Conformal coating technology has become a vital part of many electronics manufacturing operations. Coatings ensure reliability and long-life operation of the finished electronic assembly under adverse conditions. A conformal coating (a dielectric material) protects the circuit assemblies against contamination. These coatings provide a secure envelope around a circuit board and its components while acting as a barrier against moisture, fungus, dust, salt spray, and other environmental contaminates. The coating materials also act to immobilize various types of particulate on the surface of the PCB assembly and function as a protective barrier. When applied properly, conformal coatings also enhance a circuit’s reliability by elimination of detrimental conditions (such as leakage from high impedance) while allowing for the tighter spacing of circuit traces required by high component densities.

The EMPF Demonstration Factory uses the Concoat Systems DC 2002 Dip Coater (Figure 6-1). Concoat Systems is a British company which has specialized in application engineering chemical compounds for the electronics industry for nearly 20 years.

Conformal coatings are applied over electronic circuitry in a thin layer (typically a few mils). They are usually applied by dipping, spraying, or simple flow coating. Conformal coatings prevent corrosion of conductors and solder joints, and they minimize dendritic growth and the electromigration of metal between conductors. The use of conformal coatings protects circuits and components from abrasion and solvents. Also provided are stress relief and protection of the insulation resistance of the circuit board.

In the past, due to the cost of the coating material and its application to the board, only the most expensive boards or those with especially demanding needs for reliability were coated (mostly for military use). With advances in application and process abilities, the cost of using conformal coatings has come down. Additionally, as circuit sizes decrease and components become increasingly delicate, the demand for protective coatings has increased.

Types of conformal coatings
The physical and chemical properties of various coating materials differ and thereby offer varying degrees of protection. There are five basic types of conformal coatings:

1. Acrylic
2. Epoxy
3. Urethane
4. Paraxylene
5. Silicone-based materials

Acrylic
Acrylic coatings are typically solvent-based and easily repaired. They are usually low cost, tough, hard, and transparent. Along with good pot life, they exhibit low moisture absorption and have short drying times. However, this type of coating does not demonstrate
resistance to either abrasion or chemicals.

Epoxy
Epoxy coatings are very hard, usually opaque, and good at resisting the effects of moisture. Epoxy is usually available as a two-component thermosetting mixture. It possesses excellent chemical and abrasion resistance but can cause stress on components during thermal extremes. Epoxy is fairly easy to apply but nearly impossible to remove without damaging components.

Urethane
Urethane coatings are tough, hard, and exhibit excellent resistance to solvents. Along with excellent abrasion resistance and low moisture permeability, they offer good low-temperature flexibility; however, their limited high-temperature capability and lack of reparability often prevent their use.

Paraxylene
Paraxylene coatings are highly uniform and yield excellent pin coverage. Their limitations include high cost, sensitivity to contaminants, and the need for a vacuum application technique.

Silicone
Silicone coatings range from elastoplastic (tough, abrasion-resistant) to soft, elastomeric (stress-relieving) materials. Silicones are typically used in high-temperature environments. Silicone coatings are easy to apply; have low toxicity; offer good resistance to moisture, abrasion, and humidity; and can withstand a wide range of temperatures. Although their dielectric strength is less than that of other types of coatings, the ease of application and excellent adhesion to previously-applied coatings allows for the build up of a thicker film, which improves dielectric strength. They possess the following characteristics:

• Heat cure or RTV cure
• Useful over a wide range of temperatures
• Good moisture and humidity resistance
• Processing versatility
• Easy reparability
• Low toxicity

Coating process
Before coating a PCB assembly, it must be cleaned and de-moisturized within 8 hours of conformal coating. De-moisturizing may be accomplished by an oven bake at 93°C +/- 5.5°C, for a minimum of 4 hours. The coating material is applied using a method that will yield complete coverage without excessive filleting or runs. Common coating methods include spraying, brushing, dipping, or a combination thereof. Chemical vapor deposition is used to coat with paraxylene.

The Concoat Systems DC 2002 Dip Coater extracts the PCB assembly from the conformal coating dip at a controlled rate. The entire PCB assembly is dipped into the holding tank. Controlled removal from the conformal coating dip provides a uniform thickness.

The EMPF has used the DC 2002 to conformal coat a communications module for military aircraft. The coating protects the assembly against condensation from humidity and other fluids (hydraulics, water coming in from an open cockpit window, etc.) which could cause the communication unit to fail. The unit is part of an intercom system which is used in utility helicopters, enabling the crew to communicate within the aircraft, with the ground crew, and with other aircraft through radio interconnections.

For more information about the Concoat Systems DC 2002, please contact Jeff Stong at the EMPF at (610) 362-1200, extension 224 or jstong@aciusa.org.