Broadband Measurement of Dielectric Properties of Substrates up to 67 GHz using a Coaxial Air Line

This paper presents a useful low-cost wideband dielectric characterization technique for packaging substrates using a coaxial air line technique up to 67 GHz. To the authors' knowledge, this is the first demonstration of dielectric properties up to 67 GHz using coaxial air lines. The proposed method to extract the material properties is reference-plane-invariant (RPI) which combines Nicolson-Ross-Weir (NRW) and Baker-Jarvis methods. This method does not require that the material under test (MUT) be centered into the air line, relieving stress on the measurement setup. The method is validated using air as the MUT and compared with different calibration methods. The SOLT and offset short technique are used with a Keysight 85058B calibration kit to extract the properties of RT/Duroid® 5880. The measurement results from 10 GHz to 67 GHz show εr ranging from 2.16 to 2.22 and tanδ ranging from -0.002 to 0.006, excluding the Fabry-Perot resonance at 37.5 GHz.

I. Introduction

Circuit and system designers require accurate dielectric properties of the substrates for successful broadband designs. Broadband characterization of materials can be achieved using transmission line-based or free-space techniques [1]. Transmission line methods are challenging to extract accurate material properties. Generally, the resonator technique is more accurate than non-resonant methods but has the limitation of providing the properties at a single frequency [1]. As an example, Rogers RT/Duroid® 5880 is quoted at 10 GHz using the IPC- TM-2.5.5.5 clamped stripline resonator [2]. This industry adopted method is fast, repeatable and accurate, which is often used by manufacturers of microwave laminates to provide data at a single frequency point, only. Recently, a Fabry-Perot (F-P) open resonator [3] has shown dielectric properties at discrete frequency points over a broad band.