Process For Ceramic Composites

Abstract

A process is provided by which metal, or other non-ceramic material, is inset into a ceramic structure to produce a composite structure having properties not attainable by previously available processes.

Description

This invention relates to a process for the insetting of metallic or other non-ceramic compositing materials into ceramic structures followed by firing to give integral composite ceramic articles. In particular, the invention relates to a process for the insetting of metallic via-holes in one or more layers of ceramic in composite substrates.

It is widely known to produce substrates from ceramics, such as alumina, for the attachment of electrical devices giving ceramic substrates which are commonly referred to as printed circuits although that term is possibly better reserved for circuits on polymeric bases. In fact, the circuitry of ceramic substrates is usually applied by silk screening processes on the ceramic base either before firing, in which case both are fired together, or subsequent to firing. In the later case, a further step is necessary to sinter the metallic circuitry and deformation of the ceramic may occur. Leads passing from one side to the other of a single ceramic sheet or from one sheet to another in multilayer structures pass through holes known as via holes.

In the screening operation, the filling of the via holes normally occurs as the result of capillary action or it may be assisted by application of suction or vacuum. This procedure is generally satisfactory for very small holes except that the ultimate density of the metal after firing may be less than desired and that bubbles may be occluded so that there is incomplete filling of the holes. In the case of larger holes, the procedure of screening is quite inadequate because the contraction in volume occuring during the drying of the paste results in incomplete filling of the holes often with a friable residue which may be badly cracked and generally unsatisfactory. Furthermore, adhesion of the screened metal to the ceramic base may be inadequate. If there is a thinning of the metallic conductors or incomplete filling of the via holes, the electrical conductivity of parts of the circuitry may be quite limited.

It is an aim of this invention to provide conductors in ceramic composites having uniform density of metal regardless of the size thereof. Other aims and objects will become apparent hereinafter.

In accordance with the above and other aims and objects of the invention, it has been found that a highly valuable process for the production of ceramic composites is the simultaneous punching of tapes of the basic ceramic and of the compositing material so that the piece punched out of the latter tape transfers to and is lodged in the basic ceramic tape. The tapes referred to are thin self-sustaining sheets of uniform thickness of the requisite ceramic and compositing materials respectively combined with a small amount, usually less then 20 percent by weight, of a thermoplastic polymeric binder.

The compositing material may be referred to as a "non-ceramic" although in some circumstances such materials might be considered as refractory ceramics. These materials are of two types, metallic and non-metallic. The term "non-ceramic" is thus employed particularly to include materials such as metals but also embraces materials different from the basic ceramic composition. Some materials, e.g., titanates, may be considered as ceramic in certain applications and as "non-ceramic" in a different application. Because the materials employed are generally sintered at elevated temperatures, they may also be referred to as sinterable refractories which are thermally compatible. Not more than one will usually be metallic and at least one will usually be electrically non-conductive.

The volume percentage occupied by the components of the respective green tapes is apparently an important criterion in that they should be relatively closely matched. In general, each material used will fire to at least 80 percent of theoretical density and preferably 90 percent or more depending in part on the particle sizes used, and the extent of sintering at the firing temperature.

In this process it is necessary that the male die be applied to the non-ceramic, e.g., metallic, tape so that pieces are punched from it and lodged in the ceramic-containing tape.

The two tapes are die-stamped simultaneously to an extent sufficient only to offset the non-ceramic (e.g., metallic) tape into the ceramic tape. The two green ceramic tapes are termed "tapes" because produced in tape form by the process of U.S. Pat. No. 2,966,719. They may also be referred to as green sheets. They should be relatively closely matched with respect to the volume percentage of the respective components which will usually be at least 70 percent and is preferably higher. The non-ceramic tape may be further partially matches to the ceramic tape with respect to the coefficient of thermal expansion by the incorporation of moderate amounts of the ceramic material. The amounts will vary from 0 up to about 30 percent or so by weight of alumina or other ceramic material in a molybdenum or tungsten metal tape and broadly will be of that range in any case. Those having skill in the art will recognize that such additions may be contraindicated by reactivity characteristics or where other more important properties would be sacrificed by such additions. In the event that slight adjustments are desired, one or the other of the green tapes may be slightly further compacted to effect some changes in the particle density, volume percent, or percent of theoretical density. In general, the tapes are not highly compactable because of the polymeric binder. It must also be recognized that the two sheets will usually have to be of substantially the same thickness or caliper within the range of about 0.002 to 0.1 inches (0.05 to 2.5 mm.) at least within about 5 percent or less. Greater differences in thickness than about 5 percent may be employed to achieve specific results such as a protruding inset (non-ceramic sheet of extra thickness) or a depressed inset (non-ceramic sheet of lesser thickness). Methods of operation with these special conditions will be evident from the other teachings herein.

