Printed Circuit Board Assembly Conformal Coatings

Conformal coatings are most commonly used to protect printed circuit boards (PCBs) and electronic devices from moisture, corrosion, static, thermal shock, vibration, and even contamination.

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Generally, the assembled printed circuit board required a conformal coating finishing only when they are used in some mission-critical applications, such as aerospace, automotive, medical, military, etc. Nowadays, increasing demand in wearable electronics has lead to a higher requirement to the protection and reliability offered by conformal coating (the sweat will take problem between PCB and electricity).

Conformal coating is a specially protective polymeric film-forming material that protect printed circuit boards and electronic assemblies from harmful environmental impact such as moisture, corrosion, static, thermal shock, vibration, and even contamination. The coating formed as a thin film that “conform” to the contours of PCB and it’s components providing increased dielectric resistance, operational integrity, and reliability.

What is a Conformal Coating ?

Generally, conformal coatings are made of a polymeric material such as resin, sometimes mixtured with one or more solvents to enable the material to dispense and flow properly.

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The best type of coating material to choose is subject to the exact protection requirement for electronic device and the specific environment that the electronic board will be located. Additionally, some important factors need be considered such as method of application, ease of repair and rework, etc.

Typical Conformal Coatings

Typically, conformal coatings are 1-part systems with a resin base, and maybe diluted with either chemical solvent or water in rare cases. As these coatings are semi-permeable, therefore they do not fully have water-proof or seal the coated electronic devices. The purpose of the coatings is to provide environmental and mechanical protection to significantly improve the durability, longevity and reliability of the electronic devices, and it is friendly to apply and repair.

Coatings can be categorized by their base resin:

Acrylic Resin (AR) – Relatively, AR coating is a low-cost solution, can provide good overall production, and also are easy to apply, remove and repair. They have excellent dielectric strength, fair moisture, and ideal abrasion resistance. AR is also the easiest material to remove and repair by a variety of solvents, often with no need of agitation. This feature makes field repair and rework both more practical and economical. That is to say, acrylic coatings are not effective protective material against solvents and solvents vapors, like common jet fuel fumes in the aerospace applications. Also, this coating is UV light traceable for inspection purposes.

Silicone Resin (SR) – SR coatings can provide extremely excellent protection performance at a very wide temperature range. Silicon resin provides outstanding chemical resistance, salt spray and moisture resistance, it is very flexible as well. Silicon conformal coating has rubbery nature so it is not abrasion resistant, but this property enable it to have good feature of resilient against vibration stress. Also, silicon coatings are widely used in high humidity environments such as outdoor advertisement signage. However, coating removal can be a high challenging, requiring multiple conditions and tools such as: specialized solvents, long soak time, and agitation from an ultrasonic bath or a brush.

Urethane (Polyurethane) Resin (UR) – Urethane conformal coatings are well known as it’s excellent features of moisture and chemical resistance. Also they are both solvent-resistant and abrasion-resistant, hence they are very difficult to remove. Like silicone coatings, to fully remove the urethane resin coating it commonly requires specialized solvents, long soak time, and agitation from an ultrasonic batch or a brush. Generally, urethane conformal coatings are specified for most aerospace applications where exposure to fuel vapors is a critical concern.

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Automatic Conformal Coating Process

Why Use Conformal Coating?

Today, more and more complex electronics have to be exposed to harsh environments, so it is vital to have a necessary protection to ensure their normal function, durability, reliability, and longevity. Conformal coatings can provide proven protection against salt sprays, moisture, solvents and aggressive chemicals. What’s more, it is proved that a suitably chosen coating material can help reduce the effects of vibrations and mechanical stress on the electronic circuit board and its performance in serious temperature ranges. Conformal coatings are widely used in the electronic filed to protect the product, as well as enhance the reliability. Therefore, it diminish the destructive impact of early field failure and also reduces potential costs.

Is Conformal Coating Necessary?

Whether or not a coating needed to your electronic devices depending on the exact environment that the electronics will be used in, the sensitivity of the electronics, the durability and reliability required. Generally, the electronic devices manufactured and assembled by IPC class 3 are more likely to be coated due to their critical nature. Also, the increasing developing in miniaturization electronics and wearable electronics has raised up a higher demand for extra protection and reliability offered by conformal coating.

Is Conformal Coating Waterproof?

Typically conformal coatings can provide a moisture resistant layer of protection, but cannot be considered waterproof. As coatings are semi-permeable, so they can not fully seal the coated electronics or fully water-proof. They protect the electronics device from environmental exposure, improving the durability and reliability, and being friendly apply and repair.

