DIELECTRIC FILTER AND PRODUCTION METHOD THEREFORE

Abstract: A dielectric filter for use in various communication equipment, a production method therefor, and a package member obtained by packaging the filter, for preventing peel of an end portion of an electrode. The filter has a construction wherein a through-hole (2) is so formed as to penetrate through a dielectric member (1) from its upper surface to its lower surface, an external electrode (3a) is disposed on the outer peripheral surface of the dielectric member (1) other than its upper surface, an internal electrode (3b) is disposed inside the through-hole (2), and island-like input/output electrodes (6) having a non-electrode formation portion (5) of the external electrode (3a) on the outer periphery thereof are disposed on the outer periphery of the dielectric member. Furthermore, an electrode protective film (8) is disposed so as to cover the outer peripheral end of the input/output electrodes (6), the non-electrode formation portion (5) of the external electrode (3a) disposed outside the input/output electrode (6) and the inner peripheral end of the external electrode (3a) in contact with the non-electrode formation portion (5).

Description: BACKGROUND OF THE INVENTION The present invention relates to a dielectric filter used in communications equipment and a method of manufacturing the dielectric filter. Also, the present invention relates to a protection film for covering the edges of the filter's electrodes Conventionally, dielectric filters have an island-like input/output electrode, isolated from an external electrode, disposed on the outer side face of dielectrics having multiple through holes from the top to bottom. One problem faced with a conventional dielectric filter is the peeling of the outer edge of the input/output electrodes and the inner edge of the external electrode of the dielectric filter, which may occur due to external stress, such as heat and twist applied to printed wiring boards when the dielectric filters are mounted to the printed wiring boards. Reference is made to JP-181002 relative to which the present invention is characterised. Reference may also be made to EP-A-0246042, JP-183439, JP-A-4198039 and JP-A-4103202. BRIEF SUMMARY OF THE INVENTION Aspects of the invention are defined in the claims. In one embodiment, the present invention can prevent the peeling of the outer edge of the input/output electrode and the inner edge of the outer electrode, which may occur due to external stress, such as when heat is applied thereto. The protective film covers the outer edge of the input/output electrodes and the inner edge of the outer electrode of the dielectric filter. The invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying figures. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING Fig. 1 is a perspective of a dielectric filter of a preferred embodiment of the present invention. Fig. 2 is a sectional view of an input/output electrode of the dielectric filter of Fig. 1 depicting a state of application of an electrode protection film. Fig. 3 is a plan view of an input/output electrode of the dielectric filter of Fig. 1. Fig. 4 is an exploded perspective view depicting the mounting of a preferred embodiment of a dielectric filter on a printed wiring board. Fig. 5 is a front section view of the dielectric filter of Fig. 4 of the present invention mounted to a printed wiring board. DETAILED DESCRIPTION OF THE INVENTION A preferred embodiment of a dielectric filter of the present invention is described below with reference to the drawings. Fig. 1 is rectangular dielectrics, or dielectric material, 1 made of BaTiO 3 -system ceramic. The dielectric material 1 has three cylindrical through holes 2 extending from a top face (the open end) to a bottom face (the short-circuit end), an Ag-system external electrode 3a encasing the dielectrics on the outer side faces and bottom face of the dielectric material, an Ag-system internal electrode 3b on the internal face of each of the through holes 2, and capacity forming electrodes 4. Each capacity forming electrode 4 is connected to an internal electrode 3b, on the top face or open end. A pair of C-shaped non-electrode portions s are disposed in a portion of external electrode 3a formed on an outer side face, at the open end, of the dielectric material 1. An island-like input/output electrode 6 is formed inside each of the C-shaped non-electrode portions 5, facing the through holes 2. An electrode protection film 8 is provided to cover the outer edge of the input/output electrode 6 and the inner edge of the cut-away portions of the external electrode 3a formed on the outer side face. Fig. 3 shows in greater detail how the corners 7 of the edge of the input/output electrode 6 and the edge of the external electrode 3a are curved to prevent a concentration of stress on the corners 7. The electrode protection film 8 is provided to cover the outer edge of the input/output electrode 6, the non-electrode portion 5, and the inner edge of the external electrode 3a. Thus, with the provision of the electrode protection film 8, the edges of the input/output electrode 6 and the external electrode 3a, which would otherwise be subject to peeling, can be protected. The electrode protection film 8 is formed by applying a glass paste mixture of glass-ceramic and amorphous