What is PCB Conformal Coating?
When it comes to the world of PCBs, you may hear of the term PCB conformal coating? But what exactly is PCB conformal coating? As the name suggests, PCB conformal coating is the application of a thin film of some protective material to a circuit board. The coating protects the entire board and its parts. These parts include the component leads, exposed traces, solder joints, and exposed metal.
The conformal coating shields the metal parts from fungus, moisture, spray, and other environmental contaminants.
The use of PCB Conformal Coating
The primary purpose of PCB conformal coating is to shield electronic components from environmental factors and corrosion;
• To serve as an insulating material of a PCB while minimising the ecological pressure during PCB assembly. It helps in eliminating the need to have complex housings;
• To protect PCBs from instances of heat, moisture, fungus, and airborne contaminants such as dirt and dust.
Advantages of PCB Conformal Coating
There are plenty of benefits that come with PCB conformal coating as listed below:
Insulation and heat resistance
Light in weight
Protection of PCB components
Reduce environmental hazards to PCB boards and prevent the entry of pollutants
Less footprint of PCB components
Enhanced product reliability
Extends the life of a printed circuit board
Very easy to apply
Can PCB Conformal Coating be Waterproof and Moisture-proof?
It’s not easy for a PCB conformal coating to be both waterproof and moisture at the same time. The primary function of a PCB coating is to shield the board from dust, temperature extremes, chemicals, and mostly, from moisture. But, PCB conformal coatings do have their limits. Most layers, regardless of whether they are thick or thin, do provide moisture protection with a lot.
The importance of PCB Conformal Coating
PCB conformal coating is necessary, especially across the printed circuit board industry. We would still have PCBs minus the conformal coating. Unfortunately, such printed circuit boards will encounter a short shelf-life. There will also be some potential damage when in use. That’s not to forget the complete failure of the equipment that relies on them.
As mentioned earlier, conformal coating shields printed circuit boards. It shields them from a lot of damaging effects. Remove them from the scene, and we have boards with boards whose shelf-life and functionality won’t be as desirable as intended. With a conformal coating, there’s an assurance of board protection at all times.
PCB Conformal Coating Type
The conformal coating that PCB designers use when layer printed circuit boards exist in two major groups. There are traditional protective coating and a modern protective coating. This section highlights several types of conformal coating. It also touches on their advantages and disadvantages, among others.
• Traditional protective coating
An apt example of a standard traditional protective coating is Parylene. For the longest time possible, PCB designers used it to coat PCBs. There are some advantages and disadvantages that PCB makers noticed about the traditional protective coating. A better explanation is in the table below.
|Very cheap to purchase||Very poor in terms of thermal management|
|Easily available||Too hard when undertaking rework|
|Good wear||Very heavy on printed circuit boards|
Besides traditional protective coating, there are also other types of PCB coating worth as highlighted below:
• Polyurethane resin (UR)
This type of resin finds a lot of appreciation in the PCB industry based on its excellent chemical and moisture resistance. Additionally, polyurethane resin is abrasion-resistant, making them very hard to remove. They are ideal for aerospace applications where exposure to fuel vapour is standard.
Excellent dielectric properties
Excellent moisture resistance
Good resistance to moisture
Long complete cure time
• Epoxy resin (ER)
These are mainly available in a two-part compound and tend to create some of the hardest PCB coatings. Unlike others, they are challenging to remove the moment the process of curing is over. They find a lot of use in building and construction applications.
Excellent humidity resistance
Good dielectric properties
High chemical and abrasion resistance
Complex mix ratios
Difficult to remove and rework
• Acrylic resin (AR)
These acrylic polymers are soluble in solvents, and they provide both general protection and good elasticity. Acrylic resins find heavy applicability in bathtub liners, aquarium windows, and cellphone display screens.
No shrinkage during curing
Easy to repair and rework
Easy to apply and remove
Low abrasion resistant
Not suitable for harsh environments
• Silicone resin (SR)
These are single component compounds often ideal for electronics meant to work in extreme temperate conditions. They are also common in the making of coatings and paints.
Performs well under extreme temperatures
Excellent chemical resistance
Good corrosion and humidity resistance
Very difficult to remove
Localised repairs only
• Parylene (XY)
Chemical vapour deposition is the application process of this type of coating. When heating perylene, it becomes gas, and after cooling, it polymerises and subsequently turns into some thin film. Equipment such as moisture barriers and electrical insulation relies on them a lot.
