Relative Permittivity of PCB Substrates

If you’ve paid attention to refraction, then you know something about the physics of relative permittivity. The semiconductor industry has managed to continue scaling to smaller technology nodes by using materials with high dielectric constant (so-called high-k dielectrics), but can you see similar benefits in your PCB with similar substrate materials? What about the use of low-k dielectrics?To get more news about Isola 370HR PCB, you can visit pcbmake official website.

The answer is not so simple, and you’ll need to weigh tradeoffs involved in using materials with different dielectric constant. There are other properties of the substrate to consider as well, and these properties are unrelated to the substrate’s relative permittivity.With most boards, designers have been able to get away with designing on FR4 thanks to some creative design decisions. These days, there are many more options for PCB substrates that provide a number of benefits in specific applications. If you’re looking for ease of thermal management in a harsh environment, ceramic substrates may be your best bet for heat dissipation without additional active cooling measures. If your focus is on reducing losses on long transmission lines, there are many high speed laminates available that are specialized for low loss in particular frequency ranges.

The frequency dependence of relative permittivity and loss tangent are important points to consider as you should aim to optimize the behavior of your board over the relevant frequency range. Relative permittivity can vary widely from ~2 to above ~10 over a range of frequencies. The same applies to the imaginary part of the relative permittivity. If you look at data on these quantities in a variety of materials, you’ll find that most manufacturers only quote a value at specific frequencies, or they don’t even specify a frequency. In this case, you’ll have to look to the research literature, take your own measurements with your proposed substrate material, or use a model like the wideband Debye model or Kramers-Kronig relations to model the real and imaginary relative permittivity.
In general, dielectric loss, which is proportional to the imaginary part of the relative permittivity, is also proportional to the frequency of an electromagnetic wave propagating in a material. This is why attenuation is normally plotted as a linear function of frequency, although this is technically incorrect as the imaginary permittivity is also a function of frequency as shown above. Peaks in the imaginary relative permittivity spectrum arise due to different polarization mechanisms that occur in different frequency ranges. These mechanisms form the basis for calculating the refractive index of a material in different frequency ranges.

The table below shows some representative values that can be taken as objectively accurate somewhere in the 100 MHz to 1 GHz range. One should note that the value for typical FR4 varies by up to ~15%, depending on the fiberglass weave pattern and fill factor. The routing direction across the fiberglass also affects the effective dielectric constant a signal would see as it travels on a trace.
The relative permittivity of your PCB substrate material will also vary with humidity and temperature. Some FR4 weaves have high adsorptivity toward water as they can be very porous. As water has a high relative permittivity (~80) and conductivity, any PCB on FR4 that operates in a very humid environment will have greater losses at higher speed/higher frequency. A higher ambient temperature also increases losses in general as it increases the polarization dephasing in any material. This is quantified using a thermal coefficient of Dk (TCDk); lower TCDk values are preferred over the widest range of operating temperature as possible.

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