Conformal Coating Elements and Characteristics

It is important to determine the types of threats a system will be subjected to when deciding on the best conformal coating. The list of chemical systems and filler materials used in conformal coatings is very broad. Some chemical systems contain acrylics, elastomers, natural or synthetic rubbers; epoxy resins, water-based resins, silicone compounds, or volatile organic compounds (VOCs). Others contain bismaleimide (BMI) resins, phenolics, or formaldehyde resins. Commonly used chemical systems also include polybutadiene, polyester, vinyl ester, polypropylene (PP), polysulphide, and polyurethane (PUR). In terms of filler materials, some conformal coatings contain aramid fiber, chopped fiber, carbon powder, or industrial graphite powder. Other products contain glass fillers, metal fillers, or inorganic compounds. Unfilled conformal coatings are also available. Typically, these raw materials are used as starting components in the production of finished coatings. Once the coating is applied, there are several curing technologies available. Typically, thermo-set plastics and thermo-set resins are cured using heat or heat and pressure. Vulcanization, a thermo-setting reaction, uses heat and/or pressure in conjunction with a vulcanizing agent to produce materials with greatly increased strength, stability, and elasticity. Some polymer resins or compounds cure or vulcanize at room temperature. Others cure with radiation, electron beam irradiation, visible light, or ultraviolet light. Single-component curing systems consist of a resin that hardens through the application of heat or through a reaction with surface moisture. Two-component and multi-component curing systems consist of two or more resins and a hardener, crosslinker, activator or catalyst.

Selecting the Right Conformal Coating

There are several parameters to keep in mind when selecting the right conformal coating. The product’s application environment is obviously one of the keys, but so are the physical characteristics of component assembly. To what types of contaminants will the assembly be subjected? What is the severity and duration of contact? Is mechanical stress or heat a factor? How delicate are the mounted component leads and connections on a PCB? Selecting conformal coatings requires an analysis of physical, mechanical, thermal, electrical, and optical properties. Physical properties include viscosity and gap fill, the space between the material and substrate. Mechanical properties include tensile strength, tensile modulus, and elongation. Thermal properties such as temperature range, thermal conductivity, and coefficient of thermal expansion (CTE) are important considerations also. Electrical properties for conformal coatings include electrical resistivity, dielectric strength, and dielectric constant. Conformal coatings are not restricted to the environments described here. Many special purpose applications make use of conformal coatings, such as electrical power and high voltage products including generators, transformers, circuit breakers, and motor assemblies. Specialized conformal coatings meet military specifications (MIL-SPEC) MIL-I-46058, IPC-CC-830, IPC-4101, MIL-STD-1188, and are suitable for many applications. Flame retardant materials resist ignition or reduce the spread of flames when exposed to high temperatures. Flexible or dampening materials form layers that can bend without cracking or de-laminating. In fact, flexibility is of primary concern where PCBs are concerned and where PCB real estate is critical. The differences in coefficient of thermal expansion (CTE) between a non-flexible conformal coating and the PC board and its mounted components may lead to damage of light-gauge leads and connections. This effect is particularly a problem on boards that experience repeated temperature cycling. 2