Microminiature Monolithic Ferroceramic Transformer

Abstract

A monolithic microminiature transformer element comprising at least two separate contiguous windings of conductive metal film, each having an interconnecting magnetically permeable core of refractory material, immersed in a rectangular block of stratified refractory material. The element is a contiguous mass of ceramic material, especially alumina, primary and secondary transformer windings formed by printing or photoetching loops on a rectangular piece of ceramic sheet. A method for making this transformer element wherein paths of conductive material are deposited onto thin unsintered magnetically permeable ceramic sheet with holes for interconnection therein to form windings of said transformer and wherein said holes are aligned and said sheets are laminated such that upon sintering said metal forms a contiguous conductive path and provides a primary or secondary winding for said transformer, and wherein said path is immersed in a contiguous block of ceramic.

Description

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to the inventor of any royalty thereon.

BACKGROUND OF THE INVENTION

This invention relates generally to monolithic microminiature components for use in the assembly classified microcircuitry using substrates as a basis onto which the various types of components are mounted. Less generally this invention relates to a monolithic microminiature transformer element and a method for making the same. Specifically this invention relates to a microminiature monolithic transformer element with a magnetically permeable core comprising a magnetically loaded ceramic material and a process for making in mass production large quantities of this device at very economical prices. Moreover, it is related to that class of devices classified as transformers which possess primary and secondary windings wound around an inner core having the property of significantly enhancing the magnetic field. Among the classes of microelectronic manufacture to which this invention applies are thick film technology, thin film technology, and hybrid multichip technology. In any case, the invention is to be used where reduction in size is required; a large quantity is to be produced; and cost is an essential factor.

The most closely related device to this particular invention are spiral inductors which are made by the application of thick and thin films and by subtractively etching the inductor. A helical miniature inductor and method related to the invention herein is discussed in copending application, "Ceramic Inductor and Method for Making Same," having Ser. No. 230,247. A number of approaches to making microminiature transformers have evolved. The most advanced approach and the one most related to this particular invention is a monolithic transformer comprising fine wire wound onto iron rods and coated with an epoxy in order to fix the position of the wire to maintain the characteristics of the device. Many problems exist with such a structure, one of which is a possibility of the epoxy breaking down or a particular piece of wire becoming loose due to the various heat processes necessary to attach the particular component into electronic circuitry. Such flaws in this type of device under abnormal physical conditions can cause severe changes in the circuit characteristics. This is especially true in military applications where hardware comprising electronic microminiature circuitry is subjected to large accelerations and shocks.

There are other designs of monolithic microminiature transformers essentially the same as the one previously described. The difference is that these devices are molded into plastic, and naturally, they are the common structures for transformer elements which are not microminiature and which are not monlithic in the sense of this invention in that the various parts of the inductor are not chemically bonded or thermally fuzed to each other. In general, in the past transformers in microminiature form-factors and form-factors easily applicable to hybrid microelectric circuits have been extremely expensive and in relationship to the cost of other functional components to be used in the circuits and therefore offers a cost factor against the use of such components.

Another inherent disadvantage in the structure of the aforesaid components is the nature of the conductive terminations formed on the device for interconnecting it into the particular circuit for which it is needed. In most cases, such as that of a plastic device is best only to epoxy such a device to the substrate using a conductive epoxy material because of the problems involved in raising the temperature of such a device to that necessary for soldering into the circuit. The difference in thermal coefficient between the plastic and the metal terminations at the ends thereof usually preclude the use of high or even moderate soldering temperatures for interconnecting such devices into the circuit. These differences in thermal coefficience are extremely cumbersome when soldering microminiature devices. In general, in assembling hybrid circuits it is desirable to reflow solder chip components. If it is not possible to reflow solder chip components into circuits, the next feasible alternative is wire bonding for interconnecting the component into the circuit. Either of these methods precludes other sensitive semiconductors from having to be reheated many times. All devices in use today, except twinspiral-type transformers deposited onto substrates, have significant thermal mismatches in materials.

