RF and high-speed digital designers often compare data sheets from different circuit material manufacturers while seeking to find the proper material for their applications. Of course, a careful study comparing the different data sheets is highly recommended. However, when the details of circuit material data sheets are considered, several exceptions could cause confusion among those unfamiliar with this process. Understanding the test methods and their testing conditions for each property is critical in recognizing whether a direct data comparison of two different materials is valid.
To begin with, one common high-frequency circuit material property to consider in most applications is Dk (dielectric constant or relative permittivity). Many PCB designers see Dk as a straightforward property, but when you evaluate the different test methods, conditions and influence the material has on each test method, the results may not be as one would assume.
As an example, the same piece of material can be tested in two different tests and achieve two different Dk values, and both values may be correct. One reason why that statement can be true is since most materials used in the PCB industry are anisotropic, which means that the Dk is not the same on different axes. Some test methods will evaluate the material in the z-axis (thickness axis) only and other test methods will evaluate the x-y plane of the material for the Dk property. When a material is anisotropic, there should be a different Dk in the z-axis than in the x-y plane.
Another example is related to the normal frequency-dependence of the Dk in high frequency materials. As a general statement, an increase in frequency will cause the Dk to decrease slightly when tested within the microwave range of frequencies or within the lower millimeter-wave range of frequencies. Again, it is possible to test the same material in two different tests, which are using the same test method and get two different answers for Dk and both answers are correct. That can be true due to the frequency-dependence of materials and when using the same test method but at different frequencies, the Dk value should be different.
There are also similar accuracy concerns for comparing data sheets as it relates to dissipation factor (Df). The main concern regarding Df is typically the test methods being compared at different frequencies. Df is frequency-dependent and with an increase in frequency, the Df should increase. Sometimes data sheets will show the Df values at 2 GHz or 2.5 GHz when targeting applications in that frequency range and then other data sheets will report Df for the material when tested at 10 GHz. The Df should be higher at 10 GHz than 2 GHz, so the designer needs to make sure the frequency and test method is the same when comparing Df values for different materials.
To read this entire column, which appeared in the June 2018 issue of Design007 Magazine, click here.