The present invention is directed to a method for the manufacture of solid electrolyte capacitors comprising a sintered anode member composed of a valve metal, an oxide layer arranged on the anode member and serving as a dielectric, a semiconducting electrolyte layer composed of manganese dioxide that is produced by repeated immersion into a solution containing manganese nitrate and subsequent pyrolytic decomposition a graphite layer arranged on the manganese dioxide layer, and a soft-solderable layer arranged on the graphite layer.
The sintered members, composed of a suitable valve metal such as, for example, tantalum or niobium, have their inside and outside surface coated with an oxide layer that functions as a dielectric in an anodic oxidation process. The manganese dioxide layer in and on the sintered members serves as a cooperating electrode. The contact to the negative terminal of the capacitor is produced by the graphite layer located thereupon and a soft-solderable layer (composed, for example, of silver conductive lacquer). A wire of the valve metal sintered into the sintered member functions as the positive terminal.
Tantalum solid electrolyte capacitors are distinguished by a high useful life given simultaneously low temperature dependency of the capacitance, residual current, loss factor, and impedance. The most reliable embodiment of the capacitors is a hermetically sealed construction wherein the capacitor is completely protected against moisture in a metal housing by soft solder and a terminating glass pane.
When the capacitors are housed in a plastic housing, an absolute protection against moisture is not provided and there is a higher mechanical stressing due to the thermal expansion of the housing. Plastic envelopes fundamentally exhibit a water vapor diffusion. As a result thereof, potentially disturbing ions from the diffused water can be activated.
It is therefore necessary to make the internal structure of plastic-enveloped capacitors more resistant to the influences of moisture. To measure the reliability of the capacitor, the residual current of the capacitor after long-term tests with strict climatic conditions can be used. For example, the residual current after a long-term test at 85.degree. C. and 95% relative humidity nominal voltage operation without drop resistor, should not exceed 30.mu. A measured at room temperature given a 20.mu. F/20 V capacitor.
Beneficial for this purpose is that the low reactivity of the tantalum pentoxide layer alloWs the use of electrolytes having a higher conductivity, this resulting in low equivalent series resistances. Given tantalum solid electrolyte capacitors, however, the cathode coating must have an adequately thick layer of manganese dioxide in order to protect the dielectric against direct contact with graphite and conductive lacquer. This is due to the fact that if the graphite or conductive lacquer is located directly on the tantalum pentoxide layer lead, shorts will occur even given a few percent of the activation voltage.
In manufacturing the manganese dioxide layers, the shaped sintered members are immersed into manganese nitrate in a known fashion, whereby the dielectric surface is moistened with manganese nitrate. This is followed by a thermic decomposition (pyrolysis) of the manganese nitrate at temperatures between 200 and 350.degree. C., whereby MnO.sub.2 arises in and on the sintered members. A multi-layer MnO.sub.2 coat, that provides an adequate spacing between the highly conductive graphite layer and the following soft-solderable layer, is formed by repetition of this pyrolysis (8 through 15 times) in air, that is either dry or contains water vapor.
Manganese dioxide layers manufactured by these traditional methods are adequate for typical uses. These capacitors exhibit good results, in long-term tests having elevated temperature and operating voltage above the nominal voltage (125.degree. C., U.sub.N, 105.degree. C., U.sub.N. 1.35), but, without atmospheric humidity; only about 1% of capacitors treated in this manner had a residual current greater than 30 .mu. A.
However, given long-term tests with high atmospheric humidity (1,000 hours at 85.degree. C. and 95% relative humidity, U.sub.N), these capacitors exhibit a diminished reliability (about 10% of the capacitors had a residual current greater than 30 .mu. A, whereby the majority part thereof exhibited a short).
The sporadically occurring outages that occurred during the long-term tests in dry air, represent typical premature outages that can be attributed to latent production defects or impurities in the materials. But, the outages in long-term tests having high atmospheric humidity are based on errors in the structure of the capacitor. The number of outages frequently rises above an acceptable amount given an increasing dwell time in humidity. For comparably manufactured capacitors having a comparable charge and nominal voltage, but that are tightly integrated in a metal housing, long-term tests have not demonstrated any influence of humidity on the capacitors.
