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Selection, best practice and future trends of conformal coatings

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Conformal coatings are protective lacquers used within the electronics industry to protect printed circuit boards (PCBs) against failures caused by exposure to a variety of environmental conditions including airborne ionic material, atmospheric moisture and electrostatic attraction of dust.  

PCBs in more challenging environments must be protected from chemicals, extreme humidity or even corrosive atmospheres such as salt mist. Conformal coatings by their very definition are formulated to ‘conform’ to the contours of the board, protecting all areas to which they are applied and extending the working life of the PCB.  

Conformal coatings require good electrical properties, low moisture permeability and a degree of mechanical durability for efficacy, and can also offer good chemical resistance and flame retardancy. In all cases, the conformal coating should adhere well to the board materials, and maintain adhesion as well as flexibility across a wide temperature range.  

Conformal coatings are selected based on the application with each coating offering characteristic properties.  

For instance, acrylic coatings combine reasonable price with good environmental protection, retaining clarity and resisting darkening and hydrolysis during extended exterior exposure. Their limited solvent resistance however, makes them suitable for re-work but not chemical resistance.  

Polyurethane coatings in contrast, offer excellent chemical resistance and superior protection in harsh environments, also maintaining flexibility at very low temperatures and can operate within a broad operating temperature range, typically -65ºC to +130ºC.  

Silicone coatings may be chosen to offer protection in applications that also require thermal stability. Due to their silicon-oxygen backbone they offer protection over temperature ranges as broad as -100ºC to +300ºC, depending on their structure.  

Silicone alkyds are intermediates of pure silicones and alkyds, exhibiting greater flexibility, hardness, and thermal stability than alkyds, but lack the high thermal and oxidative resistance of unmodified silicones. These conformal coatings can operate in intermediate temperature ranges from -70ºC to +200ºC.

Acrylated polyurethanes or acrylics, along with some epoxy chemistries are typically utilised in UV curing systems but also present challenges with adhesion and flexibility during formulation.  

The application method and curing processes must also be considered during the selection process of conformal coatings.  

Conformal coatings can be applied for PCB cleaning via a number of methods including brush, dip and spray methods. While brushing is the easiest form of application, it requires a degree of skill from the operator to ensure an even, bubble-free surface. Dipping can be carried out either manually or via a dip coating machine, where the entire PCB is immersed in the coating.  

Spraying can be carried out in a number of ways including aerosol cans for small scale production, spray guns for medium scale coating facilities and selective coating equipment for automated production lines.  

Brushing and selective spray methods allow localised coating application without the need for masking. Selective coating machines are quite popular in the conformal coating industry as they offer increased productivity and reduce waste in addition to lowering VOC emissions in enclosed, ventilated systems.  

Commonly used conformal coatings tend to be solvent based whereby the base resin of one of the chemistry types is dissolved in organic solvents and blended with various additives to optimise the performance of the cured coating.  

The solvents reduce the viscosity to bring the coating within a workable range, with the coating drying by a simple solvent evaporation. Though solvent based conformal coatings are extremely versatile, they also contain harmful VOCs, which not only impact the environment but also affect health.  

Alternatives to solvent based conformal coatings include UV cure, water-based and moisture cure coatings.  

UV cure coatings can be cured rapidly in line by exposure to UV lamps. Processing of these high viscosity coatings can be difficult and may also require major changes to a production line.  

Water based conformal coatings can be successfully applied using dipping, brushing or spraying techniques and cured at room temperature, offering excellent protection for PCBs. Water based conformal coatings can take a while to reach their full properties, and acceleration using IR or conventional ovens can lead to cracking in the cured film.  

Moisture cure coatings cure by exposure to atmospheric moisture and come in the form of 100% solids silicones.  

Electrolube’s 100% solids polyurethane coatings offer many desirable properties including excellent protection in high humidity and corrosive environments, flame retardancy and good electrical properties. These coatings can also be modified to offer a range of viscosities suitable for all application types and can be accelerated using heat or IR cure immediately after application.  

Electrolube is continuously expanding and developing its conformal coatings range to offer greener alternatives to solvent based coatings and meet the diverse application needs of the electronics industry.  

Electrolube is a division of the HK Wentworth Group and a global manufacturer of electro‐chemicals.

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