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72 DESIGN007 MAGAZINE I OCTOBER 2021 ere are many different test methods which can be used to determine the Dk (dielectric constant or ε r ) and Df (dissipation factor, Tan Delta, loss tangent) for circuit materials. IPC has 12 different test methods to determine Dk for materials. Additionally, there are material Dk test methods defined by other industry organizations, universities, and many com- panies. I have a book on microwave material characterization which has over 80 different test methods to evaluate circuit materials for Dk and Df. e bottom line is, there is no per- fect test method, and an engineer should use the test method that most closely emulates their end-use product. In this column, I'll give an overview of test method concepts and dis- cuss common test methods which are used to determine Dk and Df. Overview of Test Methods to Determine Material Dk and Df Test Methods Concepts ere are two general categories for test methods used for high frequency circuit materials: material test methods and circuit test methods. e material test methods typi- cally use fixtures to evaluate the raw dielectric material. is type of testing is evaluating the raw material only and without circuit fabri- cation variables. e other category is using circuit test vehicles to extract the Dk (and sometimes Df ) based on circuit performance. Since the accuracy of the material test meth- ods are dependent on fixture variations and the circuit test methods are dependent on circuit fabrication variations, the Dk/Df val- ues extracted from these two different types of test methods should not be expected to be the same. Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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