Abstract:
A terminal frame including plural pairs of terminal sections arranged in rows and columns is prepared. Each pair includes a flat anode terminal section and a flat cathode terminal section disposed in the same plane with their tip ends being spaced from and facing each other. Plural capacitor elements are prepared. Each capacitor element has a cathode layer on substantially entire outer surface thereof, and a tantalum lead. The capacitor elements are placed on the frame. The tantalum lead of each capacitor element is coupled to one major surface of the anode terminal section of one pair of terminal sections, with the cathode layer of that capacitor element coupled to one major surface of the cathode terminal section of the same pair. The frame with the capacitor elements mounted thereon are coated with resin in such a manner that all of the capacitor elements are encapsulated in the resin, while at least a portion of the opposite major surface of each anode terminal section and at least a portion of the opposite major surface of each cathode terminal section are left exposed. After that, the frame and the resin encapsulation are cut to separate the capacitor elements with the anode and cathode terminal sections into individual chip capacitors.

Description:
This invention relates to a method for manufacturing a chip capacitor, e.g. a chip-type tantalum capacitor. 
     BACKGROUND OF THE INVENTION 
     Japanese Examined Patent Publication (KOKOKU) No. SHO 55-47449 B2 published on Nov. 29, 1980 and Japanese Examined Patent Publication (KOKOKU) No. HEI 1-29050 B2 published on Jun. 7, 1989, both of which are owned by the assignee of the present application, disclose a method of manufacturing chip-type tantalum capacitors. 
     A According to the method disclosed in Japanese Examined Patent Publication No. SHO 55-47449 B2, a terminal frame including pairs of cathode and anode terminal portions disposed spaced from each other along a line are arranged in rows and columns is prepared. Capacitor elements each including a cathode layer disposed to cover the outer surface of the capacitor element and an anode lead extending from one end thereof are prepared. The cathode layer of each capacitor element is connected to the cathode terminal portion of an associated one of the cathode-anode terminal portion pairs of the terminal frame, with the anode lead of that capacitor element connected to the anode terminal portion in the same pair. Then, all the capacitor elements, the cathode terminal portions and the anode terminal portions in respective rows are molded in a resin. Thus, the cathode and anode terminal portions except those portions connected to the frame are embedded in the resin. After that, the cathode and anode terminal portions are separated from the terminal frame, and, also, the resin is cut along lines between adjacent ones of the capacitor elements to thereby complete chip-type capacitors. 
     According to a method disclosed in Japanese Examined Patent Publication No. HEI 1-29050 B2, chip-type capacitors are manufactured by punching plural pairs of generally rectangular anode and cathode terminal portions arranged in line in an electrically conductive metal sheet, with an opening disposed between each pair of anode and cathode terminal portions. The anode and cathode terminal portions are pushed upright with the bases of the respective terminal portions connected to the metal sheet. Respective sides of the raised rectangular terminal portions are inwardly bent. Capacitor elements are disposed between respective pairs of raised terminal portions. Metal chips welded on tip ends of anode leads of respective capacitor elements are connected to inner surfaces of respective ones of the inwardly bent anode terminal portions, and cathode layers disposed on outer surfaces of respective ones of the capacitor elements are connected to inner surfaces of respective ones of the inwardly bent cathode terminal portions. Resin is placed in the spaces between respective anode and cathode terminal portion pairs to cover the capacitor elements disposed between them. After that, the anode and cathode terminal portions are separated from the conductive metal sheet to complete chip-type capacitors. 
     According to Japanese Examined Patent Publication SHO 55-47449 B2, resin molding is provided for each row of capacitor elements. In addition, since the cathode and anode terminal portions are embedded in the resin, they occupy a relatively large amount of the volume of the chip capacitor mold, which impedes downsizing of such chip capacitor. 
     According to Japanese Examined Patent Publication No. HEI 1-29050, both anode and cathode terminal portions have parts located below the capacitor element. However, since the anode terminal is at a location remote from the cathode layer, it is not possible to reduce the length of the chip capacitor. 
     Therefore, an object of the present invention is to provide a method of manufacturing chip capacitors in a small size. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a terminal frame is first prepared. The terminal frame includes plural pairs of terminal sections arranged in rows and columns. Each terminal section pair includes a flat anode terminal section and a flat cathode terminal section, which are arranged in the same plane with their tip ends facing to each other with a spacing disposed between them. The terminal frame may be formed by etching or punching an electrically conductive material, e.g. a metal sheet or metal foil. 
