The high frequency electrical properties of interconnects on a flexible polyimide substrate including the effects of humidity

Flexible circuit board materials can be used to advantage in radio frequency and high-speed digital systems but an obstacle to their use is the lack of availability of information on the electrical properties of materials to high frequencies and, in particular, the variation in dielectric constant and loss tangent as a function of frequency. This makes accurate electromagnetic simulation of high frequency flexible interconnects difficult. The variation of the electrical properties of these materials as a function of environmental parameters, such as humidity, is also unknown at higher frequencies. This paper has, using microwave resonators, investigated the electrical properties from 2 GHz to 18 GHz of a polyimide flexible circuit board material saturated at 25% RH and at 85% RH relative humidity levels. Rigid circuit board materials FR4 and CER-10 were also measured as reference materials. The relative permittivity, εr, total loss, αT, and loss tangent, tan δ, have been extracted from the measurements for each material. The strong influence of conductor losses on overall losses when using thin materials such as flex at high frequency has also been evaluated and quantified in these measurements. In addition to the resonators used for measurement of material electrical properties, microstrip transmission lines were also included on each test sample and their s-parameters were measured at the same time and under the same conditions as the resonators. Comparisons between the measured electrical performance of the microstrip transmission lines and simulations of the lines based on the extracted material parameters show a high degree of correlation, indicating the validity of both the use of the resonator approach and overall loss measurement methodologies.