In this study, highly filled high-temperature thermoplastics (polyethersulfone [PES], polyphenylensulfide, polyetherimide [PEI], and polyetheretherketone [PEEK]) are used as insulating substrate for printed circuit boards (PCBs). Talc has been added to the thermoplastics to adjust their coefficient of thermal expansion (CTE) to the CTE of the copper circuits, thus reducing the possibility of failure of the PCB owing to thermal stress. The dielectric properties of the substrates were analyzed between 10 MHz and 1 GHz, depending on filler fraction and water absorption. An increase of filler fraction resulted in an increase of dielectric constant e 0 . As expected, the absorption of water molecules led to an increase of both tan d and e 0 . Moreover, the combination of filler and absorbed water resulted in a strong increase of the dielectric loss factor at low frequencies. Finally, theoretical approaches with fitting parameters could be employed to precisely describe the measured properties between 0.8 and 1 GHz. This study shows that most of the materials investigated here, namely highly filled PPS, PEI, and PEEK, are suitable for high-frequency PCB applications.

This work focuses on the development of foamed high temperature thermoplastic substrates for printed circuit boards. For this application it is necessary to achieve mean cell diameters smaller than 30 µm in order to be able to realize vias and high packaging densities (miniaturization). Different additives as nucleating agents, namely macro- and micro-crystalline talc, silica, calcium carbonate, and wollastonite, were melt-compounded with polyetherimide using a twin-screw extruder. Foamed samples are prepared by foam extrusion using a slit die and CO2 as physical blowing agent. The aim of this study is to analyze the influence of the mean particle size and the particle surface tension on the mean cell diameters. Therefore, the shape of the additives, the foam morphology, and the elongational viscosity were considered. The additives with a suitable particle size and surface tension exhibit a positive influence on the foam morphology, resulting in smaller cell diameters (<30 µm), a narrower cell size distribution and a foam density lower than 900 kg/m3. If the mean particle diameter of the nucleating agents is lower than 0.6 µm in this study, no nucleation effect could be observed. This is related to the fact that no heterogeneous nucleation occurs, if the particle diameter is too small. If the mean particle diameter of the used additives is larger than 1.5 µm, which could be demonstrated in this study in case of polyetherimide, then the additive acts as nucleating agent and heterogeneous nucleation occurs. Furthermore, it was observed that the mean cell diameter was affected by the different surface tensions of the studied nucleating agents.