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July 2016 • The PCB Magazine 27 isolate the interactions of the waves with the material under test from any parasitics intro- duced by the test fixture. Another significant challenge at these high frequencies is that cur- rent is concentrated at the "skin" of metal sur- faces. As frequencies increase, the microstruc- ture of metal surfaces contributes more signifi- cantly to overall loss or degradation, and makes it nearly impossible to isolate the impact of the dielectric losses separate from the metal. Introduction In an effort to potentially determine stan- dardized test methods at these frequencies, seven members of IPC D-24C Task Group devel- oped a round-robin to measure ɛ r and tan δ for various PCB materials using different methods of their choosing and compare results. First, this paper details the problem followed by a description of the various evaluation meth- ods being considered; each method is described with sufficient information to allow for third party replication. Next, the results from each labs independent dielectric property character- izations are presented and subsequently com- pared. Finally, this paper will discuss each meth- ods pros and cons and any conclusions or next steps. Each test lab participant measured ten cir- cuit board material samples up to the highest frequency for which they could provide valid data. Each participating test lab measured mate- rial from the same lot. The circuit board materi- als for testing were constrained to the following general properties: • 0.5 oz Copper Clad (18 μm thick) • Dielectric Thickness: 100-150 μm • Relative Permittivity (ɛ r ): 2.0 – 4.0 • Loss Tangent (tan δ) <= 0.005 Ten materials of various constructions from multiple manufacturers were provided for char- acterization. Table 1 presents these materials and their general properties while assigning each material an arbitrary designator. The ɛ r of each was measured using eight dif- ferent methods and where it is demonstrated in this paper: Microstrip Transmission Line Methods: 1. Extraction from impedance (ɛ r only) 2. Group delay extraction from phase (ɛ r only) 3. Differential phase length (ɛ r only) Free Space Transmission Method: 4. Free space quasi-optical (ɛ r only) ROUND ROBIN OF HIGH-FREQUENCY TEST METHODS BY IPC-D24C TASK GROUP (PART 1) Table 1: Circuit Board Materials Tested.