Method for manufacturing a solid electrolytic capacitor

Cathode and anode sides of a plurality of solid electrolytic capacitors are connected by simultaneous electric welding. The welding step is effected to connect an anode lead of a lead frame to the anode electrode of a capacitor body and simultaneously connect a cathode lead of the lead frame to the cathode conductor layer of an adjacent capacitor body. The welding electrode for the cathode lead exerts moderate force to the capacitor bodies using a spring function of the capacitor lead. The simultaneous welding for the adjacent capacitor bodies and the moderate force prevent electrical and mechanical damages of the insulator layer of the solid electrolytic capacitors during the welding.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a method for manufacturing a solid 
electrolytic capacitor and, more particularly, to a process for connecting 
a cathode conductor layer with a cathode lead in a solid electrolytic 
capacitor. 
(b) Description of the Related Art 
Some solid electrolytic capacitors use a valve-function metal such as 
tantalum. Fabrication of such a solid electrolytic capacitor (may be 
referred to as electrolytic capacitor or simply capacitor hereinafter) 
includes a step for connection between a pair of electrodes, which include 
an anode electrode and a cathode conductor layer, and a pair of external 
lead wires, which include an anode lead and a cathode lead. In a 
conventional fabrication process, the connection between the anode 
electrode and the anode lead is usually effected by electric welding. On 
the other hand, the connection between the cathode conductor layer and the 
cathode lead is effected by several methods, which involve problems to be 
solved. A variety of proposals and studies have been made to solve the 
problems. 
Examples of the connection between the cathode conductor layer and the 
cathode lead include the use of a conductive adhesive, such as a silver 
paste, wherein conductive fillers such as silver particles and an adhesive 
such as epoxy resin are admixed. FIG. 1 shows a sectional view of an 
example of conventional solid electrolytic capacitors fabricated by using 
the conductive adhesive. The capacitor comprises a capacitor body 11, an 
anode electrode 12 protruding from the top surface of the capacitor body 
11, and a conductor layer 13 constituting a cathode electrode formed on 
the side and bottom surfaces of the capacitor body 11. The cathode lead 16 
is electrically connected and fixed to the cathode conductor layer 13 by a 
conductive adhesive 23. 
The conventional capacitor of FIG. 1 requires separate steps for connection 
on the anode side (between the anode lead 15 and the anode electrode 12) 
effected by electric welding and for connection on the cathode side 
(between the cathode lead 16 and the cathode conductor layer 13) effected 
by the conductive adhesive 23, resulting in a lower throughput of the 
capacitors. In addition, the connection on the cathode side requires a 
heat treatment for a certain length of time, which further lowers the 
throughput of the capacitors. 
Examples of the connection on the cathode side include the use of a 
soldering technique. A soldering technique using a pulse thermal treatment 
is described in Publication NO. JP-A-1990-46715, which includes in essence 
the steps of mounting the capacitor body on a pre-heated hot plate, and 
removing the capacitor body therefrom after a given length of time to 
thereby apply a pulse thermal energy to the capacitor body. The waveform 
of the pulse thermal energy has a rapid rise, a rapid fall and a high 
amplitude. 
The soldering connection on the cathode side requires separate operations 
for the anode side and cathode side, similarly to the case of the 
conductive adhesive. Also, the soldering technique requires a relatively 
large length of time, for example, several seconds, for the pulse heating, 
which also lowers the throughput of the capacitors. 
A proposal is made which uses the electric welding for the connection on 
the cathode side. Examples for the electric welding include a parallel gap 
welding in which welding current flows between a pair of parallel welding 
electrodes disposed in the vicinity of the connection. FIG. 2 shows a top 
plan view of the capacitor during the parallel gap welding, which is 
disclosed in UM Publication No. JP-B-1987-2762. A cathode lead 16 running 
parallel to the bottom surface of the capacitor body 11 in spaced 
relationship therewith is connected to the capacitor electrode (not shown) 
by a fuse 24, which is welded by the parallel gap welding technique for 
bridging the cathode lead 16 and the cathode electrode. The parallel gap 
welding using the electric welding, however, generally requires a 
relatively large space for the welding, which is not suited to a 
small-sized product. 
Another electric welding known in other technical fields applies a pressing 
force between a pair of welding electrodes sandwiching therebetween an 
element to be welded. If this technique is used in fabrication of a 
capacitor, the pressure is applied directly to the capacitor body to 
thereby damage the insulator film etc. of the capacitor body thereby 
increasing the leakage current between the electrodes. Accordingly, the 
characteristics or reliability of the capacitor is lowered. 
SUMMARY OF THE INVENTION 
In view of the forgoing, it is an object of the present invention to 
provide a method for fabricating a reliable, smallsized solid electrolytic 
capacitor with a high throughput. 
