The components used in printed circuit boards (PCBs) are sensitive to a host of environmental conditions that can have far-reaching consequences and ruin the entire electronics assembly. The correct application of conformal coatings ensures the reliability and long shelf life of these components.
By Potshangbam July
Considering their affordability and reliability, conformal coatings are the most pragmatic choice to protect PCBs and electrically insulate the components from harsh environmental stress. They form a thin polymeric film and can be applied either by spray coating, brushing, dip coating, etc, and usually take up little space. Much like how the human skin functions, they shield the components from impairment by filtering out multiple harmful external elements like moisture, salt, dirt, fungi, etc. They significantly improve the performance and prolong the life span of PCBs. Despite their light application, conformal coatings are very effective, and prevent the premature breakdown of the components.
Benefits of conformal coatings
Conformal coatings are also a protection against extreme temperatures. The electronic parts in the PCBs are highly vulnerable to these conditions. When they are left exposed without any protection, the parts start to deteriorate at an alarming speed, and worse, the performance of the board is hindered considerably. In such cases, conformal coatings play a protective role. Besides, they are lightweight and flexible, and conform easily to the irregular contours of the board. They are applied as thin films, typically 50 µm thick, ranging from 25-75µm, and possess insulating properties that protect against shocks (thermal and mechanical), and simplify the complex process of encapsulating the circuit board. Chris Palin, EMEIA (Europe, Middle East, India, Africa) manager for HumiSeal Europe, says, “The basic purpose of a conformal coating is to extend the operating life of a PCB that functions in high moisture or corrosive chemical environments. The benefits are reduced field returns caused by failures related to the operating environment and increased product reliability. On the design side, conformal coatings allow an 80 per cent reduction in track spacing compared to an uncoated board.”
Types of coatings
There are different types of conformal coatings to choose from, such as acrylic resin, silicone resin, epoxy resin, urethane resin and parylene resin. Their functions and attributes are largely influenced by the chemistry of the conformal coating. Among these options, acrylic coatings are very popular as they do not involve a complex application process, and also possess good moisture and fungus resistant properties. In addition, they do not shrink while curing and are very affordable. However, acrylic coatings are best avoided for high temperature applications and when the curing period needs to be quick.
Epoxy conformal coatings are rigid. They have good abrasion and moisture resistance, and perform well in harsh conditions. Their drawbacks are that they are difficult to remove and require a soldering iron for rework or repair. Besides, they shrink during the curing process.
Silicone resin coatings are single-component compounds that provide excellent protection in extreme temperatures, though are not recommended for use in temperatures below 125 degrees Celsius.
Also, reworking or removing silicone coated circuits is challenging due to their strong chemical resistance.
Conformal coatings are used in many areas, such as consumer electronics, industrial, military and medical devices; however, reliance on this product is increasing particularly in the automotive sector.
Automotive industry: Electronic systems are being increasingly used in this domain; so the application of conformal coatings ensures their protection from gasoline vaporisation, salt fog or brake fluid, and in turn, the long-term reliability of the automobiles.
Consumer electronics: Conformal coatings are used in home appliances to tackle the chemical substances, toxic external environments, contaminants in offices and homes, etc.
Navigation: In this field, the equipment is prone to damage when in contact with salt water and fresh water. Conformal coatings hinder the process of corrosion on the exterior of the equipment and under water.
Medical care: Conformal coatings are very effective in protecting the electronic devices from erosion. External chemical agents are very harmful for these components.
With regard to emerging application areas, Palin from HumiSeal Europe points out that there will be demand in the wearable electronics segment, apart from the demand for higher reliability coatings for autonomous vehicles.
Choosing the right conformal coatings
Selecting the right conformal coating is a long and complex process, for which there are certain key factors to be considered and analysed, before coming to a conclusion. With many choices available, the first parameter to be assessed is the operating temperature range, as this will help to decide the chemistry used in the coating. In extremely high temperatures, a silicon based coating is recommended, and acrylics or urethanes can be used if temperature resistance is less critical.
Environmental considerations are equally important since the component to be coated is likely to be subjected to multiple unfavourable elements. Once the environment conditions are figured out, we will get to know the type of protection the components need. The application process should be selected only after thorough research; otherwise, the wrong combinations can lead to acute problems and cost implications. Other factors to consider include chemical resistance, the curing process, international and national standards, the cost, etc.
Conformal coating standards
It makes sense to find out more about the product standards and specifications as this will help to differentiate between good and bad options. When comparing and analysing products, being well informed helps you to understand the price and quality differentials, and also prevents you from buying inferior quality products. Quality and reliability are the two most important aspects of any product. To yield the best results and performance, the materials should be thoroughly tested and meet the different sets of specifications. There are various standards for conformal coatings, but the two most common standards are MIL-I-46058C and UL746E. The other standards that are recommended include IPC-CC-830B, IEC 61086 and UL 94V0.
Giving more insights on this aspect, Palin explains, “There are many international and OEM specific standards, but none of them cover everything. I would suggest creating your own conformal coating standard that suits your device and its operating environment. You can do this by simply selecting the relevant tests from existing international standards and combining them with your own specific standard.”
The global conformal coatings market is currently witnessing significant growth with rising demand from the automotive and electronics sectors. The application process is getting smarter with shorter application times, less material being used, only the required areas of the board getting coated, etc. Coatings are now available in gel form for dam and fill applications. According to a new report by Grand View Research Inc., the global conformal coatings market is expected to reach US$ 15.73 billion by 2024. The market is highly competitive owing to the presence of numerous medium and large scale manufacturers and suppliers. On the Indian front, with initiatives like Make in India and increasing FDI, the electronics industry is rapidly expanding and, in turn, so is the market for conformal coatings.