glass. One of the advantages of amorphous glass is its high strength. However, amorphous glass is likely to remelt under high temperature. This disadvantage of amorphous glass can be counteracted by mixing amorphous glass with glass-ceramic, which has good temperature characteristics. Furthermore, as shown in Fig. 2, the outer end face 9 of the input/output electrode 6 and the inner edges 10 of the external electrode 3a are curved. If the outer end face 9 and inner edges 10 were to have sharp edges, the electrode protection film 8 applied to such edges would become thinner and the effect of the electrode protection would be degraded. Edges of such end faces are rounded off to prevent a degrading in the effectiveness of the protective film 8. A method of manufacturing the dielectric filter according to the present invention is described below. First, in forming the external electrode 3a and the input/output electrodes 6, silver paste is applied to the dielectrics 1, on the face where the electrode protection film 8 will be formed, by means such as screen printing. The edges of the printed silver paste (the outer end face 9 and the inner edges 10) are sharp at this point. The silver paste is then heated up to approximately 850°C to form the electrodes on the dielectrics 1. The silver paste melts and the edges of the electrodes become curved during the heat treatment. Then, after sintering, the finished end faces 9 and inner edges 10 become rounded, as shown in Fig. 2. Next, glass paste is printed to cover the outer end face of the input/output electrode 6, the non-electrode portion 5, and the inner end face of the external electrode 3a. Printed glass paste is sintered under the same heating conditions as those for the silver paste described above. During this process, the input/output electrode 6 and the external electrode 3a under the glass paste remelts. This allows the input/output electrode 6 and the external electrode 3a to mix with the glass paste. Consequently, the bonding strength between the input/output electrode 6 and the electrode protection film 8, and between the external electrode 3a and the electrode protection film 8, is strengthened. Moreover, the silver paste contains a glass component for bonding the dielectric material 1 and the electrodes 6 and 3a. A part of the glass component, which is contained in the edges of the input/output electrodes 6 and external electrode 3a, combines with the glass paste during remelting, and further improves bonding strength. Fig. 4 and Fig. 5 illustrate a product of the present invention with the dielectric filter mounted on a printed wiring board 11. Conductive paste, such as solder paste 13, is applied to the area of the input/output electrodes 6 and external electrode 3a, and heated to permit the solder to flow. As a result, a product is formed wherein a dielectric filter 12 is mounted on a printed wiring board 11. In particular, the printed wiring board 11 is formed by providing a copper electrode 15 on the surface of an epoxy substrate 14. Contact electrodes 17 are formed by a method such as etching the copper, thereby exposing non-conductive portions 16. To form the product, solder paste 13 is applied to the contact electrodes 17 and a mounting area 18, which is the approximate size of the outer side face forming a portion of the external electrode 3a. Then, the dielectric filter 12 is placed over a portion of the contact electrodes 17 and the mounting area 18, and the input/output electrodes 6 are connected to the contact electrodes 17, respectively. Heat is applied to permit the solder to flow and bond the dielectric filter 12 to the printed wiring board 11. Fig. 5 is an end view looking at the open face of the dielectric filter 12 and a cross-section of the printed wiring board 11. Depicted in Fig. 5, are the input/output electrodes 6 and the electrode protection film 8 of the dielectric filter 12. Also, Fig. 5 shows the contact electrodes 17 of the printed wiring board 11. An area of the input/output electrodes 6 is larger than that of the contact electrodes 17. This is because, as mentioned previously, the electrode protection film 8 is formed on the outer edge of the input/output electrode 6. The area of an input/output electrode 6 is enlarged for the portion covered with the electrode protection film 8. Fig. 5 also illustrates that the electrode protection film 8 covers both the outer edge of the input/output electrodes 6 and the inner edge of the external electrode 3a. Consequently, as also shown in Fig. 2, a dent 18 is formed between the input/output electrodes 6 and the external electrode 3a. This dent provides an intentional space 20 between the printed wiring board 11 and the electrode protection film 8 so as to prevent short-circuiting of an adjacent input/output electrode 6 with an external electrode 3a by spreading of the solder paste 13 between the printed wiring board 11 and electrode protection film 8 due to capillary action. According to the present invention, the electrode protection film covering the outer edge of the input/output electrodes and the inner edge of the external electrode on an outer area of the dielectric filter prevents the peeling of the outer edge of the input/output electrode and inner edge of the external electrode, which peeling may otherwise occur due to external stress, such as heat.