In the case of the twin spiral type transformer when one spiral is coupled to another spiral type winding printed on a substrate such as alumina or mylar the coupling between the two spirals are not significantly enhanced because of the lack of a common closed magnetic circuit. Usually there is free space above the spiral unless some particular type of coating is applied. One problem with spiral coaxial transformers are twin-spiral transformers printed onto to very thin sheets of material is a high probability of changes in the Q factor by bending of the substrate. Another problem of the spiral is interconnection. When both terminations of the spiral are on the same side of the substrate one of the terminations must be crossed over the respective turns of that particular spiral to interconnect the spiral to the outside circuitry. This degrades operation of the device. Even if terminations are brought out on opposite sides of the substrate the problem of traversing the respectwve windings is still present.

The term "refractory material" is used herein to mean a substance that will not melt, decompose, or materially change under the processing conditions involved in forming the device herein described. Refractory material is generally classified into four broad groups. The group of utility here includes the polycrystalline materials such as ceramics and includes, for example porcelains, steatites, aluminas, and ferrites. The present invention is described with reference to these ceramics and, more particularly, thin sheets of alumna with ferrites mixed therein. However, it should be understood that tbe present invention is equally applicable to the other ceramic materials. The spirals comprising the secondary and primary windings of the transformer of this particular invention may be formed from a paste of glass, high melting point metal such as platinum and gold, and a decomposable fluid suspending agent, or surfactant, applied to the refractory oxide by any convenient method, for example, by dipping, brushing, or spraying. The relative amount of material within the paste may vary over fairly wide limits. The main consideration is that the metal content be sufficiently high to insure that the resulting metal film after processing is continuous. The amount of fluid used as a suspending agent depends upon the method of application. If spraying is used, a relatively thin suspension is required. If brushing or "squeegee" screened processes are employed, thicker paste suspension should be such as to insure good conductivity of the deposition.

Magnetic materials usable for this device include 2-81 permalloy and carbonyl iron insulated powders, and ferroxcube III sintered powder.

Generally, a mean-particle size range which is suitable is 0.5 to 25 microns for the paste, with the preferred range being between 0.5 and 15 microns. Smaller particles are equally satisfactory. For the glass flux, a glass which fuzes and bonds to the ceramic at a temperature below the melting point of the metal and resists reduction under the usual processing conditions should be used. Glasses having these properties are readily compounded for mixtures of silica (SiO.sub.2) and various combinations of the oxides of sodium (Na.sub.2 O), calcium (CaO), barium (BaO), magnesium (MgO), aluminum (Al.sub.2 O.sub.3), boron (B.sub.2 O.sub.3), potassium (K.sub.2 O) and phosphorus (P.sub.2 O.sub.5), among other elements. Table I is illustrative of some suitable glasses which can be conveniently compounded from typical oxides specified as to kind and amount in the table. The table is not intended to be exhaustive of suitable glasses but indicates a general composition of some readily fuzable nonreductible glasses. It is noted that this table encompasses many common types of glasses such as borosilicates, phosphates and silicates.

TABLE I ______________________________________ Melt Ingredient: Parts by Weight ______________________________________ Li.sub.2 O 0-15 Na.sub.2 O 0-25 CaO 0-10 BaO 0-20 MgO 0-2 Al.sub.2 O.sub.3 0-35 SiO.sub.2 5-80 B.sub.2 O.sub.3 0-30 K.sub.2 O 0-5 P.sub.2 O.sub.5 0-80 ______________________________________

In the preparation of the glasses, the ingredients are smelted together in a furnace at a temperature sufficient to melt but not volatilize the constituent oxides, for example, between 1100.degree. centigrade and 1500.degree. centigrade, until a mass of uniform quality has been obtained. The melt is fritted by pouring into cold water, and the resultant frit is ground to the fineness desired. It is desirable for the glass particles to be finally divided, for example, on the order of 1/2 micron to 25 microns particle size, so that the paste mixture will, under the processing conditions, result in a continuous metal layer adherently bonded to the ceramic.

