What Is Dielectric Constant

Are you wondering about the dielectric constant? We'll get right on that, but first, you'll have to learn about dielectrics.

Dielectrics

So, what are dielectrics?

A dielectric is a substance that acts as an electrical barrier. To emphasize their lack of conductivity, these materials are more commonly referred to as insulators. However, the term "dielectric" is more commonly used to refer to insulators because of a unique trait they all have in common: polarization.

The charges in a conductor are pushed by the potential difference between the two poles of a battery when the battery is connected to the conductor, resulting in a current that flows slowly through the material.

The outside electrons inside a conductor aren't bound to any specific nucleus, so they're free to move about the material and generate this effect. No electric current may travel through it because the charges in a dielectric are valence electrons trapped within the atoms of a crystal or polymer.

However, charges still feel a pull from the electric field. Individual electrons are still bound to their mother atoms, but they tend to congregate on the positive-charge side.

What Is Dielectric Constant

Dielectric materials can hold electrical charges despite their low conductivity. It does not conduct electricity when placed in an electric field. This is because no free electrons are floating around in the substance. That quality is what sets electrical insulators apart from conductors.

Instead, the dielectric's positive and negative charges are pushed in opposite directions, with the former moving in the direction of the electric field. Polarization is the process through which a dielectric's electric field weakens because charges are separated.

A parallel-plate capacitor's capacitance, or its ability to store opposing charges on each plate, is improved when a dielectric is added between the plates. However, this effect disappears when a vacuum is created between the capacitor's plates. Therefore, the dielectric constant of any given dielectric substance is guaranteed to be higher than the dielectric constant of a vacuum.

Even though dielectrics are not insulators, the two are sometimes used interchangeably. These are the key distinctions between the respective materials.

FR4 Dielectric

FR4 substrate is often used as a laminate for printed circuit boards. Most users are unaware of the influence of laminate material on electrical performance indices such as signal integrity, propagation delays, etc. FR4 substrate dielectric material is essential for posing serious challenges to high-frequency signal applications. Frequency is a significant parameter that causes changes in the FR4 substrate dielectric constant. There is a drop in the FR4 substrate dielectric constant when it is used in high-frequency or high-speed circuit boards due to the polarization of the material. It is important to understand the substrate material properties when designing PCBs for mission-critical applications.

Understanding FR4 Dielectric Constant and Dispersion

All materials have some electric permittivity—a characteristic that describes the speed that an electrical signal travels through a material and the amount of electrical charge that material can store in a given volume. The electric permittivity of free space (vacuum) is:

Ɛ0 = 8.854 x 10-12 Farads/meter

Signal propagation speed occurs relative to the speed of light in a vacuum, which is defined in terms of a relative permittivity, or dielectric constant. The dielectric constant of FR4 ranges from 3.8 to 4.8, depending on the glass weave style, thickness, resin content, and copper foil roughness.

In addition to the dielectric constant of FR4 materials, the arrangement of traces and planes on a PCB laminate determine the effective dielectric constant for signals traveling in an interconnect. Signals traveling on surface microstrip traces or coplanar waveguides will have speed that is determined by some effective dielectric constant, which then depends on the geometry of a trace. In order to determine the appropriate impedance and propagation velocity for signals on a PCB trace, your PCB design software needs to include a stackup design utility with data for a range of standard PCB laminate materials, including FR4.

Dissipation in FR-4 Dielectric Constant

The limitations also become apparent when using standard FR4 materials with RF applications. On one hand, dielectric stability may suffer across the higher frequencies. Along with a higher dissipation factor (Df) and higher insertion losses at microwave frequencies of 1 GHz to 15 GHz, traces used on FR4 have a larger attenuation at RF frequencies. Further, the FR4 thickness affects the effective dielectric constant of the PCB and…the greater impedance matching required for RF circuits. When working with RF circuits, the selection of high-performance FR4 that can handle multiple lamination cycles and has good reliability characteristics works as a solution for those problems.

• Measurements of FR4 dielectric constant can be difficult as the results will vary with respect to the measurement method.
• Impedance calculations for common trace geometries, such as microstrips and striplines, require accurate wideband Debye dispersion models for FR4 dielectric constant.
• Balancing FR4 dielectric constant with PCB laminate thickness and trace width is a difficult problem, but the right stackup manager can help you produce accurate impedance and propagation delay calculations.

Useful Electrical Properties of FR4

FR4 material is flame retardant and moisture resistant and possesses several advantageous electrical properties.
 

FR4 Substrate Dielectric Constant and Reliability

FR4 substrate dielectric constant variations can cause a few issues. The FR4 dielectric constant varies with the thickness of the substrate, copper foil roughness, resin content, and glass weave style. The stability of the FR4 insulating property is subject to deterioration under the influence of high power, voltage, or heat. As the temperature varies, the FR4 substrate dielectric constant becomes unstable, leading to impedance instabilities in the board. FR4 substrate dielectric constant variations pose serious challenges when used in controlled impedance boards.

