Method of manufacturing a chip-type electrolytic capacitor

Abstract: A chip-type electrolytic capacitor (1) has a capacitor element (2) impregnated with an electrolytic solution, a cylindrical case (3) for containing the capacitor element (2), a sealing member (4) for sealing the cylindrical case (3), and an insulating board (5) having a mounting surface (5c) to be fixed to an electronic circuit board. The vapor pressure of the electrolytic solution is regulated to a predetermined value to prevent deformation of the cylindrical case (3) and the sealing member (4) by the heat of the fixing operation.

Description: This invention relates to a method of manufacturing a chip-type electrolytic capacitor which is to be used in various electronic apparatuses. In recent years, according to needs of small size, thin size and multifunction type for an electronic apparatus, an electronic circuit board is required to mount an increasingly large number of electronic devices such as a chip type aluminum electrolytic capacitor on a fixed region of the electronic circuit board. Therefore, the electronic device is being required to attain small size and small height. Furthermore, the electronic device is also required to be mounted on the electronic circuit board by a surface mounting method. A conventional chip type aluminum electrolytic capacitor is disclosed in the Japanese published unexamined patent application (TOKKAI) No. Sho 59-211214. In this chip type aluminum electrolytic capacitor, a capacitor element impregnated with an electrolytic solution is contained in a cylindrical case having an opening. The capacitor element has a pair of lead members connected to an anode foil and a cathode foil, respectively. The pair of lead members protrude from the opening in a direction parallel with the axis of the cylindrical case through the below-mentioned sealing member. Furthermore, end parts of the lead members pass through a pair of holes of the below-mentioned rectangle-shaped insulating board. And then the end tip parts of the lead members are bent at perpendicular directions to the axis so as to be contained in a pair of recesses of the rectangle-shaped insulating board. The cylindrical case is made of metal such as aluminum, and a diameter of the cylindrical case is between 3 mm and 10 mm. The opening is formed at one end of the cylindrical case, and the other end of the cylindrical case is closed. In order to prevent evaporation and leaking out of the electrolytic solution, the sealing member is disposed in the opening of the cylindrical case after the capacitor element is contained in the cylindrical case. The sealing member is made of elastic material such as rubber or butadiene rubber, and a peripheral part of the sealing member is held by the inner surface of the cylindrical case. Thereby, the capacitor element is held in the cylindrical case at a predetermined position. A rectangle-shaped insulating board is made of a resin such as polyphenylene sulfide, having a heat resisting property, and put on the opening so as to close the opening. The rectangle-shaped insulating board has a mounting surface to be fixed to the surface of the electronic circuit board such as a printed circuit board. Furthermore, as has been explained in the above, the rectangle-shaped insulating board has the pair of holes for the lead members, and has the pair of recesses for containing the end parts of the lead members. When the end parts of the lead members are contained in the recesses, the rectangle-shaped insulating board is fixed to the cylindrical case. It is known that the end parts of the lead members are fixed to an electronic circuit on the electronic circuit board by reflow soldering when the chip type aluminum electrolytic capacitor is mounted on the electronic circuit board by the surface mounting method. However, in the conventional chip type aluminum electrolytic capacitor, vapor pressure of the electrolytic solution is not particularly controlled because of the temperature applied during mounting on the electronic circuit board. Furthermore, elements of the electrolytic solution (i.e., an electrolyte and a solvent) are selected without taking account of volatility. In particular, the electrolyte is made of ammonium salt, and the solvent is made of ethylene glycol. Moreover, in the conventional chip type aluminum electrolytic capacitor, there is no consideration of a material such as water, which has a lower boiling point and is part of the electrolytic solution. There is a problem that the conventional chip type aluminum electrolytic capacitor is mounted in an inclined posture on the electronic circuit