Method of manufacturing ceramic laminate

Abstract: Disclosed is a method of manufacturing a ceramic laminate which is adapted to manufacture a laminated ceramic capacitor, for example. This method comprises the steps of stacking a plurality of ceramic green sheets (41) containing ceramic powder and a first binder on a base and forming a metal paste film (42) containing metal powder and a second binder for providing an internal electrode on an upper major surface of a prescribed one of the ceramic green sheets (41) during the stacking step. In such a method, a junction member (47) containing a solvent which can commonly dissolve the first and second binders is prepared in order to join the plurality of ceramic green sheets with no application of pressure, to be applied between adjacent pairs of the plurality of ceramic green sheets (41), which are stacked with each other. The junction member (47) joins the adjacent pairs of ceramic green sheets with each other through chemical function.

Description: The present invention relates to a method of manufacturing a ceramic laminate. BACKGROUND OF THE INVENTION A component such as a laminated ceramic capacitor or a ceramic multilayer substrate, for example, having laminated structure of ceramic sheets provided with an internal electrode which is interposed in at least one interface, has been generally obtained by applying ceramic slurry on one surface of a carrier film through a doctor blade method or the like and drying the same, separating a resulting ceramic green sheet from the carrier film, printing metal paste on the ceramic green sheet by screen printing or the like and drying the same, stacking a desired number of such ceramic green sheets to obtain a laminate, and pressurizing the laminate along the direction of stacking under appropriate conditions. This laminate is cut if necessary, and thereafter fired to obtain a sintered ceramic laminate. Fig. 11 is a sectional view showing a ceramic multilayer substrate 1 which is obtained basically through the aforementioned steps. Referring to Fig. 11, the substrate 1 comprises a plurality of ceramic layers 2 to 9 and internal electrodes 10 to 14 which are provided in interfaces between specific ones of the ceramic layers 2 to 9. A plurality of conductor films 16 to 18 are formed on one major surface 15 of the substrate 1. The conductor film 16 is electrically connected with the internal electrode 10 through an internal hole 19 passing through the ceramic layer 2. Further, two through hole connecting portions 20 and 21, for example, are defined to pass through the substrate 1, so that the through hole connecting portion 20 electrically connects the conductor film 17 and the internal electrodes 10, 12 and 14 with each other while the other through hole connecting portion 21 electrically connects the conductor film 18 and the internal electrodes 11 and 13 with each other. When the ceramic multilayer substrate 1 shown in Fig. 11 is obtained by the aforementioned method, however, the following problems are caused: A laminate of ceramic green sheets prepared to obtain the laminated structure of the ceramic layers 2 to 9 is pressurized in a stage before firing, as hereinabove described. Relatively high pressure is applied for such pressurization, to easily cause distortion of the green ceramic laminate, including metal paste films for providing the internal electrodes 10 to 14. In general, such distortion is so ununiformly caused that it is difficult to obtain the green ceramic laminate, including the metal paste films, in designed dimensions. Thus, such a green ceramic laminate frequently deviates from the designed dimensions, to cause inferiority in yield. Ununiform distortion of the green ceramic laminate in the aforementioned pressurizing step causes a significant problem particularly in the ceramic multilayer substrate 1 shown in Fig. 11, for example, which requires high position accuracy for the internal electrodes 10 to 14, the internal hole 19 and the like. Further, dried ceramic green sheets for providing the ceramic layers 2 to 9 are basically different in material composition from dried metal paste films for providing the internal electrodes 10 to 14. Sufficient junction strength cannot be attained by compressing such members of basically different materials under mechanical pressure, and hence a finished product obtained upon firing is reduced in rupture strength and resistance against thermal shock. Even delamination may result in an extreme case. In order to obtain large capacitance in a laminated ceramic capacitor, for example, a ceramic layer located between each pair of internal electrodes is most typically reduced in thickness. Referring to Fig. 12, a ceramic green sheet 22 is so thinned that its physical thickness 23 is substantially equal to physical thickness 25 of a metal paste film 24 upon drying. When such ceramic green sheets 22 are stacked with each other, the thickness 25 of the metal paste film 24 partially formed on one major surface of each ceramic green sheet 22 cannot be neglected such that, as shown in Fig. 13, relatively large stress remains