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Overview
Conformal coatings are thin polymer films applied to the surface of printed circuit boards (PCBs) as a protective measure against moisture, chemicals, temperature, and other environmental factors. Conformal coating is designed to “conform” to the shape of PCBs and its components, providing coverage in all areas. This protective layer is critical to ensuring long device life cycles and high reliability especially in harsh environments. The use of conformal coatings is especially important in the automotive, aerospace, military, and medical industries to provide a high level of reliability [1]. However, with the rapid miniaturization of components and component spacing, conformal coatings are also becoming more important in consumer and mobile electronic industries [2].
The key advantages of conformal coating are its protection against moisture and gas, metal migration, insulation resistance, handle and abrasion resistance, and biological resistance [5]. Different types of conformal coatings will provide different levels of protection and should be selected to match the level of environmental resistance required. The primary traditional classifications of conformal coatings are acrylic (AR), epoxy (ER), silicone (SR), polyurethane (UR), and Parylene (XY) [3][4]. Other types of coatings outside of the traditional ones such as UV coatings, ultra-thin coatings, and styrene block co-polymer coatings can also be used to provide board protection.
There are various methods to apply a conformal coating onto a PCB. The common application methods are dipping, spraying, brushing, and selective coating machines. Each application method will have advantages and disadvantages, and careful consideration of the coating type and the intended application method is necessary. A conformal coating should properly cover the entire assembly and any sharp edges or leads present on components [4]. Defects in conformal coatings such as orange peeling, delamination, etc. may occur after application due to coating and substrate compatibility, substrate cleanliness, application methods, and curing conditions.
Advantages of Conformal Coating
Conformal coatings provide high levels of resistance to moisture, electrical insulation, corrosion, and fungus.
Moisture Resistance
Conformal coatings are designed to provide resistance to moisture that could lead to corrosion on PCBs. Moisture protection will vary depending on the type of conformal coating used, the degree of cure of the coating, presence of impurities, and degree of adhesion onto the surface of the PCB. Conformal coatings can provide high levels of resistance to moisture but are not completely impermeable [5].
Insulation Resistance & Metal Migration
Conformal coatings prevent metal migration through the suppression of moisture absorption and leakage current on PCBs. Metal migration between closely spaced conducting lines can occur when moisture, impurities, and a bias are all present, potentially causing catastrophic shorting and failure of PCBs. Application of conformal coating over tightly spaced components is important to minimize the risk for potential metal migration and maintain a high insulation resistance [5].
Corrosion Resistance
Conformal coatings provide corrosion resistance by creating a barrier to the penetration of corrosive factors such as gases, moisture, salt, and chemicals. Gases such as oxygen and hydrogen sulfide can cause oxidation of metal surfaces or tarnishing of silver surfaces while carbon dioxide and sulfur dioxide can form acids with the additional presence of moisture. Conformal coatings also provide protection to electronics that are exposed to marine environments where the risk of corrosion from salt exposure is higher [5].
Biological (Fungal) Resistance
The decomposition of polymers in conformal coatings by microorganisms can have significant impacts on its physical and electrical properties. Degradation of conformal coatings by microbes can subsequently lead to corrosion. Microbial resistance is particularly important for PCBs that will be subjected to high humidity, tropical temperatures, or contact with soil. Most conformal coatings will either be composed of synthetic polymers and thus inherently resistant to microbial growth or will have antimicrobial chemicals added to suppress growth [5].
Classifications of Conformal Coatings
The primary traditional classifications of conformal coatings are acrylic (AR), epoxy (ER), silicone (SR), polyurethane (UR), and Parylene (XY). Other types of conformal coatings include ultra-violet light (UV) curing coatings, ultra-thin coatings, and styrene block co-polymer coatings. New types of conformal coatings are continuously being developed to meet new reliability requirements.
Acrylic
Acrylic conformal coatings cure rapidly, reaching tack-free conditions in minutes, are fungus resistant, and provide a long pot life. Since acrylic conformal coatings typically do not undergo polymer cross-linking, they give off little to no heat throughout cure, reducing the risk of thermal damage to heat-sensitive components. Acrylic coatings provide high humidity resistance and can easily be reworked with stripping solvents or direct soldering [4][5].
Silicone
Silicone conformal coatings are known for its high temperature resistance and wide operating range, making it especially useful in applications where extreme temperature cycling is common. Most silicone coating systems are cured through moisture or heat, with moisture-cured systems being accelerated with mild heat. Cured silicones are relatively soft under normal conditions, allowing for easier rework and removal, but become extremely hard when subject to shocks. Compared to other coating chemistries, silicone coatings have a higher moisture permeation rate, resulting in the need to apply thicker layers for adequate protection [4][5].
