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 Newly manufactured and cleaned printed circuit boards will generally perform well electrically, but performance will quickly deteriorate due to adsorption of atmospheric moisture, contamination of the surface with airborne ionic material, electrostatic attraction of dust to the surface etc. Conformal coatings are designed to protect printed circuit boards and related equipment from their environment. These coatings ‘conform’ to the contours of the board allowing for excellent protection and coverage, ultimately extending the working life of the PCB. The use of conformal coatings is important in automotive under-bonnet applications, particularly in safety critical areas, such as military, aerospace andindustrial applications.
The ideal conformal coating will have performance requirements that include good electrical properties, low moisture permeability, good chemical resistance and mechanical durability. It should adhere well to all the board materials, for example, the basic board, solder resist, copper and solders and also the numerous materials used for the components that populate the board. In many applications good thermal properties are important, with flexibility retained at low temperatures and a high retentionof mechanical properties at elevated temperatures. Controlling VOC emissions
The EU Solvents Emissions Directive covers defined operations such as the manufacture of coatings, coating activities (such as PCB conformal coating) and surface cleaning. The threshold limit value for solvent quantity is 5 tonnes per year for coating activities such as PCB conformal coatings. Manufacturers whose consumption of solvent falls below these thresholds fall outside the scope of the directive. Occupational exposure limits (OELs) in the workplace will still be apparent, however. It is therefore clear that both the manufacturer and end user of conformal coatings are affected by the Solvents Emissions Directive. So, why all the fuss? The emissions of VOCs have to be controlled due to their effect on the environment and human health. VOCs contribute towards the formation of groundlevel ozone, a major component of smog. Such pollution can have many detrimental effects on the environment, in particular, damaging forests and vegetation. VOCs when not managed properly can also cause health problems. Over exposure causes them to act as irritants and in worst cases carcinogens. It is therefore fair to say that the formation of ground level ozone is a serious air pollutionproblem. Ozone is not emitted directly but is formed from the photochemical interactions of VOCs and nitrogen oxides. The only significant process which forms ozone is the photolysis of NO2, therefore ozone is in a photostationary state. When VOCs are present in the atmosphere they react to form radicals which either consume NO or convert it to NO2. Increasing the level of VOCs in the atmosphere increases the conversion of NO to NO2 and in turn causes ozone levels to increase. The absence of VOCs would therefore significantly reduce the amount of ozone formed. Solvent based coatings
Traditionally, the most widely used conformal coatings tend to be solvent based and are available in several distinct chemical types; acrylic, silicone (conventional and silicone alkyds), epoxy and polyurethane. The properties of a conformal coating stem from the selected base resin and the various additives included optimising the performance of the cured coating. Organic solvents are used to dissolve the base resin and reduce viscosity to bring the coating within a workable range. As such, the coating dries by a simple solvent evaporation and in some cases can be further heat- or moisture-cured to initiate cross linking, further enhancing the coating properties. Solvent based conformal coatings are extremely versatile and can be applied in many ways, such as dipping, spraying and brushing. By simply adjusting the solvent level the viscosity of the coating can be tailored to the required application method. In the case of spraying for example, the standard coating can be diluted within reasonable limits using a suitable solvent in order to obtain a low working viscosity without affecting coating performance. Spraying equipment has seen increasing development over recent years. It is now possible to use select spray technology which applies coating to a PCB in only the required areas, eliminating the need for masking. Solvent based coatings have been an integral part of this development and perform extremely well in such applications. Spraying technology has advanced to include film coating and swirl coating amongst others, allowing for a wider viscosity range to be utilised in this equipment and in some instances allow for a reduction in solvent use. UV cure coatings
Other types of specialist conformal coatings are available on the market today. UV cure coatings can be cured rapidly in line by exposure to UV lamps – these coatings are especially useful where high volume board production is concerned. Areas under components that are shielded from the UV radiation can be slow to cure, giving rise to the so-called shadow effect. This can be minimised by altering the chemistry to incorporate a secondary curing process, of which many types are available. The secondary step is triggered by the exotherm from the UV cure which leads to a thermal cure taking place under the shadow cast by components. A range of UV cure coatings are available, including 100 percent solids and water based materials, which have been developed with the intention of replacing solvent based coatings. The chemistry used in the UV cure systems limits the performance of the cured coating to some extent and generally properties are not as good as those of the long established solvent based materials. Water based coatings