The present invention provides a method for manufacturing a solid 
electrolytic capacitor comprising the steps of preparing a capacitor body 
having an anode electrode protruding from the capacitor body and a cathode 
conductor layer formed on the capacitor body, connecting an anode lead to 
the anode electrode, making a first portion of a cathode lead in contact 
with the cathode conductor layer with an intervention of a solder layer, 
making first and second welding electrodes in contact with a second 
portion of the cathode lead adjacent to the first portion and the cathode 
conductor layer, respectively, pressing the first welding electrode toward 
the second welding electrode to thereby press the first portion to the 
cathode conductor layer, supplying welding current between the first 
welding electrode and second welding electrode to connect the cathode 
conductor layer to the cathode lead with the solder. 
The preferred embodiment of the present invention comprises the steps of 
preparing a lead frame having a plurality of lead pairs each including an 
anode lead and a cathode lead opposed to each other, each cathode lead 
having a first portion and a second portion adjacent to the first portion, 
preparing a plurality of capacitor bodies each having an anode electrode 
protruding from the each of the capacitor bodies and a cathode conductor 
layer formed on the each of the capacitor bodies, arranging the capacitor 
bodies correspondingly to the lead pairs to make the anode leads in 
contact with the anode electrodes of the respective capacitor bodies and 
to make the first portions of the cathode leads in contact with the 
cathode conductor layers of the respective capacitor bodies with an 
intervention of respective solder layers, making first and second welding 
electrodes in contact with one of the anode leads and the second portion 
of one of the cathode leads, respectively, making a third electrode in 
contact with the anode electrodes and the cathode conductor layers, 
pressing the first and second welding electrodes toward the third welding 
electrode to thereby press the one of the anode leads to a corresponding 
one of the anode electrodes and the first portion of the one of the 
cathode leads to a corresponding one of the cathode conductor layers, 
supplying welding current between the first and second welding electrodes 
and third welding electrode to connect the one of the anode leads to the 
corresponding one of the anode electrodes and the first portion of the one 
of the cathode leads to the corresponding one of the cathode conductor 
layers. 
In accordance with the method of the present invention, solid electrolytic 
capacitors having excellent characteristics and a high reliability can be 
fabricated with a high throughput and at a low ratio of the defective 
products. 
The above and other objects, features and advantages of the present 
invention will be more apparent from the following description, referring 
to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be described in detail with reference to the 
accompanying drawings, wherein similar constituent elements are designated 
by the same reference numerals throughout the drawings. 
FIGS. 3A to 3E show a capacitor in an actual process according to a first 
embodiment of the present invention. In FIG. 3A, the capacitor before 
connection comprised a capacitor body 11 made of tantalum particles and 
configured as a pellet, i.e., prism in this embodiment. An anode electrode 
12 was inserted to the pellet 11 during the shaping thereof and extended 
from the top surface of the pellet 11. The surface of the pellet 11 was 
covered by a tantalum oxide film (not shown), a semiconductor layer (not 
shown) and a cathode conductor layer 13 consecutively, with the cathode 
conductor layer 13 being the outermost film. 
The cathode conductor layer 13 contained an epoxy-resin based conductive 
material in an concentration more than about 60 weight percent of the 
total material. The epoxy-resin based material may include conductive 
fillers made of metallic particles such as silver or copper dispersed 
therein and capable of being soldered. The cathode conductor layer 13 had 
a thickness of approximately 50 mm. 
The side surface and bottom surface of the capacitor body 11 was then 
covered, as shown in FIG. 3B, by a solder layer 14 by immersing the 
capacitor body 11, in a batch process, into a melted solder having a high 
melting point. The solder layer 14 had a thickness of approximately 50 mm. 
A lead frame having a plurality of pairs of anode lead 15 and cathode lead 
16 was prepared beforehand for a plurality of capacitor bodies 11 by 
etching or punching a metallic thin plate. The anode lead 15 is straight, 
as shown in FIG. 3C, whereas the cathode lead 16 has an offset "H" between 
the first portion 16a for connection with the cathode conductor layer 14 
and the second portion 16b adjacent to the first portion 16a. The lead 
frame was positioned such that the anode lead 15 was in contact with the 
anode electrode 12 and the first portion 16a of the cathode lead 16 was in 
contact with the cathode conductor layer 14, as shown in FIG. 3C. The lead 
frame may be formed from 0.10-0.15 mm thick nickel silver or 42-alloy and 
has a resilient or spring function required for the following welding 
step. 
The anode lead 15 and cathode lead 16 in the lead frame were then 
electrically welded to the anode electrode 12 and cathode conductor layer 
13, respectively, as shown in FIG. 3D. The welding was effected 
simultaneously for the cathode side of the capacitor body 11 shown in FIG. 
