Co-Authors and contributors
- Dr Lee Hitchens, Nexus
Conformal coatings have varying electrical properties, dependent on the chemistry of the coating.
These properties include
- Dielectric Constant
- Dissipation Factor
- Dielectric Withstand Voltage
- Insulation Resistance
- Moisture Insulation Resistance
- Q Resonance
Each are addressed below in detail
Dielectric Constant and Dissipation Factor
Conformal coatings have dielectric properties that affect electrical parameters. If they are not taken into account during the design, then the circuit board may not function as desired.
The Dielectric Constant and Dissipation Factor are a measure of how much the conformal coating may slow down electromagnetic propagation or how much energy an electromagnetic signal may loose.
The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space.
The permittivity of a material is the ability of a material to store electrical energy in an electric field.
Low dielectric constant conformal coatings ensure minimal energy loss in RF signals. They are ideal for for high-speed and high frequency electronics.
High dielectric constant conformal coatings may cause excessive energy loss in RF signals.
The dissipation factor (DF) is a measure of loss-rate of energy of a mode of oscillation (mechanical, electrical, or electromechanical) in a dissipative system.
The Ideal Solution for Dielectric Properties
The perfect solution is to have the dielectric constant and dissipation factor of the conformal coating match the substrate material. This is often not possible.
Therefore, conformal coatings applied to circuit boards may change the dielectric properties of the electronics.
Dielectric constant and dissipation factor are frequency dependent. According to the IPC CC 830 Handbook, standard test frequencies for materials are 1 KHz and 1 MHz.
High dielectric constant materials are often used in DC power applications, since there is no frequency effect at DC.
Dielectric Withstanding voltage (DWV)
Dielectric Withstanding Voltage is the maximum electric field that it can withstand without breaking down (i.e., without experiencing failure of its insulating properties).
If a voltage is applied to a conformal coating and gradually increased, eventually it will breakdown, causing the electrical resistance to catastrophically fail.
Two types of failure mechanism can occur.
- The first is a high-speed break down due to the conformal coatings inability to dissipate the electric energy fast enough.
- The second is a longer process, where the electric field breaks down through the Corona effect. In circuit boards, a corona discharge is an electrical discharge brought on by the ionization of a fluid (in normal cases the air around the circuit) surrounding the coating that is electrically charged.
The Insulation resistance of a conformal coating is the electrical resistance of the coating.
The higher the insulation resistance, the better the material is as an insulator.
This is normally measured at ambient conditions (ie low temperature and humidity).
Moisture Insulation Resistance
Moisture insulation Resistance (MIR) is the insulation resistance of the conformal coating when exposed to elevated conditions of temperature and humidity.
A conformal coating with a low MIR value may not be a suitable coating to protect a circuit board in a high moisture environment.
IPC-TM-650, Test Method 188.8.131.52 is an example of a Moisture Insulation Resistance test from the IPC standards.
Q Resonance (Quality Factor)
The Q Factor relates to the damping effect of the coating on the electromagnetic frequencies experienced.
When circuits operates at high RF frequencies, the conformal coating may affect the response of the circuit as its dielectric properties change with frequency.