board. The reason is as follows: When the end parts of the lead members are fixed to the electronic circuit of the electronic circuit board by the reflow soldering, the chip type aluminum electrolytic capacitor is in an atmosphere of a high temperature which is more than 200°C. As a result, the electrolytic solution is evaporated in the cylindrical case, and the internal pressure in the cylindrical case is increased. Thereby, deformations are made in the sealing member so that the sealing member is expanded toward the outside of the cylindrical case. Accordingly, warps are generated on the mounting surface of the rectangle-shaped insulating board by the deformations. As a result, the mounting surface is inclined against the surface of the electronic circuit board, so that the conventional chip type aluminum electrolytic capacitor is mounted inclined on the electronic circuit board. The above-mentioned problem is likely to occur specially in the conventional chip type aluminum electrolytic capacitor having a large diameter of the cylindrical case. The reasons are in more detail as follows: (1) The vapor pressure of the electrolytic solution causes the increase of the internal pressure in the cylindrical case, and is dependent on the temperature applied during mounting. For example, even though the diameter of the cylindrical case varies between 3mm and 10mm, it is experimentally confirmed that the vapor pressure of the electrolytic solution is not changed for the same temperature. (2) The thicknesses of the cylindrical cases and sealing members used for the cylindrical case diameters ranging between 3 mm and 10 mm are substantially constant. Therefore, the strength of each cylindrical case and each sealing member is almost the same for the internal pressure in the cylindrical case. (3) However, since the peripheral part of the sealing member is held by the inner surface of the cylindrical case, the central part of the sealing member is most deformed to expand outward of the cylindrical case by the internal pressure in the cylindrical case in the case that the thickness of the sealing member and the strength of the sealing member are the same. This deformation of the central part becomes larger in proportion to the increase of the diameter of the cylindrical case. (4) In the conventional chip type aluminum electrolytic capacitor, the vapor pressure of the electrolytic solution is not regulated in accordance with the increase of the diameter of the cylindrical case. Therefore, when the internal pressure in the cylindrical case is increased during reflow soldering, the deformation of the central part of the sealing member gradually increases in proportion to the increase of the diameter of the cylindrical case, and the sealing member deforms the rectangle-shaped insulating board. Thereby, warps are generated in the mounting surface of the rectangle-shaped insulating board by the deformations. As the diameter of the cylindrical case becomes larger, the mounting surface is inclined against the surface of the electronic circuit board. As has been explained in the above, it is undesirable that the conventional chip type aluminum electrolytic capacitor is mounted in inclined posture on the electronic circuit board. If inclinations occur to the chip type aluminum electrolytic capacitor, there is a problem that the chip type aluminum electrolytic capacitor is not securely connected to the electronic circuit. US-A-4,715,976 discloses an electrolyte solution for an electrolytic capacitor. The electrolyte solution may be prepared by adding carboxylic acid and an aqueous solution of quaternary ammonium hydroxide to a solvent, and dehydrating the resulting solution. In accordance with the present invention, there is provided a method of manufacturing a chip-type electrolytic capacitor, comprising: preparing an electrolytic solution consisting essentially of 5 wt% to 50 wt% by weight of quaternary ammonium salt or amine salt as an electrolyte and γ-butyrolactone as a solvent, water not being used for preparing said electrolytic solution; configuring a capacitor element (2) impregnated with said electrolytic solution, said capacitor element (2) having a pair of lead members (2d, 2d'); and encapsulating said capacitor element (2) in a cylindrical metal case (3) having an opening (3a) by sealing said opening (3a) with a sealing member (4) and positioning at said opening (3a) an insulating board (5) having a mounting surface (5c) for fixing to an electronic circuit board, such that said lead members (2d, 2d') are exposed at said mounting surface (5c); wherein the sum of the water contents of said electrolytic solution