3D and the anode side for the adjacent capacitor body not shown, by the 
current flowing between the top cathode electrode 17K and the bottom 
electrode 17C for welding and by the current flowing between the top anode 
electrode 17A and the bottom electrode 17C for welding, each top electrode 
17K or 17A sandwiching the welding portion of the cathode side or anode 
side in association with the common bottom electrode 11C. 
FIG. 4 shows the step of FIG. 3D for welding the anode and cathode leads of 
the lead frame 30 to the electrodes of the capacitor bodies. The top anode 
electrode 17A for welding the anode side is positioned at the anode side 
of one of the capacitor bodies, i.e., the second capacitor body 11b as 
viewed from the left in the figure. The top cathode electrode 17K for 
welding the cathode side is positioned at the cathode side of the third 
capacitor body 11c located at the right to the second capacitor body 11b. 
The offset positional relationship between the top electrodes 17A and 17K 
allows the simultaneous welding of the anode side of the second capacitor 
body 11b and the cathode side of the third capacitor body 11c without 
problem. 
After one-pitch movement of the top welding electrodes 17A and 17K in the 
left as shown by the arrow 31, the anode side of the first capacitor body 
11a and the cathode side of the second capacitor body 11b can be welded 
simultaneously, followed by subsequent one-pitch movement and next similar 
welding. Although, the connection by the present embodiment does not 
effect simultaneous welding of the anode side and cathode side of a single 
capacitor body, the presence of a large number of the capacitor bodies 
allows substantially a single welding step per a single capacitor body. 
The offset arrangement of the top anode electrode 17A and top cathode 
electrode 17K for welding also prevents the reduction in reliability of 
the resultant capacitors, which may otherwise occur due to the offset 
timing of the large welding voltage to a single capacitor body. 
Specifically, if the electric welding is simultaneously effected to the 
anode side and cathode side of a single capacitor body, an offset timing 
of application of the welding voltage which is likely to occur between the 
anode side and cathode side raises the electric field in the insulator 
film of the capacitor body to thereby damage the insulator film. Further, 
the positional relationship between the top welding electrodes and the 
bottom welding electrode allows a sufficient space for welding in a 
small-sized capacitor unlike the parallel gap welding. 
Referring back to FIG. 3D, the top cathode 17K and bottom electrode 17C are 
shown to be in contact with the cathode lead 16 and the cathode solder 
layer 14 of the capacitor body 11, respectively. The top cathode electrode 
17K for welding is in contact with the second portion of the cathode lead 
16 at the location about 1 mm apart from the edge of the capacitor body 
11. At this step, the anode electrode 15 of the capacitor body 11 under 
the welding for the cathode side is not in contact with the top anode 
electrode 17A for welding to prevent the excess voltage from being applied 
between the anode electrode 12 and the cathode conductor layer 13. 
An insulator layer 19 was inserted, between the bottom welding electrode 
17C and the second portion 16b of the cathode lead 16 onto which the top 
cathode electrode 17K for welding is pressed, before applying the pressing 
force and supplying the welding current. The pressing force of about 500 
gram was applied to the top cathode electrode 17K against the insulating 
layer 19 and bottom electrode 17C. Moved distance of 0.5 mm for the top 
cathode electrode 17K by the pressing force provided a suitable 100 to 200 
gram pressure to the capacitor body 11, which did not damage the tantalum 
oxide film of the capacitor body 11. 
Subsequently, a pulse welding current is provided from outside to generate 
a joule heat for melting the solder layer 14, thereby connecting the 
solder layer 14 with the cathode lead 16 electrically and mechanically. 
The welding current had a 30 to 40 ampere pulse amplitude and a 
1-millisecond pulse duration. The pulse welding current flowing between 
the anode electrode 12 and the anode lead 15 is similar to the pulse 
welding current for the cathode side. The length of time required for the 
welding was 0.3 to 1.0 seconds per a capacitor, including the length of 
time for lowering and raising the top welding electrodes 17K and 17A. In 
this embodiment, the cathode lead 16 is provided with a plurality of 
protrusions thereon having a 0.05 to 0.1 mm height at the location where 
the cathode lead 16 is in contact with the solder layer 14. 
A molding resin 20 is then provided to cover the entire surface of the 
capacitor body 11, allowing the free ends of the anode lead 15 and cathode 
lead 16 to protrude from the mold resin 20. The anode and cathode leads 15 
and 16 are cut to a suitable length and subjected to a bending operation 
to provide the final product of FIG. 3E. 
FIG. 5 shows flowcharts for showing a comparison of the length of time 
required to fabricate 1000 capacitors in the present embodiment and that 
in the conventional method using the conductive adhesive. Fabrication of 
the capacitor bodies, molding of the same and a final test are common to 
both the present embodiment and the conventional method. Accordingly, only 
the length of time for the connection step for connecting the leads and 
electrodes is shown for comparison, with the length of time for 
fabrication of the capacitor body, molding thereof and final test being 
omitted therein. 