How To Apply Conformal Coating?

Generally, the following factors should be considered when creating a conformal coating application process:

Production capacity requirements –The comprehensive prep work and the speed of the conformal coating process can drive the production method.

Circuit board design requirements – A lot of connectors, solvent-sensitive components, unintended PAD, and other variables can directly affect the coating process.

Pre-coating processing – Generally, taping and masking before coating is necessary to protect unintended surfaces from accidental coatings.

Quality requirements – Most mission critical electronics that require a high degree of durability and reliability will require more automated and repeatable application methods.

Common application methods for conformal coatings:

Brushing – Brushing is mostly used for rework and repair. By this way, conformal coating is applied to specific area on the circuit board by a brush, not generally the entire printed circuit board. Although it is low-cost, but brushing is both a highly variable and labor intensive method, so it is more conveniently and economical for small production runs.

Dipping – this is a common conformal coating method that widely used for high volume PCB assembly. Usually masking is necessary before circuit board are coated. Dipping is only practical while there is coating requirement on both sides of the circuit board. Immersion speed, immersion time, withdrawal speed and viscosity will directly determine the resulting coating film formation.

Selective coating – This type of coating typically made by an automated spray system that actually can be programmable, with a robotic spray nozzle applying the conformal coating to specific areas on the printed circuit board. This is widely used for high volume assembly as it eliminates the laborious making process and improve the efficiency.

Automated spraying – This is done through a programmed spray systems to move printed circuit boards on a conveyor under a reciprocating spray head.

Manual spraying – For small volume production when equipment invest is not available, conformal coating can be done by handheld spray gun or an aerosol. This is time-consuming and may need to be masked. And, the quality and the consistency of outcome determined by the operator’s professionalism, therefore cosmetic variations are common from board to board.

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What A Thickness Should I Apply Conformal Coating?

The thicker the protective coating is not the better. Conversely, thinner coating provides the best production possible but minimizes other issues like heat entrapment, excess weight, etc. The typical conformal coating thickness is between 25-127um (1 to 5mils).

Generally, there are 4 ways to measure a conformal coating thickness:

Wet film thickness gauge – Which is a gauge that has a series of notches and teeth – each tooth having a calibrated length. It is to measure the thickness of any liquid applied coatings while it is still wet in the surface. The gauge can be directly placed into the wet film, the measurement is then multiplied by the percent-solids of the coatings material to approximate  the thickness of dry coating.

Micrometer test – Commonly, taking measurements at several locations on the printed circuit board before (un-coated) and after coating (coated). The exact cured coating thickness is subtracted from the un-coated thickness and divided by 2, then providing the thickness on each side of the circuit board. Then calculating the standard deviation of the measurements to determine the uniformity of the coating. This type of measurement is most accurate on a hardness coating that do not have any deformation under pressure.

Eddy current probes testing – Using a test probe to directly measure the thickness of a coating by creating an oscillating electromagnetic field. The thickness measurements is very accurate and non-destructive, but can be limited depending on the availability of a metallic surface or metal backplane under the coating,and the open contact zone available on the test circuit board. No measurement will be made if no metal under the test area. And the probe must sit flat on the test area (i.e. a highly density assembled circuit board), or else the the measuring readings will be inaccurate.

Ultrasonic thickness testing gauge  –  It is to use ultrasonic waves to measure the coating thickness, so it does not require a metal surface like eddy current probes. Thickness is calculated from how long that the sound pulse take to travel from the transducer, through the coating, onto the surface of the board, and then travel back through the coating to the transducer. A conductive medium, like water or propylene glycol, is needed so as to provide good contact with the surface. Generally, this measurement option is popularly considered a non-destructive test unless there come up a concern with the conductive medium affecting the coating.

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How Long Does It have a Conformal Coating To Be Cured?

The exact time that the conformal coating can dry depends on the type of the resin, curing method, and the thickness of the coating, differently from seconds to days. Depending on the type of the resin chosen, several types of curing mechanisms are available:

Evaporative Cure Mechanism – Also called liquid carrier evaporating, and the coating resin is finally left behind. Typically this curing method is coupled with another mechanism such as moisture cure. Generally printed circuit boards can be handled within one hour, but it will take some days to fully dry-when the coating return to its final properties.

