformal" in that they conform to the contours of the assembly. Major types of conformal coatings include urethane, silicone, acrylics, epoxies, and parylene. Adding Harsh Environment Conformal Coating improves and extends the operating life of the product and ensures security and reliability of performance. The Effects of Conformal Coatings The effects of chemical exposure and gas permeation, as well as mechanical and/or thermal shock, can be substantially reduced and even eliminated by the application of a suitable coating. Conformal coatings are used in many industries and applications including printed circuit boards (PCBs), and generators, transformers and other equipment that are used to create or transmit electrical power. MTBF can be positively influenced by conformal coatings, even in less demanding environments, so understanding the benefits and limitations of a variety of conformal coating polymers is critical for the manufacture of industrial electronics systems. In addition to the above, some conformal coatings provide flexibility and dampening layers that can survive flex. For example, in today’s circuit boards, space is often critical and component size continues to shrink, making attachment more challenging. A rigid coating may actually damage the substrates it was designed to protect. The differences in coefficient of thermal expansion between a non-flexible conformal coating, PC board and mounted components may lead to damage of some leads and connections. This effect is especially noticeable on boards that experience repeated temperature cycling. Specialized conformal coatings are needed to meet more demanding applications such as military specifications (MIL-SPEC) MIL-I-46058 and MIL-STD-1188, and for oil, gas and petrochemicals. The widely used IPC-CC- 830, and IPC-4101 specifications are more for protection against moisture. Conformal coatings use many different chemical systems and filler materials. Some chemical systems contain acrylics, elastomers, natural or synthetic rubbers; epoxy resins, water-based resins, silicone compounds, or
volatile organic compounds (VOCs). Others contain bismaleimide (BMI) resins, phenolics, or formaldehyde resins. Commonly used chemical systems also include polybutadiene, polyester, vinyl ester, polypropylene (PP), polysulphide, and polyurethane (PUR). In terms of filler materials, some conformal coatings contain aramid fiber, chopped fiber, carbon powder, or graphite powder. Other products contain glass fillers, metal fillers, or inorganic compounds. Unfilled conformal coatings are also available. Typically, these raw materials are used as starting components in the production of finished coatings. There are several curing technologies for conformal coatings. Typically, thermo-set plastics and thermo-set resins are cured using heat or heat and pressure. Vulcanization, a thermosetting reaction, uses heat and/or pressure in conjunction with a vulcanizing agent to produce materials with greatly increased strength, stability, and elasticity. Some polymer resins or compounds cure or vulcanize at room temperature. Others cure with radiation, electron beam irradiation, visible light, or ultraviolet (UV) light. Single component curing systems consist of a resin that hardens through the application of heat or a reaction with surface moisture. Two- component and multi-component curing systems consist of two or more resins and a hardener, crosslinker, activator or catalyst. With the growing demand for longevity and reliability in electronic assemblies, engineers are increasingly taking advantage of the benefits of conformal coatings to protect boards and components in operating all types of applications. Selecting a Conformal Coating Selecting conformal coatings requires an analysis of physical, mechanical, thermal, electrical, and optical properties. Physical properties include viscosity and gap fill, the space between the material and substrate. Mechanical properties include tensile strength, tensile modulus, and elongation. Thermal properties such as temperature range, thermal conductivity, and coefficient of thermal expansion (CTE) are also important considerations. Electrical properties for conformal coatings include electrical resistivity, dielectric strength, and dielectric constant. Optical properties include index of refraction, a measure of the speed of light in a material, and transmission. IPC/Mil specifications classify conformal coatings into types by the cured chemistry of the coating. The type for multifunctional materials are based on the type of chemistry: Type AR-Acrylic, Type ER-Epoxy, Type SR- Silicone, Type UR-Polyurerthane or Urethane and Type XY-Paraxylylene. * Acrylic: Generally the easiest of the coatings to handle. Thermoplastic lacquer base means the coating is easy to apply, easy to remove and repair. Moisture resistance comparable to urethane and silicone, but poor resistance to petroleum solvents and alcohol. Dielectric >1500 volts/mil. Temperature range -59C to 132C * Silicone: Good thermal shock resistance due to flexibility. Also easy to apply and repair, although overall removal may be challenging. Moisture resistance is similar to urethane and acrylic. Dielectric withstand may be somewhat lower than for the other coatings (1100 volts/mil), but flexibility of coating allows for much thicker film build than comparable acrylic or urethane coating. Temperature range -65C to 200C * Urethane: Hard, durable coating that offers excellent abrasion and solvent resistance. Similar moisture resistance to acrylic and silicone, but significant shrinkage during curing and extremely hard film may stress components. A difficult coating to apply and nearly impossible to remove. Rework may be accomplished by burning through coating with soldering iron (using appropriate safety precautions due to isocyanates in the cured film) on local areas, but stripping of large areas or whole boards is nearly impossible. Temperature range is as the acrylic above. * Epoxies: Excellent resistance to moisture and solvents. Usually a two-part thermo-setting coating. Coating shrinks during curing, leaving hard, difficult-to-repair film. May stress components. Due to the extreme solvent resistance of the film, coating is virtually impossible to strip. * Paraxylylene: A two-part Paraxylylene coating is typically very uniform and yields excellent pin coverage. Limitations include high cost, sensitivity to contaminants and the need for a vacuum application technique.