     A plurality of capacitor elements are prepared. Each capacitor element includes a cathode layer covering the outer surface of the capacitor element, and an anode lead extending outward from the element. The cathode layer includes a flat surface portion at least in its bottom surface. The capacitor element may have a shape of generally rectangular parallelepiped or a semicylindrical shape. At least the bottom surface of the capacitor element is flat. The cathode layer is present also on this flat bottom surface. The anode lead extends out of an end of the capacitor element and may be columnar or planar in shape. Alternatively, it may have a shape of foil. 
     One such capacitor element is disposed in association with each terminal section pair. Then, the anode lead of the capacitor element is electrically and mechanically connected to first one of major surfaces of the anode terminal section of the terminal section pair. The cathode layer of the capacitor element is connected to a first major surface of the cathode terminal section of the same terminal section pair which is located in substantially the same plane as the first major surface of the anode terminal section. Then, the terminal frame and the capacitor elements mounted on it are entirely covered with a resin so as to embed all of the capacitor elements, leaving exposed at least a portion of the second, opposite major surface of each anode terminal section and at least a portion of the second, opposite major surface of each cathode terminal section, to thereby form a capacitor assembly of chip capacitors each including a capacitor element having an anode lead and a cathode layer connected to associated anode and cathode terminal sections and a resin coating covering them. The covering may be carried out by means of a molding machine capable of molding the entire terminal frame and all of the capacitor elements, e.g. a transfer mold machine with a cavity capable of accommodating the terminal frame in its entirety, or a screen printing machine, or an injection mold machine. The capacitor assembly is cut, by means of, for example, a dicing machine, into individual chip capacitors. 
     The capacitor assembly may be separated into individual chip capacitors by forming slits having a width between adjacent chip capacitors of the capacitor assembly. If the capacitor assembly is separated by forming single thin cut lines in the terminal frame between adjacent chip capacitors, a larger amount of resin will be left on the resulting individual chip capacitors, so that the chip capacitors cannot be small in size. By separating the capacitor assembly by forming slots with some width, the amount of resin to be left on the resulting chip capacitors can be reduced, which helps further downsizing of chip capacitors. 
     When disposing respective capacitor elements in association with anode and cathode terminal sections, an insulator may be disposed on the tip end of each anode terminal section, and the cathode layer of each capacitor element is placed in contact with the tip end of the associated anode terminal section and with the associated cathode terminal section. The insulator may be an insulating tape or insulating paint, e.g. insulating ink. In addition, the proximal end of each anode terminal section may be disposed in the vicinity of the tip end of the anode lead of the associated capacitor element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a longitudinal cross-section view of a chip-type tantalum capacitor made in accordance with the present invention, and 
     FIG. 1B is a schematic front elevational view illustrating a general structure of the capacitor shown in FIG. 1A with some portions removed. 
     FIG. 2 is a flow chart illustrating a process of manufacturing chip-type tantalum capacitors according to an embodiment of the present invention. 
     FIG. 3 is a perspective view of a terminal frame used in the manufacturing process shown in FIG.  2 . 
     FIG. 4 is a perspective view of the terminal frame shown in FIG. 3 with insulating tape pieces, electrically conductive adhesive, and tantalum leads mounted in place thereon. 
     FIG. 5 is a perspective view of the terminal frame shown in FIG. 4, with cathode layers of respective capacitor elements secured by the conductive adhesive and with tantalum leads of the capacitor elements welded to the associated tantalum wires previously mounted on the terminal frame. 
     FIG. 6 is a schematic perspective view of a capacitor assembly inclusinf the terminal frame shown in FIG. 5 and a resin covering the terminal frame. 
     FIG. 7 is a schematic perspective view of the capacitor assembly shown in FIG. 6 after it is cut into individual chip capacitors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B schematically show a chip-type tantalum capacitor made by a method in accordance with an embodiment of the present invention. 
     A chip capacitor  2  includes a capacitor element, e.g. a tantalum capacitor element  4 , which may be prepared in accordance with any known process. The capacitor element  4  is generally rectangular parallelepiped and has a top surface  4   a,  a bottom surface  4   b,  side surfaces  4   c  and  4   d  and end surfaces  4   e  and  4   f.  Any other shape may be employed for the capacitor element  4  provided that the bottom surface  4   b  is flat. 
