Wide Band Measurement of Dielectric Properties of Electronic Assembly Materials Inside an LTCC Fluidic Structure

Assembly materials such as underfillers or glob top epoxies are typically not specified regarding their dielectric properties for frequencies higher than 1 MHz. However, their behavior should be known for a wider frequency range in order to implement the appropriate parameters for RF and microwave simulations and designs. Typical methods to measure permittivity and loss tangent are based on parallel plate capacitor measurement with an impedance or network analyzer (up to about 1 GHz), S-parameter measurement of filled waveguides, coaxial transmission lines, or resonance methods (e.g., split post resonator, slit cavity resonator, ring resonator, open resonator). Most of these methods require specific sophisticated sample preparation. The paper describes a novel method based on suspended or inverted microstrip evaluation in a 3D LTCC structure. Suspended and inverted microstrip lines have lower insertion losses than standard microstrip lines due to the air gap between the line and the ground plane (reduced dielectric losses). Low loss structures are necessary to be able to measure low loss dielectrics. Such suspended or inverted microstrip lines can be easily achieved in LTCC by implementing a cavity structure. Inlets and outlets allow for the cavity to be filled with fluids after an initial S-parameter measurement of line properties (i.e., impedance, insertion loss, phase velocity). Measuring is repeated once the assembly material is cured. The change in impedance, phase velocity, and insertion contains the information about the material under test. Its properties are derived by curve fitting methods with a 3D electromagnetic field simulator. It is also possible to implement line resonators instead of through lines. In the latter case, the resonant frequency shift and the quality factor contains the material information. The procedure is demonstrated on a multilayer LTCC substrate based on low loss DP 9k7 and a commercial underfill material.