Method for manufacturing multi-layer ceramic capacitor

A method for manufacturing multi-layer ceramic capacitors wherein the internal electrodes are formed by introducing molten metallic material into the void layers of a ceramic dielectric body. The method comprises of applying and baking paste of silver or platinum/silver on the opposite end surfaces of ceramic dielectric body to form first external electrodes and introducing molten metallic material into void layers through injection openings which are in communication with the void layers and open at side surfaces of the ceramic dielectric body to form internal electrodes. The ceramic dielectric body can be filled with molten metal for relative short time and under low pressure as compared with a prior art ceramic dielectric body having porous penetrable barriers.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for manufacturing multi-layer a 
ceramic capacitors wherein the internal electrodes are formed by 
introducing molten metallic material into the void layers formed in the 
ceramic dielectric body of the capacitor, and more particularly, to a 
method for manufacturing a multi-layer ceramic capacitor wherein after 
applying and baking the paste used for forming the external electrodes at 
the opposite end surfaces of the ceramic dielectric body, a molten 
metallic material is introduced into the void layers to form the internal 
electrodes. The molten material is injected through openings located at 
the side surfaces of the ceramic dielectric body. Further, these openings 
are in communication with the void layers in the ceramic dielectric body. 
In general, multi-layer ceramic capacitors have been in widespread use 
because their dielectric constant and electrostatic capacitance are 
relatively high compared to the other types of capacitors. 
FIG. 1 exemplifies a section of a prior-art multi-layer ceramic capacitor 
in which a noble metal is replaced with cheap base metal in order to 
reduce the cost associated with the manufacturing of the multi-layer 
ceramic capacitors. 
As illustrated by the drawing, the multi-layer ceramic capacitor has a 
plurality of internal electrodes 3 formed in void layers 2. The void 
layers are formed by stacking and sintering a plurality of thin ceramic 
dielectric sheets, each of which is printed with a carbon powder-based 
paste. A pair of external electrodes 4 and 4' are each connected to the 
prescribed internal electrodes 3 at opposite end surfaces of the 
multi-layer ceramic capacitor where the internal electrodes 3 are exposed 
to the outside. The term, "void layers," as used herein is intended to 
mean layers which are free or essentially free of dense ceramic material, 
and hence, are subject to being filled by molten metallic material to form 
the internal electrodes. 
The multi-layer ceramic capacitor described above is manufactured as 
follows. First, a paste, which is prepared by mixing ceramic powder, such 
as carbon, alumina or barium titanate (BaTiO.sub.3) powder with organic 
solvent and resin, is printed on each of the ceramic dielectric sheets 
where the internal electrodes 2 should be formed. Then, a plurality of 
ceramic dielectric sheets are stacked, fired at a temperature of 
500.degree. C. and kept at that temperature for a length of time 
sufficient to burn and remove the carbon powder, organic solvent and resin 
which are printed thereon. Thereafter, the stacked ceramic dielectric 
sheets are again heated to a temperature of 1100.degree.-1400.degree. C. 
and kept at this temperature for two hours prior to being cooled down. 
Thus, void layers 2 are formed in the ceramic dielectric body 1. In 
addition, crystal grains are formed in the ceramic dielectric body 1, 
giving it optimum sintering density and electrical properties. Although 
the void layers 2 prepared by the sintering procedure may vary in 
thickness due to viscosity of the paste and size of the mesh used in a 
screen printing procedure, the thickness of the void layers 2 are usually 
in a range of about 3-10 .mu.m. 
Thereafter, molten metallic material is introduced into the void layers 2 
of the ceramic dielectric body 1 to form internal electrodes 3. The molten 
metallic material primarily used for forming the internal electrodes 3 is 
Pb or an alloy thereof, or Sn or an alloy thereof. Pressure in a bath 
containing the molten metallic material should be kept at about 3.0 MPa in 
order to introduce the molten metallic material into, and completely fill, 
the void layers 2. The molten metallic material is then cooled down. 
After the molten metallic material in the void layers 2 is completely 
cooled, the external electrodes 4 and 4' are applied to each of the end 
surfaces of the ceramic dielectric body 1 and baked. By this process, the 
external electrodes are electrically connected to the internal electrodes 
3 where they are exposed to the outside. Thus, the prior art manufacturing 
process of the multi-layer ceramic capacitor is completed. 
However, a significant problem exists with the above-mentioned known 
manufacturing process for multi-layer ceramic capacitor. The problem is 
associated with the procedure for forming the external electrodes after 
the molten metallic material for forming internal electrodes is introduced 
into the void layers. 