     A cathode layer  6  is disposed over the entire outer surface of the capacitor element  4 . An anode lead; e.g. a tantalum lead  8  of the capacitor element  4  extends out of the end surface  4   e,  and a plastic washer  10  is fitted over the tantalum lead  8  and is in surface contact with the end surface  4   e  of the capacitor element  4 . Thus, there is provided no cathode layer on the end surface  4   e  at the portion which the plastic washer  10  is in contact with. The tantalum lead  8  is pillar-shaped. For example, it may be cylindrical. A planar cathode terminal  12  and a planar anode terminal  14  are disposed beneath the bottom surface  4   b  of the capacitor element  4 . 
     The cathode terminal  12  is located offset toward the end surface  4   f  with its outer end located near the end surface  4   f.  In the illustrated example, it is located slightly outward of the end surface  4   f.  The inner end of the cathode terminal  12  is located slightly offset from the longitudinal center of the capacitor element  4  toward the end surface  4   f.  One of two opposing major surfaces,  12   a,  of the cathode terminal  12  is disposed close to and substantially in parallel with the bottom surface  4   b  of the capacitor element  4  and is connected to the cathode layer  6  on the bottom surface  4   b  by means of electrically conductive  10  adhesive, e.g. silver paste  16 . 
     Similarly, the anode terminal  14  is disposed at a location offset toward the end surface  4   e.  The outer end of the anode terminal  14  is close to the outer end of the tantalum lead  8 . In the illustrated example, it is slightly outward of the outer end of the tantalum lead  8 . The inner end of the anode terminal  14  is slightly offset from the longitudinal center of the capacitor element  4  toward the end surface  4   e.  The inner end of the anode terminal  14  is spaced from the inner end of the cathode terminal  12 . 
     One of two opposing major surfaces, namely, a major surface  14   a,  of the anode terminal  14  is disposed close to the bottom surface  4   b  of the capacitor element  4  and is substantially in the same plane as the major surface  12   a  of the cathode terminal  12 . The major surface  12   a  is secured to the cathode layer  6  on the bottom surface  4   b  of the capacitor element  4  with an insulator, e.g. a piece of insulating tape  18  disposed therebetween. Thus, the anode terminal  14  is secured to but insulated from the cathode layer  6 . 
     A connector, e.g. a tantalum wire  20 , is disposed between and in contact with the tantalum lead  8  and a portion of the anode terminal  14 . The tantalum wire  20  has a columnar or cylindrical shape and extends perpendicular to the length of the tantalum lead  8 . The tantalum wire  20  is connected to the tantalum lead  8  and the anode terminal  14  by welding. 
     The capacitor element  4 , the tantalum lead  8 , the tantalum wire  20 , a portion of the anode terminal  14  and a portion of the cathode terminal  12  are covered by or encapsulated in an encapsulation  22  of resin, e.g. epoxy. As is seen from FIG. 1A, the encapsulation  22  is such that a larger portion of the other major surface  14   b  (i.e. the surface opposite to the major surface  14   a ) of the anode terminal  14  and a larger portion of the other major surface  12   b  (i.e. the surface opposite to the major surface  12   a ) of the cathode terminal  12  are left exposed. 
     Further, as is seen from FIG. 1A, the cathode terminal  12  and the anode terminal  14  are planar elements, and they lie only beneath the capacitor element  4  and do not extend on the lateral sides of the element  4 . Accordingly, the proportion of the volume occupied by the cathode and anode terminals  12  and  14  to the volume of the entire chip capacitor  2  can be small, so that the chip-type tantalum capacitor  2  can be small in size. Furthermore, since the inner end of the anode terminal  14  extends beneath the capacitor element  4 , the length of the anode terminal  14  extending outward of the end surface  4   e  of the capacitor element  4  can be short, which also makes it possible to downsize the chip capacitor  2 . In addition, since the anode terminal  14  has a relatively large area, reliable soldering of the chip capacitor  2  to a printed circuit board is provided. 
     The chip-type tantalum capacitor  2  may be manufactured, for example, in the following manner. FIG. 2 is a flow chart exemplifying how to manufacture the capacitor  2 . 
     First, a terminal frame  30  is prepared (STEP S 2 ). The terminal frame  30  includes an outer framing  32  as shown in FIG.  3 . Within the boundary defined by the framing  32 , a plurality of strap-like members  34  spaced from each other by a predetermined distance extend in parallel between a pair of opposing parallel longer sides  29 - 1  and  29 - 2  of the framing  32 , and a plurality of similar strap-like members  35  spaced from each other by a predetermined distance extend in parallel between the other pair of opposing parallel shorter sides  28 - 1  and  28 - 2  of the framing  32  in the direction perpendicular to the strap-like members  34 . Thus, a plurality of rectangular windows  36  are formed in matrix by the strap-like members  34  and  35  and the framing  32 . 
