Rigid and flexible circuit boards consist of layers of different materials. Particularly, the different thermal expansion coefficients of thermoplastic polymers and metals can lead to problems during application. The investigations presented are focused on the high-temperature thermoplastic polyetheretherketone (PEEK), as it has a remarkable potential for novel flexible circuit boards. The work shows the significant thermal expansion reductions that can be reached in a thermoplastic polyetheretherketone (PEEK) film by the incorporation of different contents of calcium carbonate or talc fillers and compares their effectiveness. Particularly the influence of filler geometry on the thermal expansion is studied in detail. The effect of the filling on mechanical properties and the dimensional stability of the PEEK films under thermal load is investigated quantitatively.