It can be important to balance dielectric strength and thermal performance when deciding on coatings for insulation and heat dissipation. Thermal conductivity, Κ, λ, or κ, measured as W/m·K, is a key factor and measures the rate at which heat flows away from a heat source towards cooler regions. The use of an optimized coating can enhance thermal performance.
Dielectric strength quantifies a material’s performance as an electrical insulator. Higher dielectric strength provides better insulation. Dielectric strength is measured as volts per unit of thickness, like volts per mil (V/mil) or kilovolts per millimeter (kV/mm).
The challenge for designers is balancing dielectric strength and thermal performance. Materials with higher thermal conductivity tend to have low dielectric strength, and conversely.
Common materials used for conformal coating of printed circuit boards (PCBs) include acrylics, polyurethane, silicone, epoxy, UV-cured coatings, and parylene. They offer a wide range of performance tradeoffs in terms of dielectric strength (electrical), cost, chemical resistance, cure times, temperature resistance and thermal conductivity, and ease of use (Figure 1).
It’s often necessary to modify the base material to achieve significant thermal conductivity. For example, silicone can support thermal conductivity, but in its base form, it’s a thermal insulator. By adding thermally conductive fillers like ceramic particles or metal powders, silicone’s ability to conduct heat increases significantly, making it useful for applications like heat dissipation in electronics.
Application considerations
Excessive temperatures can degrade an insulating material, significantly reduce its dielectric strength, and potentially lead to thermal runaway or electrical failure. Determination of the relative importance of dielectric performance versus thermal performance in each application is an important starting point for identifying the optimal coating.
The use of fillers and additives can help optimize performance. For example, adding thermally conductive, but electrically insulating, fillers like boron nitride (BN) to polymers can improve thermal performance while maintaining dielectric integrity. However, adding metallic or conductive fillers will increase thermal conductivity but reduce dielectric strength.
Optimize the coating thickness to meet application needs. Since dielectric strength is proportional to coating thickness, the use of a thicker coating can improve dielectric performance. A thicker coating will also increase thermal resistance, reducing heat dissipation. It’s often necessary to test a variety of coating thicknesses to determine the optimal solution for a given application.
Filler factors
Fillers can provide a path to optimizing the performance of conformal coatings, but it’s not that simple. For example, adding non-conductive electrical ceramic fillers like alumina or silica to polymers can boost thermal conductivity.
If too much filler is needed to achieve the target thermal conductivity improvement, it can reduce the mechanical integrity of the coating in the form of reduced flexibility or increased brittleness, making application of the coating more challenging and the result less reliable.
Once the optimal filler mixture is identified, achieving a uniform distribution of the filler is critical. Any inconsistencies in the filler distribution, especially any voids, will create weaknesses in the dielectric performance and can result in localized electrical breakdowns.
Advanced strategies
One way to avoid filler distribution problems is to use advanced materials like nanofillers and engineer the microstructure of the coating to improve its dielectric and thermal performance through enhanced filler dispersion, reduction in defects, and the integration of tailored conductive pathways.
The use of nanofillers based on graphene, BN, or carbon nanotubes can create efficient thermal conduction networks within the coating matrix. Another approach is to create aligned crystalline chains or columnar grain structures to optimize heat transfer. However, many of these approaches involve costly materials and/or complex production and application processes.
Electrical devices and PCBs are complex electrical and thermal environments. Optimizing the overall system performance involves more than an optimized coating. It often requires the use of computational analysis tools like the finite element method (FEM) for precise temperature distribution analysis and integrating effective active cooling systems to manage heat at its source (Figure 2).
Summary
Coatings can be used to enhance dielectric isolation and thermal performance of PCBs. There are various tools designers can use to achieve optimal results, including material selection, the use of various types of fillers, and the use of advanced nanofillers and engineered microstructures of the coating. In the final analysis, both dielectric isolation and thermal performance are system-level considerations, and coatings are only one dimension of a complex design challenge.
References
A Comprehensive Guide to Material Insulation Properties, Atlas Fibre
Best Coating for Dielectric Strength, Specialty Coating Systems
How To Maintain Dielectric Strength, United Resin
Industrial Coating for Dielectric Strength: 4 Crucial Things to Consider, Advanced Industrial Coatings
Optimizing Electrical & Electronic Insulation Properties for Epoxies, Master Bond
PCBA Protection for Extreme Cold: A Technical Guide for Siberian Environments, Venture Electronis
Review of enhancing thermal conductivity in polymer-based dielectrics as passive components, Progress in Polymer Science
The Complete Guide to Dielectric Coating Testing, Battery Tech Association
Thermal Coatings for Critical Applications: Advanced Materials and Performance Characteristics, Modus
Thermal Insulation Coatings: Controlling Heat Flow with a Functional Coating, Coatings Tech
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