     A terminal structure pair including one cathode terminal section  38  and one anode terminal section  40  is provided in each window  36 . Each of the cathode terminal sections  38  extends into an associated window  36  from one of the strap-like members  35  or the shorter side  28 - 1  of the outer framing  32 . Each cathode terminal section  38  has a neck portion  38   a  and a generally rectangular, enlarged head portion  38   b  formed integral with the distal end of the neck portion  38   a.  Similarly, each of the anode terminal sections  40  extend from one of the strap-like members  35  or the other shorter side  28 - 2  into an associated window  36  in the direction toward the associated cathode terminal section  38 . Each anode terminal section  40  has a neck portion  40   a  and a generally rectangular, enlarged head portion  40   b  formed integral with the distal end of the neck portion  40   a.    
     Thus, one cathode terminal section  38  and one anode terminal section  40  are in each window  36 . The center lines of the neck portions  38   a  and  40   a  in each window  36  extending in parallel with the longer sides  29 - 1  and  29 - 2  of the outer framing  32 , are coincident with each other and extend through the midpoints of the opposing shorter edges of that window  36 . The center lines of the head portions  38   b  and  40   b  in each window  36  extending in parallel with the longer sides  29 - 1  and  29 - 2  are coincident with the center lines of the associated neck portions  38   a  and  40   a.  The distal edges of the head portions  38   b  and  40   b  are spaced from each other by a predetermined distance. 
     The terminal frame  30  may be prepared by, for example, punching or etching an electrically conductive, thin metal sheet. In FIG. 3, only twenty terminal structure pairs, i.e. twenty cathode terminal sections  38  and twenty anode terminal sections  40 , are shown only for an illustrative purpose, but actually several hundreds of terminal structure pairs are formed in one terminal frame  30 . 
     In STEP S 4  (FIG.  2 ), the distal end portion of the head portion  40   b  of each anode terminal section  40  is made insulative by, for example, bonding an insulating tape  18 , as shown in FIG.  4 . In place of the insulating tape  18 , a patch of insulating ink may be screen-printed. 
     Next, a quantity of electrically conductive adhesive, e.g. silver paste,  16  is applied over a region along a distal edge of the head portion  38   b  of each cathode terminal section  38  (STEP S 6 ), as shown in FIG.  4 . The application of the adhesive may be achieved by screen printing with a screen printing machine or by potting with a high-precision dispenser. 
     Next, a connector or tantalum wire  20  is welded to the proximal end of the head portion  40   b  of each anode terminal section  40  (STEP S 8 ). The welded wires  20  are also shown in FIG.  4 . 
     It should be noted that the order of performing STEPs S 4 , S 6  and S 8  can be changed. 
     Along with STEPs S 4 , S 6  and S 8 , a plurality of capacitor elements  4  are prepared. The capacitor elements  4  may be prepared in any one of suitable known processes, and, therefore, it is not described in detail how to manufacture them. 
     Each of the capacitor elements  4  is disposed in such a manner that the anode lead  8  of the capacitor element  4  contacts with the tantalum wire  20  on the associated anode terminal section  40 , and that the cathode layer  6  on the bottom surface  4   b  of that capacitor element  4  is brought into contact with the insulating tape  18  on the head portion  40   b  of the anode terminal section  40  and with the conductive adhesive  16  applied on the head portion  38   b  of the associated cathode terminal section  38  (STEP S 10 ) as shown in FIG.  5 . 
     Next, the tantalum leads  8  of the respective capacitor elements  4  are welded to the associated tantalum wires  20  of the anode terminal sections  40  (STEP S 12 ). Also, the cathode layers  6  are made to be bonded to the respective cathode terminal head portions  38   b  by the conductive adhesive  16  (STEP S 14 ). 
     After that, the terminal frame  30  is provided with a coating  42  of a resin, e.g. an epoxy resin (STEP S 16 ). The coating  42  is provided by using a machine capable of handling the entire terminal frame  30 , e.g. by screen-printing with a screen-printing machine or by transfer molding with a transfer mold machine. The coating is provided in such a manner that the bottom surfaces of the cathode terminal sections  38  and the bottom surfaces of the anode terminal sections  40 , which respectively correspond to the major surfaces  14   b  and  12   b  of the anode and cathode terminals  14  and  12  of the capacitor element  4  shown in FIGS. 1A and 1B, are located in the same plane and are left exposed. 