The physical, mechanical, thermal, electrical, and optical properties of the above types of conformal coating, along with the need for long term, reliable protection of sensitive circuits and components when exposed to harsh environmental conditions in the oil, gas and petrochemical industry, suggest a silicone type of conformal coating. Silicone’s properties, and ability to function over a wide temperature and humidity range as a durable dielectric insulation, as a barrier against environmental contaminants, hydrocarbons, benzenes, and gas permeation, and as a stress-relieving shock and vibration absorber, make it the best solution. In addition to sustaining their physical and electrical properties over a broad range of operating conditions, silicones are resistant to ozone and ultraviolet degradation and have good chemical stability. Most silicones contain significantly less solvent than organic coating and are available in a wide variety of cure systems. Silicones also offer better repair ability and, in the manufacture of electronic devices, allow the option to salvage or reclaim damaged or defective units as they can be removed from substrates and circuitry by scraping or cutting, or by using solvents or stripping agents.
Silicone for conformal coating is best for protecting electronics such as those found in industrial networking products. Data from available tests indicates that the thickness of the silicone should vary depending on the types and the permeability of corrosive elements likely to be found in the application.
Test Results To meet corrosion and harsh environmental conditions for conformal coating, current thinking is that coatings of the thicknesses below should be considered, and were used in testing.
Due to the physical properties of silicone and the method of deposition, silicone forms a pin-hole free film on the subject substrate. Therefore, it provides an excellent barrier to both liquids and gases. The table below lists the permeability characteristics of silicone and other types of conformal coatings.
Moisture vapor permeability values have been measured at thickness below 0.1 micron. Normalized to equivalent thickness, the values are the same for all thicknesses. Assuming that silicone at 0.001 inch is pinhole free, then silicone films at 0.1 micron (~ x 10-6 inch) are also pinhole free.
GAS PERMEABILITY MOISTURE VAPOR TRANSMISSION cc-mil/100 in2 - 24 hours * gm-mil/100 in2 - 24 hours **
Diluted inorganic reagents had little effect on the silicones, and acids at 10 percent concentrations had virtually no effect at room temperature, nor, except for chromic, was there any effect at 75ºC. Concentrated acids at
room temperature (23ºC) had little effect. Under severe conditions, 75ºC for 30 minutes, all acids had a measurable effect ranging from 0.7 percent swelling with hydrochloric to 8.2 percent with chromic. Additionally, nitric acid under these same severe conditions caused severe degradation. Both concentrated nitric and sulfuric acids caused some discoloration.
Testing by conformal coating manufacturers indicates that silicones were insoluble in all common solvents. Silicone, it was found, could be dissolved in high boiling liquids such as X-chloronaphthelene or benzoyl benzoate at temperatures above 150ºC. However, these solvents are seldom encountered in the electronic industry. Of greater importance are those solvents and reagents used in processing, especially in cleaning of components and assemblies.
Conclusion: Tests performed by coating manufacturers have determined that in order to meet the Harsh Environmental condition in the refining or chemical process of hydrocarbons, the minimum thickness requirements for polymer conformal coating should be 12.5 mils or greater, and that silicone had the most attractive properties. Requirements to be met included chemical resistivity, gas permeation, salt spray corrosion, ingress of moisture, fungus, dust and other environmental contaminants; while providing consistent physical, mechanical, thermal, electrical, and optical properties which meet or exceed IPC/IEC/MIL standards for survivability in utility and heavy industrial environments. The tests also took into consideration specific environmental hazards such as the effects of H2S gas and other corrosive agents on the SMT electronic components, and determined the level of repair ability, long term reliability over a wide temperature and humidity range as durable dielectric insulation,, and the ability to act as a barrier against environmental contaminants and as a stress-relieving shock and vibration absorber.
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