and said capacitor element (2) inside said cylindrical case (3) is regulated to 10% or less by weight of the quantity of said electrolytic solution, said regulating including a desiccation process to regulate the water present through adsorption and absorption, and adjusting the quantities of said electrolyte and solvent to regulate the water present through chemical bonding and reaction. Thus the vapor pressure of the electrolytic solution is regulated so as to prevent deformations of the cylindrical case and the sealing member when the mounting surface is mounted on the electronic circuit board. In other words, when the chip-type electrolytic capacitor is exposed to a high temperature and the internal pressure in the cylindrical case is increased by the vapor pressure of the electrolytic solution during fixing of the mounting surface on the electronic circuit board, it is possible to prevent deformation of the cylindrical case and the sealing member, and thereby prevent generation of warps on the mounting surface. As a result, the capacitor can be mounted on the electronic circuit board without inclining thereto. Non-limiting embodiments of the invention will now be described with reference to the accompanying drawings, in which:- FIG. 1A is a perspective view showing a chip-type aluminum electrolytic capacitor manufactured using the method of the present invention; and FIG. 1B is a cross sectional view which is taken on line IB-IB of FIG.1A. In FIGs. 1A and 1B, a chip type aluminum electrolytic capacitor 1 comprises a capacitor element 2 configured by cylindrically winding an aluminum anode foil 2a, an aluminum cathode foil 2b and intermediating separators 2c, 2c' of paper impregnated with an electrolytic solution. The anode foil 2a and the cathode foil 2b are convolutely wound so as to face each other through the separators 2c, 2c' provided between the anode foil 2a and the cathode foil 2b. A pair of lead members 2d, 2d' are connected to the anode foil 2a and the cathode foil 2b, respectively. A substantially cylindrical case 3 encapsulates the capacitor element 2. A sealing member 4 is disposed in the cylindrical case 3 between the capacitor element 2 and an opening 3a of the cylindrical case 3. A rectangle-shaped insulating board 5 is mounted on the opening 3a of the cylindrical case 3. The lead members 2d, 2d' extend out of the cylindrical case 3 via the sealing member 4 and a pair of holes 5a, 5a' of the rectangle-shaped insulating board 5, respectively. Furthermore, the end parts of the lead members 2d, 2d' are bent at perpendicular directions to the axis of the cylindrical case 3 so as to be contained in a pair of recesses 5b, 5b' of the rectangle-shaped insulating board 5, respectively. The cylindrical case 3 is made of metal such as aluminum, and the diameter of the cylindrical case 3 is between 3mm and 10mm on a commercial basis. The opening 3a is formed at one end of the cylindrical case 3, and the other end of the cylindrical case 3 is closed. The sealing member 4 is made of elastic material such as rubber or butadiene rubber, and disposed between the capacitor element 2 and the opening 3a for preventing evaporation and leaking out of the electrolytic solution. A peripheral part of the sealing member 4 is held by the inner surface of the cylindrical case 3 by making a dent in the outer surface of the cylindrical case 3. Thereby, the capacitor element 2 is held in the cylindrical case 3 at a predetermined position. The rectangle-shaped insulating board 5 is made of a resin such as polyphenylene sulfide, having a heat resisting property, and put on the opening 3a so as to close the opening 3a. Thereby, the rectangle-shaped insulating board 5 electrically insulates the cylindrical case 3 from an external electronic circuit board (not shown). As has been elucidated in the above, the rectangle-shaped insulating board 5 has the pair of holes 5a, 5a' for the lead members 2d, 2d' and the pair of recesses 5b, 5b' for containing the end parts of the lead members 2d, 2d'. Furthermore, the rectangle-shaped insulating board 5 has a mounting surface 5c to be fixed to a surface of an electronic circuit board such as a printed circuit board. When the end parts of the lead members 2d, 2d' are bent and contained in the recesses 5b, 5b', the rectangle-shaped insulating board 5 is fixed to the cylindrical case 3. In the case that the chip type aluminum electrolytic capacitor 1 is mounted on the electronic circuit board by a surface mounting method, the end parts of the lead members 2d, 2d' are fixed to an electronic circuit on the electronic circuit board by reflow soldering or the like. In this