With continued miniaturization of electronics and their circuitry, the necessity of conformal coating has skyrocketed. And choosing the ideal type of coating and application method for your application is crucial.But weeding through the vast amount of information online is daunting.
Well, not anymore:
In this post, we’ ll give you all the information that you will need to identify the ideal conformal coating for your application’s requirements. And if you’re searching for something specific, use the index to skip to the information you need.
Conformal coating is a specialty polymeric film forming product that protects circuit boards, components, and other electronic devices from adverse environmental conditions. These coatings ‘conform’ to the irregular landscape of the PCB providing increased dielectric resistance, operational integrity, and protection from corrosive atmospheres, humidity, heat, fungus, and airborne contamination such as dirt and dust.
There are a number of options for coating technologies, and the best option should depend mainly on the protection required. Application method and ease of rework are also important factors, but should generally be considered secondary to the protective performance needed.
Traditional Conformal Coatings
What we are calling “traditional” conformal coatings are 1-part systems that have a resin base, and may be diluted with either solvent or (in rare cases) water. Traditional coatings are semi-permeable, so do not fully water-proof or seal the coated electronics. They provide resistance to environmental exposure, which increases PCB durability while still keeping application and repair processes practical.
These categories are based on the basic resin of each coating. The chemistry determines the major attributes and functions of the conformal coating. Choosing the proper conformal coating for your application is determined by operational requirements of the electronics.
- Acrylic Resin (AR) – Acrylic conformal coating provides fair elasticity and general protection. Acrylic conformal coating is recognized for its high dielectric strength, and fair moisture and abrasion resistance. What generally distinguishes acrylic coating from other resins is ease of removal. Acrylic coatings are easily and quickly removed by a variety of solvents, often without the need of agitation. This makes rework and even field repair very practical and economical. On the other hand, acrylic coatings do not protect against solvents and solvent vapors, for example that might be typical for pumping equipment. Acrylic coatings can be considered basic entry-level protection, because they are economical and protect against a broad-level of contamination, but not best-in-class for any characteristic except possibly dielectric strength.
- Silicone Resin (SR) – Silicone conformal coating provides excellent protection in a very wide temperature range. SR provides good chemical resistance, moisture and salt spray resistance, and is very flexible. Silicone conformal coating isn’t abrasion resistant because of its rubbery nature, but that property does make it resilient against vibrational stresses. Silicone coatings are commonly used in high humidity environments, like outdoor signage. Special formulations are available that can coat LED lights without color shift or reduction of intensity. Removal can be challenging, requiring specialized solvents, long soak time, and agitation like from a brush or an ultrasonic bath.
- Urethane (Polyurethane) Resin (UR) – Urethane conformal coating is known for its excellent moisture and chemical resistance. They are also very abrasion resistant. Due to this and their solvent resistance, they are also very difficult to remove. Like silicone, full removal generally requires specialized solvents, long soak time, and agitation like from a brush or an ultrasonic bath. Urethane conformal coating is commonly specified for aerospace applications where exposure to fuel vapors is a common concern.
The scope of the rest of this post is concerned mainly with what we call “traditional” conformal coatings, but we’ll cover other coating types here to provide a complete picture of the available options.
- Epoxy Conformal Coating
Epoxy resins (ER) are usually available as two‐part compounds and create a very hard coating. The epoxy conformal coating provides very good humidity resistance and isn’t generally permeable like conformal coatings are. They also have high abrasion and chemical resistance. Typically, they are very difficult to remove once they are cured, and are not as flexible as the other materials. Epoxy coatings are common in potting compounds, which in contrast to conformal coatings, completely covers the electronics in a solid and level layer of material.
- Parylene Conformal Coating
Parylene conformal coatings is a unique type of coating applied by vapor phase deposition. It provides excellent dielectric strength and superior resistance to moisture, solvents and extreme temperatures. Because of the vapor deposition method, parylene coatings can be applied very thin and still provide excellent circuit board protection. Removal for rework is very difficult, requiring abrasion techniques, and without access to vapor phase deposition equipment, recoating with parylene is impossible.