in portions 27 and 28 corresponding to edges of the metal paste films 24 upon pressurization of a laminate 26 of the ceramic green sheets 22. Such stress causes delamination or insufficient resistance against thermal shock after firing, to restrict reduction in thickness of the ceramic layers for obtaining large capacitance. In the case of manufacturing a laminated ceramic capacitor, for example, metal paste films for providing internal electrodes are generally printed on ceramic green sheets and dried, and then the ceramic green sheets are stacked with each other in an aligned state. However, when such ceramic green sheets are reduced in thickness as hereinabove described, mechanical strength thereof is also reduced. Thus, it has been extremely difficult to register the ceramic green sheets in response to requirements in steps of printing the metal paste films, stacking the ceramic green sheets and the like. Even if carrier films are employed for holding thin ceramic green sheets in such registration in order to compensate for mechanical strength, complicated and high-priced equipment is required for a registration mechanism, in order to handle basically thin substances. Figs. 14 and 15 show the so-called print lamination method, which is adapted to solve the aforementioned problem. This method basically repeats steps shown in Figs. 14 and 15. Referring to Fig. 14, for example, a squeegee 31 is driven along the arrow to act on a metal paste member 30 which is placed on a screen 29, thereby to form a metal paste film 33 for providing an internal electrode. Then, as shown in Fig. 15, another squeegee 36 is driven along the arrow to act on a ceramic slurry member 35 which is placed on a screen 34, thereby to form a green ceramic layer 37 for covering the metal paste film 33. Respective ones of such ceramic slurry members 35 and metal paste members 30 are repeatedly printed and dried to obtain a desired laminate. However, the aforementioned print lamination method has the following problems: First, a green ceramic layer formed by printing is higher in degree of defectiveness than a sheet formed through casting by the doctor blade method or the like. Thus, it is necessary to reduce the degree of defectiveness by repeating the printing step a plurality of times particularly for forming a green ceramic layer to be held between internal electrodes. This leads to reduction in productivity. Further, thickness of a ceramic layer located between internal electrodes must be adjusted by merely controlling printing conditions. Such control is relatively difficult in practice. In addition, the thickness of the ceramic layer cannot be easily managed if printing must be repeated a plurality of times in order to obtain one ceramic layer as hereinabove described. Thus, capacitance deviates from a designed value, to cause reduction in yield. In order to attain sufficient mechanical strength required for a laminated ceramic capacitor, for example, upper and lower portions of a laminate must be covered with ceramic outer layers having no internal electrodes. However, thickness of a ceramic layer which can be formed by screen printing is several to tens of micrometers at the most. Thus, if hundreds of micrometers are required for each of the upper and lower outer layers, the number of printing times is extremely increased to cause reduction in productivity. In both of the aforementioned methods of forming ceramic green sheets through casting by the doctor blade method or the like and the method of forming green ceramic layers by the print lamination method, pores and pinholes may be defined in the ceramic layers upon reduction in thickness. Thus, a capacitor may be reduced in voltage resistance, for example, or a short may be caused across internal electrodes, in an extreme case. US-A-4008514 and IBM Technical Disclosure Bulletin, vol. 18, no. 6, Nov. 1985, pp. 1857-1858 were cited by the European Patent Office during examination. The US patent describes a method of making a ceramic capacitor in which method a first pattern of electrodes is defined on a release surface, a thin layer of green ceramic is cast on the pattern and a second pattern of electrodes is defined on the other surface of the ceramic layer. A thicker ceramic layer is then adhered to the thin strip. The IBM document similarly describes a method of manufacturing a multilayer ceramic in which ceramic layers are bonded together by way of a bonding ply. SUMMARY OF THE INVENTION The present invention aims to provide a method of manufacturing a ceramic laminate, and more specifically, to provide a method of manufacturing a laminate by stacking ceramic green sheets, which can advantageously solve the aforementioned problems. According to the invention there is provided a method of manufacturing a ceramic laminate, the method comprising: stacking a plurality of ceramic green sheets each comprising a ceramic powder and a first binder, a plurality of junction members, and a plurality of metal paste films each comprising a metal powder and a second binder, the junction members being stacked between adjacent green sheets and between