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Design007-Jun2018

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48 DESIGN007 MAGAZINE I JUNE 2018 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 differ- ent data sheets is highly recommended. How- ever, 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 rela- tive 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 anisotro- pic, 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 fre- quency materials. As a general statement, an Considerations for Comparing Material Data Sheets Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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