Extracting Wideband Dielectric Properties of Stacked Packaging Substrates Using Coaxial Airlines

This article is a study of dielectric characterization of packaging substrates using a 1.85-mm coaxial airline. To the authors' knowledge, this is the first time that wideband dielectric properties have been demonstrated up to 67 GHz using a single coaxial airline. The algorithm used to extract the material properties is Chalapat's reference-plane-invariant (RPI) method. The measurement system and the extraction method are validated using air as the material under test (MUT) with SOLT and offset calibration. Traditionally, a sample length of less than 0.5λ is recommended for material extraction. However, this lowers the accuracy of the material properties. We utilized the advantage of RPI to stack fixed thickness samples to increase the MUT length. This produces a flat response and improves the accuracy of the extracted dielectric constant and loss tangent. The HFSS simulation and the Debye model have been used to compare dielectric properties with measurement using this airline.