adjacent green sheets including metal paste films formed in prescribed patterns on one of the green sheet major surfaces by printing, and thereby forming a laminate, characterised in that the junction member comprises a solvent which commonly dissolves said first binder and said second binder. The plurality of ceramic green sheets containing the ceramic powder and the first binder are not necessarily of the same materials with respect to the ceramic powder and the binder, but may be prepared from different materials. The present invention enables a plurality of ceramic green sheets to be stacked to obtain a laminate through chemical junction by a junction member applied between adjacent pairs of the ceramic green sheets, with application of no high pressure. Thus, the laminate is protected against ununiform distortion, thereby to prevent reduction in yield caused by deviation in capacitance etc. The junction member contains a solvent which can dissolve both of the first binder contained in the ceramic green sheets and the second binder contained in the metal paste films, thereby to increase junction strength between the ceramic green sheets themselves and between the ceramic green sheets and the metal paste films. Thus, rupture strength after firing is improved, while resistance against thermal shock is enhanced. Further, defectiveness such as delamination is reduced. The junction member can contain ceramic slurry, in order to fill up pores and pinholes upon application to the ceramic green sheets. Thus, ceramic layers located between internal electrodes, for example, which are obtained from the metal paste films, can be reduced in thickness with no reduction in voltage resistance and no short across the internal electrodes, thereby to obtain a small-sized capacitor having large capacitance. Further, also advantageously solved is the problem that thickness of a ceramic green sheet is reduced in a laminated ceramic capacitor, for example, to be substantially identical to that of a metal paste film for providing an internal electrode such that the thickness of the metal paste film partially formed on the ceramic green sheet for providing the internal electrode cannot be neglected to result in residual stress, causing delamination and reduction in resistance against thermal shock, around edges of the metal paste film upon pressurization of the laminate. This problem is solved because there is no need to positively pressurize the laminate under high pressure, while the junction member can advantageously absorb difference in thickness between a part of the ceramic green sheet provided with the metal paste film and the remaining part at need. Although a large amount of binder is generally required in the prior art to adhere metal paste films to ceramic green sheets in a step of pressurizing an unfired laminate, the amount of such a binder can be reduced, thereby to reduce thickness of the metal paste films in response. This feature of the present invention is also effective to solve the aforementioned problem concerning difference in thickness between parts of the ceramic green sheets caused by formation of the metal paste films. Further, although a large amount of plasticizer must be generally contained in ceramic green sheets which are subjected to a pressurizing step as in the prior art, the amount of such a plasticizer can be reduced, whereby the ceramic green sheets can be increased in thickness and reduced in contraction upon drying, to improve the quality of a finished product. No pressurizing step is required, unlike the prior art, whereby the cost required for such a pressurizing step can be saved to extremely reduce the manufacturing cost. Further, the metal paste film is formed on the ceramic green sheet by printing in a state positioned through the base, whereby no misregistration of the metal paste film is caused by a registration error in stacking of the ceramic green sheets while no high accuracy is required for alignment of the stacked ceramic green sheets. Thus, a positioning mechanism employed for stacking the ceramic green sheets may be simplified in structure, to require no high-priced equipment. The above and other features of the invention are set forth with particularity in the appended claims and together with other objects, aspects and advantages thereof will become more apparent from consideration of the following detailed description of an embodiment of the invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 7 are enlarged sectional views sequentially showing typical steps included in an embodiment of a method of manufacturing a laminated ceramic capacitor according to the present invention; Figs. 8 and 9 are enlarged sectional views for illustrating another embodiment of the present invention; Fig. 10 is an enlarged sectional view for illustrating still another embodiment of the present invention; Fig. 11 is a sectional view illustrating a ceramic multilayer substrate 1, which is a component employing a ceramic laminate obtained by the present invention; Figs. 12 and 13 are diagrams for explaining problems of a first prior art example