Gideon Analytical Labs received four tantalum capacitors identified as failing from two vendors, Kemet (T491B225K035AT) and AVX TAJB225K035RNJ. A tantalum capacitor is a type of electrolytic capacitor. It is constructed of a pellet of tantalum metal as anode, covered by an insulating oxide layer that forms the dielectric (manganese dioxide), surrounded by conductive material as a cathode (carbon with a silver epoxy interface). Tantalum electrolytic capacitors exploit the tendency of tantalum to form a non-conductive protective oxide surface layer. A tantalum capacitor consists of tantalum powder pressed into a pellet shape as one “plate” of the capacitor, with the oxide as a dielectric, and an electrolytic solution or conductive solid as the other “plate”. The dielectric layer thus can be very thin. This gives tantalum capacitors an advantage over electrolytic capacitors in having a high capacitance per unit volume and weight.
These capacitors have lower equivalent series resistance (ESR), lower leakage current, and higher operating temperature than other electrolytic capacitors.
Since tantalum capacitors are considerably more expensive than any other commonly used type of capacitors, they are used in applications where the small size or better performance is important. They also have no known failure mechanism. If they do fail, it is by voltage surge or contamination, that is if the manufacturing process is acceptable.
There were no external mechanical or assembly marks on any of the packages suggesting abuse or mishandling. A number of experiments were run and capacitors were re-tested afterward to observe the effects of the experiment. In both of the above capacitors, Gideon found the epoxy encapsulation to be insufficient in sealing out normal cascading cleaning system. This moisture ingress caused the DCL (DC leakage current) to increase way beyond the manufacturers' specification. In a dye penetration test, the dye covered 90% of the carbon area and rendered this particular with the worst DCL. Call Gideon Analytical Labs if you have problems with tantalum capacitors. 845-255-5356.