     When transfer-molding is employed for coating the terminal frame  30  with the above-described components, including the capacitor elements  4  placed on the terminal frame  30 , all of the capacitor elements  4 , the anode terminal sections  40  and the cathode terminal sections  38  are placed in a single cavity. In prior art transfer molding, capacitor elements must be individually placed in separate cavities, which raises the costs for the equipment and requires a long manufacturing time. Furthermore, prior art provides poor dimensional precision, no degree of freedom of dimensions, and low effective use of materials. In contrast, according to the present invention, all of the capacitor elements  4 , the anode terminal sections  40  and the cathode terminal sections  38  are placed in a single cavity, and, therefore, such disadvantages seen in prior art can be eliminated. 
     A capacitor assembly including the terminal frame  30  with the capacitor elements  4  mounted thereon provided with the coating  42  is shown in FIG.  6 . In case that resin adheres to the bottom surfaces of the cathode and anode terminal sections  38  and  40  (which bottom surfaces are the major surfaces  14   b  and  12   b  of the anode and cathode terminals  14  and  12 , respectively, of the completed chip capacitor  2 ), it must be removed by means of, for example, a honing machine or micro-sandblasting machine so that the major surfaces  14   b  and  12   b  of the anode and cathode terminals  14  and  12  can remain exposed through the encapsulation  22  or coating  42 . 
     The terminal frame  30  with the capacitor elements  4  and the other components mounted thereon is coated with the resin by screen-printing, the thickness of the resin coating  42  is adjusted by means of a surface grinding machine. 
     Although the bottom surfaces of the cathode and anode terminal sections  38  and  40  may be kept flat, they may be provided with portions offset toward the respective top surfaces so as to improve the solderability of the completed capacitors  2  to printed circuit boards. Such offset portions can be formed by a dicing machine or micro sandblasting machine. 
     Then, the terminal frame  30  with the capacitor elements  4  mounted thereon, i.e. the capacitor assembly, is severed (Step S 18 ). Thus, the encapsulations  22  are formed from the coating  42 . For example, a dicing machine may be used to cut the coated terminal frame  30  with the capacitor elements  4  mounted thereon along horizontally and vertically extending sets of two lines spaced apart from each other as shown dashed in FIG.  6 . The dicing machine not only cuts the resin coating  42  but also separates the cathode terminal sections  38  and the anode terminal sections  40  from the outer framing  32  and the strap-like members  34  and  35 . When the coating  42  is cut along the two spaced apart line sets, slots are formed between adjacent chip capacitors  2  as shown in FIG.  7 . The spacing between the two lines or the width of the slots is as large as possible, but to such an extent that neither the capacitor elements  4  nor the tantalum leads  8  are exposed. Thus, the proportion of the volume of the completed chip capacitor  2  occupied by the encapsulation  22  can be small, which means the completed chip capacitor  2  can be small in size. 
     After that, the exposed surfaces, including the major surfaces  12   b  and  14   b,  of the cathode terminals  12  and the anode terminals  14  of the respective chip capacitors  2  are plated with, for example, solder or tin. The plating of the exposed surfaces of the terminals  12  and  14  facilitates soldering of the chip capacitors  2  to printed circuit boards or the like. In addition, solder can creep up along the sectional or end surfaces of the terminals, so that the chip capacitors  2  can be secured to the boards more reliably. Furthermore, the plating of the sectional end surfaces of the terminals  12  and  14  can prevent rusting of the terminals from the sectional end surfaces. The exposed surfaces of the anode and cathode terminal sections  40  and  38  may be plated prior to the cutting of the coating  42 . 
     In the above-described embodiment, the columnar tantalum wire  20  is used as the connector for connecting the anode terminal section  40  to the tantalum lead  8 , but the connector  20  can be of a shape other than columnar shape. Instead of using the separate connector or tantalum wire  20 , the portion of the head portion  40   b  of the anode terminal section  40  which faces the lead  8  may be raised to contact with the lead  8  so that the anode terminal section  40  can be connected directly to the tantalum lead  8 . Alternatively, instead of using the connector  20 , the tantalum lead  8  may be led out from the capacitor element  4  at a location closer to the anode terminal section  40  so that they can be connected directly each other. Another alternative is to use an Ω-shaped connector, which is placed over the tantalum lead  8  from above so as to bring it in contact with the lead  8  with the lower ends of the legs contacting the anode terminal section  40 .