reflow soldering, it is known that a cream solder is painted on the electronic circuit, and the lead members 2d, 2d' are put on and temporarily fixed to the cream solder. Thereafter, in order to melt the cream solder, the electronic circuit board having the chip type aluminum electrolytic capacitor 1 is put in a soldering bath heated to an atmosphere of a high temperature (hereinafter referred to as a "mounting temperature") of 200°C or above by irradiation of infrared rays or hot-air blowing. As a result, the chip type aluminum electrolytic capacitor 1 is fixed to the electronic circuit board by the cream solder. Thus, since the capacitor 1 is exposed to the high temperature of more than 200°C, the electrolytic solution is evaporated in the cylindrical case 3, and the internal pressure in the cylindrical case 3 is increased by vapor pressure of the electrolytic solution. However, the vapor pressure of the electrolytic solution at the mounting temperature is regulated to a predetermined value to prevent deformations of the cylindrical case 3 and the sealing member 4. That is, the electrolytic solution is composed of a non-volatile material or a material having a high boiling point. An electrolyte is made of amine salt or quaternary ammonium salt, and a solvent is made of γ-butyrolactone. Furthermore, the electrolytic solution has been regulated so as to have such a low boiling point content as not to make a high vapor pressure. Representative of the material having the low boiling point generally is water, which has the boiling point of 100°C (at 1 atmospheric pressure). Water is not used for preparing the electrolytic solution of the chip type aluminum electrolytic capacitor. However, water is contained in the electrolytic solutionfor the following reasons: (1) Water exists in the elements of the electrolytic solution by means of chemical bond. For example, the electrolyte includes water in its chemical constitution. (2) Water is generated in the electrolytic solution as a result of reaction between the electrolyte and the solvent during preparation of the electrolytic solution. (3) Water exists in the electrolytic solution as a result of absorption after the electrolytic solution is prepared. Furthermore, in the inside of the cylindrical case 3, water exists in capacitor element 2 as a result of physical adsorption. That is, water is adversely adsorbed by the separators 2c, 2c'. On the contrary to such undesirable water, in this embodiment, water content contained inside the cylindrical case 3 is regulated to a predetermined quantity (hereinafter referred to as a "regulated quantity of water") as follows: (1) With regard to water resulting from the above-mentioned chemical bond and reaction, the water is regulated by adjusting quantities of the electrolyte and solvent. (2) With regard to water resulting from the above-mentioned adsorption and absorption, the water is regulated by performing a desiccation process. Test results of the chip type aluminum electrolytic capacitor manufactured by the above-mentioned process are shown in Table 1. The test was performed on six working examples wherein materials of the elements of the electrolytic solution and the regulated quantity of water are varied as shown in Table 1. The working example No.6* is a comparison working example in accordance with the prior art. The regulated quantity of water is measured by a moisture meter, and is equal to the sum of water contents of the electrolytic solution and the capacitor element 2. As shown in Table 1, the regulated quantity of water is shown by a proportion of the quantity of the electrolytic solution. In this test, the test results are obtained by means of the following steps I and II: (I) For each working example, 1000 samples of the chip type aluminum electrolytic capacitor were mounted on an electronic circuit board by reflow soldering. Rated values and size of the chip type aluminum electrolytic capacitor are as follows: Rated voltage is 50 V. Rated capacitance is 47 µF. The diameter of the cylindrical case is 10 mm. Furthermore, heat conditions in the reflow soldering were as follows: (1) Preheating condition: temperature is 160°C. (Duration is 120 seconds.) (2) Heating condition: temperature is 200°C. (Duration is 30 seconds. ) (3) Peak temperature is 230°C. (Duration is under 5 seconds.) (II) Each sample was inspected to see if it was inclined relative to the electronic circuit board because of warps in the mounting surface of the capacitor. The number of inclined capacitors was counted. Working example No. Materials of the elements of the electrolytic solution: the electrolyte / the solvent Regulated