The FR4 substrate dielectric constant is frequency dependent and offers non-uniform values in high-speed or high-frequency circuit board applications. The characteristics of the circuit and the performance of the board fluctuate with the dielectric constant variations, which are not preferable in high-frequency applications.

The transmission line or interconnect size is also influenced by FR4 substrate dielectric constant. When designing smaller boards, it is essential to choose a PCB substrate material with a higher dielectric constant.

The Dielectric Effect of the Media on Radar Messaging

The sensor in radar technology sends out brief microwave pulses into the liquid. The microwaves are sent out, bounce off the liquid's surface, and return to the sensor. The sensor calculates a delay using the microwave's known speed and the time it requires for the signal to return.

When the dielectric constant is high, the microwave signal is easily reflected since the electric flux density is also high. The higher electric flux density leads the microwave signal to scatter off of the medium rather than pass through it.

The microwave signal may be unable to return to the sensor if it passes directly through a medium with a low electric flux density. Consider the difference in bounce between a ball on a firm surface and one on a soft, less dense surface. When bounced off a denser surface, the ball will go farther than it would off a less dense one.

The dielectric constant of the media is crucial because it aids the microwave signal in reflecting off the medium and back to the sensor.

Capacitor Performance and the Influence of Dielectric Constant

When voltage is applied to a capacitor, the dielectric material inside the device becomes polarized. As a result, negatively charged electrons are shifted slightly towards the positive terminal, and the electric field is weakened.

Although the electrons do not go a sufficient distance to generate a current, the process nonetheless produces an impact essential to the functioning of capacitors. When the voltage is cut off, the polarization of the dielectric material is lost. In contrast, if the substance in question has weak molecular bonding, it may retain its polarized state even after the voltage is turned off.

The electric field created by applying voltage to a capacitor stores energy. Each type of dielectric substance has its own unique capacity to store electrical energy. A capacitor becomes more or less polarized as a result of an applied voltage. This polarization affects the capacitor's ability to store electrical energy.

More energy can be stored in a material with a high dielectric constant than in one with a low one. A material's electric susceptibility indicates how readily it will become electrically polarized when subjected to an applied electric field. The electric susceptibility of an ideal dielectric is high.

The dielectric constant is an important factor when choosing a dielectric material for a capacitor. The amount of capacitance that a capacitor is capable of depends on the dielectric constant, which is expressed in farads per meter. The capacitance of a capacitor is determined by various factors, including the electrode separation and the effective plate area; however, dielectric materials with high dielectric constants are typically utilized for this purpose.

What Polarization Means

Putting an external electrical field through a dielectric causes electrical dipoles to form inside the dielectric. This process is known as polarization.

The four forms of polarization are as follows:

• Polarization of electrons
• Polarization of ions
• Polarization of charges
• Polarization of space

Differences Between Active and Passive Dielectrics

Dielectrics that can be readily adapted for the storage of electrical energy are called active dielectrics, whereas those that can only store a limited amount of such energy are called passive dielectrics. Piezoelectric materials are active dielectrics, while glass is a passive dielectric.

How to Calculate Dielectric Constant?

The dielectric constant is the ratio of the capacitance generated by two metallic plates separated by an insulator to the capacitance induced by the identical plates separated by air or a vacuum.

An insulating material with a larger dielectric constant is required to meet the high capacitance requirements of E&E applications. On the other hand, a lower dielectric constant is preferable if a material is going to be utilized solely for insulating reasons.

Determine the Dielectric Constant Using This Formula

The formula for the dielectric constant is K = ϵ/ϵ0

• ϵ is supposed to be the substance's permittivity
• ϵ0 is supposed to be the permittivity of empty space.
• K is the ratio of any two similar things. It is a quantity without dimensions or units.

 Dielectric constants measure a material's ability to hold an electric charge and vary widely across different types of materials. It is a dimensionless electrical property.

• Capacitance is measured by sandwiching a sample between two metal plates. The material is not placed between the electrodes in the second set of readings. The dielectric constant is defined by the ratio of these two numbers.
• Variable frequencies between 10Hz and 2MHz are typically used for the test.
• The measurement requires a flat sample that is bigger than the 50mm diameter of the circular electrodes.

Dielectric Constant of Plastics

We will move on to the role of dielectric constant; however, you must learn the difference between polar and non-polar plastics first.

Differences between Polar and Non-polar Plastics

The structure of a polymer is crucial to understanding its dielectric properties. A polymer's electrical properties are determined by its structure, which in turn determines whether or not the polymer is polar.

When the distribution of electrons is unbalanced, dipoles are formed in polar polymers. These dipoles tend to line up when an electric field is applied.

Comparatively, the molecules of non-polar polymers are symmetrical and authentically covalent. Because they lack polar dipoles, an electric field does nothing to align their dipoles when it is present. However, a small amount of polarization of electrons happens as a result of the electrons' transient motion in the direction of the electric field. Both the dielectric constant and the resistivity of these polymers are quite low.

