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Conformal coatings – a simple look at a complex PCB practice. By Steve Hook.

Conformal coatings are applied during the manufacture and assembly of PCBs to protect the circuitry and components. The performance and condition of the PCB are safeguarded against moisture, corrosion, dust, chemicals and temperature, as well as where extreme climate or environmental conditions will lead to electronics failure if not suitably coated.

There are numerous coatings available, varying in degrees of shielding from a protective lacquer, used mainly for the light protection of commercial PCB assemblies, to a high specification coating that gives a level of protection suitable for the products targeted at military and aerospace applications.

If a PCB is left unprotected, damage and failure is likely. Design engineers can specify a conformal coating by taking into consideration not only the environmental conditions the particular assembly will encounter, but also whether any PCB components are likely to be affected by the coating – crystals, for example. Coatings are between 25 and 200µm thick and offer good dielectric properties for breakdown voltages, insulation and moisture.

Potting, the process of applying resin into a component part for structural rigidity, to keep out all moisture or to simply protect any reverse engineering, can be used in conjunction with a conformal coating. While the potting of individual parts is determined at the design stage, so too is the requirement to coat the whole PCB or assembly. Both achieve the same goal – keeping contaminants out of the circuitry and to protect the reliability and service life of the product.

Conformal coatings prevent ionisable contaminants, such as salts, residue from fingerprints and other potential oxidising catalysts, from undermining the integrity and quality of the product. It is important to understand that conformal coatings 'breathe' and any moisture in the air passing through membranes may react with soluble contaminants under the conformal coating. Therefore, degreasing and surface contamination removal prior to coating is critical, not only to ensure the coating adheres as expected, but also to make sure no impurities could cause problems once a good conformal coating is applied.

It is important to remember that some coatings are organic in nature and so can be penetrated by water molecules. Polyparaxylylenes types are the exception; known as 'Parylenes', these coatings – which typically increase the cost per PCB and are not suited to volume manufacturing – are applied using vacuum deposition.

Conformal coatings are based upon various chemistries. Aside from Parylenes, these include: acrylic, epoxy, polyurethane, silicones and fluoro based amorphous polymers. Each has different properties, allowing engineers to determine the type and process that best answers the coating demands for the product in question. These questions include:
• What are the likely design factors and range of the product – such as temperature, humidity and dust?
• What level of protection is expected and from which environmental elements?
• Are the chemical, mechanical and electrical factors of the coating suitable for and compatible with the components, parts and conditions in which the product needs to operate?
• Are there any physical factors which need to be considered regarding the coating's thermal expansion characteristics? Will these constrain the choice of coating?
• Does the coating need to conform to a MIL-SPEC standard, IPC standards such as IPC A-610 and IPC-CC-830, or ISO 10993 for biological evaluation medical devices?
• Further considerations on the choice of conformal coating include:
• Drying times in relation to speed of application during assembly,
• The type of manufacturing; accounting for rework, pilot, batch, low and high volume manufacturing, and
• Quality and cost relating to the product and the coating

Selecting the wrong conformal coating type or application process will impact the product's long term reliability. This can also affect the cost per unit, with too little or too much processing during manufacture and unnecessary extra material costs. The choice of coatings is extensive and includes solvent, water, solvent less silicones and solvent less UV curable versions.

Low volume manufacture has typically used simple brush and spray application methods and, dependent on the coating type/application environment, some coatings may require dipping. The quality of brush applications is akin to painting a car using a brush, rather than by applying it using an aerosol, or even better, a spray gun in a clean spray booth.

High volume manufacture uses automated processes to cope with the demands of mass manufacturing. Robotic application is also used when selective coatings and measurable thicknesses of conformal coatings are demanded. The slightest pinhole in an applied lacquer enables potential contaminants to void the whole conformal coating, thus leading to failures.

Some coatings will allow for curing and drying at room temperature (air drying film formation) while others, either for the protection standard to be achieved (ISO 10993, for example) or, may require a fume extraction chamber and fully controlled drying oven to accelerate curing time for faster manufacturing. This could be inline ovens and conveyors, or curing in batches.

Manual application by brush or aerosol does demand a level of experience to know when a sufficiently thick coating has been applied to all areas. A good coating can be verified simply by visual inspection; the coating is glossy and any missed areas can be readily seen. Others containing a fluorescent dye can be inspected under ultraviolet light.

UV conformal coatings, often used in automotive and consumer electronics, have gained in popularity as their chemical make up has advanced greatly to allow rapid curing, simple processing, resistance to thermal cycling and the ability to follow environmental standards for recycling.

Conformal coatings can also be used to manipulate the performance of an electronic circuit. Voltage rating, for example, can be increased by using the coating's insulating properties which may enable a stronger electric field than, for example the high altitude thin air it may find itself in.

Occasions when size, weight and space are vital, COB assemblies can use the 'glob-top' as encapsulation. This blob of resin doubles as a conformal coating and protects the chip's delicate wire-bonding structure – but only the chip is coated, the rest of the PCB may require an additional conformal process.

Part of the 4E Futures community of companies, flexible manufacturing company Batchbuild and design house MLE have been applying conformal coatings for nearly 15 years in military, medical and aerospace products. Meanwhile, recently acquired Stock Electronics, which has produced motor control products for outdoor and harsh environments for 30 years, continues to use a range of conformal coatings across its product portfolio.

Steve Hook is head of group marketing for 4E Futures (www.4efutures.com).

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Steve Hook

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