Choosing a coating—the prime considerations
First, determine the expected operational temperature range for the circuit board – the highs and the lows. Should this be greater than 150-160°C, for example, it is almost certainly an application for a silicone, rather than an acrylic or polyurethane conformal coating. Also, consider the temperature excursions; if thermal shock or thermal cycling is not taken into consideration, it could lead to cracking, severely compromising a coating’s protective capabilities.
Second, what degree of chemical resistance is required? Acrylic materials, while easily removable when reworking, are generally highly susceptible to attack by solvents. Polyurethane materials, on the other hand, provide more chemical resistance but are generally not amenable to rework. Assess whether immersion or splash resistance is required and whether the coating may be exposed to heated solutions of potential contaminants, which will increase their ability to act as a solvent.
Third, consider what level of corrosion protection is required. Humidity generally only becomes problematic when condensation occurs, which would require close attention to the thickness and coverage of the coating. But do remember, while a thicker coating might provide superior protection in condensing environments or where salt-spray or corrosive gases are present, anything more than the 50 micron target thickness may be prone to cracking under conditions of thermal shock or thermal cycling.
Reasons to consider solvent-free materials
Selecting solvent-free technology is a balance of ethics, performance and process. Ethically, solvent-free materials are a smart choice because solvent emissions will be drastically reduced, if not eliminated, and workforce health will be better protected, ensuring easier compliance with local legislative requirements. Moreover, the energy required for curing these materials is significantly lower than that needed for solvent based materials, resulting in reduced energy bills and reduced CO2 emissions.
From a performance point of view, solvent-free materials can be applied slightly more thickly, improving coverage and protection. Easier to process and more readily compatible with rapid manufacturing operations, solvent-free formulations are often technically superior, and therefore more able to meet the demands of challenging applications in the automotive and aerospace sectors where increased condensation and thermal shock resistance is required.
The cure mechanisms
The cure mechanisms of the main classes of coating materials are: drying, oxidative, moisture, heat, chemical and UV. Your choice will depend on a variety of factors such as the performance requirements of the application and the physical constraints, including the maximum permissible cure temperature and the time allotted for curing.
Acrylic polymers in the solvent can be air dried; once the solvent has evaporated, the residual coating is physically dry and no further reaction mechanisms are needed. Heat is often used to speed up solvent evaporation, but care must be taken to avoid solvent-entrapment and bubble formation.
Oxidative cure coatings based on solvent-based alkyd chemistry are dried as above, before undergoing a reaction with atmospheric oxygen, which initiates a cross-linking reaction that further develops the coating’s protective properties. Cure times are typically much longer than for physically drying products, often requiring many hours at 80°-90°C to develop optimum properties.
Moisture curing coatings are available in both silicone and polyurethane chemistries, in solvent-based and solvent-free formulations. The materials absorb water from environmental humidity, which initiates cross-linking. These materials are used widely because they don’t require any extra curing processes, although heat can be used to accelerate the reactions, if required.
Heat-curing conformal coatings are largely silicone based and require a minimum temperature of 100°-110°C for 10-15 minutes to achieve full cure. The main advantage of these materials is that they require no additional time to develop properties and are considered to be virtually 100 per cent reacted, enabling coated boards to be safely bagged without fear of outgassing.
With chemical-cure coatings (such as urethanes and silicones), reactive oligomers are mixed with cross-linking materials immediately prior to application. Once these two coatings are mixed together in the correct ratio with a suitable catalyst, a chemical reaction occurs to produce a dry, cured coating. Again, heat can be introduced to increase throughput.
UV curable materials cure extremely rapidly (in seconds) when exposed to UV radiation of a suitable wavelength and intensity. However, risk of shadowing by tall components means that a secondary cure mechanism, such as by heat or moisture, is often necessary.
When coating failure is not an option
When coating failure is not an option, it is important to consider the two foremost failure mechanisms: corrosion and loss of insulation (leading to short circuits). Corrosion is a complicated, diffusion controlled, electro-chemical process that takes place on an exposed metal surface, usually in the presence of water and ionic contaminants. Cleaning prior to applying conformal coatings will go a long way in eliminating these two pre-requisite conditions for corrosion.
Conformal coatings help prevent the formation of electrolytic solutions by acting as moisture barriers. However, small voids in the coating that expose a PCB’s metal surfaces can actually accelerate corrosion under the right environment. The challenge when applying conformal coatings is to achieve good coverage and adhesion to the complex, three-dimensional topography of a PCB.
Poorly performing coatings also risk loss of insulation at the PCB surfaces when water condenses in combination with ionic impurities to form conductive pathways between PCB tracks. Without doubt, condensation can severely test the insulation resistance of a coating.
Protection against condensation, immersion and salt-spray
The greatest test of conformal coating performance is posed during power-up under wet conditions, whether this is due to condensation, immersion or salt-spray. Water with soluble impurities is electrically conductive and, finding any weak spots in a coating, will eventually lead to short circuits at the PCB surface. In order to provide protection in these circumstances, it is essential to achieve 100 per cent defect-free coverage of the PCB’s metal surfaces, and this poses a real challenge for both the material itself and the application process.
Fortunately, a new class of conformal coating materials dubbed ‘2K’ enable a much greater thickness and perfect application coverage, resulting in a higher level of protection. Indeed, the performance advantages of 2K materials have been positively demonstrated in the harshest tests that these materials can be subjected to, including powered condensation testing and powered immersion testing in salt water.