Moisture Cure – Primarily found in silicone and some urethane systems. The coating material reacts with ambient moisture to form the polymer coating. Usually, moisture curing is coupled with evaporative curing. When carrier solvents begin to evaporate, moisture chemically reacts with resin to initiate final curing. Generally it will take several days to fully cure.

Heat Cure – Heat cure mechanisms is a scientifically formulated coating can be used with either single or multi-component 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 accelerate the system curing. This would be helpful when one cure mechanism is insufficient to gain the required or expected cure properties.

UV Cure – It is a photochemical process that instantly cures under ultraviolet light, can offer very fast production throughputs. They are 100% solid systems without any carrier solvents. UV curing is line-of-site, therefore, a secondary curing mechanism is needed under components and in shadow areas. Curing time is near instantaneous when exposed under ultraviolet light, but in shadowed areas, it may takes some days to fully cure. And UV cured coatings are more difficult to rework and repair, and require UV curing equipment and UV radiation protection for process operators.

Conformal Coating Removal

While there is a repair or rework on coated PCB, you may be required to remove a conformal coating from the printed circuit board to perform the related procedures. Generally, the methods and materials used to remove coatings are determined by the type of coating resins and the size of the coating area, and can impact the time required. The basic methods as specified by IPC are:

Solvent

Peeling

Thermal/Burn‐through

Grinding/Scraping

Micro Blasting

Solvent Removal– Although most conformal coatings can be dissolved in solvent, however it must be sure that the solvent used will not damage parts or components on the circuit board. Acrylic is extremely sensitive to solvent, hence it is the fastest and easiest coating to remove with solvent. Urethane and silicone coatings are the least sensitive to solvents, so it will need more soak-time and may require brushing to thoroughly remove the coating. And, parylene cannot be removed with solvent.

Peeling – Some conformal coatings can be peeled off from the printed circuit board. This is mainly a characteristic of some flexible conformal coatings and some silicone conformal coatings.

Thermal/Burn-through – A common technique of removing a coating is to simply burn through the coating with a soldering iron as the board is reworked or repaired. This method works well with most forms of conformal coatings.

Microblasting – Micro‐abrasive blasting is one of the most effective and safest methods to remove conformal coatings on delicate electronic devices. The removal is done by using a concentrated mix of soft abrasive and compressed air. This process is easy and very convenient while the removal on small areas of the conformal coating. It is most commonly used when removing epoxy and Parylene coatings.

Micro‐abrasive blasting conformal coating removal

Grinding/Scraping– This is a method to remove the coating by abrading the printed circuit board. This method is more effective and popular used in removing harder conformal coatings, such as epoxy, parylene and polyurethane. This method is only used as a option of last resort, as serious damages could be incurred.

Available Industry Certifications for Conformal Coatings

Certifications are used to specify conformal coatings from general-purpose varnishes and shellacs. There are several categories of user and industry specifications, but the two major certifications are most commonly referenced: but there are two major certifications most commonly referenced:

IPC-CC-830B / MIL-I-46058C

IPC-CC-830 revision B is a civilian version conformal coating qualification standard, the previous standard is MIL-I-46058C,which was inactive in November 1998. IPC-CC-830B is a battery of tests that includes appearance, UV fluorescence, insulation resistance, fungus resistance, flexibility, flammability, moisture and insulation resistance, thermal shock, and hydrolytic stability,and mostly used by board fabricators, OEM design engineers, and coatings suppliers. 

UL746E

Underwriters Laboratories (UL) is officially considered a reliable and credible safety certification body worldwide, and UL certification is a common requirement for consumer goods throughout the world. UL746E testing for both the flammable safety and electrical safety of coated electronic devices. For electrical safety, there is a series of testing similar to IPC-CC-830B, but with a cycling current loading to constantly measure the failure of the isolative properties of the coatings, so as to evaluate whether a conformal coating can withstand sudden electrical surges and maintain its dielectric integrity. The flammability testing uses the UL 94 standard like IPC-CC-830B, which involves attempting to light the cured coating with an open flame, then observing the sustainability of the flame.

Once a conformal coating has passed UL746E, it can be registered with UL and get 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 must be retested annually.

The Most Common Conformal Coating Defects?

The following are the 7 most common causes of conformal coating defects we see, along with recommendations for how to avoid these:

De-wetting – De-wetting happens when a liquid conformal coating do not evenly coat the surface of circuit board . Particular areas of the coatings may bead up while other area appear correctly coated. De-wetting usually occurs because of non-ionic contamination, commonly from the manufacturing, handling or transport process.

