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40 DESIGN007 MAGAZINE I FEBRUARY 2023 Electrical characterization of high frequency circuit materials can be done in a variety of ways. Some characterization methods use a fixture to test the raw dielectric material, while other methods use circuits. e Dk and Df results shown on material datasheets are commonly obtained by a fixture test method. However, when comparing results from circuit evaluations, the Dk results from the fixture tests occasionally do not agree with the Dk results of the circuit tests. e reasons for these differences vary and are oen situation dependent. In general, however, a fixtured test method will not have the same variables which will impact the Dk extraction, as compared to a circuit test. When using a circuit test, the Dk extraction will be affected by various aspects of PCB fabrication processing, such as conductor width control, copper thickness variations, final plated finish, and copper surface roughness. A typical fixture test method will not have these PCB variations; instead, it may have vari- ations due to entrapped air, fixture alignment variations, electrical coupling differences, and some tolerancing effect due to the machining of the fixture components. Additionally, the electromagnetic fields applied to the material under test can vary across test methods. e different field orientation also can be problem- atic when evaluating anisotropic circuit mate- rial. Most circuit materials used in the PCB industry are anisotropic, meaning that the Dk is not the same on all three axes of the material. If the electrical characterization to be done is related to a PCB being built in large volume, the best test method and/or best test vehicle will be the one that is most like the actual cir- Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION Test Vehicles for PCB Electrical Material Characterization

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