The procedure of the invention offers the advantage of producing metallic insets which are of essentially the same particulate density and hence have about the same permeability to gases as the ceramic itself. Hermeticity is thus more readily achieved particularly when a range of particles sizes are used. The process of the invention further offers the advantage of being able to provide relatively thick buried or exposed electrical conductors which therefore have relatively lower resistance than conductors produced solely by screening operations. The process of the invention also permits the use of compositing materials, i.e., other than the base ceramic, which are non-metallic such as, for example, titanates, beryllia, ferrites, as well as other useful materials for electrical, thermal or magnetic properties. Furthermore, the processes of the invention and green items produced employing this process may be further handled using conventional operations such as screen printing, punching, cutting, and the like. The process of the invention is particularly adapted for insertions having a width of at least 0.002 inches (0.05 mm.) and upward.

As pointed out, the process of the invention is not limited with respect to the use of particular materials for ceramics and non-ceramics but it is illustrated herein particularly in terms of alumina and tungsten composites. The alumina may be of better than 90 percent purity, for example, a 94 percent alumina containing additions of greater or lesser amounts of talc, clay, and/or calcium carbonate. A typical tape will contain about 90-95 percent of alumina of 90 percent or greater purity in a polymeric binder such as plasticized polyvinyl butyral. Plasticizers include polyalkylene glycol ethers, dioctyl phthalate and other relatively non-volatile materials conventional to the field of polymers. With molybdenum or tungsten as the conductors, higher purity alumina firing at higher temperatures may be used. Firing of these composites requires a non-oxidizing atmosphere and temperatures of about 1650.degree. C. A typical tungsten tape may include about 95 to 99 percent by weight of tungsten and alumina together with a plasticized polyvinyl butyral binder. Both tapes contain about 70-80 volume percent of solids. Similar tapes are made using molybdenum and other metallic and non-metallic compositing materials.

Those skilled in the art will readily recognize that the compositions used together must be thermally compatible in the sense of being firable together. Some sintering must occur in the higher melting before the other is melted entirely. Ceramics are commonly typified by alumina. As noted above, non-metallic materials which may be employed in the process of the invention include, but are not limited to, such materials as titanates, beryllia, and ferrites. Some of these may be employed together. The metals which are most used are molybdenum and tungsten because they are compatible with alumina. It is contemplated that composites may include three or more component compositions.

It is possible to use metals which sinter at lower temperatures with suitably maturing ceramics. It is also contemplated to use substantial proportions of materials melting at temperatures below that employed for firing if they are combined with a sufficient amount of higher melting metal so that the metallic portions of the structure maintain shape during firing and do not bead up or run out of the piece. Thus, a structure of molybdenum impregnated with a lower melting metal may include molten metal in a molybdenum matrix at the firing temperature but will solidify on cooling.

In the production of green tapes which are used in the process of the invention, organic binders are employed which are thermoplastic polymeric materials which depolymerize at temperatures well below the ultimate temperature of firing. Depolymerization at least below 500.degree. C. is necessary and maximum temperatures of about 350.degree. C. are preferred. A useful illustrative polymer is polyvinyl butyral which may be plasticized with any desired plasticizer as may be needed for convenience in handling. Generally plasticizers are of relatively low volatility. In general, the green tapes which are used are described as having a leathery consistency. They are generally more or less tough at ambient temperatures and soften at more elevated temperatures so that they will adhere under mild pressure and heating. They should be self-sustaining as normally used. It will be recognized that the process will give scrap metallic tape and that scrap can normally be reprocessed to give fresh tape.

The process is now further described by reference to the accompanying drawings wherein:

FIGS. 1 and 2 show a top and side view respectively of a small plug-in package unit produced employing the process of the invention.

FIG. 3 shows the underside of the package unit of FIGS. 1 and 2.

FIG. 4 shows the cross section along line 4-- 4 of FIG. 3.

FIG. 5 shows the bottom side of a differently designed package unit from that shown in FIG. 1 and;

FIG. 6 shows the cross section along line 6-- 6 of FIG. 5.

FIGS. 7, 8 and 9 show the successive positions of the dies in the process producing the article of FIGS. 1 - 4 inclusive.

FIG. 10 shows the underside of the male die employed in FIGS. 7 - 9.

FIG. 11 shows a green sheet produced in the process of FIGS. 7 - 9 inclusive before screening and FIG. 12 shows the same sheet after the screening operation.

FIGS. 13, 14 and 15 show the successive position of the dies and;

FIG. 16 shows the configuration of the male die for production of one of the two green sheets employed in the article of FIGS. 5, 6 and 7.

FIGS. 17, 18 and 19 show by the die positions of the process and FIG. 20 shows the male die configuration for production of the second sheet employed in the article of FIGS. 5 and 6.

