Engineering:Conformal coating

Short description: Coating used on PCBs

Conformal coating is a protective, breathable coating of thin polymeric film applied to printed circuit boards (PCBs). Conformal coatings are typically applied at 25–250 μm to the electronic circuitry and provide protection against moisture and other harsher conditions.

Coatings can be applied in a number of ways, including brushing, spraying, dispensing, and dip coating. A number of materials can be used as conformal coatings, such as acrylics, silicones, urethanes and parylene. Each has its own characteristics for different purposes. Many circuit board assembly firms can coat assemblies with a layer of transparent conformal coating, which is used as an alternative to potting.

Conformal coatings are used to protect electronic components from the environmental factors they are exposed to. More recently, conformal coatings are being used to reduce the formation of whiskers, and can also prevent current bleed between closely positioned components.

Conformal coatings allow trapped moisture in electronic boards to escape while maintaining protection from contamination. These coatings are not sealants, and prolonged exposure to vapors will cause transmission and degradation to occur. There are typically four classes of conformal coatings: acrylic, urethane, silicone, and varnish. All of which allows for a closer conductor spacing.

Applications

Conformal coatings of PCBA where it is UV-cured

Precision analog circuitry may suffer degraded accuracy if insulating surfaces become contaminated with ionic substances such as fingerprint residues, which can become mildly conductive in the presence heightened humidity. A suitably chosen material coating can reduce the effects of mechanical stress and vibrations on the circuit and its ability to perform in extreme temperatures.

For example, in a chip-on-board assembly process, a silicon die is mounted on the board with an adhesive or soldering process, then electrically connected by wire bonding, typically with 0.001-inch-diameter gold or aluminium wire. The chip and the wire are delicate, so they are encapsulated in a version of the conformal coating called a blob top. This prevents accidental contact from damaging the wires or the chip. Another use of conformal coating is to increase the voltage rating of a dense circuit assembly. An insulating coating can withstand a much stronger electric field than air, particularly at high altitudes.

Except for parylene, most organic coatings are easily penetrated by water molecules. A coating preserves the performance of electronics primarily by preventing ionizable contaminants such as salts from reaching circuit nodes and combining with water to form a microscopically thin electrolyte film. The coating is more effective if all surface contamination is removed first, using a highly repeatable industrial process such as vapor degreasing or semi-aqueous washing. Pinholes would make contact with circuit nodes and form undesired conductive paths.

Methods

The coating material can be applied by brushing, spraying, dipping or selectively coating by robots. Nearly all modern conformal coatings contain a fluorescent dye to aid in coating coverage inspection.

Brush coating

This works by coating the material onto the board and is suitable for low volume application. The finish tends to be subject to many defects such as bubbles. The coating also tends to be thicker.

Spray application coating

Conformal coating spray booth

This coating can be completed with a spray aerosol or spray booth with a spray gun and is suitable for low and medium-volume processing. The coating application may be limited due to 3D effects. The masking requirements have less penetration. This method can be done in spray booths for medium-scale production.

When conformal coatings are applied to a PCB, they have a tendency to slump. The first layer of a coating can give a thin edge on the corners of components.

Conformal coating dipping

Conformal coating dip system

This coating is a highly repeatable process. If the printed circuit board (PCB) is designed correctly, it can be the highest volume technique. The coating penetrates everywhere, including beneath devices. Therefore, many PCBs are unsuitable for dipping due to their design.

The issue of thin tip coverage where the material slumps around sharp edges, can be a problem, especially in a condensing atmosphere. This tip coverage effect can be eliminated by either double dipping the PCB or using several thin layers of atomized spraying.

Selective coating by machine It works by using a needle and atomized spray applicator, non-atomized spray or ultrasonic valve technologies that can move above the circuit board and dispense/spray the coating material in select areas. Flow rates and material viscosity are programmed into the computer system controlling the applicator so that the desired coating thickness is maintained. This method is effective for large volumes, provided that the PCBs are designed for the method. There are limitations in the select coat process, such as capillary effects around low profile connectors that suck up the coating accidentally.

