METHOD OF MANUFACTURING DIELECTRIC BLOCK FILTERS

Field of the Invention

10 This invention relates to dielectric block filters.

More particularly, this invention relates to an improved method of manufacturing these filters.

Background of the invention

15

Ceramic block filters are well known in the electronic art. U.S. Patent No. 4,431 ,977, shows a perspective view of one ceramic block filter in FIG. 1. Another ceramic block filter is shown in U.S. Patent No.

20 4,879,533. Of the several issued U.S. patents on ceramic block filters, virtually all are coated with a layer of metal which heretofore has been applied using a spraying technique.

Metallizing ceramic blocks by spraying them with a

25 silver, other metal-bearing paint has several drawbacks. Spraying wastes material merely because much of the material is lost to the atmosphere. These losses associated with spraying increase the cost of the metallization process because in some instances up to

30 fifty percent of the metallic spray is lost. Atmospheric discharging of metal particulates during a spraying metallization process causes additional atmospheric pollution, and poses potential health risks to workers in a manufacturing facility.

The spraying process itself also suffers certain drawbacks. In virtually all ceramic block filters, the interior surfaces of small diameter holes in the blocks must be thoroughly coated with metal to ensure proper electrical performance. Holes are frequently re-sprayed to insure that their complete metallization.

An improved process or method by which ceramic block filters can be manufactured that reduces the problems associated with spraying metals onto a ceramic block would be an improvement over the prior art. Such a process preferably would be more cost effective, would be an improvement to the environmental hazards caused by spraying, and would ensure more thorough metallization of small diameter interior passages in ceramic blocks.

Summary of the Invention

There is provided herein an improved method of manufacturing dielectric block filters that is comprised of the step of coating the surfaces of the dielectric block filter with a layer of electrically conductive material, by immersing the block into a predetermined liquid. This predetermined liquid, which is commercially available, is selected to have certain predetermined characteristics such that it thoroughly coats the exterior surfaces of the block and small diameter holes in the ceramic blocks with a conductive layer, adheres to the ceramic surfaces yet is not so viscous that it fills small diameter holes, and, in at least one embodiment, can withstand temperatures required to fire ceramic.

Brief Description of the Drawings

FIG. 1 shows a perspective view of a prior art block filter; FIG. 2 shows a cross-sectional view of the filter shown in FIG. 1

FIG. 3A shows the steps of the process used in the preferred embodiment of the invention;

FIG. 3B shows the steps of the process used in an alternate embodiment of the invention;

FIG. 4 shows the steps of the process used to form a ceramic block;

FIG. 5 shows subprocess steps used in the immersion of the block; and FIG. 6 shows steps used in the finish processing of a dipped block filter.

Description of a Preferred Embodiment

FIG. 1 shows a perspective view of a prior art dielectric block filter (5). This filter is shown in FIG. 1 herein, is similar to that shown in U.S. Patent No. 4,431,977. The filter (5) is comprised of a block of ceramic (1), which ceramic may include substances such as alumina, barium titanate, tin zirconium titan ate, neodymium titanate, or other materials with suitable electrical characteristics. The block (1) has formed in it, a plurality of holes (2), the interior surfaces of which are metallized, and which holes extend completely through the top surface (203) of the block as shown in FIG. 2. Since the top surface (203 ) of the block (5) is unmetallized, the metallization within the holes (2), which is electrically coupled to metallization on the bottom (205 in FIG. 2) of the block, is electrically equivalent to a length of short-circuited transmission

line at some particular frequency, which is well-known in the art.

FIG. 2 shows a cross-section of the block filter (5) in FIG. 1. It can be seen that hole (201), like other holes in the block (1), extends completely through the block through the top surface (203) to the bottom surface (205) and is metallized throughout its length. A metallization layer (204) completely coats the interior of the hole (201), the bottom surface (205) and the side surfaces, but does not cover the top surface (203), which is unmetallized to provide an electrically isolated conductor at the top (203) of the block.

Manufacturing a dielectric block filter such as the filter shown in FIG.1 requires several process steps, not including the steps required to attach the cable connectors (3). An uncured ceramic powder is typically pressed into the desired shape for the block. This pressed block of powder is then calcined and sintered, using well-known techniques. After sintering (or firing), the block is then metallized by spraying and then electrically tuned if necessary, typically by either removing ceramic and/or metal, or by adding more metal. (Some dielectric filters use so-called patterning, which is the application of predetermined metal patterns on one or more sides of the block to alter the filters electrical behavior.)

Referring to FIG. 3A, the improvement in manufacturing lies in the process steps shown in FIG. 3A. A dielectric block is first formed (10), typically using well-known process steps, such as those depicted in FIG. 4.

