The dielectric constant is an indispensable factor in PCB impedance design, which refers to the ability to increase the energy storage capacity of the material relative to vacuum, which is an inherent electrical property of the material itself.
With the high-speed development of circuit boards to high frequency, the accuracy of controlling the impedance of transmission lines is becoming higher and higher, and the dielectric constant value provided by board manufacturers is generally measured by the resonant cavity method, and the dielectric constant is affected by frequency and has differences at different frequencies.
It is important to use values in the design to improve the accuracy of the impedance and to provide a dielectric constant value that is accurate and matches the design model used. In addition, how to measure and select the dielectric constant needs to attract the attention of board manufacturers and PCB manufacturers.
What is a dielectric constant
The dielectric constant describes the properties of an insulator in an electric field.When the medium is applied to the electric field, it will produce an induced charge and weaken the electric field, and the ratio of the original applied electric field (in vacuum) to the electric field in the medium is the relative dielectric constant (permittivity) . The dielectric constant refers to the product of the relative dielectric constant and the absolute one in the vacuum.
Dielectric properties refer to the properties of savings and losses of electrostatic energy under the action of electric fields, usually expressed in terms of dielectric constant (Dk) and Dissipation factor ( loss tangent angle tanδ). The dielectric constant is commonly called Dk and Df in the PCB industry. Both permittivity and dielectric constant can be used to indicate its meaning.
What are the characteristics of the dielectric constant
The dielectric constant is mainly related to the molecular structure and arrangement of the material itself, so it is an inherent property of the material itself and generally does not change. However, for mixed materials, it is more complicated. For example, if the material absorbs water after being left for a period of time, it will cause a change in the dielectric constant.
The dielectric constant of most materials is direction-independent, and we call it isotropic materials. There are also some materials that are different, such as some woven materials, and the dielectric constant parallel to the woven surface and perpendicular to the woven surface is not the same, which is called anisotropic material. There is also a special class of materials that are also anisotropic, such as ferroelectrics and vector liquid crystals. The dielectric constant of a material is a function of frequency, that is, the dielectric constant is different at different frequencies.
Commonly used PCB substrate and their dielectric constant
The dielectric constant of PCB substrates is important because when a signal is transmitted on the PCB, both speed and signal completeness are affected by the dielectric constant. The reason why people often ignore this parameter is that when the manufacturer selects different materials to make the PCB board, the dielectric constant has been determined, such as the dielectric constant of the PCB board of Shengyi Technology (SYTECH) company is 3.7, and other company is 4.2.
Among the circuit board substrates, the dielectric constantεof the PTFE substrate is the lowest, typically only 2.6~2.7, while the dielectric constant of general FR4 ( glass cloth epoxy resin substrate) is 4.6~5.0, therefore, the signal transmission speed of the Teflon circuit board is much faster than FR4 (about 40%).
The Teflon board has a loss factor of 0.002, which is 10 times lower than the FR4’s 0.02, and the energy loss is much smaller. In addition, PTFE is called the “king of plastic “, and its electrical insulation performance is excellent, chemical stability and thermal stability are also good, so high-frequency and high-speed PCBs use Teflon or other substrates with low dielectric constant.
Polyflon, Rogers, Taconic, Arlon, Meclad can provide substrates with dielectric constants of 2.10, 2.15, 2.17, 2.20, and their dielectric loss factors are 0.0005~0.0009 at 10GHZ. PTFE substrate has good performance, but its processing process into circuit boards has a completely different process from traditional FR4. In recent years, in addition to substrates with a dielectric constant of 2.15 and 2.6, Rogers RO4000 and GIL1000 series with a dielectric constant of 3.38, 3.0, 3.2, and 3.8 have also been used.
What does the dielectric constant affect
The dielectric constant is not very easy to measure or define and is easily affected by the medium, test method and frequency, humidity and temperature, and it also affects other things as well. There are three main impacts:
①. It has an impact on the electrical signal. The dielectric constant of a capacitor has another name “permittivity”, and the permittivity of a material affects the capacitance of the capacitor.
②. The propagation speed of electromagnetic waves propagates at the speed of light. We often say that the speed of light is 299792458, but this speed of light is the one at which electromagnetic waves travel in a vacuum. When electromagnetic waves propagate in the dielectric material, the speed slows down, and how much slower it is determined by the dielectric constant.
③. The characteristic impedance of the microwave transmission line. Microwave transmission line is a concept in microwave technology used to transmit microwave signals. The most important index of microwave transmission line is its characteristic impedance, and the characteristic impedance of various microwave transmission lines is related to the dielectric constant of the dielectric filled in it.
How to calculate the dielectric constant
The dielectric constant (Dk) is a very important parameter in the SI field and is closely related to the transmission line characteristics, especially when simulating, therefore, we must ensure that the value of Dk is correct. Since many times the material parameters provided to us by the manufacturer may be parameters of one material, or parameters obtained without compression testing, such parameters cannot be used for simulation, which will lead to large errors. Therefore, we must obtain a reasonable dielectric constant by measuring (measuring TDR to calculate Dk).
The speed of signal propagation in the medium is related to the dielectric constant of the material, so we can use this relationship to calculate the dielectric constant of a particular material. Taking FR4 material as an example, when the signal is transmitted in the FR4 and copper pressed PCB, the speed can be expressed as:
Thereinto:
v : The speed at which the signal propagates through the PCB material
c: The speed of light
Dk: Dielectric constant
TD: The time it takes for a signal to travel on a transmission line
From the above equation after a simple mathematical operation:
Test and standard of dielectric constant
Ink dielectric constant, PCB dielectric constant, plastic dielectric constant, ceramic, metal, glass, PCB, alumina, epoxy resin, graphene, film, fabric, carbon fiber, silicon dioxide, PTFE, pulverized coal, resin.
Standard of dielectric constant:
ASTM A893/A893M-2003 (2015)
ASTM D3380-2010
BS 4542-1970
ASTM D1531-2006
BS 4542-1970 (R2007)
What is PCB dielectric constant
The dielectric constant is commonly called Dk and Df in the PCB industry.
Dielectric properties refer to the properties of savings and losses of electrostatic energy under the action of electric fields.
What's the kind of PCB substrate?
paper base, glass fiber cloth base, composite base (CEM series), multilayer board base and special material base (ceramics, metal core base, etc.).
What affects the PCB dielectric constant?
The PCB dielectric constant will beaffected bythe medium, test method and frequency, humidity and temperature.
What does the dielectric constant affect
It has an impact on the electrical signal, the characteristic impedance of the microwave transmission line and the propagation speed of electromagnetic waves.
Conclusion
Now many electronic products, such as communication electronic products, the working frequency continues to increase, requiring that the dielectric loss of the printed circuit board substrate for packaging and interconnection should be reduced as much as possible; The volume of electronic products is gradually reduced, which requires that the dielectric constant of the substrate needs to be improved.
