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26 The PCB Magazine • July 2016 Problem Statement Accurate values of relative permittivity (ɛ r ) and loss tangent (tan δ) are important charac- teristics for designers and fabricators in predict- ing electrical performance of circuits at high frequencies [1] . The most common method for evaluating these parameters at frequencies up to 10 GHz is described in IPC-TM-650-2.5.5.5 [2] . This method is equivalent to ASTM-D-3380 [3] . This method excites a stripline resonator at both ends with the dielectric under test comprising most of the volume. The stripline is created by establishing intimate contact using a constant clamp force. This method is highly repeatable and is optimized for QA testing at a specific fre- quency. This method is not well suited for char- acterizing at frequencies higher than 10 GHz. Both analog and digital applications now commonly excite dielectric materials at fre- quencies well above 10 GHz. Measurements at higher frequencies are especially challenging for many reasons. For instance, the wavelength of radiation at 30 GHz is <10 mm in air and <5 mm in FR4. This makes it more challenging to by Glenn Oliver, Jonathan Weldon, et al. DUPONT* This paper was originally published in the pro- ceedings of IPC APEX EXPO, Las Vegas, Nevada, February 2016. It won the Best Paper Award for the conference. Abstract Currently there is no industry standard test method for measuring dielectric properties of circuit board materials at frequencies greater than about 10 GHz. Various materials vendors and test labs take different approaches to deter- mine these properties. It is common for these different approaches to yield varying values of key properties like permittivity and loss tan- gent. The D-24C Task Group of IPC has devel- oped this round robin program to assess these various methods from the "bottom up" to de- termine if standardized methods can be agreed upon to provide the industry with more accu- rate and valid characteristics of dielectrics used in high-frequency and high-speed applications. Round Robin of High-Frequency Test Methods by IPC-D24C Task Group (Part 1) FEATURE