When contaminants left on the substrate are incompatible with the coating, it will lead the surface to resist spreading in those contaminated areas. Generally, the conformal coating will bead up and move away from the below contaminants:

Process oils
Flux residues
Mold-release
Fingerprints

Whether contamination come up on the substrate during PCB assembly or are introduced during soldering process, all must be thoroughly removed with careful cleaning. The best way to avoid de-wetting is to ensure the substrate material is spotless prior to apply the conformal coating. Select low-residue materials to control the process more effectively.

De-lamination – De-lamination occurs when a conformal coating lifts away from the substrate, and the area below it will be exposed. Loss of adhesion between the conformal coating and the substrate can cause coating partial or complete lifting. In most cases, de-lamination can not be immediately observed and just noticed when the parts put in use. That’s why prevention beforehand is vital for this defect.

De-lamination most caused by following incentives:

Contaminants existing on the surface of the laminate
Lack of compatibility between the coating and the substrate material
Coating applied too thick to the substrate
Moisture between the coating and the substrate
Improper curing of the conformal coating
Insufficient drying time between coats

To help prevent de-lamination:

Thoroughly clean the board before coating it
Choose a different coating material
Reduce the coating thickness
Reduce force drying
Choose a less permeable coating material
Allow adequate time between coats
Apply a “primer” material known to bond with the substrate and the conformal coating material.

Air bubbles – Air could be probably introduced into the coating when you mix or form the spray system. Any other coating method like brushing can also take air bubbles into the coating when there is insufficient tack time between coats.

Common causes of bubbles are:

Coating applied with incorrect equipment settings or pressure
Contaminants on the surface of the substrate
Coating applied too thick to the substrate
High viscosity of the coating material
High temperature during curing
Coating cured too quickly

To help avoid these problems:

Use a lower viscosity version of the conformal coating
Ensure the coating is applied only to the recommended thickness
Apply several thin coats, allowing bubbles to dissipate between layers
Blend coats applied with a brush so they flow easily into all areas of the substrate.

More bubbles and voids – Most of the bubbles are caused by trapped solvent, which evaporating through the coating layers. If the coating thickness is too thick, or if the coating is cured too quickly, the coating surface will skin over while there is still solvent underneath that is vaporizing up, bubbles caused in the top layer.

Fish eyes – Small circular zone, highlighted by a “crater” in the center, which usually come up during spray application or shortly thereafter. Generally, this can be caused by water or entrained oil in the sprayer air system and is commonly seen when using shop air. Prevention comes in the form of a well-maintained filtration system to scrub any moisture or oil from entering the sprayer.

Orange peel – Orange peel occurs when the coating is textured or uneven, often appearing dull and looks like the skin of an orange. Mottled textures in the conformal coating may be just an cosmetic issue and non-critical. However, they usually imply a process flaw that must be corrected.

There can be multiple causes for orange peel:

High viscosity of the coating material from incorrect thinners
Low air pressure leading to uneven atomization
Improper application of the coating materials
Insufficient coating applied to the substrate
Coating applied too thick to the substrate
Not enough time curing

To mitigate the effect of orange peel:

Optimize the spraying technique to manufacturer specifications
Choose a different thinner to reduce the viscosity
Apply the coating to the recommended thickness
Increase the “flash-off time” for the solvents to evaporate before turning up the temperature to speed the curing process

Cracking – Cracking usually occurs when a smooth surface of coating fractures into segments. The cracks between the segments leave the area below exposed to potential contaminants. Because there exists a higher risk of water, moisture, and debris reaching the board level, cracking could lead to other conformal coating failures.

Common causes for cracking are:

Coating applied too thick to the substrate
Insufficient curing time between coats
Too high temperature during curing
Too quickly coating curing
Operating temperature too high or too low

To prevent cracking:

Lower the curing temperature
Allow additional drying time at room temperature
Apply the coating to specified thickness levels
Choose a coating with a wider effective temperature range
Choose a more flexible coating

Generally, a staged evaporation rate is ideal to prevent the cracking. Use a lower cure temperature over a longer period time if cracks persist in the substrate. Two-step accelerated cure method is recommended, this allows the more volatile solvents to cure at a more controlled rate and at a lower temperature. Meanwhile, slower solvents are flashed off at a subsequent period of increased temperature.

WellerPCB® Conformal Coatings

WELLER offers a broad selection of high quality conformal coatings to our printed circuit board assemblies. Contact us now and we can help you find the best conformal coating for your electronics devices.

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