As the molten material used to form the internal electrodes cools down, it 
tends to shrink. This shrinkage causes the molten metal to recede from the 
end surfaces of the capacitor, thereby creating a gap between the end 
surfaces of the capacitor and the outermost end surfaces. The gap makes it 
difficult for the metallic paste used to form the external electrodes to 
be electrically connected to the metal which forms the internal electrodes 
in the void layers. 
Furthermore, in order for the external electrodes to adhere to the ceramic 
dielectric body, it is necessary to bake the metallic paste used to form 
the external electrodes at a temperature of 600.degree.-800.degree. C. 
Since 600.degree.-800.degree. C. is higher than the melting point of the 
metal components used to form the internal electrodes, it is impossible to 
prevent the outward leaking of the metal components of the internal 
electrodes because they are again melted during the baking procedure. 
An attempt to solve the leakage problem of the metal components in the void 
layers is disclosed in the U.S. Pat. No. 4,584,629. In the patent, the 
ceramic dielectric body is subjected to a sputtering or a plating 
procedure to form a metallic film at each of the end surfaces thereof 
before the molten metallic material for forming internal electrodes is 
introduced into the void layers. This prevents the molten metallic 
material introduced into the void layers from leaking outwardly and 
thereby, improves quality of connection between the internal and the 
external electrodes. 
However, the above-mentioned method for forming metallic film requires 
complicated procedures such as a vapor deposition or a plating procedure. 
These procedures make it necessary to mask the ceramic dielectric body in 
order to control the formation of the metallic film only at both end 
surfaces of the dielectric body, thereby causing the manufacturing process 
to be cumbersome. 
In addition, it is necessary to heat the paste for forming the external 
electrodes at a temperature higher than 600.degree. C. or above, which is 
higher than the melting point of the molten metal for internal electrodes. 
Thus, it is impossible to prevent the molten metallic material used for 
forming the internal electrodes from leaking outwardly since it will be 
melted by the high baking temperature. 
Also, further attempts to overcome the above problems are disclosed in U.S. 
Pat. Nos. 4,071,880 and 4,652,967. In the above patents, porous penetrable 
barriers are applied to both end surfaces of the ceramic dielectric body 
where the openings which are in communication with the void layers are 
located. The porous penetrable barriers are comprised of either metal or 
ceramic. After the porous penetrable barriers are applied, a molten 
metallic material is introduced into the void layers through the porous 
penetrable barriers to form the internal electrodes. 
If metal porous penetrable barriers are used, they may be used as a part of 
the external electrodes. On the other hand, if ceramic barriers are used, 
they must be ground away until the internal electrodes are exposed. After 
which, the external electrodes are applied to the ground barriers. 
However, there are problems associated with the usage of either the ceramic 
or metal porous barriers. In the case where ceramic porous penetrable 
barriers are used to form the external electrodes, it is difficult to 
grind them due to minute size of the ceramic capacitor. 
On the other hand, it is substantially more difficult to introduce the 
molten metallic material into the void layers through the metal porous 
penetrable barriers since they are significantly less permeable compared 
to their ceramic counterparts. Consequently, injecting pressure and 
injecting time of the molten metallic material must be increased in order 
to sufficiently introduce the molten metal into the void layers if metal 
barriers are used. 
However, increasing the injecting pressure and injecting time of the molten 
metallic material used to form the internal electrodes causes other 
problems. Due to the increased pressure and time, the molten metallic 
material introduced into the void layers also permeates the ceramic 
dielectric body through defects of surfaces of the void layers so that 
essential thicknesses of the ceramic dielectric layers between the 
internal electrodes become small. As a result, internal insulation 
resistance of the ceramic capacitor is decreased while the potential for 
shorts of the internal electrodes is increased. 
Also, the combination of using metal porous penetrable barriers comprised 
of silver or platinum/silver as external electrodes, and lead or an alloy 
presents a problem. Because injection pressure and time of the molten 
metal must be increased, the silver in the porous barriers is subjected to 
leaching and removed by the molten lead. As a result, the electrical 
conductivity of the metal porous penetrable barriers is substantially 
decreased, thereby causing functions of the metal barriers as external 
electrodes to be lost. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above-described problems 
occurring in the prior art methods for manufacturing ceramic capacitors. 
An object of the invention is to provide a method for manufacturing 
ceramic capacitors in which injection openings are formed to be in 
communication with the void layers of a dielectric ceramic body used to 
produce the internal electrodes so that both the injection pressure and 
injection time of the subsequent injection process are substantially 
decreased. 