for the present invention; and Figs. 14 and 15 are diagrams for explaining a second prior art example for the present invention. DESCRIPTION OF THE EMBODIMENTS Methods of manufacturing a laminated ceramic capacitor embodying the invention will now be described in detail. As shown in Fig. 1, ceramic slurry is applied to one surface of a carrier film 40 by the doctor blade method or the like to form a ceramic green sheet 41, which is thereafter dried. As shown in Fig. 2, a base 43 is prepared to be employed for carrying out a stacking step. The base 43 comprises a rigid plate 44, which is adapted to facilitate location of substances to be stacked in the later stacking step. In order to repeat heating operation for drying and cooling operation in the stacking step, the rigid plate 44 is preferably formed of a material having a small thermal expansion coefficient, such as super invar of nickel steel, whose thermal expansion coefficient is about 1/100 of that of stainless steel. Alternatively, the rigid plate 44 may be formed of glass, ceramic or the like. An elastic layer 45 of silicone rubber, for example, is formed on the rigid plate 44, in order to absorb irregularity of the rigid plate 44 in the stacking step. This elastic layer 45 has thickness of about 1 to 5 mm, and hardness of about 40 to 80. A resin film 46 of polyethylene telephthalate, for example, is formed on the elastic layer 45. The stacking step is carried out upon preparation of the base 43. In this embodiment, the ceramic green sheet 41 contains barium titanate ceramic powder containing about 13 percent by weight of an acrylic binder, and is formed into a sheet by employing ceramic slurry which is prepared by dissolving such a material in a delay solvent such as BCA (butyl carbitol acetate). It has been confirmed that the following stacking step can be similarly carried out by employing another ceramic material such as an alumina ceramic material or by employing another binder such as a cellulose or vinyl binder. First, a junction member 47 is applied on the base 43 by screen printing, pad printing, the doctor blade method or the like, as shown in Fig. 3. This junction member 47 is prepared from slurry having the same raw material system and the same binder system as the ceramic green sheet 41. Then, the ceramic green sheet 41 prepared as shown in Fig. 1 is stacked on the junction member 47, to contain no air. Then the carrier film 40 is separated from the ceramic green sheet 41. The carrier film 40 may be separated before stacking of the ceramic green sheet 41. Thereafter the junction member 47, the base 43 and the ceramic green sheet 41 are dried at a temperature of 150°C for about two minutes, for example. The dried substance is then cooled. The aforementioned step is repeated by a desired number of times, to obtain a lower outer layer block 48 of ceramic green sheets 41 provided with no metal paste films 42, as shown in Fig. 4. Then, the base 43 holding the outer layer block 48 shown in Fig. 4 is positioned in a screen printer. In such a positioned state, metal paste films 42 are formed in prescribed patterns on an upwardly directed major surface of the uppermost ceramic green sheet 41, as shown in Fig. 5. The metal paste films 42 are formed by driving a squeegee 56 against a metal paste member 55, which is placed on a screen 54, along arrow. The method of printing the metal paste films 42 is not restricted to screen printing, but another method, such as pad printing, for example, may be employed. The metal paste films 42 thus printed are dried and then cooled. A drying step similar to that for the metal paste films 42 is also applied to the junction member 47, as hereinabove described. Thus, the junction member 47 may be dried simultaneously with the metal paste films 42 in the same step. In relation to this, the metal paste films 42 are preferably successively dried upon formation in advance of subsequent processing so that the patterns thereof are not damaged, while the junction member 47 can be dried in an absolutely arbitrary stage. Then, as shown in Fig. 6, another junction member 47 is applied over the entire surface of the outer layer block 48, which is provided with the metal paste films 42 shown in Fig. 5. This junction member 47 is similar in application method and composition to the junction member 47 described with reference to Fig. 3. Then, another ceramic green sheet 41 shown in Fig. 1 is stacked on the junction member 47, to contain no air. Thereafter the carrier film 40 is separated. The ceramic green sheet 41 shown in Fig. 6 may be stacked after separating the carrier film 40. The step of forming the metal paste films 42 shown in Fig. 5 and that of applying the junction member 47 and stacking the ceramic green sheet 41 shown in Fig. 6 are repeated by desired number of times. No high accuracy is required for stacking the ceramic green sheets 41 on those printed with the metal paste films 42. In other words, the ceramic green sheets 41 may be stacked in such position accuracy that subsequently printed metal paste films 42 are not spread beyond the margins of precedingly provided