A solid electrolytic capacitor is provided that contains a casing material that encapsulates the capacitor element. The casing material is formed from a curable resinous matrix that has a coefficient of thermal expansion of about 42 ppm/° C. or less at a temperature above the glass transition temperature of the resinous matrix. Further, the capacitor exhibits an initial equivalence series resistance of about 200 mohms or less as determined at an operating frequency of 100 kHz and temperature of 23° C., and the ratio of the equivalence series resistance of the capacitor after being exposed to a temperature of 125° C. for 560 hours to the initial equivalence series resistance of the capacitor is about 2.0 or less.