Solid electrolyte capacitor and manufacturing method therefor

Abstract: In a solid electrolyte capacitor, an improvement wherein the solid electrolyte is a mixture of at least two kinds of 7,7,8,8-tetracyanoquinodimethane complexes having cations different from each other which is obtained by cooling the mixture in a liquid state, the mixture being so stable to a high heat applied to the capacitor upon mounting the same on a circuit board using reflow solder. Further, a method for aging solid electrolyte capacitors is also disclosed wherein a higher aging temperature is employed.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a manufacturing method for a solid electrolyte capacitor and more particularly for an organic semiconductor solid electrolyte capacitor using 7,7,8,8,-tetracyanoquinodimethane (hereinafter referred to as "TCNQ") complexes as an electrolyte. RELATED ART JP-A-60214265 discloses a new material for use as an organic electrically conductive compound, capable of exhibiting physical property values as an independent compound not as a mixture and expected to produce various electronic chemical or photoelectronic chemical results. The new material may comprise TCNQ. For example two or more different complexes consisting of Nitrogen-containing heterocyclic compound cations and TCNQ are mixed and disolved in an organic solvent while heating and crystals are deposited in the above mentioned solvent to afford the aimed composite complex. GB-A-2113916 discloses a solid electrolyte compacitor wherein the solid electrocyte is solidified by cooling a TCNQ-salt form a liquid state. The TCNQ-salt remaining so stable in its liquid state as not to substantially decompose before solidification has occured. EP-A-0152082 discloses a dielectric oxyd film formed on the surface of a metal anode, and a solid-electrolyte layer formed on the dielectric oxyde film. The electrolyte layer is made by melting and solidifying a mixture of an isopropylisoquinolinium tetracyanoquinodimethane complex salt and a lactone compound. One of inventors of the present invention has proposed various organic semiconductor solid electrolyte capacitors using a TCNQ complex as an electrolyte. For example, in the United States Patent No. 4,580,855, there is disclosed a solid electrolyte capacitor wherein a TCNQ complex of N-substituted isoquinoline of which the substituent at N atom is an alkyl group is used as an electrolyte. The solid electrolyte capacitors of this type exhibit excellent high frequency characteristics and, therefore, are widely used for switching power sources or the like. In recent years, according to the necessity of miniaturization of electronic apparatuses, capacitors of this type are also demanded to miniaturize as chips which are mountable directly on a printed circuit board using reflow solder. In general, the TCNQ complex is too weak to resist the heat applied upon soldering capacitor of chip type and deterioration of performance of the capacitor such as an extroadinary increase of the leakage current or the like is invited. Accordingly, in order to improve the heat resistivity of the TCNQ complex, some TCNQ complexes having melting points higher than those of the conventional TCNQ complexes have been developed. However, TCNQ complexes having been developed until now exhibit improved heat resistivities but the electrical conductivities of them are considerably lowered in comparison of that of the conventional isoquinoline TCNQ complex when solidified by cooling after melted once. Due to this, they have disadvantages in that the excellent high frequency characteristic which is the most important property of the capacitor using a conventional TCNQ complex is lost. SUMMARY OF THE INVENTION Accordingly, one of objects of the present invention is to provide organic semiconductor solid electrolyte capacitors which exhibit excellent high frequency characteristics even if high heat were applied to them upon soldering. Another object of the present invention is to provide TCNQ complexes for solid electrolyte capacitors which are able to yield both the high heat resistivity against soldering and the excellent high frequency characteristics to the capacitors. A further object of the present invention is to provide a method for manufacturing solid electrolyte capacitors using at least one TCNQ complex which is able to yield a high heat resistivity to them. In order to achieve these objects, according to the present invention, there is provided a method for manufacturing a solid electrolyte capacitor according to claim 1. The dependent claim is related to a further advantageous development of the present invention. When said mixture of TCNQ complexes having different cations is solidified by cooling after being liquefied by heating, the equivalent series resistance (hereinafter referred to as E.S.R.) thereof is reduced to about one tenth to one half of the