Colourless and transparent
No need for curing time
High dielectric strength
Best solvent at extreme temperatures
• Challenging to get rid of
• Not suitable for long-term outdoor exposure
Application areas of PCB Conformal Coating
Initially, the conformal coating did find a lot of applicability in printed circuit boards that belonged to high-tech environments only. However, such is not the case as of today, with the application areas of PCB conformal coating continuing to grow in leaps and bounds. At this point, we are going. We will focus on the use or application of PCB conformal coating. Typical application areas of PCB conformal coating include:
7.1 Automotive industry PCB
Looking across the automotive industry, you’ll notice something different. Electronic systems are lately finding a lot of use across this domain. Thus, the use of printed circuit board conformal coating does a lot here. It ensures that there’s the protection of automobiles’ crucial parts, especially instances of salt fog and gasoline vaporization. All these and many more ensure the long-term reliability of motors.
7.2 Consumer Electronics PCB
Besides the automotive sector, printed circuit board conformal coating finds applicability elsewhere. For instance, it protects circuits meant for home appliances. The application of a conformal coating to home appliances solves many problems. With PCB conformal coating in your home appliances, rest assured of better functioning.
7.3 Navigation product PCB
Salty sea-water and freshwater may cause damaging effects to the circuits that are in the sailing equipment. Here, the material is prone to some damage the moment they come in contact with water. But all is not lost thanks to PCB conformal coating procedures. A conformal coating prevents corrosion speeds. It reduces corrosion at the exterior of the device and also underwater components.
7.4 Medical device PCB
The medical field is another area that relies on PCB conformal coating. Across this industry, conformal coatings seem to be playing an essential role. For instance, it protects electronic medical devices from cases of corrosion. Chemical agents are shared across the medical field and tend to be harmful to equipment that this industry relies on daily. But, with conformal PCB coating now with us, there’s the avoidance of such problems.
7.5 PCB for military products
Electronic equipment meant for military applications operate under the most demanding conditions. Military equipment must be in a position to withstand constant and heavy vibrations. Also, they should be able to work well under extreme temperature conditions. To protect such equipment from damage, must PCB conformal coating. Thanks to PCB conformal coating, it’s now possible to protect military equipment. It’s far easier to protect them from instances of oxidation and corrosion.
7.6 New high-tech product PCB
High-tech fields, such as science, technology, engineering, and mathematics, often use high-tech equipment. In a laboratory setting, for instance, you’ll find high-tech equipment. Some include shakers, test chambers, stirrers, and vacuum chambers are common. All these have essential components that must function without failure. With PCB conformal coating, there’s the avoidance of any challenges that could occur. It is because it’s possible to apply PCB conformal coating here too.
How to Choose the Right PCB Conformal Coating
Having to select the right PCB conformal coating is one of the most tedious and complex processes. It’s because there are some crucial factors that you may have to analyze before you conclude. Failure to consider essential aspects is risky. For instance, you may end up purchasing a coating that won’t suit your needs. Having said this, here are a few essential factors to consider. These factors are critical when choosing the right PCB conformal coating for your project:
• Consider the PCB board operating temperature, the highest point, and the lowest point
The first parameter that you may need to assess concerns the operating temperature range. The reason for this is since it will affect the chemistry that you’ll have to apply when coating. For instance, in high temperatures, it’s advisable to use a silicon-based layer. Also, use urethanes and acrylics. It is if you are dealing with less critical temperature resistance.
• Degree of chemical resistance
This one also matters a lot. It is easy to remove Acrylic materials for rework. But unfortunately, they are prone to attacks by solvents. Polyurethane materials tend to provide more chemical resistance, but not easy to amend.
• Level of corrosion protection
How far can the conformal coating in terms of protecting your equipment in terms of corrosion? In short, you have to ensure that you purchase only the best layer known for being robust in resisting instances of corrosion.
• Long Shelf Life
Shelf life is how long a product continues to remain useful on the shelf under the specified storage, display, and conditions of distribution. You do not want to shop for products that will go bad in a few days. Unfortunately, some paints can’t guarantee long shelf life. Silicone conformal coating is one of the best choices based on its long shelf life.
• Performance and Engineering Characteristics
Before you can go about selecting the type of coating that you intend to use, make sure that it can’t cause stress on the components, especially on occasions of thermal extremes.
