Conformal Coating

Conformal Coating

Overview
Conformal coatings are specially formulated lacquers designed to protect PCBs and related equipment from the environment. This improves and extends the working life and ensures security and reliability of performance. These coatings 'conform' to the contours of the board and its components, creating a thin (25-50 microns) layer which is both lightweight and flexible. This protects circuitry from hazards such as chemicals (e.g. fuels, coolants etc.), vibration, moisture, salt spray, humidity and high temperature. For circuit boards that are not conformally coated these conditions can cause corrosion, mould growth, and current leakage, resulting in board failure.

Conformal coatings allow higher power and closer track spacing as they prevent current leakage. This, in turn, enables designers to meet the demand for miniaturization. Both transparent and pigmented coatings improve the appearance of PCBs - the latter ensures a high degree of secrecy by camouflaging the components and layout.

The Electrolube team consists of some of the world's foremost experts in the formulation and application of conformal coatings designed to meet international approvals. The range of products currently available comprises acrylics, silicones, polyurethanes and solvent free coatings.

Electrolube also offers a full range of ancillary products to complement the use of their conformal coatings. These include thinners and strippers, peelable coating masks and thixotropic materials for the effective coating of lead ends. A specially designed Dip Coating Machine is also available. It provides the ideal method of applying conformal coatings.

TYPES OF CONFORMAL COATINGS
Acrylics
Acrylic conformal coatings provide good flexibility and all round protection. As one part systems, they are easy to apply and easy to remove. Some acrylics meet military standards, they dry quickly rather than cure and are solvent removable.

Silicones
Silicone conformal coatings are available in solvent resistant and solvent removable forms. They perform at higher temperatures, and provide excellent chemical and salt spray resistance. The solvent resistant products are more difficult to remove although they can be soldered through. They cross-link with time but for maximum performance heat curing may be required. Silicones have remarkable electrical insulation properties making them ideal for high power and highly populated boards.

Polyurethanes
These can be one or two part systems. The most useful products for coating applications are single part systems. They offer excellent moisture and chemical resistance and good electrical characteristics. They are not easily removed once cured.

Ultra-Violet Curing
Coatings that use UV energy to initiate the curing process are solvent-free. This means that they are non-flammable, do not contain VOC's and they comply with current and proposed environmental legislation. They provide high chemical and heat resistance, flexibility and gloss, and give excellent adhesion and coverage on lead ends. Curing times are a fraction of other coatings (seconds), greatly reducing production costs by increasing production speeds.

Aqueous
Water based coatings are non-hazardous, but because of the evaporation rate of water may require longer drying times and generally require curing at elevated temperatures (90°C). As water is the only solvent, they are VOC free.

Fluoroacrylics
These specialty acrylics have been introduced to the market because of their extremely low surface energy. This means that any moisture will be instantly repelled. They are applied in extremely thin, weak films, making masking unnecessary as the film is easily removed by mechanical action e.g. inserting an edge connector.

As the film has low strength it does not offer the same level of protection as conformal coatings, but for high volume production where moisture resistance is the main concern, fluoroacrylics offer enormous benefits.

Typical Applications
Initially, conformal coatings were only used in 'high-tech' applications. As consumers become more concerned with quality and reliability, and electronic assemblies become more widely used in everyday life, conformal coatings offer manufacturers the ability to improve the quality of their products while eliminating costly warranty failures. Typical areas of use include:

Domestic and Commercial
Conformal coatings provide protection for electronic circuitry in a wide range of domestic appliances and offer resistance against:

Water and detergents (e.g. washing machines and dishwashers)
High humidity (e.g. microwave ovens )
Chemical atmospheres (e.g. washer/dryers)
Office and home hazards (e.g. computers)
External environments (e.g. burglar alarms)

Automotive
The automotive industry specifies conformal coatings to protect circuitry from attack by agents such as petrol vapor, salt spray and brake fluid. The use of electronic systems in vehicles continues to grow rapidly, therefore, the use of conformal coatings becomes essential to ensure long term reliability. Conformal coatings are used in applications:

Under the hood (e.g. engine management systems), and
In passenger compartments (e.g. onboard computers)

Aerospace
The stringent environmental demands of the aerospace industry where rapid compression and decompression will affect the performance of circuitry, necessitates the extensive use of conformal coatings. The coatings are used in both pressurized and depressurized areas:

Depressurized (e.g. fly-by-wire systems)
Passenger compartments (e.g. air-conditioning systems)

Marine
Both fresh and salt water marine environments will attack electronic circuitry. Conformal coatings are ideal for the protection of equipment used on the surface, but where equipment is to be installed in a submerged location, the added protection of a resin will almost certainly be required. Typical applications include:

Navigation systems
Radar control systems

Medical / Industrial
There are numerous areas where a conformal coating will be required for environmental protection:

Tool protection while in storage to prevent corrosion.
Agriculture machinery to prevent attack from agrochemicals and farmyard waste.
Pacemakers, where it is vital to ensure continuous performance.

Selection Criteria
With such a wide range of conformal coatings, all with different features and benefits, it is important that the correct coating and method of application be chosen. The main questions to be addressed are:

Working environment
Electrical requirements
Board layout
Method of application
Rework and repair

Working Environment
Controlling or predicting the exact operating conditions is difficult to achieve without expensive, sophisticated test chambers, so the solution is to protect the circuit completely from the hazards it may face. The physical demands on equipment range from sudden decompression, as an aircraft climbs to the constant vibration of an office printer; so flexibility and the ability to expand and contract are important features in a conformal coating. Checks should be made to ensure these characteristics are maintained when sudden pressure and temperature changes take place, and that the material will retain these properties throughout the lifetime of the equipment

Electrical Requirements
A conformal coating should exhibit high dielectric strength and breakdown voltage. The minimum required dielectric strength of the coating can be determined from the inter-track separations and the potential difference between adjacent tracks.

Board Layout
The design of the board should include consideration of the placement of components that must not be coated. These components include connectors, IC sockets, adjustable potentiometers and test points. If the design allows, these components should all be placed on one edge of the PCB to enable the board to be coated with the minimum of masking.

If this is not possible, Electrolube provide a fast curing peelable coating mask to protect the relevant components. Component legs should be cropped to the minimum length to ensure that a coherent film is formed.

Methods of Application
Conformal coatings may only fully protect the assembly if the board is thoroughly cleaned prior to application. This ensures that the coating will adhere to the surface and that corrosive residues are not trapped on the PCB. The main application methods include:

Brushing is a popular application technique and can give excellent results on even surfaces.
Spraying with aerosols is convenient for repair or small production runs, and spray guns are suitable for larger production. However, both methods can give "shadowing" (voids under components) and the effectiveness can depend on operator accuracy.
Automatic Dip Coating is excellent for medium volume production. Dipping ensures complete coverage and there is no overspray or material wastage.
Selective Coating using a programmable XY table is ideal for high volume coating to eliminate masking.

Dip Coating Machine
The Electrolube Dipcoater provides a reliable, constant method for the coating of PCBs and other components. Entirely pneumatic, with no electrical parts, the machine is safe to use with flammable materials, and the automatic dip cycle provides a smooth insertion and controlled withdrawal rate which eliminates air entrapment and ensures an even film.

Rework and Repair
If the assembly requires repair, consideration must be given to the ease of removal of the coating. Most coatings may be safely soldered through and some can be easily removed in a standard solvent. Electrolube can also supply powerful solvents and strippers to remove the most stubborn, aged coatings.

Conclusion
Reliability is the key requirement for today's electronic equipment and conformal coatings provide a cost effective solution.

back to home page