Solid electrolytic capacitor containing an interfacial coating

A solid electrolytic capacitor having a length in a longitudinal direction, comprising:

• a capacitor element having opposing front and rear surfaces, opposing upper and lower surfaces, and opposing side surfaces, wherein the capacitor element contains a sintered porous anode body that includes tantalum, a dielectric that includes tantalum pentoxide and that overlies the anode, a solid electrolyte that overlies the dielectric, and an external coating that overlies the solid electrolyte, wherein the solid electrolyte includes one or more inner layers containing an intrinsically conductive thiophene polymer and one or more outer layers containing pre-polymerized extrinsically conductive thiophene polymer particles and a polymeric counterion, and further wherein the external coating contains an extrinsically conductive thiophene polymer, a polymeric cation, and a cross-linking agent;
• an anode lead extending from the front surface of the capacitor element in the longitudinal direction, wherein the anode lead is a wire, wherein at least a portion of the wire that is adjacent to the front surface of the capacitor element is free of any thiophene polymer from the solid electrolyte or the external coating;
• an anode termination that is in contact with the anode lead at a connection region, wherein a distance between the connection region and the front surface of the capacitor element is from about 1 to about 2 millimeters, and further wherein the ratio of the distance between the connection region and the front surface of the capacitor element to the length of the capacitor is about 0.15 to about 0.5;
• a cathode termination that is in electrical connection with the solid electrolyte;
• a casing material containing a resinous matrix that includes an epoxy resin and an inorganic oxide filler, wherein the casing material encapsulates the capacitor element and anode lead and leaves exposed at least a portion of the anode termination and cathode termination for external contact;
• and
• an interfacial coating that includes a fluoropolymer formed from a fluoroalkyl (meth)acrylate, wherein the interfacial coating is in contact with the casing material;
• wherein the solid electrolytic capacitor demonstrates an initial equivalent series resistance of about 200 mohms or less when measured at a frequency of 100 kHz and a temperature of about 23° C. and an initial capacitance of about 30 nanoFarads per square centimeter or more when measured at a frequency of 120 Hz and a temperature of about 23° C., further wherein the solid electrolytic capacitor has a ratio of an equivalent series resistance and a capacitance after being exposed to a temperature of about 125° C. for 240 hours to the initial equivalent series resistance and the initial capacitance of about 2.0 or less.