BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method of producing an anode for a capacitor.
In its preferred implementation, the capacitor is a chip capacitor. However, the present invention can also readily be applied to other capacitors, such as for example, capacitors without housings. A capacitor without a housing is of low structural height and is integrated, for example, in a hybrid circuit. However, the following text assumes that the capacitor according to the invention is a chip capacitor.
Chip capacitors, in particular tantalum chip capacitors, are distinguished by a high volume-specific capacitance-voltage product, also known as the “CV product.” This means that in these capacitors the value of the volume-related product of capacitance and voltage which can be applied to the capacitor is particularly high. Further advantageous properties of chip capacitors include a stable thermal behavior and frequency response, a low residual current and a small loss factor.
Due to these excellent properties, in particular tantalum chip capacitors are used for numerous applications in a very wide range of fields. New applications, demanding conditions of use and an increasing tendency toward miniaturization in electronics mean that the demands imposed on chip capacitors are becoming ever greater.
A pertinent prior art assembly is illustrated in FIGS. 24-26. FIG. 24 shows a diagrammatic section through the structure of a conventional tantalum chip capacitor. FIG. 25 shows a side view of the anode body of that chip capacitor, and FIG. 26 is a plan view of the anode body.
The prior art chip capacitor comprises an anode body 1, a dielectric 2, and a layered cathode 3, which form an actual capacitor element.
In addition, there is a housing 4 which is responsible for important protective functions for the capacitor element.
A tantalum wire 5, which in the interior of the housing 4 is connected to a first metal connector 6. The wire 5 leads to the capacitor element comprising the anode body 1, the dielectric 2, and the layered cathode 3. By means of a conductive adhesive 8, the layered cathode 3 is connected to a second metal connector 7. The second metal connector 7, similarly to the metal connector 6, leads out of the housing 4.
Chip capacitors of this nature are produced in different sizes of housing 4, usually with standardized basic surface area dimensions and structural heights. Consequently, if a higher CV product is to be achieved, the volume taken up by the capacitor element or the anode body 1 contained therein must be increased.
Owing to the use of the tantalum wire 5 in the anode body 1 (in this respect see, in particular, FIGS. 25 and 26) as the anode-side conductor, the utilization of the housing can scarcely be increased further. This is because the free end of the tantalum wire 5 is welded to the metal connector 6 which, in the finished chip capacitor, is intended to provide electrical connection to an electronic circuit on a printed-circuit board, together with the other metal connector. In a design of this nature, the distance between the capacitor element and the housing wall is particularly great especially on the positive side. The distance between the positive metal connector 6 and the capacitor element or the anode body 1, which is formed by the tantalum wire 5, can scarcely be reduced further, for manufacturing reasons. In other words, the volume of the housing is only insufficiently utilized in the prior art chip capacitor.
East German Patent DD 215 420 discloses a tantalum chip capacitor in which an anode conductor is embedded in a two-part anode body produced by extrusion. That anode body and anode conductor preassembly is then sintered. The use of two pre-extruded partial anode bodies has the drawback that it is impossible to achieve an exact form fit between the anode body and the anode conductor, owing to manufacturing tolerances. Consequently, the electrical contact between the anode body and the anode conductor is impaired.
German published patent application DE 36 34 103 A1 discloses a tantalum capacitor in which a tantalum powder is pressed around a wire anode conductor. That capacitor has the drawback of a small contact area between the wire anode conductor and the anode body. The result is an increased resistance in the capacitor which may have an adverse effect on the electrical characteristics of the capacitor. This is an undesirable effect.
U.S. Pat. No. 3,903,589 discloses a tantalum capacitor, the anode of which is produced by immersing the anode conductor in a dispersion containing metal powder. When the anode conductor is pulled out of the dispersion, a drop remains hanging from the anode conductor, which is then dried and sintered. That tantalum capacitor has the drawback that the anode body cannot be produced with a defined geometry. Because of the absence of an optimized anode geometry and the broad tolerances, that prior art capacitor is subject to poor volume utilization.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method of producing an anode for a capacitor which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which has a large contact area between the anode conductor and the anode body, the anode body of which is of fixedly predetermined shape, and in which there is good electrical contact between the anode body and the anode conductor.
With the above and other objects in view there is provided, in accordance with the invention, an anode for an electrolytic capacitor, comprising:
a flat anode conductor; and
an anode body of a continuously deformable material molded onto the anode conductor and solidified into a fixedly predetermined shape.
In other words, the anode has an anode body of fixedly predetermined shape and a flat anode conductor. The anode body is molded onto the anode conductor from a continuously deformable material which can be solidified.
There is also provided, in accordance with the invention, a capacitor, comprising:
the anode according to the above summary, wherein a second end segment of the anode conductor is shaped into a first terminal connector;
a dielectric enclosing the anode body; and
a layered cathode disposed on the dielectric and connected to a second terminal connector.
In other words, the anode body of the capacitor is surrounded by a dielectric, and a layered cathode is provided on the dielectric. A further end section of the anode conductor is shaped into a first terminal connector and the layered cathode is connected to a second terminal connector.