Water vapor from the air tends to be absorbed by polar polymers. There is an increase in the dielectric constant and a decrease in the resistivity when water is present. Temperature increases cause a quickening in the motion of polymer chains and the alignment of dipoles. This results in greater dielectric constant values for polar polymers.

Non-polar polymers retain their properties even when exposed to heat and water.

Now the question is, what is the dielectric constant for plastic materials?

A plastic's dielectric constant is the ratio of the charge that can be kept in a plastic sandwiched between two metal plates to the charge that can be kept when the plastic is replaced by air or vacuum.

Dielectric Constant of Dry Air

The dielectric constant of dry air is very small. The dielectric can abruptly start conducting electricity, a phenomenon known as a dielectric breakdown. Nevertheless, the breakdown is temporary since air reverts to its normal dielectric condition as the high electric field is removed. This feature protects not only metals but also several other materials from being permanently damaged.

Air's dielectric constant is nearly identical to vacuum. This explains why a capacitor's capacitance is not increased by being exposed to either vacuum or air. When the electrical current is increased, the dielectric characteristics of a solid dielectric material like polyethylene or glass are gradually destroyed.

Dielectric Constant and its Influencing Factors

A material's dielectric constant changes as a function of temperature and current frequency. The value is also affected by other factors.

1.      Temperature

A dielectric material's electric field is affected by its temperature because of the previously mentioned effect of charge separation or polarization. Polarization and, by extension, the dielectric constant are sensitive to temperature.

For instance, water's dielectric constant decreases from 80 to 55 when heated from 0 to 100 degrees Celsius. As a result, the dielectric constant decreases as temperature increases.

2.      Voltage

Value of the dielectric constant drops when DC voltage is applied. However, the dielectric constant rises in response to an alternating current voltage.

3.      Frequency

Another factor that impacts the dielectric constant is the frequency of the applied voltage.

The dielectric constant is a non-linear function of frequency; as frequency increases, the constant drops at an ever-increasing rate. The loss of electrical power rises sharply at very high frequencies. In part because of this, materials having low levels of dielectric constant are favored for use in high-frequency settings.

4.      Humidity

The material's dielectric strength falls when its relative humidity rises, making its dielectric constant decreases as the relative humidity increases.

Dielectric Constant and Its Many Uses

The dielectric constant is a vital quantity to consider when choosing the dielectric material for a capacitor or any situation where a material is needed to inject capacitance into an electrical circuit or printed circuit board.

In addition to its obvious use in representing the dielectric material, the dielectric constant has many additional practical applications.

Equipment used in electrical substations, including rheostats, transformers, radio frequency transmission lines and communications, and energy storage devices. Here are some examples of widely used applications of dielectric constant.

1.      Dielectric Constant for Energy

Capacitors rely heavily on the relative permittivity of energy sources, so understanding this property is crucial. The electric field strength inside a dielectric material with a high relative permittivity is diminished when the material is subjected to an electric field. A procedure is used to boost the capacitance of an existing capacitor layout. Dielectric layers are present in printed circuit boards beneath the linked conductors.

2.      Dielectric Constant for Communication

The RF transmission lines are the primary users of dielectric constant. In the case of a coaxial cable, polyethylene is commonly employed as an insulator between the inner conductor and the outside shield. When housed within waveguides, which are themselves employed as filters. Optical fibers are the most well-known type of dielectric waveguide. Doped impurities in this waveguide's construction allow for more exact regulation of the relative permittivity of its cross sections. These are widely utilized in optical transmission modes due to their ability to regulate the refractive index of the material.

3.      Dielectric Constant for Environment

The relative permittivity of a material is a crucial component in how sensitive a material is to environmental changes. Temperature, humidity, and barometric pressure swings have profound effects on the atmosphere. Alterations in capacitance result from shifts in relative permittivity. Sensors will be able to detect and measure these alterations. Since atmospheric pressure remains relatively constant, changes in the surrounding environment are typically the result of variations in temperature and humidity

You may calculate the relative humidity by utilizing engineering formulas and data on capacitance and temperature.

4.      Dielectric Constant for Chemistry

Static permittivity is a measure of a solvent's relative polarity, which indicates how polar it is in terms of chemicals. Let's look at an example of water. Since water is extremely polar, its relative static permittivity is 18.10 at 200 degrees Celsius, as is well known. The relative permittivity of n-hexane at 200 degrees Celsius is 1.89, making it a nonpolar solvent.

Conclusion

The capacitor's conductive plates are separated by a dielectric. What a component's qualities are like is largely determined by its insulating substance. How much energy a capacitor can store in response to an applied voltage is determined by the dielectric constant of the material.

Polarization results from an electric field being applied to a dielectric substance. When an electric field becomes polarized, its strength is diminished. The dielectric constant is often specified at a particular frequency and temperature, as the permittivity varies with both. A material's dielectric constant is also typically reported in relation to the permittivity of the vacuum. Contact us to get further assistance in this regard.