In accordance with one aspect, the present invention provides a method for 
manufacturing multi-layer ceramic capacitor in which the dielectric body 
layers is formed by stacking a plurality of dielectric sheets. The void 
layers with injection openings in which the internal electrodes are to be 
formed are produced by applying a carbon combustible paste on each of the 
dielectric sheets prior to being stacked together. The stacked dielectric 
sheets are then sintered to produce a dielectric body with injection 
openings at both end surfaces which are in communication with the void 
layers. 
The electrodes of the multi-layer ceramic capacitor are formed in the 
following manner. The first set of external electrodes are formed by 
applying a paste to both end surfaces and then baked. The internal 
electrodes are formed by introducing a molten metal material into the void 
layers through the injection openings. The second set of external 
electrodes are formed by applying Ni or an alloy thereof onto the first 
set of electrodes, thus producing a multi-layer ceramic capacitor. 
In accordance with another aspect, the present invention provides a method 
for manufacturing a multi-layer ceramic capacitor in which the dielectric 
body layers is formed by stacking a plurality of dielectric sheets. The 
void layers with injection openings in which the internal electrodes are 
to be formed are produced by applying a carbon combustible paste on each 
of the dielectric sheets prior to being stacked together. The stacked 
dielectric sheets are then sintered to produce a dielectric body with 
injection openings at both end surfaces which are in communication with 
the void layers. 
The electrodes of the multi-layer ceramic capacitor are formed in the 
following manner. The first set of external electrodes are formed by 
applying silver or platinum/silver paste to both end surfaces and then 
baked. The second set of external electrodes are formed by applying Ni or 
an alloy thereof onto the first set of electrodes by plating, vapor 
deposition or paste baking procedure. The internal electrodes are formed 
by introducing a molten metal material into the void layers through the 
injection openings. Thus a multi-layer ceramic capacitor is produced. 
These and other objects, features and advantages of the invention will 
become more apparent upon a reading of the following detailed 
specification and drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A method for manufacturing multi-layer ceramic capacitors according to the 
present invention will now be described by referring to FIGS. 2A through 
2E in the accompanying drawings. 
As shown in FIG. 2A, the ceramic body of the capacitor is formed by 
stacking together a plurality of dielectric sheets 5, each having a 
thickness of 20-100 .mu.m. The void layers in the dielectric body are 
formed by applying a carbon paste 6 onto each of the dielectric sheets by 
a screen printing or a painting procedure (See FIGS. 2B to 2C). The carbon 
paste used to form the void layers consist of carbon powder and 5-50 wt. % 
alumina or barium titanate (BaTiO.sup.3) mixed with organic solvent and 
resin. 
Furthermore, only one end edge of the carbon paste 6 is applied up to the 
end portion of the dielectric sheet 5 while the other three edges are 
positioned within the interior of the dielectric sheet 5. Also, a portion 
of the carbon paste 6 is extended to one of the side edges of the ceramic 
sheets 5 to form projection 7, where the injection opening is to be 
formed. The length 1 of projection 7 is preferred to be 1/100-1/10 of the 
length L of the carbon paste 6. 
The dielectric sheets 5 with the paste 6 are then stacked one-by-one so 
that the exposed edge portion of the paste 6 are positioned alternately in 
the dielectric layers. In addition, the carbon paste faces upwardly and 
the projections 7 of the adjacent carbon sheets 6 are positioned in 
opposite directions of each other. 
FIGS. 2B and 2C represent a ceramic dielectric body in which the ceramic 
sheets are fired and sintered. After the ceramic dielectric body 1, as 
mentioned above, is heated to a temperature of 500.degree. C. or less, it 
is then fired to burn and remove the organic material in the carbon paste 
6. The result of which is the formation of void layers 2 with injection 
openings 8 in the ceramic dielectric body 1 
Next, the fired ceramic dielectric body 1 is sintered at a sintering 
temperature of 1100.degree.-1400.degree. C. so that fine ceramic crystal 
grains are formed on the ceramic dielectric body 1, thereby giving it 
excellent electrical property. 
FIG. 2D shows a ceramic dielectric body on which the first external 
electrodes are applied. After the ceramic dielectric body 1 is sintered, a 
paste made of silver, platinum/silver or the like is then applied onto 
both end surfaces. After which, the ceramic body is baked, thus, forming 
the first external electrodes 4a and 4a' which serve as a part of external 
electrodes. 
FIG. 2E, represents a ceramic dielectric body in which molten metal for 
forming internal electrodes is introduce. The ceramic dielectric body 1 on 
which the first external electrodes 4a and 4a' have been applied is dipped 
into a bath containing molten Pb or an alloy thereof, or Sn or an alloy 
thereof so that the molten metal is introduced into the void layers 2 
through the injection openings 8. As the molten metal completely occupies 
the void layers 2, it becomes coupled to the inner surfaces of the first 
external electrodes 4a and 4a'. 
