Protective Conformal Coatings
Conformal coatings can protect the performance and sustain the functionality of today's highly sophisticated electronics for a wide variety of reasons. One of the most obvious and important is the protection they generate from the effects of moisture on the operation of printed circuit boards (PCBs) or wiring boards (PWBs), and their related electronic assemblies.
Due to circumstances, PCB or PWBs may be subjected to harsh operating conditions, requiring dependable and long-lasting protection from such unavoidable environmental developments as moisture condensation, liquid abrasion caused by chemicals, salt water and other substances, as well as water spray and related liquidized obstacles to appropriate functioning. Application of conformal coatings extends component/circuit working-life, simultaneously generating enhanced performance.
Acrylic, epoxy, silicone, urethane, and parylene are typical conformal coating materials. Each coating has advantages and disadvantages in terms of their deposition method, chemical properties, physical properties, re-workability, and affordability. As such, some have less moisture resistance and are best used for other purposes.
Criteria for Coating Functionality
According to the stipulations of IPC-CC-830B, there are two categories of conformal coating functionality. Each delineates a generalized operating environment:
- Class A concerns non-hydrolytically stable conformal coatings, typified by a need for lesser moisture insulation resistance; in such cases, neither humidity nor temperature aging is required.
- Class B specifies standards for hydrolytically-stable conformal coatings; these require higher levels of moisture insulation resistance, as well as sound temperature and humidity aging testing.
Non Vapor-deposited Coatings
These conformal coatings are easily applied, generally via brush, spray, or dip-coating processes.
- The fact that acrylic coatings are easily dissolved by a number of organic solvents is helpful for making repairs, but interferes with their capacity to provide reliable moisture protection, despite generally good humidity resistance. Not hydrophobic, acrylic is incapable of generating dependable protection against moisture under operating conditions where heavy moisture is commonplace. Their moisture protection under normal conditions is generally good, however.
- Offering hydrophilic polar protection, epoxy's hard conformal coatings do not possess the same degree of hydrophobic protection as parylene; water migration beneath epoxy's conformal coating remains possible, subjecting the PCB to product corrosion and performance degradation. Swelling within the ostensibly protected area can result, as can peeling of the epoxy as a consequence, leaving the assembly unprotected.
- Although silicone offers very good adhesion to a wide range of substrates and is hydrophobic to water, its moisture control is less than optimal. The positive impact of their superior humidity resistance is compromised by an exceptionally high incidence of moisture penetration (often 500% of acrylics). Despite an operating range of -55°C/+200°C, and the capacity to endure extreme changes in temperature, silicone also exhibits negative susceptibility to moisture-concentrations in coastal salt-fog carrying the corrosive agents of sodium and chloride ions; air pollutants carrying sulfur dioxide and other corrosive agents in their mist also interfere with silicone's moisture resistance.
- Urethane provides good resistance to humidity and liquid chemicals, with high sustained dielectric properties. Popular as conformal coatings, their moisture protection is generally good, within normal, non-critical ranges of performance.
Parylene exhibits consistent thickness with true conformance to substrate contours. Its application process of vapor desposition binds the substrate to the surface, free of air bubbles and pinholes. Its overall dielectric properties are outstanding. Thermal expansion is minimal; both abrasion and chemical resistance match or exceed competitive coatings, making it suitable as a protective agent for assemblies and circuits expected to operate in harsh environments.
This includes all aspects of moisture penetration and interference. Working conditions characterized by acid rain, aggressive solvents, atmosphere pollutants, high humidity, intermittent immersion, persistent rain, snow and salt fog can be effectively controlled by protective parylene conformal coatings. The same can be said for internal environments with high moisture/liquid content. For instance, parylene coatings have been approved by the FDA for implantable medical device applications subjected to the entire range of frequently abrasive internal moisture conditions common to the human body.
Conformal coatings protect PCBs/PWBs and related electronic assemblies from environmental contamination, also providing insulation for substrate surfaces. However, their moisture protection capabilities vary. High moisture concentration is often dependent upon geographic region. Areas affected by persistent wet conditions on such outdoor electronic operating environments as electrical power stations, street-light signals, warning signs, lighthouses or wind-driven turbines will require superior, ruggedized moisture protection not all conformal coatings provide on a consistent basis.
Acrylic, epoxy, silicone and urethane coatings are negatively impacted by the presence of elements found within the mists of airborne pollutants, salt-air and similar uncontrollable weather conditions. They have shown lower moisture management in these conditions than parylene. Parylene offers superior moisture protection from pollutants, salt-air and acid rain. The fact that it is readily adaptable for bio-implantable medical devices further demonstrates its moisture control capacities, for use on medical systems that monitor and regulate bodily systems, affecting human life.