Conformal coatings are a legacy technology that protect sensitive circuitry from the outside environment; namely moisture, dust and fungus. These coatings are electrically insulating polymeric materials that increase dielectric protection for printed circuit boards (PCBs).

With circuit boards becoming increasingly miniaturized, conformal coatings add a dielectric barrier between traces, preventing arcing and corona discharge. Maintaining adhesion and dielectric integrity are often the pass/fail criteria when testing conformal coatings for particular applications. 

Fundamental Concepts of Conformal Coatings

Conformal coatings are classified by their chemical makeup and comprise of the following: acrylic, polyurethane, epoxy, silicone, parylene and synthetic rubber. There are UV curable materials although from a classification perspective, these are considered either acrylic or polyurethane depending on the chemistry of the polymer backbone. 

There are two main standards used to benchmark conformal coatings, IPC-CC-830C and UL 746E. The IPC-CC-830 standard replaced the MIL-I-46058C standard that was retired in 1997 and comprises a series of tests that essentially ensures the coating maintains a threshold protection while subjected to various stresses. The UL 746E is similar that evaluates the dielectric integrity of the coating following thermal and humidity stress. 

The need for RF Transparency in Military Aircraft Circuitry

Ancillary to the conformal coating’s protective features, additional electrical properties critical in maintaining the functionality of some systems’ circuitry must be reviewed. Aboard military aircraft, systems like active electronically scanned array (AESA), electronic warfare and high frequency communication contain dense PCB assemblies that process radio frequency (RF) input.

All these systems transmit and receive high fidelity RF signals to work properly. AESA is a radar system that uses thousands of modules to transmit and receive signals. For AESA to function, it is paramount that radio transmissions received are not attenuated, meaning there’s no RF signal loss. Here, even a small change with impedance matching across the array degrades the radar resolution.

Proper selection of conformal coatings for circuits governing systems that send and receive RF involves consideration for not only protection against moisture and corrosion but also high transparency to RF signals. RF transparency requires using materials with a low dielectric constant or dissipation factor. Specifically, the proper conformal coating must have a low dielectric constant to prevent signal loss or phase distortion, preserving signal fidelity. 

Selecting a Conformal Coating for RF Sensitive Systems

Selecting a conformal coating for circuits that run systems sensitive to RF signals is about trade-offs. In terms of the environmental protection that each coating type affords, most chemistries provide adequate protection against mild contaminants like moisture and dust. These would be the main pollutants present in controlled environments like the PCB assemblies for RF equipment. If RF transparency is prioritized, silicone materials tend to have the lowest dielectric constant with acrylics, polyurethanes and UV materials all slightly higher.

MG Chemicals have developed a complete line of conformal coatings including acrylic, polyurethane, silicone and UV curable materials. These products are certified to both the IPC-CC-830 and UL746E standards. Many of these products were measured for dielectric constant per ASTM D150, with values ranging from 2 to 3.3. Aside from extending the lifespan of PCB assemblies, these materials were engineered to preserve performance with RF sensitive electronics. 

In RF driven electronics found in military aircraft, conformal coating selection must now carefully consider not just the protection aspect afforded by the coating but also the dielectric constant to preserve the operational integrity of circuits. MG Chemicals’ conformal coating line provides options that address not only environmental concerns but also RF sensitive circuits. ​

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MG Chemicals, a CADSI and Ontario Defence Association (ODA) member, manufactures specialty chemicals for electronics and defence applications from its facility in Burlington, Ontario. ​​