Referring to FIG. 4, which shows a more detailed series of steps in the formation of a block, a ceramic material is preferably poured (12) while it is in a granular, or powder form, into some predetermined

shape in a mold, The cast powder or granular, uncured ceramic is then pressed (14), typically using a hydraulic pressing device (well known in the art) into the predetermined desired shape. The uncured pressed ceramic powder is then sintered (16) to remove binders and other materials in . the uncured ceramic. Sintering is a well-known heating process, the temperature and time duration of which are well known in the art. In the preferred embodiment, which used neodymium titanate, after pressing, the blocks are heated to provide a temperature rise of 1 degree centigrade/minute to 500 degrees C. to burn out binder. The blocks are then heated to 1325 degrees C. for 3.5 hours to sinter the ceramic. After this firing, which is done on zirconia setter plates, the blocks are ground to a desired height using a silicon carbide and water slurry in a lapping machine having two degrees of freedom. The preferred embodiment uses a slurry mix of. 280 grit size silicon carbide mixed at a volume ratio of 1 cup of grit to 1 gallon of water. (An alternate embodiment would be to use blocks which are not lapped at this stage but which are lapped or otherwise sized after being initially coated with metal. In such an embodiment, the surfaces of the block that are removed might have to be re-metallized.)

The blocks are cleaned (17) in water under ultrasonic action for approximately twenty minutes. The blocks are then finished to radius sharp corners (18). The radiused corners are achieved by mixing the blocks with a grinding media and then vibrating the blocks and grinding media together. The preferred media is 0.16" by 0.312", 22 degree angle cut porcelain ceramic cylinders impregnated with silicon carbide grit. The media is kept moist enough with water to limit dust emission. Alternate embodiments would include using a different

media such as alumina or zirconia, and tumbling the media/filter block mix in a gem tumbler. Still other embodiments might not radiusing the block edges at all but would thereby risk damaging the relatively sharp corners of the blocks and nicking the metallization layer. After radiusing the blocks' edges, the blocks are cleaned in water (not shown) under ultrasonic action for approximately twenty minutes. Different solutions, times and/or cleaning methods might be used in alternate embodiments.

Referring again to FIG. 3A, after the ceramic block is formed (10), they are immersed, (also known as being dipped) (20) in a commercially available predetermined highly thixotropic liquid (also known as a paste). While dipping blocks, the liquid (also referred to as a paste) used in the preferred embodiment is observed to insure that it has a viscosity of approximately 3200 cps. (centipoise), as measured with a Brookfield RVT viscometer with a #3 spindel, available from Brookfield Engineering Labs, Inc., Stoughton, Mass., 02007. The liquid from DuPont is comprised of 100% silver flake (as opposed to grain) suspended in a media of volatile solvent of xylene, and an emulsifiable pine oil. (Cermalioy uses silver flake in butyl acetate.) The silver flake is approximately 75% of the weight of the mixture. An additional amount of glass frit is added to promote bonding. The added glass frit is approximately 0.1 - 1.0% (by weight) of the mixture. Viscosity of the liquid can be controlled by the addition of either more solvent or more solid, as required. The liquid also includes a small amount of lead silicate glass frit.

In the preferred embodiment the liquid that was used is a commercially available proprietary product of DuPont Electronics' and is identified as product 7307D. Heraeus, Inc., Cermaloy Divisions', also produces a

useable material known as Cermalioy C8733 or C8710 (or equivalents thereof). Although the material safety data sheet from DuPont lists various other components in their product 7307D and is the preferred material, either product (or similar families of products) might contain other components, (binders for example) the identities of which might be proprietary to the manufacturers.

The liquid (paste) is held in an enclosed dip tank of approximately 80 cubic inches and is constantly recirculated at a flow rate of approximately 40 in3/minute. As mentioned above, solvent and virgin silver flakes can be added to the contents of the tank as needed to maintain optimum viscosity of the liquid and to maintain a sufficient level of the liquid in the tank to permit dipping. The recirculation keeps the liquid uniformly mixed.

In addition to the recirculation, the contents of the dip tank are constantly stirred using an automatic, slow-moving paddle that keeps the silver flakes from settling. The paddle continuously stirs the tank contents except during intervals when blocks are actually being dipped. The recirculation, stirring, and the addition of solvent and silver, are part of an effort to maintain consistent properties of the liquid (22). (See FIG. 6.)

Blocks are immersed (24) in the above-identified liquid at a controlled rate, held submerged for a predetermined time (26), and withdrawn (27) at a controlled rate. (See FIG. 5.) During their immersion into the liquid, a layer of the liquid will adhere, to the surfaces of the block, (or be otherwise coupled to the block's surfaces). After the block is withdrawn, this layer forms a substantially continuous layer of material on the blocks surfaces, a component of which is the electrically conductive silver.