The causes for the failure of the barrier effect of the amorphous tantalum pentoxide layer lie in the amorphous tantalum pentoxide layer itself or in a deficient manganese dioxide layer. Increased impurities in the oxide layer disturb the barrier effect and lead to the formation of a crystalline condition due to locally high residual current. This crystalline condition leads to a short when it extends over the entire thickness of the dielectric. When there are crystalline islands in the oxide layer, these regions are subject to outage that, however, can be potentially protected by a local conversion of the manganese dioxide into the more poorly conductive oxidation stage (MnO.sub.2 .fwdarw.Mn.sub.2 O.sub.3, .rho. (MnO.sub.2).about.1-10 Ohm.cm, .music-flat.(Mn.sub.2 O.sub.3).about.10.sup.4 -10.sup.5 Ohm.cm) via an increased resistance.
These outage mechanisms, that are due to impurities in the oxide layer, however, are not the sole cause for the outages when humidity is present. An evaluation of the tantalum pentoxide layer demonstrates that the layer is in proper order proceeding from the shaping and from its structure. Increased residual currents due to impurities or faulty structure of the oxide layers have not been identified.
The cause of the outage, accordingly, is due to the manganese dioxide layer that enables unfavorable influences on the blocking current behavior of the dielectric due to the inadequate protection the plastic housing provides against moisture.
These unfavorable influences can include particles of graphite, or of conductive lacquer, that penetrate through pores in the manganese dioxide layer and lower the resistance of the cathode coating at individual locations in front of the dielectric. Water vapor that has diffused through the housing promotes a simple diffusion (material transport due to the flow motion of larger volume elements) of graphite and silver particles that do not adhere as well and can thus lead to a short due to low drop resistance preceding the dielectric.
In the silver conductive lacquer that is used as the soft-solderable layer, the silver particles are blended in an organic bonding agent that exhibits a limited temperature loadability. An incipient thermic destruction of the conductive lacquer and, thus, a mechanical release of silver particles is already provided given a slight overheating. Water that has penetrated also leads to a better contact between the dielectric and the manganese dioxide layer, with the ultimate result that locations of the dielectric that are not contacted by manganese dioxide and may be defective are additionally contacted. When, therefore, a local void in the tantalum pentoxide layer that is otherwise not detrimental, coincides with a location of inadequate drop resistance, a potential outage location is established. Due to heating in the presence of the electrical field, locally increased residual current leads to further field crystallization and, thus, to a short.
The effect of the curing mechanism due to the transition of MnO.sub.2 .fwdarw.Mn.sub.2 O.sub.3 can be suppressed due to the regions in the manganese dioxide layer lying close to the dielectric that are contaminated by graphite, silver particles, and water. Accordingly, the high specific resistance of Mn.sub.2 O.sub.3 is lost.
For the reasons set forth above, it is therefore necessary to produce adequately thick, smooth, and nearly porefree manganese dioxide layers.
Since it is known that the dielectric layer can be damaged due to the thermic stressing of the pyrolysis, it could be concluded that the manufacture of the manganese dioxide layer can be produced by immersion into highly concentrated manganese nitrate solutions. Although fewer immersion and pyrolysis events are required for the manufacture of a defined layer thickness, adequately smooth and nearly pore-free manganese dioxide layers are not obtained. This is due to the fact that, particularly, given a pyrolysis without pre-drying, large pores are produced in the outer manganese dioxide layers due to the escape of the nitrous gas from regions lying farther inside. The risk of pore formation is less given more dilute solutions; however, as previously discussed, roughly 8 through 15 individual immersion and pyrolysis events are required for the manufacture of an adequately thick manganese dioxide layer in order to guarantee an adequate distance of the highly conductive graphite layer and of the following soft-solderable layer from the oxide layer.
Although thermic decomposition in dry air at 200 through 350.degree. C. leads to thick manganese dioxide layers having dimensions of 0.1 mm and more after a few pyrolyses, these layers are highly porous and dissolute. Moreover, due to high porosity, these layers do not provide an adequate distancing of the graphite layer from the dielectric. In contrast, decompositions in air containing water vapor at temperatures between 200 and 250.degree. C. lead to manganese layers that are smoother, more solid, and have fewer pores. However, a large number of immersion and following pyrolysis processes are necessary in order to achieve an adequate layer thickness.
Manganese dioxide layers that lead to usable electrical values of the capacitor arise after 8 through 15 - fold repetitions on the basis of alternating decomposition in dry air and in air containing water vapor. However, a large number of pyrolysis steps that deteriorate the dielectric are thereby still required.