E.S.R. of each of TCNQ complexes. In other words, the electrical conductivity of the mixture of TCNQ complexes is increased from two times to ten times when compared with that of each of TCNQ complexes. The melting point of the mixture of TCNQ complexes having been solidified after melted by heating is supposed to be lowered when compared with that of each TCNQ complex, however it is still higher than that of the conventional isoquinoline TCNQ complex. Therefore, the mixture has a melting point and a heat resistivity enough for enduring the heat applied upon mounting chips of capacitcrs on circuit boards using reflow solder. Conventionally, aging of capacitors was usually done at a temperature falling in a range between 105°C and 125 °C since the melting point of the TCNQ complex used for the solid electrolyte capacitor is ranging from about 210°C to 260 °C. Contrary to this, according to an embodiment of the present invention, the aging of capacitors is done at a temperature higher than that of the conventional aging which falls in a range defined between about 150°C and 260°C and, therefore, the heat resistivity of an insulating cover membrane which is generated to repair a leakage current is supposed to be enhanced (See Tables 3 and 4 in the present specification). Thereby, it becomes possible to prevent the leakage current from increasing even in the case that the capacitors are exposed to such a high temperature as applied upon soldering. Further, since the efficiency of aging to a flowing current (the rate of generation of the insulating membrane) in the case of an aluminium electrolyte capacitor impregnated with a TCNQ complex becomes better as an aging temperature becomes higher, it becomes possible to shorten the aging time considerably. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a capacitor element to which the present invention is applied, Fig. 2 is a sectional view of a solid electrolyte capacitor according to the present invention, and Fig. 3 is a perspective view of the solid electrolyte capacitor according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a capacitor element to which the present application is applied. At first, aluminum foils having a purity higher than 99.99% are subjected to etching treatment in order to increase the effective surface area of them. Next, they are subjected to a chemical generation processing for forming oxidized membranes (membranes of oxidized aluminum) on surfaces of the aluminum foils electro-chemically in an electrolyte liquid. An anode foil 1 comprised of the aluminum foil processed as mentioned above and an opposing cathode foil 2 are rolled up into a cylinder putting a Manila paper as a separator inbetween these foils. Thus, a capacitor element 6 is formed, as shown in Fig. 1. Reference numerals 4 and 4′ indicate aluminum leads and reference numerals 5 and 5′ indicate lead wires. Further, the capacitor element 6 is subjected to a heat treatment for carbonizing the Manila paper as the separator 3 to lower the density thereof by thinning fibers thereof. Fig. 2 is a sectional view of a capacitor of chip type wherein the capacitor element 6 shown in Fig. 1 is enclosed in an aluminum case 7 and Fig. 3 is an appearance of the capacitor of chip type. Before inserting the capacitor element, predetermined quantities of TCNQ complexes 8 are charged into the aluminum case 7 and the latter is put on a heat plate (not shown). In the present preferred embodiment, the aluminum case 7 is heated up at a temperature equal to or higher than 210°C to liquefy the TCNQ complexes charged thereinto. Thereafter, the capacitor element 6 preheated is inserted in the aluminum case 7 to impregnate the liquefied TCNQ complexes into the capacitor element 6. And, immediately, the TCNQ complexes are solidified by cooling. Thereafter, a resin 9 being not reactive with the TCNQ complexes is charged into the aluminum case 7 and the opening of the case 7 is sealed with an epoxy resin 10. The aluminum case 7 has grooves 11 formed along bottom edges thereof into which lead wires 5, 5′ are inserted. Next, examples of TCNQ complexes having melting points higher than about 230°C are shown in Table 1. In Table 1 there are also listed up electrical characteristics of capacitors made by using respective TCNQ complexes. Complex symbol (I) in Table 1 indicates a conventional TCNQ complex used in conventional solid electrolyte capacitors. Lutidine used for compositing TCNQ complexes indicated by complex symbols (A), (B) and (H) is 3.5-lutidine exactly and phenylpyridine used for compositing TCNQ complexes indicated by complex symbols (C), (D) and (E) is 4-phenylpyridine exactly. Further, specifications of the capacitor used were 25 volts in the voltage and 0.68 µF in the capacitance. Also, symbols used in Table 1 indicate values as follows; Cap; Electrostatic Capacitance (nF) at 120 Hz tan δ; Tangent of Loss angle (%) at 120 Hz