The glass and metal particles are suspended in a volatile and decomposable fluid suspending agent or surfactant and applied to the refractory oxide by any of the methods aforementioned. The relative amount of metal and glass used may vary over fairly wide limits. The main consideration is that the metal content be sufficiently high to insure that the resulting metal film after processing is continuous. Generally, between five to 50 parts by weight of metal is used by each part by weight of glass.

The fluid suspending medium serves to disperse the paste mixture in the desired pattern on the substrate and to hold the pace in this pattern until processing commences. During processing the suspending medium should volatize leaving no residue. The suspending medium should not react with the metallic or glass components of the coating composition before or during firing. To insure proper dispersion and bonding of the paste, many of the common suspending media contained two components. The first component acts as a dispersing medium for the paste and as a solvent for the second component which insures proper bonding of the pace to the "green ceramic" or refractory oxide until processing commences. Examples of suitable dispersion media which are solvents for the below listed binders are benzene; the esters of fatty acids; alcohols of low molecular weight such as ethyl, butyl, and amyl; acetates including "cellosolve acetate" (ethylene, glycol, monoethyl, ether, acetate), diethylene glycol monoethyl ether acetate; ketones such as acetone and methyl-ethyl-ketone; and higher ethers such as glycol diethyl ether. Suitable binders are, for example, the vinyl or substituted vinyl polymers.

In general, any ceramic which is resistant to the usual processing conditions may be used as a refractory ceramic substrate. The following table is illustrative of various ceramic compositions that have been successfully used. The compositions are expressed in parts by weight.

TABLE II ______________________________________ Porcelain Steatite Alumina Composition A B C D E F G ______________________________________ Feldspar 35 50 30 25 Ball Clay 15 10 8 10 Kaolin 30 30 37 40 15 Talc 60 Dolomite 2 BaCO.sub.3 17.5 MgCO.sub.3 1 7.5 SiO.sub.2 20 10 25 22 9-10 2 CaO 1 1 MgO 3-4 1 Na.sub.2 O 1-5 1/2 Al.sub.2 O.sub.3 (1) 95.5 ______________________________________ .sup.1 Remainder

In order to form a ceramic slurry with good flow properties, the aforementioned thermoplastic organics are used as flow promoting binders for the refractory oxide. The prime step is to coat the fine alumina particles with these thermoplastics. This step is facilitated by intense mixing at high temperatures in the range from 100.degree. centigrade to 400.degree. centigrade. Water emulsions of the organic plastic agents facilitate the initial mixing of the organic with the ceramic particulates, and the initial contact can be made by using an aqueous or nonaqueous slurry and solution. Removal of the volatile constituients provides an intimate mixture of the organic and the "green ceramic."

In a typical process, firing of the laminate is done in a furnace in which both atmosphere and temperature can be controlled. The firing is done in a reducing atmosphere. This firing step is carried out under conditions sufficient to volatilize the fluid suspending media, and to commence formation of a refractory ceramic-to-glass-to-metal bond. The temperature and firing time are interdependent. The fluid suspending vehicle used and the temperature required commences formation of the refractory ceramic-to-glass-to-metal bond. This temperature is dependent upon the temperature required to sinter the ceramic and to cause wetting of the refractory ceramic and at least part of the metal by the glass in the paste system. Such wetting and sintering temperatures are dependent upon the glass flux used. Temperatures ranging from, for example, 1400.degree. centrigrade to 1600.degree. centrigrade have been successfully used.

The maximum temperature is limited by the melting point of the metal while the minimum temperature is again dependent upon the wetting and the sintering temperature of the glass flux employed and the temperature required to sinter the ceramic wetted by the glass comprising the paste.

The invention described herein overcomes many of the disadvantages of the foregoing constructions. It is therefore the object of this invention to provide a new and novel process for manufacturing monolithic microminiature transformer elements with a magnetically permeable core.

It is yet another object of this invention to provide a new and novel microminiature transformer element with a magnetically permeable core which is monolithic and comprises materials having coherent thermal coefficients.

It is yet an additional object of this invention to provide a new an novel device and process for manufacturing said device which is economical and easily adaptable to high volume manufacturing used in the thick film industry.

