Conformal coating technology has become a vital part of many electronic manufacturing operations to assure 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 to the various devices on the PCB assembly. Furthermore, 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 closer circuit traces required with high component density.

The EMPF Demonstration Factory is using the Concoat DC 2002 Dip Coater. Concoat is a British company specializing 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, solder joints and minimize dendritic growth and the electromigration of metal between conductors. In addition, the use of conformal coatings protects circuits and components from abrasion and solvents. Stress relief is also provided, as well as protection of the insulation resistance of the circuit board.

In the past, due to the cost of the coating itself as well as the cost of applying the coating 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 improved. Additionally, as circuit sizes decrease and components become increasingly delicate, the need for a protective coating has become a necessity in many cases.

Types of Conformal Coatings
The physical and chemical compositions of various coatings differ and thereby offer varying degrees of protection. However, there are five basic types of conformal coating: Acrylic, Epoxy, Urethane, Paraxylylene and Silicone-based materials.

Acrylic
Acrylic coatings are typically solvent based and are easily repaired. They are usually low cost and are 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 to chemicals.

Epoxy
Epoxy coatings are very hard, usually opaque, and are 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 is nearly impossible to remove without damaging the 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. Their limited high-temperature capability and inability to be repaired often prevent their use.

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

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

Coating Process
Before coating a PCB assembly, it must be cleaned and de-moisturized within 8 hours before conformal coating. De-moisturizing may be accomplished by an oven bake at 93 o C +/- 5.5 o 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 the process used for Paraxylene.

EMPF
The EMPF is using the Concoat Systems DC 2002 Dip Coater in the conformal coating process. This model was selected for this process due to the controlled extraction rate of the PCB assembly from the conformal coating. The entire PCB Assembly is dipped into the holding tanks with a controlled removal from the conformal coating to obtain uniform thickness.

The assembly designed by ACI is a communication module for military aircraft. The conformal coating is being used to protect the assembly against condensation from humidity and other fluids (Hydraulics, water coming in from an open cockpit window etc.,) which could possibly cause this communication unit to fail. The intercom system, which is used in utility helicopters, enables the crew to communicate within the aircraft, with the ground and with other aircraft through radio interconnections. ACI is providing this assembly to be form, fit, and function compatible to the original unit in an effort to decrease the substantial cost of repairing the legacy boards.