PCB Conformal Coating Thickness
The thickness of the conformal coating is one of the essential elements that will determine the longevity of your electronic devices. A minimum coating thickness is necessary to bring about the required function or role of the conformal coating. If the conformal coating tends to be too thick, it may bring about adverse effects on the level of protection that you desire for your boards. So, what’s the importance of coating thickness? How do you go about measuring PCB conformal coating? Are there any types of coating thickness? Below is a clear explanation touching a lot on PCB conformal coating thickness.
9.1 Importance of protective coating thickness
Paints are coatings acting as protective layers. Paints also shield the surface from damages emanating from the environment. When carrying out a painting task, it’s essential to ensure that you use a high-performance coating. The thickness of the layer is critical. It will impact the performance and functionality of your equipment.
However, there is a concept that the thicker the protective coating (especially on the PCB), the better the protective effect. But again, this isn’t always the case. A thick protective layer may bring about uneven texture finish or start some cracking due to some external forces. But again, protective coating ensures the surface doesn’t suffer damage from the external environment.
But you need to remember that low-coating thicknesses, such as those below 25 µm might be risky. It might be dangerous because dielectric strength might not have been tested based on standardized methods. Also, high coating thicknesses that are above the limit rarely protect the PCB as intended.
9.2 Coating type and typical thickness
As already mentioned above, there are several types of coating, including a handful of special ones. But the five main basic categories are epoxy resin (ER), urethane resin (ER), acrylic resin (AR), parylene (XY), and silicone resin (SR). 25-250μm (micrometers) is the standard or typical thickness.
Such thickness can protect the circuitry. It shields circuits against instances of dust, moisture, temperature extremes, and chemicals.
9.3 How to measure PCB conformal coating
Achieving a coat that’s even with the right desired thickness is a big challenge. When applying a conformal coating on a PCB, getting an even thickness isn’t easy. A coat that’s too thin is risky. It may expose the electronic assembly to potential environmental risks. It is an aspect that will defeat the purpose of the coating itself.
Also, a coat that’s too thick isn’t right. It might leave the electronic components on the PCB non-functional. All this would then render the entire electronic assembly useless, and Coating thickness needs to meet the specified quality specifications. Measuring conformal coating isn’t a tasking process as such. There are several methods of measuring PCB conformal coating. But there are two most common ones.
First, there’s the dry measurement method. Here, PCBs have to be completely dry before undertaking any sort of measurement to avoid damaging the conformal film. Here, you can use several methods to assess the depth of the coating thickness. The easiest way involves the use of calipers. With calipers, you can measure the width of a printed circuit board.
You do this by noting down the actual dimensions before the application of the conformal coating. Secondly, there’s what we refer to as Wet Measurement. This type of PCB conformal coating measurement allows designers to ensure quality control of the coating thickness. Remember that designers do this while the film is still wet. Here, a wet-film gauge does the work of measuring conformal coating that hasn’t dried yet.
9.4 How to determine the thickness of PCB conformal coating
Determining the depth of PCB conformal coating is not a complicated process as such. The average PCB conformal coating thickness should be between 1 to 5 mils. But that’s not all. There are plenty of ways that designers use to determine the depth of the PCB conformal coating. Here are four ways of doing so:
• Wet film thickness gauge
Here, the measurement of wet film thickness gets achievable by using a series of teeth and notches. Each tooth contains a calibrated length. Designers place the gauge directly into the wet film and then multiply the measurement by the percentage of solids. It enables them to come up with an approximation of the dry coating thickness.
With this method, there’s the taking of measurements on a PCB at different locations. You have to do this before and after coating. To determine the thickness of the layer, you need to subtract the width of the cured layer from the uncoated measurements. After that, you divide it by 2. This way, you’ll be able to determine the thickness on a single side of the board. This type of measuring the thickness of PCB conformal coating is highly accurate.
• Eddy current probe
Here, there’s the use of test probes to measure coating thickness directly. It is through the creation of an oscillating magnetic field. Something ideal about this mode of measuring conformal coating is that it produces very accurate thickness measurements. In case there’s poor positioning of the probe on the test area, then the readings won’t be accurate.
• Ultrasonic thickness gauge
This is the measurement of coating thickness by the use of ultrasonic waves. Unlike eddy current probes, there’s no need to use metal backplanes here. The calculation of thickness begins from the moment the sound waves start traveling from the transducer, through the coating, bouncing off the boards’ surface, and back.
The Curing Mechanism of PCB Conformal Coating
Curing is essential when considering the long term reliability of the circuit board. There are several notable cure mechanisms for conformal coatings worth noting. You’ll get a better understanding, as explained below. Of importance to note is that each one of them has got its key features.