- Fluorocarbon Conformal Coatings / “Nano” Coatings
A coating is dissolved in a fluorocarbon‐based carrier solvent and applied with a spray or dip method to create a very thin coat, although not nanometer scale as the nickname suggests. They are commonly used to provide a minimal amount of hydrophobicity, which may prevent shortages from very quick exposure to water. This type of coating does not offer the level of the surface protection of other coating methods.
Once the coating type is selected, the next question is how to apply the conformal coating? That decision should be based on the following variables:
- Production throughput requirements – The prep work needed, the speed of the coating process, and how quickly the boards can to be handled after the coating process.
- Board design requirements – Connector laden designs, solvent sensitive components, and other issues impact your decision.
- Equipment requirements – If a coating is only sporadically required, tying up capital and floor space with additional equipment may not make sense.
- Pre-coating processing – Some processes require masking or taping before coating.
- Quality requirements – Mission critical electronics that require a high degree of repeatability and reliability will generally move you to more automated application methods.
The following are the application methods for traditional conformal coatings:
- Manual Spraying - Conformal coating can be applied by an aerosol can or handheld spray gun. It is generally used for low volume production when capital equipment is not available. This method can be time-consuming because areas not requiring coating need to be masked. It is also operator dependent, so variations are common from board to board.
- Automated Spraying - Programmed spray system that moves the board on a conveyor under a reciprocating spray head that applies a conformal coating.
- Selective Coating - An automated conformal coating process that uses programmable robotic spray nozzles to apply the conformal coating to very specific areas on the circuit board. This process is used in high volume processes and can eliminate the need for masking. An applicator may have
a built-in UV lamp to cure coating immediately after it is applied.
Photo courtesy of PVA
- Dipping - The circuit board is immersed then withdrawn from the conformal coating solution. Immersion speed, withdrawal speed, immersion time and viscosity determine the resulting film formation. It is a common conformal coating technique for high volume processing. A great deal of masking is generally required before the coating process. Dipping is only practical when coating on both sides of the board is acceptable.
- Brushing - Brushing is a simple application technique used mainly in repair and rework applications. The conformal coating is applied with a brush to specific areas on the board. It is a low cost but labor intensive and highly variable method, best suited for small production runs.
Conformal coatings are usually applied as very thin coatings, providing the maximum protection possible using the thinnest about of material. This minimizes heat entrapment, additional weight addition, and a variety of other concerns. Normal thickness with most conformal coatings is anywhere between 1 to 5 mils (25 to 127 microns) with some coatings applied at an even thinner level. Anything greater than this thickness is usually an encapsulate or a potting compound, which typically provide more mass and thickness to protect the boards.
There are three primary ways to measure the thickness of a conformal coating.
- Wet Film Thickness Gauge - Wet film thickness can be measured directly by using the appropriate gauge. These gauges incorporate a series of notches and teeth, each tooth has a known and calibrated length. The gauge is placed directly onto the wet film to take the film measurement. See http://www.geionline.com/wet-film-gauge. This measurement is then multiplied by the percent solids of the coating to calculate the approximate dry coating thickness.
- Micrometer - Micrometer thickness measurements are taken on the board (or on a test panel) on several locations before and after coating takes place. The cured coating thickness is subtracted from the uncoated measurements and divided by 2, providing the thickness on one side of the board. The standard deviation of the measurements is then calculated to determine the uniformity of the coating. Micrometer measurements are best taken on harder coatings that do not deform under pressure.
- Eddy Current Probes - Eddy current measurement of conformal coating thickness uses a test probe that directly measures the thickness of a coating by creating an oscillating electromagnetic field. The thickness measurements are non-destructive and very accurate but can be limited depending on the availability of a metal backplane or metal under the coating, and the direct contact available of the test sample. Without metal below the test area no measurements will be made, and if the probe does not fit flat on the test area, readings will inaccurate.
