Conformal Coatings

Conformal coatings encapsulate circuit boards and their electronic components in order to prevent the ingress of moisture, fungus, dust and other environmental contaminants. There are several basic types of conformal coatings. Electrically conductive products provide low resistivity and are often used to prevent electrostatic discharge (ESD), electromagnetic interference (EMI), and radio frequency interference (RFI). They are also used in thick film metallization and to provide electrical interconnections at both the device and circuit board level. Thermally conductive conformal coatings are applied between a heat-generating electrical device and a heat sink in order to improve heat dissipation. They form a thermally conductive layer on the substrate, either between components or within a finished product.  

Conformal coatings use many different chemical systems and filler materials. 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 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.

There are several curing technologies for conformal coatings. Typically, thermoset plastics and thermoset resins are cured using heat or heat and pressure. Vulcanization, a thermosetting 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 (UV) light. Single component curing systems consist of a resin that hardens through the application of heat or 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 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 also important considerations. Electrical properties for conformal coatings include electrical resistivity, dielectric strength, and dielectric constant. Optical properties include index of refraction, a measure of the speed of light in a material, and transmission. 

Conformal coatings are used in many industries and applications. Some products are used in the manufacture of printed circuit boards (PCBs). Other products are designed for electrical power and high voltage products such as generators, transformers, circuit breakers, and motor assemblies. Specialized conformal coatings meet military specifications (MIL-SPEC) and are suitable for many aerospace applications. Products for automotive and optoelectronic applications are also available. 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 delaminating.

• Conformal Coating Testing at Vendor Level (.pdf)
  Conformal Coatings are subjected to a wide array of both short and long term testing before their arrival at the assembler/user. This paper will familiarize you with these tests and associated...
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• Approaches to Sharp Chisels for Contaminated PCB Test
  IDI Fluxbusters are a series of probes specifically designed to penetrate heavy oxide layers, contaminates, flux and conformal coatings....
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• PCB Layout Guidelines for High Voltage Amplifiers
  As circuit size shrinks it may not be practical for today's high voltage amplifiers to adhere to the old PCB trace spacing design rules of yesteryear. As far back as 1980, Sandia National Laboratories...
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Part Numbers for Conformal Coatings