The present embodiment requires a soldering step S1, in which 1000 
capacitor bodies are subjected to bath soldering in a batch process. The 
soldering step S1 requires about 0.1 A seconds per a capacitor, wherein 
"A" represents a length of time in second for a welding step S2 for 
welding both the anode sides and cathode sides for the 1000 capacitors. 
Accordingly, the present embodiment requires only 1.1 A seconds for 
fabrication of the 1000 capacitors. 
On the other hand, the conventional method includes an adhesive coating 
step S10 for connection of the cathode side which requires A seconds for 
1000 capacitors, a welding step S11 for anode connection which requires A 
seconds for the 1000 capacitors, and a hardening step S12 for hardening 
the adhesive which requires 0.3 A seconds for the 1000 capacitors, thereby 
requiring a total of 2.3 A seconds for the 1000 capacitors in the 
conventional method. Thus, the present embodiment achieves a reduction of 
the processing time by about a half of the processing time for the 
conventional method. 
Further, the present embodiment avoids an excess pressing force or pressure 
applied to the capacitor body during the welding. The present invention is 
compared against the conventional method using the welding electrodes 
which directly sandwich the capacitor body therebetween in terms of a 
ratio of defective products having a large leakage current to the total 
products. The conventional method exhibited a 1.0 to 10 percent of the 
defective products, whereas the present embodiment exhibited a ratio of 
the defective products below 0.2 percent due to the soft pressing force. 
Thus, the present embodiment achieved a reduction of the defective rate by 
below 1/50 compared to the conventional method, showing a remarkable 
advantage of the present invention. 
Although it is described that the present embodiment uses a soldering 
material having a high melting point, the soldering material may be an 
eutectic solder or a solder having a low melting point in the present 
invention. The protrusion as formed on the cathode lead may be omitted. 
FIGS. 6A to 6E show consecutive steps in a process for fabricating a 
capacitor according to a second embodiment of the present invention. 
The capacitor body was fabricated similarly to the first embodiment, as 
shown in FIG. 6A. A lead frame having a plurality of cathode leads 16 and 
anode leads are also prepared similarly to the first embodiment, and was 
subjected to application of a creaming solder 21, as shown in FIG. 6B, 
following which the creaming solder 21 was thermally melted and 
re-solidified to form a solder layer 22 on a first portion of the cathode 
lead 16 to be in contact with the cathode conductor layer 14 of the 
capacitor body 11, as shown in FIG. 6C. The solder layer 22 on the cathode 
lead 16 had a thickness of about 150 microns. If the creaming solder is 
conductive after application of the same onto the cathode lead 16, the 
melting and re-solidifying steps may be omitted accordingly. 
The cathode lead 16 was disposed such that the solder layer 22 on the 
cathode lead 16 is in contact with the cathode conductor layer 14 of the 
capacitor body 11, as shown in FIG. 6D. The capacitor body 11 had no 
solder layer on the cathode conductor layer 14. A top cathode electrode 
17K for welding is then pressed against a second portion of the cathode 
lead 16 which is apart from the edge of the capacitor body 11, similarly 
to the first embodiment, thereby electrically welding the cathode lead 16 
to the cathode conductor layer 14 simultaneously with welding for the 
anode side of an adjacent capacitor body. After welding both the cathode 
side and the anode side, the entirety of the capacitor body 11 is covered 
by a molding resin not shown, subjected to a bending step for the cathode 
and anode leads, similarly to the first embodiment, to provide a final 
product. 
In the second embodiment, the solder layer 14 formed on the cathode 
conductor layer 13 in the first embodiment is omitted and the soldering 
for the lead frame can be effected in a pre-treatment for the lead frame 
to thereby simplify the fabrication steps in the present embodiment. 
In both the embodiments, the anode and cathode leads were simultaneously 
welded to the electrodes of the capacitor body to save the length of time 
for the welding step. The welding step may be completed within a second, 
for example. 
The welding operation is effected by a pair of welding electrodes for each 
of the anode and cathode sides, the pair of welding electrodes being 
disposed opposite to each other in the direction perpendicular to the 
moving plane of the welding electrodes or the array of the capacitor 
bodies, which saves the space for the welding operation and can be applied 
to a small-sized capacitors having a small cathode lead. 
In the electric welding of the embodiments, the pressing force is not 
applied directly to the capacitor body, which prevents the insulator layer 
of the capacitor body from being damaged. 
Although the present invention is described with reference to preferred 
embodiments thereof, the present invention is not limited thereto and 
various modifications or alterations can be easily made from the 
embodiments by those skilled in the art without departing from the scope 
of the present invention as set forth in the appended claims.