ceramic green sheets 41. Further, while the junction member 47 may be dried in an arbitrary stage as hereinabove described, it is efficient and preferable to dry each junction member 47 simultaneously with metal paste films 42 which are formed on a ceramic green sheet 41 subsequently stacked on the said junction member 47. Then, steps similar to those described with reference to Figs. 3 and 4 are carried out in order to form an upper outer layer block 49 as shown in Fig. 7. Thus, a green ceramic laminate 50 is obtained, to be subjected to a final drying step at a temperature of 100 to 120°C for about 24 to 48 hours, for example. The final drying step is adapted to dry solvents contained in the green ceramic laminate 50 as completely as possible. Junction members 47 included in the upper outer layer block 49 shown in Fig. 7 may be simultaneously dried in the aforementioned final drying step, in place of successive drying. Thereafter the laminate 50 is cut to provide a chip having prescribed dimensions, calcined and subjected to firing and then provided with external electrodes, thereby to provide a desired laminated ceramic capacitor. The aforementioned final drying step may be carried out at the same time in a calcination condition upon cutting into the chip form. In the aforementioned embodiment, the thickness of the ceramic green sheet 41 in a green state may be about 5 to 30 µm in practice. The junction member 47 may be prepared from slurry which is identical in raw material system to the ceramic green sheet 41 but is different in binder system from the same. When an aqueous binder is employed for the ceramic green sheet 41, for example, the junction member 47 may be prepared from slurry containing an organic binder. In the above manufacturing method for obtaining a laminated ceramic capacitor, the ceramic green sheets 41 contained in the laminate 50 are adapted to provide dielectric members, for example. On the other hand, ceramic green sheets employed for a laminated LC composite component, for example, are not identical in material to each other but prepared from different materials to provide dielectric sheets and magnetic sheets. Also in this case, a junction member may be prepared from a material containing a solvent which can commonly dissolve binders contained in the respective ones of such sheets of different materials. The slurry employed for the junction member is generally formed of ceramic powder, a binder, a plasticizer and a solvent. Within these materials, the solvent must be capable of commonly dissolving the binders contained in the ceramic green sheets and the metal paste films, while the materials are preferably in such ratios that the slurry can be thinly applied as uniform as possible to be easily diffused and evaporated in a drying step, so far as the ceramic green sheets can be joined with each other through dissolution of the binders. If such condition is satisfied, the junction member can be prepared only from a solvent. In the step shown in Fig. 6, the junction member 47 for joining the ceramic green sheets 41 with each other through the metal paste films 42 is applied over the entire surface of each ceramic green sheet 41. It has been confirmed by an experiment that, if such a junction member 47 is applied by screen printing, about 50 % of physical thickness of the dried metal paste films 42 appears as difference from that of parts of the ceramic green sheet 41 provided with no metal paste films 42. Thus, when the ceramic green sheets 41 are reduced in thickness or the number of stacking is increased, the outer surface of a finished product may be bulged or stress may be caused by contraction in drying and firing to remain around edges of the metal paste films 42 or the internal electrodes, to exert bad influence on the quality of the product. A method shown in Figs. 8 and 9 is employed in order to avoid such inconvenience. Referring to Fig. 8, a junction member 47 is partially applied on a first ceramic green sheet 41. That is, the junction member 47 is applied on a region except for that provided with a metal paste film 42. Then, a second ceramic green sheet 41 is stacked and a pressurizing member 51 applies pressure on the second ceramic green sheet 41 as shown in Fig. 9, so that a part of the junction member 47 enters an interface between the metal paste film 42 and the first ceramic green sheet 41. Thus, difference in thickness caused by the metal paste film 42 can be effectively absorbed by the junction member 47. Fig. 10 shows still another embodiment of the present invention, in which a junction member 47 is applied to a second ceramic green sheets 41, which is to be stacked on/joined to a first ceramic green sheet 41. Junction members may be applied to both of ceramic green sheets to be joined with each other. While the present invention has been described with reference to the drawings mainly on the case of obtaining a laminated ceramic capacitor, the present invention is widely applicable to various components comprising ceramic laminates, such as a laminated LC composite part, a ceramic multilayer substrate and the like.