It is yet an additional object of this invention to provide a monolithic trnasformer having conductive primary and secondary windings comprising combinations of spiral helical conductive paths emmersed in a magnetically permeable ceramic material with bonding paths thereon suitable for attachment of metal tabs or for wire microbonding the components to the appropriate electronic circuitry.

It is another object of this invention to provide a microminiature monolithic transformer element which has a form factor compatible with that of other components used in hybrid microelectronics.

It is still an additional object of this invention to provide a microminiature monolithic transformer element with a structure made of ceramic and cermet materials.

It is yet another additional object of this invention to provide a transformer having a surface of refractory ceramic material.

It is yet an additional object of this invention to provide a microminiature monolithic transformer with a new and novel core comprising a magnetically permeable alumina ceramic.

It is yet an additional object of this invention to provide a combination helical spiral path winding for a transformer, said path being imbedded in a magnetically permeable alumina ceramic having a rectangular solid form factor.

Yet another additional object of this invention is to provide a new and novel microminiature monolithic transformer element whose electrical paths are formed by metalization paths of thick film and thin films.

Still an additional object of this invention is to provide a new and novel method for making a microminiature monolithic transformer in which the electrical conductive paths are formed onto unfired alumina ceramic tape by a subtractive etching technique.

Still yet an additional object of this invention is to provide a new and novel design of metalization which provides a helical spiral path that provides transformation of voltage and current functions within a compact area.

These and other objects of the present invention will become more fully apparent with reference to the following specifications and numerous drawings which relate to several variations of a preferred embodiment of the invention described herein.

SUMMARY OF THE INVENTION

A monolithic microminiature transformer comprising a combination helical spiral conductive path for the primary and secondary windings, of deposited metal film immersed in a rectangular block of magnetic refractory material. The transformer is either made with a coparallel windings or intermeshed windings. The terminations of the transformer are metallized paths located thereon. A method for making this transformer wherein spirals of conductive metal are deposited onto a thin unsintered magnetically permeable unsintered ceramic sheet with holes therein for interconnection and wherein said holes are aligned and said sheets are laminated such that upon centering said metal forms a spiral helical combination path of contiguous conductive material immersed in a contiguous block of ceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

The specific nature of the invention as well as other objects, aspects, uses, and advantages thereof will clearly appear from the following description and from the accompanying drawings, in which: FIG. 1 (a) is an illustration of the process of manufacturing magnetically permeable ceramic tape.

FIG. 1 (b) is a flow chart of the method for manufacturing magnetically permeable ceramic tape.

FIG. 2 (a) is an illustration of the pattern deposition processed by which conductive paths are formed on the ceramic tape.

FIG. 2 (b) is a flow chart of the pattern deposition process.

FIG. 3 is a flow chart of the manufacturing process by substractive etching.

FIG. 4 (a) is a top view of the layers of a biaxial embodiment of the microminiature monolithic transformer.

FIG. 4 (b) is a bottom view of the layers of a biaxial microminiature monolithic transformer.

FIG. 5 (a) is an illustration of two adjacent layers of ceramic tape with conductive patterns thereon for lamination.

FIG. 5 (b) is an illustration of two adjacent layers laminated with the interconnection of the conductive metallization from one layer to the next.

FIG. 6 is a graphic illustration of a coaxial microminiature monolithic transformer showing the spatial configuration of the various spirals.

FIG. 7 (a) is a top view of the various layers of a coaxial microminiature monolithic transformer.

FIG. 7 (b) is a bottom view of the layers of a particular embodiment of a coaxial microminiature monolithic transformer.