L.C.; Leakage current {(µA after 1 minute)} E.S.R; Equivalent Series Resistance (mΩ) at 100 kHz Δcap; Rate of change in the Capacitance (%) at 120 Hz. Composition of TCNQ complex M.P. °C Electrical Characteristics Cap tan δ L.C. E.S.R. A N,N-pentamethylene(lutidinium) 2 (TCNQ) 4 250 625 14.1 0.01 5200 B N,N-hexylmethylene(lutidinium) 2 (TCNQ) 4 238 717 7.9 0.01 2300 C N-ethylphenylpyridinium(TCNQ) 2 234 714 1.63 0.01 2290 D N-n-propylphenylpyridinium(TCNQ) 2 246 672 1.45 0.03 1310 E N-isobutylphenylpyridinium(TCNQ) 2 242 727 1.45 0.01 1630 F N-bengilisoquinolinium(TCNQ) 2 238 664 2.07 0.01 2140 G N-phenethylisoquinolinium(TCNQ) 2 254 704 1.63 0.01 2480 H N-phenethyllutidinium(TCNQ) 2 267 703 2.50 0.01 1500 (I) N-n-buthylisoquinolinium(TCNQ) 2 215 716 1.09 0.01 190 Table 2 contains results obtained when capacitors were made using various mixtures of the same quantity of TCNQ complexes listed up in Table 1 and those obtained in a reflow test made under thermal conditions supposed to be caused upon mounting capacitors using reflow solder. In the reflow test, capacitors were leaved at 160 °C for two minutes and subsequently, were leaved at 230 °C for 30 seconds in a reflow furnace. It is to be noted that no significant differences in the results were observed when used either one of the mixture mixed with respective TCNQ complexes synthesized and separated beforehand and that obtained by mixing materials in synthesizing process beforehand. Further, only the mixtures of the same quantity of TCNQ complexes are mentioned in the present preferred embodiment, however, the rate of mixing of TCNQ complexes is not limitted to the above since essential effects can be obtained when it is changed to a rate such as 2:1, 3:1 or the like. TCNQ Symbol Initial electrical characteristics Electrical characteristics after reflow test Cap tan δ L.C. E.S.R. ΔCap tan δ L.C. E.S.R. A+C 687 1.19 0.08 807 -3.1 1.80 23 791 A+D 722 0.95 0.01 463 -1.7 1.17 5.0 459 A+F 690 1.17 0.09 799 -4.3 1.73 3.6 798 A+G 736 1.11 0.04 577 -1.6 1.58 8.6 573 A+H 753 1.23 0.03 435 -2.0 1.32 2.8 460 B+D 752 1.57 0.02 604 -3.2 2.14 42 746 C+G 709 1.24 0.02 1190 -2.3 1.95 16 1190 D+G 706 1.34 0.01 898 -3.7 1.55 7.4 919 E+F 681 1.48 0.05 1067 -4.5 1.68 19 1238 A+D+G 737 1.06 0.01 541 -2.4 1.47 37 537 (I) 716 1.09 0.01 190 -14.4 4.12 1890 201 As is apparent from Table 2, mixtures of at least two kinds of TCNQ complexes having cations different from each other as an electrolyte for solid electrolyte capacitors can control increase of E.S.R. and can yield capacitors having the heat resistivity endurable against the heat applied upon mounting them on printed circuit boards. Though E.S.R. of a single TCNQ complex having a high melting point which is chosen in order only to improve the heat resistivity against the heat applied upon mounting capacitors becomes 7 to 27 times that of the conventional TCNQ, for instance, isoquinoline TCNQ (I) wherein the substituent at N atom is an alkyl group, E.S.R. of the mixed TCNQ complex used in the method according to the present invention can be reduced to an order of 2.4 to 6 times that of the conventional one and, further, electrical characteristics of the capacitor after exposed to severe thermal conditions, are kept substantially unchanged from the initial electrical characteristics. Next, the method for aging solid electrolyte capacitors is explained using examples. COMPARATIVE EXAMPLE The capacitor element 6 as shown in Fig. 1 was impregnated with N-n-buthylisoquinoline (TCNQ) 2 and, then, it was sealed with the resins 9 and 10 in the aluminum case 7 as shown in Fig. 2. Specifications of the capacitor in this embodiment were 25 volts in the voltage and 15 µF in the capacitance. The capacitors were aged under various conditions (A) to (E) cited below in a state that they were inserted in holes formed on the heat plate each of which has diameter slightly larger than that of the capacitor and a depth larger than the height of the capacitor. Results of this aging test are listed up in Table 3. Ten samples were used per each aging condition and, therefore, each of values listed in Table 3 was a value obtained by averaging over ten samples. The melting point of N-n-buthylisoquinoline (TCNQ) 2 used in the preferred embodiment ranged from 210 °C to 220 °C. Further, the aging voltage to be applied to the capacitor during aging was so called reduction voltage which was decreased as the heating temperature was increased. Aging conditions: A: Heating at 210 °C for 30 seconds and applying 20 volts as aging voltage (during cooling process also) B: Heating at 180 °C for 40 seconds and applying 20 volts as aging voltage (during cooling process also) C: Heating at 150 °C for 60 seconds and applying 21 volts as aging voltage (during cooling process also) D: Heating at 125 °C for 180 seconds and applying 22 volts as aging voltage (during cooling process also) and, E: Heating at 125 °C for one hour in a temperature regulating box and applying 22 volts as aging voltage (during cooling process also). Aging condition after Aging after Leaving at High temp. with no load Cap µF tan δ % L.C. µA/30" E.S.R. mΩ Cap µF tan δ % L.C. µA/30" E.S.R. mΩ A 15.9 1.6 0.59 40 15.6 1.9 8.9 39 B 16.2 1.5 0.73 41 16.0 1.8 23 40 C 16.1 1.5 0.82 39 15.8 1.8 56 38 D 15.8 1.4 5.04 42 15.7 1.8 154 41 E 15.9 1.5 1.08 41 15.8 1.7 68 39 In Table 3, symbols indicate as follows; Cap; Electrostatic capacitance (µF) at 120 Hz tan δ; Tangent of Loss angle (%) at 120 Hz LC; Leakage Current (µA) after 30 seconds E.S.R.; Equivalent Series Resistance (mΩ) at 100 kHz No load; Applying no voltage to the capacitor In Table 3, there are also cited results of the test wherein capacitors were leaved for 3 minutes at 160 °C in stead of those of the heat resistance test for testing the heat resistivity upon soldering capacitors on a printed circuit board. It is to be noted that similar results were obtained when used N-n-propylquinoline in place of N-n-Buthylisoquinoline. EXAMPLE Capacitors were prepared using capacitor elements 6 as shown in Fig. 1 and a mixture of TCNQ complexes of the same quantity comprised of N, N-Penthamethylene (lutidinium) 2 (TCNQ) 4 and N-n-propylphenylpyridinium (TCNQ) 2 . Each capacitor 6 was enclosed by curing with a buthyl rubber molded and sealded therewith, as shown in Fig. 2. Specifications of this capacitor were of 25 volts in the voltage and 1 µF in the capacitance. These capacitors were subjected to aging under conditions (F) and (G). Table 4 shows results obtained by this aging test. In this aging test, so called reduction voltage as mentioned above was employed as the aging voltage. The melting point of the mixture of TCNQ complexes was about 240 °C. Aging conditions; F: Heating for 15 seconds by immersing capacitors by 80 % of the height in a solder of 235 °C and applying 20 volts (during cooling process also) and G: Heating at 125 °C for one hour in a heat regulating box and applying 22 volts. In Table 4, there are cited results of a solder immersing test wherein 80 % of the capacitor was immersed in a solder of 235 °C for ten minutes instead of those of the heat resistivity test for mounting capacitors using reflow solder. Aging condition after Aging after Immersing Test in Solder by 80 % Cap µF tan δ % L.C. µA/15" E.S.R. mΩ Cap µF tan δ % L.C. µA/15" E.S.R. mΩ F 1.08 2.1 0.04 510 1.04 2.8 0.82 540 G 1.12 1.8 0.06 580 1.07 3.0 430 520 As is apparent from the above mentioned, according to an embodiment of the present invention, the aging time needed in the manufacturing process of capacitors can be shortened extremely and reduction of processing steps and symplification of the aging equipment can be achieved. Further, capacitors having heat resistivities being excellent in the electrical characteristics especially in the leakage current property can be obtained. Capacitor elements of roll type are mentioned in the present preferred embodiment, however, the present invention is applicable to capacitors using anode elements made by sintering molded bodies of powder metal such as aluminum, tantalum and/or niobium and at least one TCNQ complex as an electrolyte.

Claims: A method for manufacturing a solid electrolyte capacitor comprising the steps of: liquefying by heating a mixture of at least two kinds of 7,7,8,8-tetracyanoquinodimethane complex salts (8) having cations different from each other, in which said at least two kinds of 7,7,8,8-tetracyanoquinodimethane complex salts are selected among the following 7,7,8,8-tetracyanoquinodimethane complex salts (a) 7,7,8,8-tetracyanoquinodimethane complex salt with two molecules of lutidine combined at N atom thereof with a pentamethylene group or a hexylmethylene group, (b) 7,7,8,8-tetracyanoquinodimethane complex salt with isoquinoline or lutidine combined at N atom thereof with a benzyl group or a phenethyl group, and (c) 7,7,8,8-tetracyanoquinodimethane complex salt with phenylphylidine combined at N atom thereof with a hydrocarbon having a carbon number from 2 to 4; impregnating said liquified mixture of 7,7,8,8-tetracyanoquinodimethane complex salts (8) into a capacitor element (6) having oxidised anode and cathode elements (1, 2); and solidifying by cooling said impregnated mixture of 7,7,8,8-tetracyanoquinodimethane complex salts (8). A method for manufacturing a solid electrolyte capacitor as claimed in claim 1, comprising the steps of: enclosing said capacitor in a case (7) made of heat resistive material; and aging said capacitor by heating thereof at a temperature between 150°C and the melting point of said mixture of 7,7,8,8-tetracyanoquinodimethane complex salts (8).

Inventors: KAKUMA KENJI, MIZUTOMI KATSUNORI, NIWA SHINICHI, KAKUMA, KENJI, MIZUTOMI, KATSUNORI, NIWA, SHINICHI
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1990
Main classification: H01G9
IPC codes: H01G9