• Drying – Drying involves the coating of the printed circuit board, making it fit for operators to handle it. It may only take a few minutes after the application process or even several days layers. It depends on plenty of factors.
• Oxidation – This involves the curing of conformal coatings by means or through atmospheric oxygen. Usually, the process of drying occurs first since plenty of oxidative cure coatings are solvent-based.
• Moisture – Here, conformal coatings cure with some ambient humidity in the condensation of the atmosphere in the cure system. The volume of moisture found in the air does impact the rate of cure.
• Heat – Here, conformal coatings cure in elevated temperatures. On most occasions, layers don’t begin healing unless they reach the required temperature. Something ideal about the heat cure method is that they are one of the fastest cure methods compared to room temperature cure.
• Chemistry – Also, going by the name Catalytic Cure, these are a combination of two-part material conformal coatings. These combinations start the reaction and the curing process. It’s impossible to stop the curing process the moment a combination has taken place.
• UV light – These are conformal coatings that cure under UV light. Energy and the wavelength significantly influence the ability and the rate of the surface to heal. Mostly, UV cure coatings consist of secondary cure mechanisms. The cure mechanisms ensure that any coating that fails to cure under UV light eventually cures.
How to Cover PCB Conformal Coating
There are a handful of ways that PCB designers use to cover the surfaces of their PCBs using the conformal coating. The following are some of the most common printed coating formulas that you can use:
• Manual spray – As the name suggests, this is a method of manually spraying a conformal coating onto the PCB surface. You can achieve manual application by aerosol or in a spray booth. Of importance to note here is that the result might not be perfect.
• Automatic spraying – Automatic spraying is available to buy using specialized automated or robotic equipment. The automatic spray is a bit costly. But it produces some of the best conformal coating results. Compared to manual spraying, automatic spraying is the best.
• Selective coating – Selective coating employs the use of robotic spray nozzles to effectively apply a conformal coating to some specific regions of a printed circuit board. The particular surface is a high-volume method, meaning that it is accurate and fast, applying a layer to the exact intended places of the board.
• Impregnation – This one also goes by the name saturation coating. It is a coating process that designers use to coat the porous substrates of a printed circuit board. It is ideal for PCBs with pores penetrating the entire substrate.
• Paint – Paint is more a mechanically bonded surface coating providing a barrier film between the environment and the steel. The paint system involves using several sheets of layers. Also, it is the use of different paint formulations. All this depends on the operating environment of your boards.
Determine the Failure of PCB Coating
As noted above, the coating protects printed circuit boards. With surface, PCBs are in a position to perform well at the intended operational circumstances. But, there are instances where boards fail despite having undergone coating protection.
12.1 The two most essential failure mechanisms: corrosion and insulation loss
• Corrosion: Unprotected boards tend to corrode quickly. Corrosion is one of the biggest reasons that contribute to poor board functionality, and it may also lead to total board failure. Copper is a common base material on a printed circuit bay. Copper is highly susceptible to corrosion when you don’t coat it properly. To avoid such a case, all you have to do is to ensure that you undertake proper conformal coating.
• Insulation loss: Poorly performing coatings risk the damage of insulation at the surfaces of a printed circuit board. Water can condense and combine with several iconic impurities forming conductive pathways between the tracks of the printed circuit board. Undoubtedly, condensation is a test on the insulation resistance of a conformal coating that you choose to use.
12.2 How to prevent the failure of PCB conformal coating
PCB conformal coating can fail due to plenty of reasons. PCB failure is an aspect that may render the whole coating process a waste of time. But there are several ways to follow if you want to prevent the collapse of printed circuit board conformal coating. First, you need to ensure that you apply the layer only to the recommended thickness. Don’t use too little nor too much coat on your printed circuit board.
Secondly, ensure that while applying the coat, you do so in several thin coats. The reason for this is to allow the bubbles to dissipate between the layers properly. Third, you may want to use a low viscous version of a conformal coating that you choose to use. Fourth and last, if you are doing brush coating, then blend the surface so that it flows quite smoothly into every other part of the substrate.
12.3 Reparability of PCB conformal coating
Fortunately enough, it’s possible to repair PCB conformal coating using a soldering iron. Something impressive about PCB conformal coating is its reparability function. With a soldering iron, the surface will melt away, leaving behind joints that are clear of the sheet. With other layers, you may realize the materials char. But this is a small issue that you have to discuss with your quality department.
