Conformal coatings: selection, best practice and future trends

Such conditions can include airborne ionic material, atmospheric moisture and the electrostatic attraction of dust. In more challenging environments however protection from chemicals, extreme humidity or even corrosive atmospheres, such as salt mist, may be required.

The coatings are designed to ‘conform’ to the contours of the board, protecting all areas to which they are applied and thereby extended the working life of the PCB. There are a range of properties essential for a conformal coating to possess; good electrical properties, low moisture permeability and a degree of mechanical durability are some of the key requirements, whereas good chemical resistance and flame retardancy might include some of the additional desirables, for example. In all cases, the conformal coating should adhere well to all the board materials, and maintain adhesion and also flexibility across a large temperature range.

It is essential that the correct coating is selected for the application - There are a range of chemistries available and each type offers a few characteristic properties which help to distinguish the use of one polymer over another: Acrylics combine reasonable price with good environmental protection; they retain clarity and can resist darkening and hydrolysis during extended exterior exposure. They do however have a limited solvent resistance, making them suitable for re-work but not chemical resistance. In contrast, polyurethanes offer excellent chemical resistance and superior protection in harsh environments, also maintaining flexibility at very low temperatures. Like acrylics, they are also very versatile covering a broad operating temperature range, typically -65°C to +130°C.

Silicones may be chosen where a good level of protection is required but thermal stability is vital; due to their silicon-oxygen backbone they offer protection over temperature ranges as broad as -100°C to +300°C, depending on their structure. There are concerns surrounding the migration of free silicon causing failures in electronic circuits, high prices and poor mechanical properties, however.

The properties of silicone alkyds are intermediate of pure silicones and alkyds, exhibiting greater flexibility, hardness, and thermal stability than alkyds, but ultimately lack the high thermal and oxidative resistance of unmodified silicones. Temperature ranges are also considered intermediate, ranging from -70°C to +200°C. Acrylated polyurethanes or acrylics, along with some epoxy chemistries, are typically utilised in UV curing systems. These systems will again offer a combination of properties but challenges with adhesion and flexibility are presented when formulating such products.

During the selection process, the application method and curing processes must also be considered. Conformal coatings can be applied via a number of methods including, brush, dip and spray methods. Brushing is the easiest form of application but requires a degree of skill from the operator to ensure an even, bubble free surface is obtained. 

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: For small scale production aerosol cans are typically used, 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, thereby eliminating a step in the production line.

The use of selective coating machines has become very popular in the conformal coating industry. They offer increased productivity and reduce waste as only the required areas are coated. They also offer reduced VOC emissions in enclosed, ventilated systems.

Traditionally, the most widely used conformal coatings tend to be solvent based whereby the base resin of one of the chemistry types discussed above 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 and as such, the coating dries by a simple solvent evaporation.  Solvent based conformal coatings are extremely versatile and can be applied via all the methods discussed by simply adjusting the solvent level in the coating. 

Typically over 50 per cent of a solvent based conformal coating consists of volatile organic compounds (VOCs). In today’s environmentally conscious society, VOC reductions are desirable in all industries, including conformal coating application. VOCs contribute towards the formation of ground level ozone, a major component of smog. Such pollution has many detrimental effects on the environment and in addition, when not managed properly, VOCs can also cause many health problems.

Alternatives to solvent based materials are therefore very desirable. They include UV cure, water-based and moisture cure coatings. UV cure coatings can be cured rapidly in line by exposure to UV lamps – areas under components can be slow to cure and require a secondary process, such as moisture or heat. UV cure coatings can be 100 per cent solids materials and are typically high viscosity systems.

The processing of such coatings can therefore be difficult and may also require major changes to a production line. Water based conformal coatings also exhibit issues in the processing/curing stage. They can be successfully applied using dipping or brushing techniques and curing at room temperature, offering excellent protection for PCBs.

Water based conformal coatings can also be sprayed; this requires some adjustments to the standard parameters set by solvent based coatings, however. Such 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. They can come in the form of 100 per cent solids silicones which have already been discussed above. A more recent solution, introduced by Electrolube, is the use of moisture cure, 100 per cent solids polyurethanes.

Such coatings can be formulated to offer many desirable properties, including excellent protection in high humidity and corrosive environments, flame retardancy and good electrical properties. In addition, 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.

The properties exhibited by such materials are very close and in some cases superior to the standard solvent based polyurethane types, therefore offering a true environmentally friendly solution with exceptional performance.

The electronics industry is rapidly expanding and in turn, the number of applications for conformal coating is also escalating. Coatings are being developed to meet the increasing demands of the industry by offering superior performance at an affordable cost, both in monetary and environmental terms. In addition, modifications are being made to the application processes to ensure that only the required areas of the board are coated, reducing the amount of material needed, decreasing application time and eliminating stages in the production process where possible.

The availability of many gel versions of conformal coatings for dam and fill applications are an example of this change. Over time, regulations will get tighter and the use of solvent based coatings will lessen as the industry norm. By continual expansion and development of their conformal coating range, Electrolube aim to simplify the transition away from solvent based materials, thus paving a way towards a greener future within the industry of conformal coatings.