December 9, 2020 11:51 am
Brian Chislea is senior technical service and development scientist, and Erica J. Everett is marketing manager for transportation and mobility at Dow Consumer Solutions
high humidity, thermal cycling, airborne contaminants and physical stresses that were once associated mainly with military and industrial electronics are now challenges for consumer and automotive electronics as well. Whether it is a cellphone inside a backpack or a PCB assembly inside a car, electronics can be at risk. To provide environmental and mechanical protection, conformal coatings are applied to PCBs and other electronic substrates. These thin, polymeric films are used with rigid and flexible circuit boards, sensitive electronic components such as integrated circuits (ICs), and fine pitch designs with tight fabrication tolerances. Generally, conformal coatings are not applied to radio frequency (RF) antennas and other board-level components that are sensitive to the additional capacitance of an added protective layer. Some conformal coatings are 1-part systems that require the use of solvents, chemicals that may contain volatile organic compounds (VOCs) that pose potential environmental health and safety (EHS) concerns. There are also 2-part solvent-borne systems, but they require mixing. Today, many manufacturers prefer 1-part solventless silicone conformal coatings that speed production and support sustainability initiatives. By combining the optimal coating chemistry with the right application method and curing process, it is possible to achieve higher throughputs and greater energy efficiency. Design and manufacturing PCBs can fail for a variety of reasons, but moisture and contamination are two of the most common causes. If moisture accumulates on a board’s surface or within a void in the conformal coating, corrosion and electrochemical-mitigation (ECM) between conductors may occur. Tin whiskers and dendrite growth are similar to ECM. Creep corrosion, the migration of corrosive products on the surface of a PCB, can also cause electrical shorting and current leakage. Insufficient cleaning or the improper application of conformal coatings that results in inadequately protected PCB regions are often to blame. In addition, airborne contamination from dust or debris can penetrate insufficiently coated regions. Today, many consumer devices are expected to provide improved water resistance, an established requirement in automotive applications where there is routine exposure to weather conditions. In the automotive industry, critical systems such as steering and braking must provide dependable performance. Silicone conformal coatings have been validated by the automotive industry to replace dam and fill or PCB encapsulation, a protective technique that encloses an entire device to form a “brick”. By using conformal coatings instead of PCB encapsulants, automotive engineers can enable modular designs and support vehicle light weighting without sacrificing performance. Light weighting is especially important to manufacturers of electric vehicles (EVs) and hybrids because of concerns about range or mileage. Reducing or eliminating heavy PCB housings can help, but not if PCB protection suffers. Silicone conformal coatings that are applied in multiple layers can serve as the primary defense against environmental contamination. They are also used in consumer appliances such as dryers, where heat and steam are factors. Portable devices such as smartphones and household devices such as televisions are not used in identical environments, yet both may need to resist the high humidity of the tropics or the dusty conditions associated with desert environments. Proper conformal coating selection and application can mitigate these and other risks, but it is important to use products that support both current technologies and emerging designs. For example, as more fifth generation mobile networks are deployed, more electronic designs will feature 5G electronics for faster connections, higher throughput, and greater capacity. Yet, the greater power densities associated with 5G electronics produce higher levels of heat, which can reduce the life of PCBs, solder joints, and components. Silicone conformal coatings provide reliable protection against high temperatures, support high-throughput assembly, and offer many other advantages. Getting started with conformal coating selection When selecting conformal coatings for electronic devices, it is important to start with a well-balanced chemistry. Acrylic, urethane, polyurethane, epoxy, and parylene conformal coatings all offer advantages but come with significant trade-offs. For example, acrylic resins provide good humidity resistance but have a narrow service temperature range. Urethane and polyurethane resins are hard and abrasion resistant but can raise health and safety concerns during processing. Epoxy resins offer good resistance to humidity but are harder and more susceptible to cracking and delamination. Parylene is chemically inert but supports limited throughput because its deposition system is limited by chamber size. By contrast, silicones are well-balanced. They resist chemicals, moisture and contamination and are soft, flexible, stress relieving and suitable for high-throughput manufacturing. Their hydrolytic stability is critical because 5G will require more towers, base stations and transmitters, expensive assets that are exposed to outdoor weather conditions such as high humidity and rain. Compared to other conformal coating materials, silicones allow water vapor to pass-through more quickly; however, it is liquid water – not water vapor – that causes corrosion. Over time, other types of conformal coating materials tend to absorb more water than silicones, which can increase the risk of corrosion. Importantly, silicone conformal coatings resist higher temperatures and provide longer-lasting heat resistance without a significant loss in properties. With their low modulus, silicones can also absorb some of the stress that results from the material expansion and contraction that occurs at different rates when materials have different coefficients of thermal expansion (CTE). By contrast, urethanes and epoxies have a higher modulus, higher glass transition temperature and are less stress-relieving. Silicone-based coatings also provide good adhesion to many substrates, often without the use of a primer for greater production efficiency. Formulations and automation Historically, many conformal coatings have been formulated with solvents such as benzene, toluene, ethylbenzene or xylene (BTEX) that become volatilized and produce potentially harmful airborne emissions. Today, solvent-less silicone conformal coatings are helping the electronics industry to address both EHS and environmental sustainability concerns. By reducing or eliminating VOC emissions, these solventless products also reduce the risk of fire since many solvents are flammable or combustible. Solvents are also closely regulated and may be subject to greenhouse gas reporting requirements. The use of solvents in conformal coatings can also complicate waste management and disposal for facilities managers. According to the U.S. Environmental Protection Agency (EPA), solvent wastes are typically classified as flammable materials. Therefore, solid absorbent material such as the wipes that come into contact with flammable solvents must be disposed of as hazardous flammable solid wastes. Used applicators, unused solutions and materials leftover