Tantalum capacitors have a core or anode of finely divided, sintered tantalum in an electrically conducting can filled with fluid electrolyte. The electrolyte contacts the electrically conducting can which then forms the cathode of the capacitor.
The finely divided, sintered tantalum anode is a porous, electrically conducting body having a thin, electrically insulating oxide layer on all surfaces throughout the porous body. When the anode is inserted in the electrolyte filled can, the electrolyte flows into the porous anode to form an electrical conductor spaced from the electrically conducting anode body by the thin, insulating, surface oxide. The spaced, electrically insulated conductors are a capacitor. Because the oxide layer is thin to narrowly space the electrolyte and the conducting anode body, and because the anode is finely divided to provide a large, coextensive surface area between the electrolyte and the conducting anode body, this capacitor structure has a very high ratio of capacitance to physical volume which is desirable in applications requiring a large capacitance in a small space.
Since the electrolyte functions as a conductor of the capacitor, it must be retained in the can for continued operation of the capacitor. The electrolyte is usually sulfuric acid to which the can and tantalum oxide insulation on the anode are impervious but which, in both its liquid vapor states, is highly corrosive of many other materials with which the capacitor may be used. It is therefore necessary to seal the capacitor against the escape of the liquid or vapor states of the electrolyte to maintain operation of the capacitor and prevent corrosion of adjacent materials.
It is known to seal the electrolyte in the can by crimping the can about a polytetrafluoroethylene plug placed in an open end of the can. The plug is usually grooved about its circumference to receive the crimped portion of the can for improving the seal. However, even with the addition of gasket material in the groove of the plug, the seal between the can and the plug often fails to be completely impervious to the liquid or vapor states of the electrolyte over an extended period of time.
Another sealing problem is presented by a connector which electrically connects the anode to other devices. The connector is often force-fit through the plug. However, the plug merely presses against the anode and the electrolyte may then flow between the anode and the plug to reach the portion of the plug penetrated by the connector. The electrolyte may then also escape through the force-fit hole where the connector penetrates the plug.
Although the oxide insulation on the anode is sufficient to electrically insulate the electrically conducting body of the anode from the electrolyte over their substantial, coextensive surface, contact of the cathode can and anode causes sufficient charge accumulation at the relatively small contact area to break the insulation of the oxide and short circuit the cathode to the anode. Such shorting destroys the capacitor. Therefore, the anode must be held out of contact with the can. The plug and a spacer fit between the anode and can at an end of the anode opposite the plug cooperate to hold the anode away from the can.
FIG. 1 is an exploded illustration of the several components assembled into one known tantalum capacitor. The capacitor comprises an open-ended can 10 for receiving the other components. A spacer 12 having a bottom and fingers for gripping an anode 14 is fit over one end of the anode. The anode and spacer are inserted in the can and the can filled with a fluid electrolyte (not shown). A connector 16 is electrically connected to the conducting anode body by inserting an end of the connector into an end of the finely divided anode material before it is sintered. The connector extends from the end of the anode opposite the spacer for making electrical connections to the anode. A plug 18 is force-fit over the connector and against the end of the anode. The plug has a groove 20 about a side which engages the can when the plug is positioned over the anode in the can. The can is then crimped into the groove to seal the can. A ring gasket 22 is sometimes fit in the groove 20 to improve the seal. The plug and connector cooperate to hold the anode away from the can. A cap 24 is force-fit on the connector over the plug and edges of the open end of the can folded over the cap to additionally seal the can and provide a finished outside surface to the capacitor.
Each of the described components is manufactured to very close tolerances, usually plus or minus a few thousandths of an inch. However, an accumulation of the maximum tolerance variation of each of the assembled components may still sufficiently displace certain components so as to cause failure of the capacitor. For example, an accumulation of the miximum tolerance in the bottom of the spacer, the anode, and the plug may sufficiently displace the crimp in the can from the groove in the plug to cause an improper seal or lift the plug from the anode to permit the anode to tilt into shorting contact with the can.
Each of the illustrated components of the capacitor is relatively small. For example, the lengths of cans of capacitors suitable for work for the federal government are in a range from one-fourth to three-fourths inch. Assembly of the correspondingly small individual components of the capacitor is then difficult, usually requiring extensive, relatively skilled manual labor for the exacting assembly of the small components. While such manual labor is ordinarily expensive, the highly corrosive sulfuric acid electrolyte with which the capacitors are ordinarily filled makes the work unpleasant and, therefore, additionally costly.