Film capacitors are made out of two pieces of
plastic film covered with metallic electrodes, wound into a cylindrical shaped winding, with terminals attached, and then encapsulated. In general, film capacitors are not polarized, so the two terminals are interchangeable.
A key advantage of modern film capacitor internal construction is direct contact to the electrodes on both ends of the winding. This contact keeps all current paths to the entire electrode very short. The setup behaves like a large number of individual capacitors connected
in parallel, thus reducing the internal
ohmic losses (
ESR) and the
parasitic inductance (
ESL). The inherent geometry of film capacitor structure results in very low ohmic losses and a very low parasitic inductance,
Winding — Two films are rolled together into a cylindrical winding. The two metallized films that make up a capacitor are wound slightly offset from each other, so that by the arrangement of the electrodes one edge of the metallization on each end of the winding extends out laterally.
Application of metallic contact layer ("schoopage") — The projecting end electrodes are covered with a liquefied contact metal such as zinc or aluminum, which is sprayed with compressed air on both lateral ends of the winding. This metallizing process is named
schoopage after Swiss engineer Max Schoop, who invented a combustion spray application for tin and lead