After the void layers 2 have been filled with the molten metal as mentioned 
above, Ni or an alloy thereof is coupled to the first external electrodes 
4a and 4a' to form the second set of electrodes. Ni or an alloy thereof is 
used because it can easily be soldered. Further, Ni or an alloy thereof 
are not easily subject to oxidation. The second set of electrodes 4b and 
4b' is formed on the first set of electrodes by either applying a paste, 
metal vapor deposition, electro-plating or electroless plating procedure. 
Therefore, with a series of procedures described above, a multi-layer 
ceramic capacitor according to the present invention is finished. 
The present invention described hereinbefore relates to the preferred 
embodiment of the method for manufacturing multi-layer ceramic capacitors. 
Although in the above-described method, the second external electrodes of 
layers of Ni or an alloy thereof are formed onto the first external 
electrodes only after the molten metal for forming the internal electrodes 
has been injected into the void layers, it should also be understood that 
the present invention can be carried out such that the second external 
electrodes of Ni or an alloy thereof are formed before the molten metal is 
introduced into the void layers through the injection openings to form the 
internal electrodes. 
As apparent from the above description, the multi-layer ceramic capacitor 
manufactured according to the present invention has injection openings 
through which molten metal is injected into the void openings to form the 
internal electrodes. By having injection openings, both the injection time 
and injection pressure of the molten metal bath are substantially 
decreased. 
To exemplify this, an experiment comparing the electrostatic capacitance of 
a prior art capacitor having porous penetrable barriers instead of 
injection openings with one which is made according to the present 
invention. Two different tests were conducted. In the first test, both 
capacitors were dipped in a molten lead bath for one minute with an 
injection pressure of 0.5 MPa, while in the second test, an injection 
pressure of 1.5 MPa was used. The measured electrostatic capacitance of 
both capacitors are given in Table 1. 
TABLE 1 
______________________________________ 
Injection 
Pressure Electrostatic Capacitance (nF) 
(MPa) Present Invention 
Prior Art 
______________________________________ 
0.5 45.0 4.5 
1.5 45.0 45.0 
______________________________________ 
As apparent from Table 1, the electrostatic capacitance of a ceramic 
capacitor with its void layers completely filled with molten metallic 
material is 45.0 nF. Thus, a multi-layer ceramic capacitor with metal 
porous penetrable barriers requires an injection pressure of 1.5 MPa to 
completely fill the void layers with the molten metallic material. On the 
other hand, only an injection pressure of 0.5 MPa is required to 
completely fill the void layers of a multi-layer ceramic capacitor made 
according to the invention. Also, the experiment shows that if an 
injection pressure of 0.5 MPa was used, it would require at least 50 
minutes to completely introduce the molten metal into the void layers of a 
prior art multi-layer ceramic capacitor. 
Therefore, it is appreciated that the injection time and injection pressure 
required under the present invention is substantially less compared to 
that of a prior art ceramic dielectric body having porous penetrable 
barriers. 
In addition, the present invention has an another advantage. The 
connections between the molten metal and the first external electrodes (or 
external electrodes) are entirely maintained after solidification of the 
molten metal. This is due to the fact the first external electrodes had 
been formed prior to the introduction of the molten metals. Thus, the 
subsequent introduction of the molten metal into the void layers creates a 
perfect connection to the inner surfaces of the first external electrodes. 
Also, there is an advantage of this invention if the second electrode are 
applied by electro-plating, electroless plating or vapor deposition 
procedure. The advantage is that it is possible to select the proper metal 
for forming the internal electrodes regardless of the baking temperature 
of external electrodes due to the fact that the molten metal for internal 
electrodes is introduced after the external electrodes are baked. 
However, if the metal selected for the internal electrodes has a lower 
melting point which is lower than the temperature for soldering or 
mounting the multi-layer ceramic capacitor manufactured by the method 
according to the invention on circuit boards, then the internal electrodes 
will be melted and will leak outwardly through the injection openings when 
the multi-layer ceramic capacitor is used in practice. Therefore, it is 
preferable to select metals for forming internal electrodes which have a 
melting point greater than the temperature for soldering or mounting the 
capacitor on circuit boards. Typically, this means metals having a melting 
point greater than 250.degree. C. These metals include Pb or an alloy 
thereof, or Sn alloy, such as Sn-Cu. 
Although the preferred embodiments of the invention have been disclosed for 
illustrative purpose, those skilled in the art will appreciate that 
various modifications, additions and substitutions are possible without 
departing from the scope and spirit of the invention as disclosed in the 
accompanying claims.