Claims: A method of manufacturing a ceramic laminate, the method comprising: stacking a plurality of ceramic green sheets (41) each comprising a ceramic powder and a first binder, a plurality of junction members (47), and a plurality of metal paste films (42) each comprising a metal powder and a second binder, the junction members (47) being stacked between adjacent green sheets (41) and between adjacent green sheets including metal paste films (42) formed in prescribed patterns on one of the green sheet major surfaces by printing, and thereby forming a laminate, characterised in that the junction members comprise a solvent which commonly dissolves said first binder and said second binder. A method as claimed in claim 1, comprising: preparing a base (43) for receiving the stacked green sheets, junction members and metal films. A method as claimed in claim 2, wherein said base is prepared by applying an elastic layer (45) to a rigid plate (44). A method as claimed in claim 3, wherein said base is prepared by applying a resin film (46) to the elastic layer (45). A method as claimed in any preceding claim, wherein a plurality of ceramic green sheets and junction members are stacked together to form a block (48) for stacking on said laminate. A method as claimed in any preceding claim, wherein a junction member is applied to each ceramic green sheet before said green sheets are stacked on said laminate. A method as claimed in any preceding claim, further comprising preparing each ceramic green sheet (41) on a respective carrier (40). A method as claimed in claim 7, further comprising removing said carrier from said green sheet before each green sheet is stacked on said laminate. A method as claimed in claim 7, further comprising removing said carrier from said green sheet after each green sheet is stacked on said laminate. A method as claimed in any preceding claim, comprising: forming the junction members from a ceramic slurry comprising a second ceramic powder and a third binder in addition to said solvent. A method as claimed in claim 10, wherein said second ceramic powder is the same as said first ceramic powder. A method as claimed in claim 10, wherein said second ceramic powder is different to said first ceramic powder. A method as claimed in claim 10, 11 or 12, wherein said third binder is the same as said first binder. A method as claimed in claim 10, 11 or 12, wherein said third binder is different to said first binder. A method as claimed in any preceding claim, wherein said plurality of ceramic green sheets include at least two types of ceramic green sheets, each type of green sheet containing as said first ceramic powder materials different to the materials as said first ceramic powder in the other types of green sheet. A method as claimed in any preceding claim, wherein at least one said metal paste film is applied to a ceramic green sheet as a metal pattern and the junction layer therefore is applied to the ceramic green sheet at portions of the sheet between the metal pattern. A method as claimed in any preceding claim, further comprising drying said metal paste film after formation of said metal paste film on said laminate and prior to stacking further ceramic green sheets on said laminate. A method as claimed in any preceding claim, further comprising connecting said metal films to form a laminated ceramic capacitor.

Inventors: KOHNO YOSHIAKI, SAKAI NORIO, KOHNO, YOSHIAKI, SAKAI, NORIO, KOHNO, YOSHIAKI C/O MURATA MANUFACTURING CO., LTD., SAKAI, NORIO C/O MURATA MANUFACTURING CO., LTD.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1989
Main classification: B32B18
IPC codes: H01G4, B32B18, H05K1, B28B11, H05K3