Claims: A dielectric filter comprising: a dielectric material (1) having a top face, a bottom face and four side faces; at least one through hole (2) extending from the top face to the bottom face or at least one non-through hole; an internal electrode (3b) disposed inside each of said through holes or non-through holes; an external electrode (3a) disposed on the bottom face and the four side faces; at least one island-like input/output electrode (6) disposed on a side face of said dielectric material and surrounded by a non-electrode portion (5), which is surrounded by the external electrode (3a); and an electrode protection film (8) covering an outer edge of each input/output electrode (6), the non-electrode portion (5) of the external electrode disposed around the input/output electrode (6), and an inner edge of the external electrode (3a) adjacent to the non-electrode portion (5), characterised in that : said electrode protection film (8) is dented over said non-electrode portion (5) between the outer edge of the input/output electrode (6) and the inner edge of the external electrode (3a); and the outer edge of the input/output electrode (6) and the inner edge of the external electrode (3a), opposite to the outer edge of the input/output electrode (6), each comprise an edge surface (9, 10) having a rounded-off cross-section shape. The dielectric filter of claim 1, wherein at least one corner (7) of the outer periphery of the input/output electrode (6) and at least one corner (7) of an inner periphery of the external electrode (3a) disposed outside of the input/output electrodes (6) are curved in a plane parallel to the electrodes. The dielectric filter of claim 1, wherein said electrode protection film (8) is a glass paste mixture of at least glass-ceramic and amorphous glass. The dielectric filter according to claim 1, wherein a plurality of said through holes (2) and multiple input/output electrodes (6) are provided on said dielectric material (1) at specified intervals. An assembly comprising: a printed wiring board (11); and a dielectric filter as defined in any of claims 1 to 4, wherein said dielectric filter is mounted on a surface of the printed wiring board (11). A method of protecting the electrodes of a dielectric filter that includes: dielectric material (1) having a top face, a bottom face, and four side faces; at least one through hole (2) extending from the top surface to the bottom surface or at least one non-through hole; an internal electrode (3b) disposed inside each of said through holes (2) or non-through holes; an external electrode (3a) disposed on the bottom face and the four side faces, and at least one island-like input/output electrode (6) disposed on a side face of said dielectric material and surrounded by a non-electrode portion (5), which is surrounded by the external electrode (3a), the outer edge of the input/output electrode (6) and the inner edge of the external electrode (3a), opposite to the outer edge of the input/output electrode (6), each comprising an edge surface (9, 10) having a rounded-off cross-section shape, said method characterised by the steps of: applying a glass paste to at least an outer edge of the input/output electrode (6), the non_electrode portion (5) of the external electrode, and an inner edge of the external electrode (3a) adjacent to the non-electrode portion; and heating the glass paste to melt the glass paste, so as to provide an electrode protection film (8) over the outer edge of the input/output electrode (6), the non-electrode portion (5) of the external electrode, and the inner edge of the external electrode (3a) adjacent to the non-electrode portion, the electrode protection film (8) being dented over said non-electrode portion (5) between the outer edge of the input/output electrode (6) and the inner edge of the external electrode (3a). A method according to claim 6, further comprising the steps of: prior to applying and heating the glass paste, applying a silver paste to a side face near the top face by means of screen printing to form each input/output electrode (6), the non-electrode portion (5) of the external electrode and an inner edge of the external electrode (3a) adjacent to the non-electrode portion; heating the silver paste to curve the edges of the electrodes; and sintering to round the edges of the electrodes.

Inventors: IWASAKI TOMOYUKI, TAKIMOTO NOBUHIRO, IWASAKI, TOMOYUKI, TAKIMOTO, NOBUHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H01P1
IPC codes: C04B41, H01P5, H01P7, H01P1, C03C8, H01P11