The anode according to the invention has the advantage that as a result of the entire anode body being molded onto the anode conductor in the form of a continuously deformable material, it is possible to form a homogeneous anode body which exhibits a good form fit with the anode conductor and good electrical contact with the anode conductor.
Furthermore, the anode according to the invention has the advantage that, as a result of the anode body being shaped with the aid of a continuously deformable material, it is possible to achieve any desired shape with the aid of suitable molds which are removed before or after solidification of the anode body. The material from which the anode body is formed may, for example, be a paste containing metal powder, a green film produced from the paste, or a suitable metal powder itself.
Due to the flat design of the anode conductor which is sintered into the anode body made from sintered tantalum powder, for example, a larger contact area between anode conductor and anode body is achieved compared with a sintered-in tantalum wire of the same cross-sectional area. The number of powder particles which are in contact with the surface of the anode conductor is increased, and consequently the mean length of the current paths between the dielectric and the anode conductor, which comprise tantalum particles which have been sintered together, is reduced. Consequently, it is possible to achieve reduced resistances and an increased capacitance at high frequencies.
Moreover, the use of a flat anode conductor in the anode body reduces the risk of local overheating at the transitions between the anode conductor and a fine network formed by the sintered tantalum particles when current is flowing. This is because higher current densities occur at these transitions than in the adjoining network. Local overheating of this type may be a cause of chip capacitors being suddenly and dramatically eroded.
The primary feature of the capacitor or anode according to the invention is in particular the production of a strong, large-area connection between the anode body, comprising an open-pored sintered body which forms the capacitance, and an anode conductor with a large surface area. For all these components, it is preferable to use tantalum or another suitable metal, such as niobium or a material which allows a layer with a high dielectric constant to be formed.
Furthermore, an anode in which the anode body completely surrounds an end section of the anode conductor is advantageous. In this way, it is possible to achieve optimum utilization or the surface area of the anode conductor for contact with the anode body. Furthermore, high mechanical stability of the anode is ensured in this way.
Furthermore, the invention also encompasses a method of producing an anode for a capacitor. The method comprises the following steps: molding a continuously deformable material onto a flat anode conductor and simultaneously externally shaping the material, and solidifying the material to form an anode body.
In a preferred embodiment of the method, the molding step comprises applying a paste of a binder system and a powder to the flat anode conductor, and the solidifying step comprises subsequently drying and sintering the paste.
In other words, the anode is produced by molding continuously deformable material, which can be solidified, onto a flat anode conductor with simultaneous external shaping, and is then solidified to form an anode body. The simultaneous molding of the material onto the anode conductor and the defining of the external shape of the anode body with the aid of a continuously deformable material obviates the need for complex post-machining processes for shaping the anode body.
A process in which a paste which contains a binder system and a powder is applied to the anode conductor and is then dried and sintered is particularly advantageous. In this process, the paste may be combined with the anode conductor to form an anode by means of various methods. By way of example, it is possible to use a paste which is known from document DE 199 27 909 A1 and which comprises a discrete phase containing a metal powder and a continuous phase containing organic compounds.
The capacitor according to the invention can be used as an SMD (SMD=Surface Mounted Device). The use of a paste simplifies the processing of high capacitance and ultra-high capacitance tantalum powders.
Furthermore, the invention provides a process for producing the anode according to the invention, in which a powder is pressed around a flat anode conductor, with the anode conductor projecting on one side. Then, the compact is sintered. The method according to the invention may advantageously be carried out in such a way that an anode conductor in the form of a strip-like metal sheet is pushed into a bed of the powder which is situated in a press mold, and then the pressing operation takes place.
The material tantalum which is preferably used for the anode conductor is extremely expensive, and consequently the strip used for the process should be as thin as possible. Conversely, the tantalum sheet used must be sufficiently mechanically stable to be able to be pushed into the powder bed and subsequently bent into a mechanically stable connector. Within these boundary conditions, strip-like anode conductors with a width of between 0.3 and 5 mm and a thickness of between 50 and 150 μm have proven suitable. These anode conductors have a width/thickness ratio of between 2 and 100.
The pressing of the powder may particularly advantageously be carried out by transverse pressing, that is to say by pressing transversely with respect to the direction in which the strip-like anode conductor extends.
The further manufacturing steps correspond to the steps used for the conventional manufacture of tantalum chip capacitors. Therefore, in the procedure known as forming, the dielectric is formed from tantalum pentoxide on the inner and outer surfaces of the sintered anode body. After the cathode layers have been applied, cathode terminal and housing are produced. In the configuration according to the invention, the anode connector made from tantalum, which is used as the positive electrical terminal, may undergo further treatment in order to be made suitable for soldering or adhesive bonding, and this statement also applies to the other exemplary embodiments.
As a modification to the above design according to the invention of the tantalum chip capacitor, the tantalum powder is also mixed with an additive which, due to its lubricating action, makes the pressing operation easier and more gentle on the press mold. The flow properties of the powder and the mechanical stability of the compact are also improved by the binding action of the additive. Camphor is a standard additive. Before the compact is sintered, the additive should be removed as far as possible without leaving any residues.
As an alternative to tantalum, it is also possible to use other suitable metals, such as for example niobium, or alloys of suitable metals, or other materials which are capable of forming a dielectric.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of producing an anode for a capacitor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.