[PCB DESIGN] ROGERS: Knowing the specific meaning of Dk value of circuit material can reduce design changes

In the development stage of PCB circuit, it usually goes through several circuit design iterations, including testing, redesign, reprocessing, etc. These multiple changes can lead to higher costs, and it is not uncommon for a project to undergo four to eight changes from inception to market. The conventional approach is to use circuit simulation software to do accurate simulations that can significantly reduce the number of changes and associated costs.

There are many very good simulation software tools that allow circuit designers to estimate the electrical performance of a circuit. All such circuit simulation is based on the circuit model, and is closely related to the circuit impedance, insertion loss and so on. Many properties and properties of the circuit can be obtained through circuit simulation, but there are often some differences between the actual circuit performance and the simulation results. Some of the differences are small; Some of the differences are quite striking.

Before designers can input the data into the simulation software, they need to understand the specific meaning and details of the data. Due to the unique desired performance of the circuit, all simulation models are different, so the input data may not fit the model perfectly. The inaccuracy of the simulation results may be caused by the user's input data when the model is defined.
Sometimes, it could be an oversight on the part of the user, or it could be that what the user thinks is accurate isn't. One of the potential inaccuracies is the dielectric constant (Dk) value of the PCB material used in the circuit. It may be that the Dk value itself is accurate, but users may use it inappropriately due to misconceptions about how to obtain a Dk value and what it represents.

The Dk of any dielectric material is frequency dependent. In other words, when testing the same material and using the same test method, the Dk value will be slightly different at different frequencies. Usually in the range of a few megahertz to about 5GHz~10GHz, Dk varies slightly with frequency. For most low-loss circuit materials, there is a slight negative slope change in the Dk frequency curve from 10GHz to 250GHz. Considering this frequency range and depending on the degree of polarization of the circuit material, the Dk usually decreases to 2% or less as the frequency increases. For more accurate circuit modeling, material Dk data generated at the same frequency as the circuit should be used.

Another problem is anisotropy, which is often more overlooked for Dk values used in circuit simulations. Most circuit materials are anisotropic, which means that Dk values are different on the three axes of the material. It is common for the Z-axis (thickness direction) Dk of a circuit material to differ from the X-y plane Dk of the material. The Dk values on the X and Y axes are usually similar, but the Dk values on the X, Y and Z axes are usually quite different. In addition, there are many testing methods for material Dk, some of which only evaluate the properties of the material's Z-axis and some of which evaluate the properties of the x-y plane.

If the Dk information designers use in their models is in the X-y plane (as opposed to the Z-axis), then it may not be appropriate for their particular model. It is helpful to know what test methods are used to determine Dk and how often to obtain Dk values.

For most high-frequency circuit materials, the Dk value is usually around 4 or lower, and the anisotropy is usually less significant.
In most cases, the difference between the z-axis and the Dk values in the x-y plane is less than 3%. For non-filled glass fabric reinforced circuit materials, however, the Dk difference may be much higher.

Materials with a higher Dk value (such as materials with a Dk value of 6 or higher) are more different from Dk values on the X-y plane and the Z-axis. For these materials, due to anisotropy, the Dk difference of the material is usually 5% to 15% (the actual value depends on the specific material). There are some exceptions, some high Dk materials have very small anisotropy. In any case, designers should consider anisotropy when using materials with higher Dk values.

Finally, the designer should use the Dk value from the test method most similar to its circuit design structure, and at the same frequency. Due to the limited number of test methods and the wide variety of circuit applications, it may be difficult for designers to find a good match between test methods and their models. In any case, designers should understand and try to use the generated Dk values as close to their models as possible. In addition, contact the material supplier to see if they can provide Dk information more suitable for their design.