The process quality of dip or dam-and-fill coating and non-atomized spray technology can be improved by applying and then releasing a vacuum while the assembly is submerged in the liquid resin. This forces the liquid resin into all crevices.

The differences in application methods can be seen in a comparison presentation.

Solvent and water-based conformal coatings

For standard solvent-based acrylics, air drying (film forming) is the normal process except where speed is essential. Then heat curing can be used, using batch or inline ovens with conveyors and using typical cure profiles.

UV conformal coatings

UV Inline Conveyor for curing conformal coatings

UV curing of conformal coatings is becoming important for high-volume users in fields such as automotive and consumer electronics.

UV-curable conformal coatings is thermal cycling resistance.

Moisture curing

The moisture in the atmosphere cures the resin and forms a polymer. Boards are handled between a few minutes to an hour, but take a few days to reach their final properties.

Thickness and measurement

Coating material (after curing) should have a thickness of 30–130 μm (0.0012–0.0051 in) when using acrylic resin, epoxy resin, or urethane resin. For silicone resin, the coating thickness recommended by the IPC standards is 50–210 μm (0.0020–0.0083 in).

There are several methods for measuring coating thickness, and they fall into two categories: wet film and dry film.

Wet film conformal coating measurement

Wet film gauge for conformal coating thickness measurement

Wet film measurements are for conformal coatings where the dry film thickness can only be measured destructively or through over-application of conformal coating. The wet film gauges are applied to the wet conformal coating; the teeth indicate the coating thickness.

Dry film conformal coating thickness measurement

Dry film conformal coating thickness measurement

An alternative to wet film measurement is by using eddy currents. The system works by placing the test head on the surface of the conformal coating. The measurement provides a repeatable result for thickness measurement.

Test coupons can be archived as a physical record.

When liquid water is present, it is possible for a pinhole to form in the coating. This is considered a defect and can be eliminated with appropriate steps and training. These techniques effectively "pot" or "conform" to components by completely covering them.

Inspection

Conformal Coating Inspection Booth

Conformal Coating AOI

Traditionally, conformal coating inspection has been done manually. An inspector usually examines each PCB under a high-intensity, long-wave UV lamp. The inspector checks for standards. Recent developments in conformal coating automated optical inspection (AOI) have begun to use Automated Inspection Systems, which can be camera- or scanner-based.

Selection

Incorrect selection can affect long term reliability of the circuit board, and can cause processing and cost problems.

The most common standards for conformal coating are IPC A-610 and IPC-CC-830. These standards list indications of good and bad coverage and describe various failure mechanisms such as dewetting and orange peel.

Another type of coating called parylene is applied with a vacuum deposition process at ambient temperature. Film coatings from 0.100 to 76 μm can be applied in a single operation. Coating thickness is uniform, even on irregular surfaces. Desired contact points such as battery contacts or connectors must be covered with an air-tight mask to prevent the parylene from coating the contacts. Applying parylene is a batch process which does not lend itself to high volume processing.

Coating chemistry

Acrylic

• Ease of rework
• Simple drying process
• Good moisture resistance
• High fluorescence level
• Ease of viscosity adjustment

Epoxy

• Useful to about 150 °C (302 °F)
• Harder durometer, abrasion resistance
• CTE closer to epoxy PCB substrate
• Higher Tg (Glass transition)
• Good dielectric properties

Polyurethane

• Good dielectric properties
• Good moisture resistance
• Solvent resistance
• Less reversion potential
• Abrasion resistance

Silicone

• Stable over a wide temperature range (in general, −40 °C to 200 °C; −40 °F to 392 °F)
• Flexible, provides dampening and impact protection
• Good moisture resistance
• High dielectric strength
• Low surface energy for better wetting

Poly-Para-Xylylene (Parylene)

• Excellent uniformity regardless of part geometry
• Chemical inertness
• Minimal added mass and low out-gassing
• Low environmental impact process
• Low dielectric constant

Amorphous fluoropolymer

• Low dielectric constant
• High glass transition temperature
• Low surface energy
• Low water absorption
• Solvent resistance