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74 DESIGN007 MAGAZINE I OCTOBER 2021 Another issue is test methods will extract the Dk/Df values based on the orientation of the electric fields, in relation to the material being evaluated. Generally, some test methods will test the Z-axis (thickness axis) and other test methods will evaluate the X-Y plane of the material. Since most circuit materials used in the PCB industry are anisotropic, Dk is differ- ent on each axis of the material. It is possible to test the same piece of material with two differ- ent test methods and achieve two different Dk values, and both values are correct. If the mate- rial is anisotropic, one test method is evaluat- ing the Z-axis of the material and the other test method is evaluating the X-Y plane; we should achieve different Dk values when testing the same material. A few other considerations for test method concepts are material dispersion, copper sur- face roughness, and the use of resonant or transmission/reflection techniques. All mate- rials have dispersion and that means the Dk will vary with a change in frequency. So, if the same material is tested twice using the same test method, but tested at different frequen- cies, there should be a difference reported in the Dk value. Basically, as frequency increases, the Dk value will slightly decrease. Copper surface roughness can slow the propagating wave and a slower wave will be perceived as a higher Dk value, regardless of the material Dk value. Some test methods are sensitive to the copper surface roughness and other test methods are not. Lastly, it is well accepted that a test method which uses resonance is typically more accu- rate than a test method which uses transmis- sion/reflection. e resonator test methods are usually more accurate, but they typically give Dk results at a discrete frequency or mul- tiples of a discrete frequency. Many of the transmission/reflection techniques will give Dk results vs. frequency over a wide band of frequencies. Following is an overview of several common test methods used in the PCB industry. X-band Clamped Stripline Resonator Test per IPC-TM-650 2.5.5.5C Aer the laminate is made, all copper is etched off and samples of the raw material are put inside a clamping fixture. e fixture has a very thin resonator circuit image in the middle, a ground plane on both sides of the resonator, and the material under test (MUT) is put between the resonator and the ground planes. When clamped together, the fixture has the RF structure of a stripline which is ground-signal-ground; more specific to this test method is the clamped structure, ground- MUT-signal-MUT-ground. is test method evaluates the Z-axis of the material for Dk and Df. It can be used at increments of 2.5 GHz and up to about 12.5 GHz. Typically, this test method is used at 10 GHz and is a relatively accurate test method. One drawback to this test method is the reported Dk value will sometimes be lower than the Dk of the material, due to the natu- ral issue of a clamped fixture having entrapped air (air has a Dk of about 1). Another poten- tial issue for this test method is that when test- ing material with high anisotropy (Dk is very different on all three axes), the resonant peak can be altered in such a way to be less accurate for the Dk extraction. is is typically not a concern, except for some materials with high nominal Dk values, such as having a Dk of 6 or more. Overall, this is an excellent test method for a high-volume circuit material manufac- turer to use for ensuring consistent Dk/Df properties of their material. All materials have dispersion and that means the Dk will vary with a change in frequency.