Water based conformal coatings have also been developed in response to increasing environmental pressures against the use of organic solvents. Many different types are currently available on the market. They produce cured films which provide protection for the PCB in a variety of environments. The difficulty however, lies in producing the cured coating. Water based conformal coatings can be successfully applied using dipping or brushing techniques and curing at room temperature. Water based conformal coatings can be sprayed; this requires some adjustments to the standard parameters set by solvent based coatings, however. Thick films can lead to so-called “mud cracking” and in select coating applications this has been an ongoing concern. Water-based conformal coatings also take a while to dry and acceleration using IR or conventional ovens can add to the cracking issue. Again, it can be said that the chemistry of this type of coating can restrict the operational performance of the final cured coating. Non-VOC coating
Recently, a new conformal coating (Non-VOC coating) has been introduced to the market by Electrolube Ltd, with the aim of meeting the demanding requirements set down by the EU Solvent Emissions Directive. Non-VOC coating is based on polyurethane technology. It utilises a polyurethane pre-polymer as the base resin, a 100 percent solids material in the form of a high viscosity liquid. The pre-polymer is pre-reacted polyurethane and isocyanate where some of the functional groups are blocked. Upon exposure to moisture the reaction continues to completion,forming a tough, even coating. Moisture cure and 100 percent solids chemistries have been used for many years in a wide range of applications however, previous attempts have been unsuccessful when applied to the conformal coating market. Moisture cure coatings can tend to be fast reacting and could cause blockages in machines, particularly spray equipment where a fine needle dispenser is used to apply the coating. 100 percent solids materials are high viscosity materials which can cause problems in spraying and dipping applications. They can only be applied in thick films and therefore can take a long time to cure. In most cases, however the viscosity is so high that successful coating of a delicate PCBcannot be achieved. Non-VOC coating has been created by adapting the polyurethane pre-polymer with a blend of carefully chosen diluents to ensure that all materials are reacted within the system. The diluents chosen offer a massive reduction in viscosity creating a 100 percent solids material at a sprayable viscosity, similar to that of solvent based materials, without the emission of any VOCs. The cure profile starts with the polyurethane prepolymer reacting with moisture in the air. This is then followed by a further reaction with the diluent blend, releasing only carbon dioxide from the reaction. The cross-linking nature of this coating means that the mechanical strength and abrasion resistance properties are excellent. The cured coating therefore is extremely tough and possesses excellent moisture and chemical resistance. Due to the reaction profile of Non-VOC coating, it is supplied in a ready to use formulation for sprayingapplications. According to Electrolube, it has been tested by the worlds leading manufacturers of select spray equipmentwith extremely positive results. It has been shown that it ispossible to apply the new coating via conventional methodsand with little change to the set up parameters. The coatingcan then be cured at room temperature or accelerated bythe use of an IR or conventional oven, without any delay.Non-VOC Coating offers the performance and ease of useof a solvent based coating and in addition supplies users aready to use material with no evaporation loss. Thereforethe same amount of coating will spread much furtherthan a solvent based material and does not require costlyextraction processes. Summary
Non-VOC coating is a flexible coating with excellent adhesion to a wide variety of substrates. It offers excellent mechanical and electrical properties, can be used over a wide operating temperature range and incorporates the use of flame retardants to meet UL94 V0 approval. The development of Non-VOC coating has led to a new era in conformal coating. Over time, regulations will get tighter and the use of solvent based conformal coatings will lessen as the industry norm. It is the development of products such as Non-VOC coating that pave the way to a new greener future within the industry of conformal coating. | What are VOCs? | Volatile Organic Compounds or VOCs are carbon based compounds which vaporise easily at room temperature. They are more clearly defined by the EU Solvents Emissions Directive, which states that a VOC is “any organic compound having at 20°C a vapour pressure of 0.01kPa or more, or having a corresponding volatility under the particular conditions of use.” VOCs can be naturally occurring, for example isoprene and monoterpene are two of the most common VOCs emitted by vegetation and are commonly termed Biogenic VOCs (BVOCs). Man-made VOCs come from both industrial and domestic sources including emissions from oil, gas and transportation as well as general fuel consumption and solvent use. It is the latter that is of most concern within the coatings industry. Many paint and vehicle refi ning manufacturers have reformulated their products to include lower levels of VOCs and a detailed labelling system has been put into force, ensuring that all consumers are aware of the VOCs they are using. | Spraying equipment Non-VOC coating Non-VOC coating exhibits excellent flow properties Non-VOC coating exhibits excellent edge definition Jade Bridges is the R&D Manager at Electrolube Ltd. | 