These and other objects of the present invention will become more fully apparent with reference to the following specifications and the drawings which relate to several variations of a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The manufacturing process of the present invention will be easily understood in broad aspects by reference to FIG. 1 (a) wherein there is illustrated the basic steps in the manufacture of the magnetically permeable ceramic tape. The construction principle of the multilayer ceramic transformer chip begins with the mixing of a slurry 14 of alumina powder and various binders that can be cast into thin layers 10 on a flat surface 11. However, before casting, a magnetically permeable powder 12 is added to the slurry of ceramic material 14. A doctor-blade 29 is normally used to achieve the required thinness and uniformity. Organic binders in the slurry give enough strength and flexibility to the tape 10 after drawing for removal and handling. Strips 19 are cut out of this tape and backed with insulating paper 17 and then holes are punched with the tool 16 and the resulting strips are rolled onto rolls 18. Stripping or cutting of strips is done with tool 15. In order that this particular portion of the process may be easily understood it is presented in flow-chart form. In FIG. 1 (b) step 20 consists of adding a portion of magnetic powder to the ceramic slurry 14. The proportion of the magnetic material added to the ceramic material may vary between 5 and 30 percent of the volume of ceramic material. The next step of the process 21 is to spread this slurry material by doctor blade to the desired thickness. After step 21 is accomplished the ceramic sheet material is dried, step 22, by convection or infrared or other heat transfer means. Once the ceramic sheet material is dry it acquires a plastic and rubber-like characteristic and becomes very flexible, yet not brittle. In step 23, this tape-like, rubber-like material is cut into long strips 19. In order to prevent the slurry tapes from sticking together due to temperature changes during rolling, a paper of plastic backing is appositioned, step 24, to the tape and the tape is then, in step 25, punched with holes at the required locations. The long strips of tape are next rolled onto reels, step 26. This completes the process for formation of the tape.

The next phase necessary in the production of these monolithic microminiature transformers with magnetically permeable ceramic cores is a formation of conductive paths by pattern deposition as shown in FIG. 2 (a). The reels of ceramic tape 27 are fed through a printer 30 which screens conductive pattern necessary to form the spiral windings of the transformer onto the tape 19. The tape 19 is then fed through a drying oven or apparatus 31 which removes the highly volatile components from the metallization silk screened onto the substrate 19. After the metallization is dried the paper is separated from the ceramic tape 19 and rolled onto reels 50 for reuse. The ceramic tape 19 is then fed into a compressor and laminator apparatus 35 along with reels of tape from other lines similar to the one we have just described as illustrated in 34. Sections of these sheets are aligned and stacked and the stacked structure 36 is compressed and laminated by compression apparatus 35 and the resulting compressed structure is cut from the tape sources and trimmed with cutting apparatus 37. These large compressed sheets of ceramic tape with metallization thereon contain many transformers. These laminated structures 38 are then carried by belt 41 to a sintering oven 39 which cures the ceramic block of transformers 38 at temperatures up to 1600.degree. centrigrade transforming the laminated structure 38 into a monolithic mass of conductive, ceramic, and magnetically permeable powder immersed and surrounded by the various granules and molecules of alumina ceramic. Once this essential step is completed it is necessary to use a diamond cutter 40. The above steps are illustrated in flow chart form in FIG. 2 (b). Note that the step 58 of FIG. 2 (b) indicates that metal terminations are attached to the individual transformers. Step 58 is a final increment in the production of these transformers. Usually metal tabs are attached to the various exit bonding terminations. In particular, FIG. 3 shows the various steps necessary in order to form the proper pattern and to get the proper interconnections between the various layers to compose an interconnected spiral transformer. If we were to take a transformer cut from the laminae 38 which, for instance is comprised of 5 layers of ceramic tape 19 the structure would look as indicated and illustrated in FIGS. 4 (a) and 4 (b), FIG. 4 (a ) illustrating the top view and FIG. 4 (b) illustrating a bottom view of the printed matter on the layers 19. Taking one of the dice of laminae 38 and on stacking it such that the top layer 113, the next layer 114 and so on to the bottom layer 117 are illustrated in FIG. 4 (a) and FIG. 4 (b). First looking at the top of a particular layer as illustrated in FIG. 4 (a) and then looking at the bottom of a particular layer as illustrated in FIG. 4 (b) we obtain a view of the interconnections and corresponding parts between various layers of the transformer 110. One layer 113 of the transformer in FIG. 4 (a) is shown metallization pad 111 with hole 127 therein. This hole 127 is formed therein by tool 16 and similarly for holes 128, 129, 130, 131, 136, 141, 138, 142 and 140 as illustrated in FIGS. 4 (a) and 4 (b). Conductive metallization 111 is connected to the other side of layer 113 via hole 127 which connects 111 to spiral 121 and specifically to metallization pad 133. Metallization pad 133 is connected to spiral 123 and metallization pad 134 through metallized holes 129 and 131. Metallized pad 134 of FIG. 4 (b) is connected via hole 141 and 142 to terminating bonding pad 125. The conductive path from pad 111 to pad 125 through layer members 113, 114, 115, 116 and 117 comprises the secondary winding for this particular embodiment of a biaxial microminiature monolithic transformer element 110. The conduction path from pad 112 to pad 126 through layers 113, 114, 115, 116 and 117 in which spirals 122, 119, 124 and 120 are interconnected comprises the primary winding for the microminiature monolithic transformer 110.