Epoxy
Epoxy conformal coatings are usually available as two-component systems (Parts A and B) and provide high abrasive and chemical resistance, but poorer humidity resistance. Two-component epoxies begin to cure immediately upon mixing and thus generally have a short pot life and processing window. Epoxy properties such as viscosity will also continue to change over time during the processing window. Epoxy coatings are virtually impossible to remove for rework due to the risk of attacking epoxy-based components and printed board laminate with stripping chemicals. The most effective way to rework an epoxy-coated board is to burn through the coating with a knife or soldering iron or with the use of abrasives [4][5].
Polyurethane
Polyurethane conformal coatings provide excellent chemical resistance, as well as good humidity and dielectric characteristics. Due to the high chemical resistance, polyurethane coatings are difficult to rework and costly, often requiring specialized stripper compounds. Polyurethane components are available as either one or two-component formulations. One component formulations are easy to apply but require additional time at room temperature to reach optimum properties. Two-component formulations can reach optimum properties much quicker but have a significantly shorter pot life upon mixing [4][5].
Parylene
Parylene conformal coatings are unique from other standard conformal coatings in that they are applied through vapor deposition. Through this unique process, parylene coatings provide true conformance to PCBs, even coverage, and are free of bubbles. Parylene coatings can be applied to ultra-thin thicknesses (a few microns) while still providing excellent moisture, temperature, and chemical resistance [5].
UV
UV conformal coatings are designed to cure at extremely fast speeds (on the order of seconds) and have low volatile organic compound (VOC) levels. UV coatings provide high moisture, temperature, and chemical resistance and are best fit for high volume, selective coating applications. Most UV coating formulations have a secondary cure mechanism such as moisture or heat activation to account for shadowing during the primary cure process [4][5].
Application Methods
There are numerous methods to apply conformal coating to a PCB. The most popular methods are dipping, brushing, spraying, selective coating, and vapor deposition [3]. The application method should be carefully considered as each has advantages and disadvantages.
Dipping
Dipping a low-cost application method that can provide good coverage for complex parts [5]. Dipping requires full immersion of a PCB into a vat of conformal coating, which means sensitive areas must be masked before the application process. The thickness of the conformal coating layer is dependent on the immersion and withdrawal speeds of the PCB from the vat [3]. The conformal coating bath viscosity, pot life, and risk of contamination must also be controlled during a dipping process [5].
Brushing
Brushing is a simple application method that is best suited for production in limited quantities. Since this process requires using a brush to apply the conformal coating, it is a very labor-intensive process. Brushing is primarily used for touch-up before or after rework rather than coating an entire board [3]. Consideration should be taken to prevent the introduction of air or formation of bubbles into the conformal coating during application [3].
Spraying
Manual air spraying of conformal coating uses either an aerosol can or handheld spray guns to atomize the coating with a high flow of air to create a mist that is applied to a PCB. Commonly conformal coating must be thinned to achieve the proper viscosity and spray characteristics [5]. Manual spraying should be performed in a spray booth to contain over-spray and harmful vapors [1]. The quality, coverage, and uniformity of conformal coating on a manually sprayed board is heavily dependent on the operator technique [3].
Selective Coating Machines
Selective coating machines are effective for high volume applications and provide a high level of repeatability [1]. Selective coating machines offer several advantages over manual spraying such as high throughput, repeatability, accuracy, operatory safety, and reduced emissions [1]. With selective coating, very small spray patterns can be achieved, eliminating the need for masking in most cases [1][5].
Chemical Vapor Deposition
Vapor or vacuum deposition can only be applied to a few select types of polymeric conformal coatings. Vapor deposition is mostly performed on parylene (XY) type conformal coatings. The starting material are white, non-toxic, solid dimers of p-xylene. The material is then sublimed and decomposed under a vacuum before flowing into a deposition chamber where they will re-form long chains of polymers on the PCB [5]. Since this is a gaseous process, all exposed surfaces on the PCB will be covered unless masked [3]. Film thicknesses can range from the submicron range to several mils with a high degree of accuracy and repeatability [3].
Common Conformal Coating Defects
Defects in conformal coatings can arise at any time due to material changes or improper preparation prior to coating. It is important to recognize the common conformal coating defects and how to prevent them before they occur. Unexpected rework due to conformal coating defects can cause delays in production especially if the coating is difficult to rework or must be stripped away.