- Ultrasonic Thickness Gauge – This type of gauge measures coating thickness using ultrasonic waves. It has the advantage over Eddy Current Probes because it does not need a metal backplane. Thickness is determined by the amount of time sound takes to travel from the transducer, through the coating, bounce off the surface of the PCB, and back. A complaint, like propylene glycol or water, is needed to provide good contact with the surface. This is generally considered a non-destructive test unless there is a concern with the complaint affecting the coating.
While the cure mechanism isn’t a primary criterion when selecting a coating, it has a direct impact on the type of application method that will be feasible, and the production throughput that can be expected. Some mechanisms are relatively foolproof, while others are very complex and leave room for application errors when used in an uncontrolled process.
- Evaporative Cure Mechanism - The liquid carrier evaporates, and what is left behind is the coating resin. Although very simple in theory, circuit boards usually need to be dipped at least two times to build up an adequate coating on component edges. Whether the liquid carrier is solvent or water‐based, humidity affects application parameters. Solvent systems tend to be easy to process, provide consistent coverage due to good wetting, and fast cure times. However, solvents are often flammable, so adequate ventilation and fume extraction methods are required. Using water as a carrier can eliminate the flammability concern, although they tend to take much longer to cure, and can be very sensitive to ambient humidity.
- Moisture Cure - Primarily found in silicone and some urethane systems. These materials will react with ambient moisture to form the polymer coating. This type of curing mechanism is often coupled with an evaporative cure. As carrier solvents evaporate, moisture reacts with resin to initiate final curing.
- Heat Cure - Heat cure mechanisms can be used with one or multicomponent systems, as a secondary cure mechanism for UV cure, moisture cure, or evaporative cure. The addition of heat will cause the system to polymerize, or speed the cure of the system. This can be advantageous when one cure mechanism is insufficient to gain the cure properties required or expected. However, thermal sensitivity of circuit boards and components must be taken into consideration when curing in elevated temperatures.
- UV Cure - Coatings that are cured by ultraviolet light offer very fast production throughputs. They are 100% solid systems with no carrier solvents. UV curing is line‐of‐site, so a secondary curing mechanism is needed under components and in shadow areas. UV cured coatings are more difficult to repair and rework and require UV curing equipment and UV radiation protection for workers.
On occasion, it is necessary to remove a conformal coating from the circuit board to replace damaged components or perform other reworking procedures. The methods and materials used to remove coatings are determined by the coating resins as well as the size of the area and can impact the time required.
The basic methods as cited by IPC are:
- Solvent Removal – Most conformal coatings are susceptible to solvent removal, however, it must be determined if the solvent will damage parts or components on the circuit board. Acrylics are the most sensitive to solvents hence their easy removal; epoxies, urethanes, and silicones are the least sensitive. Parylene cannot be removed with a solvent. [photo: coating_remover-pen.jpg.
- Peeling – Some conformal coatings can be peeled from the circuit board. This is mainly a characteristic of some silicone conformal coatings and some flexible conformal coatings.
- Thermal/Burn‐through – A common technique of coating removal is to simply burn through the coating with a soldering iron as the board is reworked. This method works well with most forms of conformal coatings.
- Microblasting – Micro blasting removes the conformal coating by using a concentrated mix of soft abrasive and compressed air to abrade the coating. The process can be used to remove small areas of the conformal coating. It is most commonly used when removing Parylene and epoxy coatings.
- Grinding/Scraping – In this method, the conformal coating is removed by abrading the circuit board. This method is more effective with harder conformal coatings, such as parylene, epoxy and polyurethane. This method is only used as a method of last resort, as serious damage can be incurred.
If all you are doing is replacing a component or working on an isolated area, it is common to simply burn through the coating with a soldering iron. In cases when this is aesthetically unacceptable, contamination is a concern, or components are densely spaced, there are coating removers available in pen packaging.
Certifications are an important way to distinguish general purpose varnishes and shellacs from engineered coatings designed specifically for PCB protection. Although there are dozens of user and industry specifications, the two major certifications are IPC-CC-830B and UL746E. When selecting a coating, look for the availability of 3rd party test documentation, rather than coatings with the claim that it “meets the requirements”. Both standards use the UL94 standard to judge flammability, with a V-0 rating signifying the lowest flammability potential.