Turning further to FIGS. 5 (a) and 5 (b) the intricacy of interconnecting a metallization pad 151 on the surface of one layer 150 to a metallization pad 154 on the surface of another layer 155 is described. FIG. 5 (a) shows two layers 150 and 155 of ceramic tape with metallizations 151 and 154 thereon and metallized through-hole 152 therein before lamination and interconnection. Ceramic tape 150 is positioned and aligned on top of ceramic tape 155. Appositioned onto ceramic tape 150 is a metallization connection path 151. This metallization connection path 151 is connected to metallization connections path 154 through hole 152. It is noted that hole 152 is metallized throughout. FIG. 5 (b) shows two ceramic tape layers appositioned to each other and appropriately compressed. What is shown is ceramic tape 150 on top and juxtaposed to the ceramic tape 155. Metallized hole 152 is compressed into metallized connection pad 154 forming a continuous conductive path from 151 to pad 154 with the metallization in hole 152 serving as the connecting means.

In FIG. 6 a diagrammatic cross section of a microminiature monolithic transformer having coaxial primary and secondary spiral windings is illustrated. In this embodiment the transformer is comprised of layers 76, 77, 78, 79, 80, 81 and 82. These layers aforementioned have holes therein to provide a conductive path through the monolithic block interconnecting the leads of the primary and secondary windings. In particular, the primary winding comprises complimentary spirals 86, 87, 90 and 91. The secondary winding comprises complimentary spirals 88 and 89. Leads 72 and 73 provide exit terminations for the primary winding and leads 74 and 75 provide exit terminations for the secondary winding. Tracing the primary winding from end termination 72 conductive material passes through hole 83 to conductive spiral 86 through hole 85 to conductive spiral 87 through hole 100 and holes 99 and 98 to conductive spiral 90 and then through hole 102 to complimentary conductive spiral 91 and finally through hole 97 to exit termination 73. Note that spiral winding 86 may be appositioned either to layer 76 or layer 77. Similarly for conductive spiral winding 87 which may also be attached to layer 77 or alternately to layer 78. This also holds for conductive spiral windings 88, 89, 90 and 91 because of the unique way in which the various layers are interconnected due to lamination. Secondary winding for the monolithic transformer comprises end termination 74 which passes through hole 84 and holes 92 and 93 to complimentary spiral 88 and then through holes 101 to complimentary spiral 89, then to holes 94 and 95 and 96 to termination 75.