The withdrawal rate is believed to at least partially affect the coating thickness. In the preferred embodiment, the blocks are immersed at approximateiy 0.03 inches/second, until they are fully submerged. The blocks are held submerged (26) for approximately 10 seconds, (The blocks are held submerged for approximately 10 seconds/inch of hole length.) and they are withdrawn at 0.03 inches/second. Alternate embodiments would include variations in the dipping, submersion time, and withdrawal rate to vary the silver thickness.

The liquid characteristics and dipping immersion, submersion time, and withdrawal time that provides good coating of the exterior surfaces to the blocks will typically leave excessive silver in the relatively small diameter resonator holes, especially when the aspect ration of the hole diameter to the hole length ratio is small. (Hole diameter/hole length aspect ratios of approximately 0.2 or less is considered to be small. This figure is obtained when the hole diameter is 0.06 inches and the hole length is 0.3 inches.). After withdrawal from the dip tank, the blocks are blotted on a porous, absorbent material, such as an open weave paper or cloth to clear the holes, and can optionally be optically inspected to insure that the holes are open (30 in FIG. 4 and 6). If blotting the blocks alone does not clear the holes, a vacuum can be drawn below the absorbent material while the block is blotted to assist in removing excess material. After blotting, the blocks are dried (40 in FIG. 3A) at 150 degrees C. for approximately fifteen minutes. Alternate embodiments would include varying the time and temperature for drying the silver.

In an alternate embodiment of the invention, shown in FIG. 3B, after pressing (14) the cast and uncured

powder (12), the uncured blocks could thereafter by immersed (20) and blotted (30). Thereafter, the uncured blocks and the metallization applied by dipping could both be sintered (16) to cure the green ceramic to form cured blocks, bond the metallic component of the liquid to the ceramic and in the process also dry the paste (40). The sintering typically requires heating the dipped blocks at relatively high temperatures for long periods of time using conventional processes known in the art. In such an alternate embodiment, the silver used for the paste would likely have to be a palladium silver, or other equivalent, that would fire at the same temperature as the ceramic material.

In the preferred embodiment, the entire block is coated with this thixotropic liquid during the immersion process. To produce a useable block, a finish processing step (50) is required during which the blocks have conductive material removed from specific areas so as to produce useable filters. For a block shown in FIG. 1 , conductive material is locally removed from the top, exterior surface (1), in at least the areas surrounding the holes (2) so as electrically isolate the electrically conductive material that coats the interior surfaces of the holes (2) at the top surface of the block (1). Electrically conductive material at the bottom end of the holes (2) electrically couples the bottom ends of the holes to the metallization coating the exterior surfaces of the block, which permits the metallization lining the holes (2) to resemble a short-circuited transmission line at high frequencies.

Referring to FIG. 6, the finish processing step (50) comprises at least partially removing metallization applied during the immersion process (20). It should be noted however, that alternate embodiments would include only partially dipping a block in the thixotropic

liquid so as to leave at least portions of one surface uncoated. Referring to the block shown in FIG. 1 , which is a block in the shape of a parallelpipped, such a block could be immersed up to the edge (6) at where the side surfaces meet the top surface thereby leaving the top surface unmetallized. The plurality of holes through the block (1) should each be lined with conductive material up to the level to which the block was immersed. Subsequent finish processing (50) might then only include applying a top surface metallization pattern or top surface milling (or equivalent) during which the ceramic material might be removed for example.

In the preferred embodiment the metallization that is applied on the entire surface of the block by complete immersion is locally removed (from the top surface of the block shown in FIG. 1 for example) after drying (40) the metallization firing but alternate embodiments would include removing the metallization prior to drying (40) or prior to firing (16 in FIG. 3B) as well. Still alternate embodiments might contemplate masking or applying a resist material to localized areas of the cured or uncured blocks where metallization was not wanted so as to prevent adhesion of the thixotropic liquid to the blocks during the immersion process (20). When metallization is removed after drying the paste, the finish processing step (50) shown in FIG. 3A might include abrasion, etching, machining or other processes appropriate to at least locally remove from the top surface of the block (203) as shown in FIG. 2 to at least locally remove conductive material from areas surrounding at least one hole (2) as shown in FIG. 1 , to electrically isolate conductive material on the top surface (203) so as to produce an electrically isolated single end of a length of conductive material in the hole (201) as shown in FIG. 2.

Using the methods described herein, metallization of ceramic blocks can be achieved without the drawbacks of the prior art. By not spraying the blocks, costly material is saved and metal particulates are not discharged into the atmosphere. Moreover, complete metallization of the interior surfaces of holes is more assured.

What is claimed is: