Conformal coating for ESD and other protection
Conformal coating (also referred to as electronics coating) is commonly used for electronics protection. It is applied on printed circuit boards (PCB) and related equipment ,to prevent electrostatic discharge (ESD); the sudden electricity flow between two electrically charged components. The coating additionally, protects from harsh operating environments where the components are subject to moisture or chemicals. Not only prevents conformal coating internal damage but but it also forms a barrier on the substrate protecting from air borne contaminants; in other words, the coating is not only electrically insulating but also anti-corrosive.
The coatings are nonetheless, not designed to be total sealants. Conformal coating forms a breathable protective layer which protects against the specific environment requirement but also allows any moisture trapped in the circuit board to escape.
List of conformal coating suppliers in the UK
| Manufacturer / Supplier | Address |
|---|---|
| SCH Technologies Limited (Conformal Coating Manufacturer) | Newburgh Building McLintock Way Barnsley South Yorkshire S70 6BF |
| Blundell (Conformal Coating Manufacturer) | Unit C-D, Quinn Close Seven Stars Industrial Estate Coventry CV3 4LH |
| Icon Electronics Limited (Conformal Coating Manufacturer) | Knaresborough Technology Park Manse Lane, Knaresborough North Yorkshire United Kingdom, HG5 8LF |
| 3M United Kingdom PLC (Conformal Coating Manufacturer) | 3M Centre Cain Road Bracknell, RG12 8HT |
| UK Electronics (Conformal Coating Supplier) | Fitton St Royton, Oldham OL2 5JX |
PCB conformal coating advantages
Conformal coatings are very flexible, easy to apply coating systems which are usually applied in thin layers on PCB’s and other related equipment. The conformal PCB coatings are additionally, easy to remove, repair or replace when needed. They also have several advantages which guarantee improved performance and reduce the risk of system failures. The advantages include for example:
- Insulation properties which can reduce PBC conductor spacing of over 80%
- Internal and external protection which allows simpler enclosures, thus reduces need for complexicity
- Light weight
- Complete protection from chemicals and corrosion
- Protection from environmental hazards preventing performance degradation
- Minimised environmental stress on PCB assemblies due to insulation
- Protection from thermal and mechanical shocks
The coatings are not total sealants; for they form a thin protective layer which is also breathable. This means that the coating offers protection against environmental, external hazards while letting any moisture trapped in the PCB escape.
Possible modifications
Conformal coatings are in a critical position when it comes to system functionality and reliability. Therefore, most of the conformal coating suppliers are able to create a special formulation, modifying the coating to optimize the properties for the specific application. It is also possible to enhance other characteristics.
However only coating manufacturers should blend accessory chemicals with base polymer resins.
Also plasticiers can be added to the coating in order to improve its flexibility. To improve adhesion of the film, after curing, certain solvents can be added to the conventional coating formulation.
Electronics coating applications
Conformal coatings are common in electronics applications.
Most conformal coatings are resistant to moisture and humidity, aggressive chemicals and vapours, salt sprays, drastic temperature fluctuation, vibration, and organic attacks such as fungus. Conformal coatings are suitable for many applications; they are usually applied on PCB of electronics which require maximum reliability. Therefore. the coatings are common for example in automobile, consumer electronics, aerospace and defence sectors.
Next to the industrial use, the coatings are also increasing their popularity in consumer and domestic applications which are vulnerable to field failure by environmental factors. Therefore, conformal coatings are used for example in portable devices (tablets, smartphones etc.) and other consumer electronics from computers to dishwashers.
Types of electronics coatings
Due to the many uses of conformal coatings, there are many types suitable for different applications. The most common electronics coatings are acrylic and silicone conformal coating. These two are also the easiest to apply.
The types of conformal coatings are introduced in the following table including their advantages and disadvantages.
| Conformal coating type | Advantages | Disandvantages |
|---|---|---|
| Acrylic |
|
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| Silicone |
|
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| Polyurethane |
|
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| Epoxy |
|
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| Parylene (also known as poly-para-xylylene) |
|
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| Amorphous fluoropolymers |
|
|
Application- and curing methods
Conformal coating is the most effective when it covers the complete assembly. It should additionally, offer good coverage to sharp edges, such as component leads, and to other contours such as solder joints. Sometimes also the edges of the circuit board require coating if no solder mask is present.
Conformal coatings can essentially be applied in four ways:
- Dipping – Suitable for materials which do not cure under exposure to light, oxygen or moisture
- Selective robotic coating – Suitable for all conformal coating types. However, make sure to select the correct dispense head.
- Spraying – Hand spray using either a spray booth or aerosol can. Generally suitable for all coatings
- Brushing – Requires professional and skilled operators in order to be suitable for production
After application, the coating must thus, go through a curing process; this is when the coating hardens. The possible curing methods are dependent on the coating choice, for example only parylene coating is applicable in vapour deposition polymerisation . The further possibilities are as follows.
| Curing method | Coating |
|---|---|
| Solvent evaporation | Acrylic, epoxy, polyurethane |
| Heat cure | Polyurethane, acrylic, epoxy |
| UV cure | Silicone, polyurethane, acrylic, epoxy |
| Room temperature vulcanisation | Silicone |
| Vapour deposition polymerisation | Parylene |
| Solution deposition | Amorphous fluoropolymer |
| Vacuum deposition | Amorphous fluoropolymer |
When it comes to curing, it is important that the liquid coatings properly wet the whole surface, this usually guarantees curing without surface defects. However, especially epoxies are sensitive for surface defects and they can shrink during the curing process resulting in loss of adhesion. Shrinking during curing should also be avoided whenever possible; due to the fact that it can place heavy mechanical stress upon circuit components and lead to damage and system failure.
Volume and price indication
The price of conformal coatings is usually linked to the volume, therefore, the larger the volume the lower the price per litre. The price is thus, dependent on how many circuit boards must be coated. Fortunately there is a formula which results in a calculation giving a rather accurate indication.
The layer thickness plays an important role in the calculation, therefore, the general advice is that the dry layer thickness of solvent borne coatings should be 50µm; UV curing coatings, on the other hand require thickness between 100µm and 200µm, depending on application method.
The calculation is thus as follows:
Volume per PCB (ml) = [width (mm) Xlength (mm) Xdry thickness (mm) X100 / % solid] / 1000
Cost per PCB (£/PCB) = [cost (£/L) X Volume per PCB (mL)] / 1000
Coatings – Ireland