How to remove PCB Conformal Coating
By now, you must be well aware of almost everything to do with a conformal coating. For instance, you know its types, uses, and thickness, to mention but a few. Now, it’s time you get to understand something else. It’s time to know of the best removal strategies if you want to remove or replace conformal coating on your PCBs.
13.1 Why remove the protective coating?
During the initial selection of the type of conformal coating material to use, you need to factor most efficiently to remove it should you want to do so. The process of removing the coating can bring about damaging effects to your PCB, especially if you get it wrong.
There are plenty of reasons why you may have to undertake PCB conformal removal. For instance, you may want to do so due to the wrong choice of the coating material. Also, it may be due to inadequate coating coverage or a PCB design that doesn’t need the inclusion of a coating. The reasons are plenty.
13.2 Removal method
There are several PCB conformal removal methods worth mentioning. Some of the most common ones include:
• Chemical solvent method – This is removing a PCB conformal without affecting the board and its components. Of importance to note is that there’s no particular solvent that’s ideal for all applications. The water must be safe to work with in addition to being environmentally friendly.
• Stripping method – Here, you’ll need a wide range of solvents to remove the conformal coating. This process involves swabbing and brushing the area using water until you remove the conformal coating. Stripping is effective in getting rid of different types of circuit board coatings.
• Heating method – Here, you’ll have to use a very low-temperature heat as a means of gently burning and melting the coating. If need be, you may have to use some low-temperature control tools to ensure that you don’t damage the PCB. It’s not suitable using a soldering iron.
• Grinding and scraping method – This method of removing conformal coating relies on several rubbing and scraping equipment. All this depends on the composition of the PCB coating material. A majority of people use a dental style scraper and a hand-held drill when grinding.
• Miniature sandblasting machine to remove the conformal coating – Sandblasting involves propelling finely graded abrasive powder into streams of compressed air, through some abrasive-resistant hose and out some tiny nozzles that you automatically or manually place at the workpiece. You can use this method to get rid of several coatings such as acrylic, epoxy, urethane, parylene, and silicone, among others.
How to Choose high-quality PCB Conformal Coating
All over the globe, there seems to be some sort of rapid miniaturization of electronics and circuitry. It is an aspect that has seen matters to do with a conformal coating of PCBs increasing. As mentioned above, to protect PCBs from extreme temperatures, moisture, and dust, among others, conformal coatings are necessary. But how do you go about choosing high-quality PCB conformal coating material? What are some of the things that you need to watch out for the best PCB conformal coating material?
Here’s what you need to look out for when choosing a high-quality conformal coating to guarantee you the best result:
The surface has to feature excellent flexibility
Also feature low toxicity levels and cost
It should be easily reworkable
Feature a vast range of operating temperature range
At high speeds, it should be able to cure without generating by-products
Has to shield the board from low/high temperatures and mechanical impact
It must have a long shelf life
Must be easy to spray and feature low viscosity
Safety and plenty of environmental concerns are paramount when it comes to selecting the type of conformal coating material to use. Some elements, once they get obsolete, may end up bringing some damaging effects to the environment. Besides all these, there are several regulatory bodies tasked with the mandate of ensuring that PCB coating materials are environmentally friendly:
• Occupational Safety and Health Administration (OSHA) – This is a statutory body in the US whose term is to ensure worker safety. Plenty of coatings are flammable, contain high toxicity levels, and emit fumes. OSHA ensures that all companies dealing in the production of PCBs put in place safety measures to protect its workers. OSHA provides that all PCB manufacturers have personal protection equipment for their workers and that there’s enough ventilation in the company.
• EPA (the United States Environmental Protection Agency) – In the US again, there’s strict adherence to EPA requirements by all PCB producers. For instance, the EPA has in place severe restrictions regarding chemicals that affect the ozone layer. Fortunately, though, a majority of the restricted chemicals aren’t readily available, a reason as to why matters concerning ozone depletion appear not to be a big concern as such.
• CARB (California Aviation Review Board) and other regional regulations – For the longest time possible, the CARB, in conjunction with other local authorities, has continuously played more prominent roles to ensure that they enforce environmental restrictions aimed at the protection of the environment. Some of these restrictions include VOC (volatile organic compounds) or smog-producing chemicals. The latest one is the environmental topic under discussion is the Global Warming Potential (GWP).
This article is the knowledge of PCB protective coating, 15 things you want to know about PCB conformal coating. Are you looking for the best conformal coated printed circuit board that can generally work on the market?
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