Bubbles and Blisters
Bubbles or bubbles may occur in a conformal coating when small pockets of air or moisture become trapped underneath the coating layer. Epoxy and polyimide-based circuit board laminates can absorb significant amounts of moisture so moisture and other possible contaminants must be removed prior to applying conformal coating [5]. Pinholes occur when a bubble has penetrated through the top of the conformal coating layer [7].
Capillary Flow
Capillary flow occurs when conformal coating flows from one region on a substrate to another, leaving uneven coverage when dried [7].
Cracking
Cracking in conformal coating occurs when there are internal stresses in the coating due to overcuring, temperature fluctuations during cure, or differences between expansion coefficients of the coating and the substrate. Cracking will generally occur due to shrinkage or rapid solvent evaporation of a solvent-based coating [5].
Delamination
Delamination occurs when dried conformal coating separates from the substrate, leaving voids. Delamination can be caused by poor compatibility between the surface energies of the coating and substrate, contamination, or improper curing conditions [7].
De-wetting
De-wetting occurs when a conformal coating will not flow evenly across the surface of a substrate. This can be caused by incompatibility between the surface energy of the substrate and coating due to remaining contaminants such as greases and oils [7]. Ensuring that the substrate is free of contaminants and moisture is the best method to preventing de-wetting [5].
Orange Peel
Orange peel, also known as mealing, in conformal coating occurs when the dried coating texture is uneven and has a texture resembling to that of the skin of an orange [5][7].
Important Considerations When Selecting a Conformal Coating
Conformal coatings will have different final cured properties depending on the specific type and chemical formulation. Some important film properties to consider are the cure mechanism and drying time, glass transition temperature and coefficient of thermal expansion (CTE), and electrical insulation.
Curing Mechanism
Different types of conformal coating will have their own curing mechanism and required curing conditions. Most standard conformal coatings will cure through moisture, heat, or catalytic cure. The specific curing mechanism is specified by the manufacturer and final cured properties may vary if the recommended curing conditions is not followed.
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Moisture Cure – Moisture cure conformal coatings react with ambient moisture present at the time of application. Most RTV silicone and some polyurethane conformal coatings cure through moisture [5]. The relative humidity of the environment has a direct impact on curing speed. Moisture cured conformal coatings should be purged with dry nitrogen, when possible, before storage.
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Heat Cure – Heat cure conformal coatings require elevated temperatures to begin the polymerization process. However, all solvent-based conformal coatings will have improved curing times with elevated temperature to speed up drying.
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Catalytic Cure – Catalytic cure conformal coatings cure through a reaction of two different species. Curing mechanisms such as UV or heat is generally incorporated to accelerate the rate of cure of these materials and is popular with solvent-free conformal coatings [3].
Glass Transition Temperature & Coefficient of Thermal Expansion
The glass transition temperature (Tg) of a material is the temperature or temperature range at which a polymer material reversibly changes between a glassy/brittle state to a rubbery state [3]. It is important to know the glass transition temperature for a conformal coating because depending on the intended use environment, the polymer film may cycle through the glass transition temperature, affecting film properties, and possibly leading to defects.
One important thermal property of materials is the Coefficient of Thermal Expansion (CTE). The CTE is the linear dimensional change that occurs when a material changes temperature [3]. It is important to match the CTE of the conformal coating to that of the substrate or components on the board to prevent fatigue, cracking, and even component failures [3]. It is also important to know the CTE above and below Tg since it will change due to changes in the material. For example, if conformal coating has wicked underneath a component, the thermal expansion of the coating at elevated temperatures can result in extra stress on solder joints, increasing the risk of component failure.
Electrical Insulation
Electrical insulating properties of conformal coating is critical to prevent unwanted electrical discharge and metal migration that could lead to board failure. One specification that you may have seen before when choosing conformal coatings is MIL-I-46058C. MIL-I-46058C is an inactive military specification detailing the required protective properties of a conformal coating. Although inactive, this specification may still be used by military contractors and coating manufacturers when selecting and developing conformal coatings as a MIL-I-46058C certified coating is automatically IPC-CC-830 certified, but not vice versa. Both MIL-I-46058C and IPC-CC-830 outline requirements for electrical insulation such as insulation resistance, dielectric withstanding voltage, and Q (resonance). For example, MIL-I-46058C specifies a minimum average insulation resistance of 2,500,000 megaohms and a resistance of 1,500,000 megaohms for each specimen on a coated board [6]. Conformal coatings should be selected to match the level of electrical insulation required.