IPC-CC-830B / MIL-I-46058C
This standard originated with the military standard MIL-I-46058C, which became obsolete in 1998. The civilian version IPC-CC-830B is nearly identically, so it is generally understood that if a board passes the IPC spec it will also pass the MIL spec., and visa versa. IPC-CC-830B is a battery of tests, some pass-fail and others that provide data that can be referenced and compared:
- Insulation resistance
- UV fluorescence
- Fungus resistance
- Moisture and insulation resistance
- Thermal shock
- Hydrolytic stability
Underwriters Laboratories (UL) is considered a credible and reliable safety certification body worldwide, and UL certification is commonly required for consumer goods. UL746E tests for electrical safety and flammable safety of the coated electronics. For electrical safety, there is a battery of tests similar to IPC-CC-830B, but with a cycling current load to constantly measure the failure of the isolative properties of the coating. The flammability test uses the UL94 standard like IPC-CC-830B, which involves attempting to light the cured coating with an open flame and observing the sustainability of the flame.
Once a coating has passed ULT746E, it can be registered with UL and assigned a registration number. Products certified and registered to UL746E standards can include the UL symbol (which looks like a backward “UR”). To maintain the registration, a coating much be retested annually.
Coatings can, and often are, tested to standards that only represent a portion of the whole standard. In the case of UL94, this is helpful when flammability is the main concern. Some specialty coatings may not be tested to the entire IPC-CC-830B or UL746E because they may fail portions of the test because of the nature of the product, not a reflection of the quality of the product. For example, some coatings intended to coat LEDs leave out the UV indicator to prevent color shift, but this automatically would cause disqualification under IPC-CC-830B. In other words, it is by definition impossible to pass IPC-CC-830B and have optical clarity in the UV part of the spectrum.
Of course, safety and environmental considerations should always play a part of the chemical selection and process design, but the various regulatory bodies make this even more challenging as requirements have to be interpreted and matched with product specifications.
OSHA (Occupational Safety and Health Administration) - In the US, OSHA has overriding authority over worker safety concerns. Many coatings are very flammable, and many emit fumes that have a high level of toxicity. Close attention needs to be paid to ventilation (explosion-proof when dealing with flammable fumes) and the appropriate PPE (personal protection equipment) to keep operator exposure down below the safety threshold. Flammability may be difficult to avoid without exploring more niche water-based coating materials. Newer coatings have been introduced that don’t include HAPs (hazardous air pollutants – a government classification of particularly toxic chemicals) like toluene, xylene or methyl ethyl ketone (MEK). Global Harmonized System (GHS – with those red diamond symbols) needs to be followed for labeling, which is generally taken care of by the manufacturer. Make sure safety data sheets (SDS) are readily available to operators, as they should be with any hazardous chemical in your facility.
EPA (United States Environmental Protection Agency) – In the US, EPA requirements must be followed at the national and regional level. The EPA, following the Montreal Protocol treaty, enforced restrictions on ozone-depleting chemicals. Since most of the restricted chemicals are unavailable and haven’t been used in conformal coating formulation for years, ozone depletion isn’t the current concern. If there are regional agencies (see next paragraph) that have stricter requirements than the EPA, those generally will need to be followed.
CARB (California Air Review Board) and other regional regulations – Local agencies continue to play are a larger-and-larger role in environmental restrictions. CARB was one of the early regulatory bodies, laying down VOC (volatile organic compounds – smog-producing chemicals) restrictions by product category. Other regional agencies followed their lead. Global warming potential (GWP) is the latest environmental topic of discussion.
That’s our guide to conformal coating. We hope we answered a lot of your questions. Like any challenge, selecting the best coating and coating process can be broken apart, analyzed and solved.
Now we want to turn it over to you:
What did you think of this guide? Or maybe there’s something we missed. Let us know by leaving a comment with your feedback. Techspray has experts available to help guide you all the way through the selection and qualification process.