from cleaning can also raise waste-related concerns. These challenges are not just limited to North America but are of increasing concern globally. In the European Union, Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Waste (WEEE) are just two of the directives that apply to electronic content. Depending on the nature of an electronic device or product, the EU’s End of Life Vehicles (ELV), Energy-using Products (EuP), or Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) directives may also apply. As more countries, including China and South Korea, restrict the use of BTEX solvents, global manufacturers are seeking to minimize the risks associated with prohibited materials. Solventless conformal coatings can help with material content declaration requirements and support design for environment (DfE) considerations. Solventless conformal coatings that are made of silicone support sustainability initiatives and can improve production efficiency. Because there are no solvents to evaporate, solventless silicone conformal coatings maintain their applied thickness with only minimal shrinkage and can be applied in a single pass. By contrast, other types of conformal coatings with a high solvent content require significantly thicker layers. Solventless silicone coatings are also available in lower viscosity materials for faster flow rates during automated spraying or needle dispensing. These alternatives to hand spraying support the higher volumes of electronics associated with consumer devices and automotive manufacturing. Importantly, automated dispensing also provides fast, consistent, and reliable results across shifts or manufacturing locations. Curing methods Curing provides the electronics industry with an additional opportunity for cost savings and efficiency gains. Conformal coatings use four main curing methods: evaporative, moisture, heat and ultraviolet (UV). Evaporative curing and moisture curing typically occur under ambient conditions and do not require the use of specialized equipment. Thermal curing uses infrared lamps or thermal ovens to initiate or accelerate curing. Ultraviolet curing uses a UV light source and, typically, a secondary curing mechanism for regions of the assembly that were shadowed and did not receive direct UV exposure. Evaporative drying or curing occurs at room or near-room temperatures and is a lengthy process that may require significant space for work-in-process. Typically, PCBs are dipped at least twice to achieve the desired thickness on component edges. When the liquid carrier evaporates, the coating resin is left behind. With solvent-borne conformal coatings, ventilation and fume extraction may be required for evaporative curing. Facilities may also need special permitting and a red-label room for applying solvent-borne products. Moisture curing uses the moisture in the air to initiate the curing process. These formulations can be solventless or solvent-borne. For solvent-borne formulations, carrier solvents evaporate while atmospheric moisture reacts with the coating. Since moisture curing requires a minimum of 30 percent relative humidity, facilities in cold or dry climates typically require humidification, an additional operating expense. Depending on the nature of a conformal coating, moisture curing can be used alone or in conjunction with another primary curing method, such as UV light. Heat curing subjects conformal coatings to high temperatures so that the applied heat initiates the chemical crosslinking of the polymer. Because heat curing requires the entire assembly to reach the prescribed cure temperature before the cure time starts, significant time and energy are required. Cure temperatures and durations vary, and some conformal coatings now offer rapid curing at higher temperatures of 150°C. Although heat curing improves wetting and lowers viscosity, it is important to remember that high temperatures can damage some electronic components. Heat curing is often used alone but it can also be used in conjunction with another curing mechanism, typically to accelerate curing. Ultraviolet curing requires irradiation at a specific wavelength and energy intensity. This process is very fast and occurs at room or near-room temperature. Compared to thermal ovens, UV curing equipment has a smaller footprint and uses less energy. To reach shadowed areas where UV light cannot penetrate, A newer, solventless conformal coating that is made of one-hundred percent silicone combines a low viscosity for automated spraying with a fast, primary UV cure that occurs in under a minute with broad-spectrum UV light. This product’s secondary moisture cure avoids potential issues with areas that are shadowed, or hidden, from the UV light during curing. The low modulus that characterizes this solventless silicone coating protects delicate components and its elastomeric formulation improves reliability against stress. This innovative products also has a UV indicator for automated inspection during high-throughput manufacturing. Additional considerations Electronics manufacturers often need conformal coatings that meet UL-746E, a safety standard from Underwriters Laboratories (UL) for materials used in PCBs. They may also need conformal coatings that meet the UL-94 flammability rating for plastic materials, a category that includes silicones. In addition, the industry wants products that help meet the requirements of IPC CC-830, a global standard for coating performance capabilities, quality and consistency. This self-certifying standard allows an electronics manufacturer to confirm that a conformal coating meets baseline electrical performance levels for protection from moisture and contamination. Also important is a conformal coating’s ability to pass environmental testing and environmental cycling, humidity testing and water immersion testing. If a standardized test method does not exist, electronics manufacturers may modify automotive or even aerospace standards to meet the needs of consumer devices. Planned obsolescence is a thing of the past, and smartphone shoppers and car buyers alike want safety, performance, and reliability that lasts longer than just the warranty period. Today’s electronics manufacturers have a host of concerns. The materials that are used to address these challenges must provide protection against moisture and contamination while supporting high-throughput assembly and environmental sustainability. Because heat is the enemy of reliable electronics, 5G’s greater power densities and increased temperatures are of growing concern. Energy-efficiency, worker safety, and support for automated inspection are also top priorities. Conformal coatings can help. When selecting these products, electronic designers and engineers have a choice of chemistries, formulations, application methods and curing systems. An innovative solvent-less silicone conformal coating with UV and moisture dual cure can help high-volume manufacturers to meet multiple requirements. Many other products are also available. The right partner can help with product selection and application development while continuing to provide innovative technologies like solventless silicone conformal coatings for high-throughput assembly. or UV curing is otherwise incomplete, a secondary moisture cure is often used. Manufacturers have a choice of solventless silicone conformal coatings that use this dual cure mechanism, but most of these products do not have a low enough viscosity to support spray coating for high-volume manufacturing.