Withstanding such complications to the operational environment as corrosion, fungus, oxidation, rain, salt water/mist, snow, temperature fluctuations or vibration is essential to long-term performance of electronic devices. Without suitable protection, printed circuit boards (PCBs) and similar electronics will not survive harsh environments, and malfunction. Prominent examples include:
· Aeronautic and automotive vehicles requiring PCBs that function as specified for years through an exceptional range of operating conditions.
· Defense systems that power communications/surveillance equipment and high-tech weaponry in high-stress, life-threatening performance situations.
· Personal devices -- like handheld smartphones and tablets – required for personal and commercial communication.
· Wearable devices and implants – fitness and medical trackers -- which require circuitry that won’t fail during intense exercise sessions, or when subjected to corrosive fluids within the body.
Conformal coating provides a reliably secure, non-conductive, dielectric covering, applied to protect PCBs from damage due to contamination, dirt/dust, exposure to corrosive chemicals, fungus, moisture, salt spray or other potential deterrents to component functionality.
Electronics’ Protection with Conformal Coatings
The five conformal coating materials include four liquid types -- acrylic, epoxy, silicone and urethane, and a fifth, vapor-phase polymerization parylene. Liquid coatings are applied to substrate surfaces by wet brush, dip or spray methods, which are relatively simple and inexpensive to enact; they provide generally reliable exterior film protection for PCBs.
Parylene’s unique chemical vapor deposition (CVD) process transforms solid parylene dimer into a gas that penetrates deep within the surface of electronic assemblies. PCB protection with parylene is frequently more consistent than with liquid coatings. It is also slower and can be more expensive to process, with costs running between from $100.00 to $10,000.00+ per pound, depending on the type and quality required for the coating project.
The objective is to decide which coating type best meets the requirements of your conformal coating assignment, based on the material properties of available coating types and application methods suitable to your electronic assembly.
Conformal Coating Selection
Clearly defining the performance requirements for the PCB to-be-coated includes matching the conformal film material with such factors as operational moisture levels, temperature range, and anticipated vibration. Conformal projects are well-advised to consider the practical performance benefits of each coating material to ascertain the type most suitable for the assembly and its usage:
- Easily applied acrylic resin (AR) conformal coatings dry in minutes at room temperature. They are fungus resistant, possessing as well desirable electrical and physical properties. Typically applied at thickness between 0.002 - 0.005 inches, they withstand temperatures of no more than 125° Celsius, limiting their use for many higher-heat applications. They remain a popular coating choice, because many AR-variations cure in 30 minutes, enhancing their value when a short turnaround time is the objective.
- Usually available as two-component compounds, epoxy resin (ER) systems deliver extremely durable coating with good resistance to damage from chemicals, high-level abrasion, and humidity. However, ER can shrink during polymerization, and temperature extremes diminish its stress resistance. The same rugged, long-lasting films that distinguish epoxy performance makes them difficult to rework and repair, so application processes need to be enacted carefully.
- Silicone resin (SR) coatings are a popular choice for automotive applications because they maintain high-level performance in upper-temperature environments of 200°+ Celsius. They also provide good corrosion and humidity resistance. Because it can be applied in thicker layers than other coatings, SR is exceptional at dampening vibrations during operation. Unfortunately, silicone provides lower resistant to abrasion and solvents than other conformal films.
- Delivering reliable chemical and humidity resistance, urethane resin (UR) coatings also provide excellent dielectric properties for long periods of time, better insulating signal traces from circuits placed in near proximity, a property that supports PCB miniaturization. Withstanding chemical solvents, UR also mitigates tin whisker growth. However, solvent-resistance makes it difficult to remove or rework. Long-term functionality in high-heat/vibration environments is limited.
- A non-liquid coating material, parylene (XY) uses a unique CVD application process that delivers an exceptionally uniform, pinhole-free conformal film for PCBs. Parylene can be applied to virtually any surface and objects of any shape, including glass, metal, paper, resin, plastics, ceramics, ferrite, and silicon. XY’s chemical inertness enhances its utility for implanted biomedical devices. Parylene is expensive and difficult to rework; batch-sizes are small compared to other conformal coatings.
Most of these conformal coatings will provide at least some degree of protection for electronics, but benefits vary according to material, its application style, the PCB’s purpose and operational environment. These basic properties offer useful guidelines for deciding which film material is best for your coating assignment.
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