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Measuring the Impact of Test Methods for High-Frequency Circuit Materials



Measuring the Impact of Test Methods for High-Frequency Circuit Materials
How can two different test methods provide Dk measurement that are different? When the methods measure the same circuit material in two different directions.
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Authored By:


Technical Marketing Manager, Advanced Connectivity Solutions
Rogers Corp., 100 N. Dobson Rd., Chandler, AZ 85224

Summary


High-frequency materials are characterized by several important parameters, including the dielectric constant or relative permittivity (εr) and the dissipation factor (Df). For those parameters to have meaning for circuit designers, they must be measured using methods that deliver accurate, repeatable results. However, circuit materials are characterized by many different test methods: For measuring εr, IPC has 12 different test methods. Many other organizations, including ANSI and IEEE, have their own test standards for characterizing εr. Each measurement method provides detailed insights into the properties of the material it is testing, and the results of each test method may be correct, but they also may not agree. What follows is a review of different methods for measuring εr along with useful modifications and even some suggestions for new Dk measurement approaches.

How can two different test methods provide Dk measurement results that are correct but different? It can occur when the two methods measure the same circuit material in two different directions. Some Dk measurements are performed through the thickness or z-axis of the material while some are performed across the length and width or x-y plane of the material. Since most high frequency materials are anisotropic, the two different measurements will typically provide different results at a given frequency, such as 10 GHz, although both Dk values are correct.

Conclusions


In short, different material measurement approaches can yield different results for essential circuit material parameters, such as Dk and Df. Measurements can be made using raw material tests on materials with no conductive layers as well as circuit-based tests using basic circuits, such as transmission lines or resonators, fabricated on a MUT. But because test approaches are different, and employ different structures made differently, they have different variables affecting measurement accuracy resulting in different values of Dk for the same material.

Initially Published in the SMTA Proceedings

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