Novel substrate materials for recyclable printed circuit boards (PCB) were developed using a foamed high-temperature thermoplastic (polyetherimide, PEI). These innovative printed circuit boards show numerous advantages (e.g. weight reduction, inherent flame resistance, high frequency-suitability, continuous foam extrusion processing etc.) compared to conventional employed thermosetting composite substrates. With regard to the lamination process with a copper foil, a thermoplastic film adhesive is necessary to guaranty good adhesion during the soldering process. For this the surface roughness of the substrate is of a decisive importance. Due to its low viscosity during the processing, a deep filling of the substrate surface roughness happens with the thermoplastic adhesive film. Thereby voids under the adhesive layer, being visible on the copper surface and voids between adhesive and copper can occur. The last causes undercutting by etching liquids, which can lead to a reduction of the circuit path width or in the worst case to rupture of the circuit path. Scientifically, it is essential to understand the influence of the processing parameters (mass and die temperature, throughput, screw speed), the process materials (blowing agent and additive) and parameters on the surface roughness. The results show an influence of the die temperature, the throughput and screw speed. An increase of the die temperature reduces the surface roughness due to a higher gas loss, cell collapse and coalescence at the substrate surface. This leads to a higher resulting foam density. The influence of throughput and screw speed results in an increased surface roughness because of higher local shear rates, stresses and surface instabilities. The foam density remained nearly constant. Accompanying, rheological properties e.g. by means of capillary rheometer in dependency of shear rate and temperature could confirm the occurrence of flow instabilities. Furthermore the influence of the amount of the blowing agent, the roll gap and roll temperature on the surface roughness, foam density, mean cell size and cell density of the foam-extruded polyetherimide is discussed.