In FIGS. 7 (a) and 7 (b) are illustrated a top view and a bottom view, respectively, of the coaxial construction of the monolithic transformer as illustrated diagrammatically in FIG. 6. Tracing the conductive path of the primary winding of the microminiature monolithic transformer illustrated in FIGS. 7 (a) and (b ) we start from conductive pad 191 of transformer 200 which is connected to square spiral 195 through hole 171. Note that spiral 195 terminates in conductive hole 172. And, metallized hole 170 is connected to metallized pad 171 terminating spiral 195. Conductive hole 172 interconnects spiral 195 to complimentary spiral 196, spiral 196 terminating in conductive pad 173. Conductive pad 173 interconnects to conductive hole 174, conductive hole 175 and through conductive hole 176 to conductive pad 177 interconnected to spiral 197 and terminating in conductive hole 178 which is interconnected to complimentary spiral 198 which terminates in conductive pad 179. Conductive pad 179 is interconnected to terminating pad 193 via conductive hole 180. The above denotes the path of the primary winding for the transformer 200 illustrated in FIG. 7 (a) and 7 (b). It is useful to note that kovar strip tabs may be welded to pads 191 and 192 and extended in the same direction. Similar kovar tabs extending in the opposite direction of those attached to pads 191 and 192 may be attached to pads 193 and 194 either by welding, soldering or other metal to metal joining means. Now to trace the secondary winding of the transformer 200 in FIGS. 7 (a) and 7 (b) we begin with conductive path 192. Conductive pad 192 is connected to spiral 199 via holes 181 and layer 201, hole 182 and layer 202 through hole 183 and layer 203 through the conductive spiral 199 to the conductive interconnecting pad 184. Interconnecting pad 184 is connected to spiral 208 via interconnecting hole 185. Spiral 208 interconnects interconnection hole 185 to conductive interconnecting pad 186 which in turn is connected to the terminating pad 194 on layer 207 via holes 187, 188 and 189.

It is to be understood that barium titanate (BaTiO.sub.3) may also be used in the slurry to enhance the properties of the refractory ceramic material.

The inventor wishes it to be understood furthermore that he does not desire to be limited to the exact details of construction shown and described herein for obvious modifications will occur to a person skilled in this art.

Claims

What is claimed is:

1. A microminiature transformer comprising:

a. a monolithic block comprised of a plurality of appositioned strata thermally fused together, each stratum comprised of sintered refractory ceramic dielectric material;

b. an electrically-insulated magnetically-permeable powder dispersed in said strata;

c. individual conductive metal primary spirals deposited on some of the surfaces of said strata, said primary spirals on each said stratum being coaxially located and enclosed by said block;

d. individual conductive metal secondary spirals deposited on some of the surfaces of said strata, said secondary spirals on each said stratum being coaxially located and enclosed by said block;

e. a first hole in each said stratum for housing a means for interconnecting each primary spiral;

f. a second hole in each said stratum for housing a means for interconnecting each secondary spiral;

g. said means for interconnecting each primary spiral comprising a metal film deposited on the inside cylindrical wall of each of said first hole;

h. said means for interconnecting each secondary spiral comprising a metal film deposited on the inside cylindrical wall of each of said second hole.

2. The microminiature monolithic transformer element of claim 1 wherein said primary spirals comprise a primary transformer winding and wherein said secondary spirals comprise a secondary transformer winding and wherein said dispersed powder provides a magnetically permeable flux link for said windings.

3. The microminiature transformer of claim 2 further comprising at least four deposited metal bonding pads, two of which are located on one face at said block, two more of which are located on the opposite face of said block, whereby one parallel and coaxial pair of said bonding pads form the terminations of the primary winding and another parallel and coaxial pair of said bonding pads form the terminations of the secondary winding.

4. The microminiature transformer of claim 3 wherein said powder is selected from the group consisting of 2-81 permalloy and carbonyl iron.

5. The microminiature transformer of claim 4 wherein said primary and said secondary spirals are coaxially oriented.

6. The microminiature transformer of claim 4 wherein said primary spirals are coaxially arranged about a first axis and wherein said secondary spirals are coaxially arranged about a second axis, said second axis being located a distance away from said first axis greater than the sum of the combined maximum radii of said primary and said secondary spirals.

7. The microminiature transformer of claim 5 wherein said strata are of equal thickness.

8. The microminiature transformer of claim 5 wherein said metal spirals, said metal film, and said metal bonding pads are thermally matched to said strata.

9. The microminiature transformer of claim 6 wherein said strata are of equal thickness.

10. The microminiature transformer of claim 6 wherein said metal spirals, said metal film, and said metal bonding pads are thermally matched to said strata.

11. The microminiature transformer of claim 1 further comprising melted glass frit dispersed in said conductive metal spirals and said metal film, whereby said frit provides a ceramic-to-metal bond.