quantity of water (wt%) Test results (No. of samples) 1 quaternary ammonium salt 20 (wt%) / γ-butyrolactone 80 (wt%) 2 (wt%) 0 2 amine salt 22.5 (wt%) / γ-butyrolactone 77.5 (wt%) 2 (wt%) 0 3 amine salt 22.5 (wt%) / γ-butyrolactone 77.5 (wt%) 5 (wt%) 0 4 amine salt 22.5 (wt%) / γ-butyrolactone 77.5 (wt%) 10 (wt%) 0 5 amine salt 22.5 (wt%) / γ-butyrolactone 77.5 (wt%) 15 (wt%) 7 ammonium salt 12 (wt%) / ethylene glycol 88 (wt%) 2 (wt%) 105 For the working examples Nos. 1, 2, 3 and 4 of Table 1, it is found that the chip type aluminum electrolytic capacitor 1 is mounted on the electronic circuit board without inclining thereto. For the reason as has been explained in the above, the vapor pressure of the electrolytic solution is regulated to the predetermined value to prevent deformations of the cylindrical case 3 and the sealing member 4. Therefore, when the internal pressure in the cylindrical case 3 is increased by the evaporation pressure of the electrolytic solution, it is possible to prevent deformations of the cylindrical case 3 and the sealing member 4 and, thereby, it is possible to prevent generation of warps on the mounting surface 5c. When the regulated quantity of water is more than 10 wt% of the quantity of the electrolytic solution, e.g. as shown in the example No. 5 which is not an embodiment of the invention, it is the case that the chip type aluminum electrolytic capacitor 1 is mounted inclined on the electronic circuit board. This is because the internal pressure in the cylindrical case 3 exceeds the value for generating deformations of the cylindrical case 3 and the sealing member 4 by evaporation of the water and, thereby, warps are generated in the mounting surface 5c. Accordingly, in the present invention the regulated quantity of water is 10 wt% or less by weight of the quantity of the electrolytic solution, and is further preferably 5 wt% or less by weight thereof. In the above-mentioned test, according to the inventor's analysis of the degree of warping, when the chip type aluminum electrolytic capacitor 1 was mounted inclined on the electronic circuit board, the internal pressure in the cylindrical case 3 was roughly between 30 atmospheric pressure and 60 atmospheric pressure (at temperature between 180 °C and 220 °C). In the case that the electrolytic solution is composed of quaternary ammonium salt and γ-butyrolactone in the working example No. 1 of Table 1, there is used quaternary ammonium salt in a range between 5 wt% and 50 wt% by weight, and γ-butyrolactone in a range between 95 wt% and 50 wt% by weight depending on prepared weight of quaternary ammonium salt. in the case that the electrolytic solution is composed of amine salt and γ-butyrolactone in the working examples No. 2 --- 4 of Table 1, there is used amine salt in a range between 5 wt% and 50 wt% by weight, and γ-butyrolactone in a range between 95 wt% and 50 wt% by weight depending on prepared weight of amine salt. Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure.

Claims: A method of manufacturing a chip-type electrolytic capacitor, comprising: preparing an electrolytic solution consisting essentially of 5 wt% to 50 wt% by weight of quaternary ammonium salt or amine salt as an electrolyte and γ-butyrolactone as a solvent, water not being used for preparing said electrolytic solution; configuring a capacitor element (2) impregnated with said electrolytic solution, said capacitor element (2) having a pair of lead members (2d, 2d'); and encapsulating said capacitor element (2) in a cylindrical metal case (3) having an opening (3a) by sealing said opening (3a) with a sealing member (4) and positioning at said opening (3a) an insulating board (5) having a mounting surface (5c) for fixing to an electronic circuit board, such that said lead members (2d, 2d') are exposed at said mounting surface (5c); wherein the sum of the water contents of said electrolytic solution and said capacitor element (2) inside said cylindrical case (3) is regulated to 10% or less by weight of the quantity of said electrolytic solution, said regulating including a desiccation process to regulate the water present through adsorption and absorption, and adjusting the quantities of said electrolyte and solvent to regulate the water present through chemical bonding and reaction. A method in accordance with claim 1, wherein said regulated sum of water is 5% or less by weight of the quantity of said electrolytic solution. A method in accordance with claim I or 2, wherein said case (3) is made of aluminum.

Inventors: NAKATA TAKUMI, NAKATA, TAKUMI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: H01G9
IPC codes: H01G9, H05K3