Referring now to FIGS. 1 and 2, it will be seen that the plug-in unit illustrated has a mounting pad 12, connection fingers 14, collar 16, base 10 and prongs 18. The prongs 18 are shown with an enlarged head portion 20 which is attached by brazing, soldering, or welding to the underside of base 10. The underside view is shown in FIG. 3. The cross section of the plug-in unit along line 4--4 is shown in FIG. 4 and indicates that the pad 12 completely penetrates base 10 as does also the via hole 22 to the lower side of which the prong 18 is attached as indicated. The surface of ring 16 may be provided with a metallic coating (not shown) for purposes of attaching a suitable lid.

A somewhat modified design of this plug-in package is shown in FIGS. 5 and the cross section thereof along line 6--6 shown in FIG. 6. In FIG. 6, it will be seen that pad 12 penetrates only half way through base 10 and that contact fingers 14 also penetrate to the same depth through the base. This is attained by constructing base 10 in two layers which are then joined together. In the one layer, the contact fingers 14 and pad 12 are inserted through a sheet of green ceramic of suitable thickness and through the other sheet the via holes 22 are punches. Subsequently the two are joined in correct register and pressed to consolidate. Because this consolidation makes the ceramic material essentially integral, no line of demarcation separating the two parts is shown in this drawing. The production of these two parts is illustrated in FIGS. 13 - 20.

Referring now to FIGS. 7, 8, 9 and 10, this describes a method for the production of the base piece used in FIGS. 1, 2, 3 and 4. Male die 30 which is shown in its underside view in FIG. 10 is provided with pins 32 and central square portion 34 and mating female die (30), now shown separately, is provided with square opening 44 and holes 42 corresponding to parts 34 and 32 respectively of the male die. In FIG. 7 these are placed with respect to a green ceramic sheet 50 and green non-ceramic and, in this case, metallic sheet 52. It will be seen that these two sheets are of substantially the same thickness. Exemplary of such sheet materials would be as indicated above, alumina 94 percent in a binder of a few percent of polyvinyl butyral and molybdenum metal to which may be added up to about 30 percent of the 94 percent alumina also in a similar or different binder, both being to an extent of approximately 70 - 80 percent by volume in the tape compositions. In FIG. 8 it will be seen that die 30 has been advanced to the point where the male member has penetrated through the metallic tape 52 and forced slugs 60 and 62 corresponding to the square pad and via holes respectively from metallic tape 52 into ceramic tape 50. At the same time, slugs 70 and 72 from the ceramic tape are forced into the female die. In FIG. 9 the male die has been withdrawn leaving the metallic slugs lodged in the ceramic tape. The two tapes are now separated and, as noted hereinabove, the metallic scrap can be reprocessed. The slugs knocked from the female die are of insufficient value to warrant recovery and moreover may bear some contamination with metal which would be undesirable in the ceramic base.

Referring to FIG. 11 is seen sheet 50 with plugs 60 and 62 corresponding to a pad and via holes lodged in it. The sheet is shown with broken edges to emphasize that these drawings are diagramatic to the extent that several such pieces may be made simultaneously using dies which are multiples of the single one shown in FIG. 10.

FIG. 12 shows the green sheet of FIG. 11 on which a pattern has been screened to provide the contact fingers of FIG. 1 and connect them to the via holes 62 and which has been cut out before application (not shown) of ring 16 in FIGS. 1 - 4.

FIGS. 13 to 20 illustrate the operations for producing the plug-in package of FIGS. 5 and 6 by forming two ceramic layers which are then combined by basically conventional methods. First, male die 80 in FIG. 16 having central square portion 84 and connector pins 86 and female die 82 are positioned respectively over and under metallic sheet 90 and ceramic sheet 92 as shown in FIG. 13, then male die 80 is advanced by the thickness of sheet 92 to lodge slugs 94 and 96 from sheet 90 in sheet 92 and die 80 is then withdrawn. The sheets 90 and 92 are then separated (not shown).

Likewise, in FIGS. 17 to 20, male die 100 in FIG. 20 having pins 104 and female die 102 are positioned with respect to metallic sheet 110 and ceramic sheet 112, and male die 100 is advanced by the thickness of sheet 112 and withdrawn thereby lodging plugs 106, corresponding to via holes, in sheet 112. The sheets are separated and sheets 92 and 112 are brought together in correct register and joined. Alternatively rings or collars 16 may be added before cutting from the sheets. After firing of the ceramic pieces, prongs 18 are attached by brazing to the undersides of the via holes.

It will be seen by those skilled in the art that the process of the invention may be employed with different combinations of leathery tapes to provide ceramic composites having extensive fields of utility.

Claims

What is claimed is:

1. In a process for production of a ceramic composite, the steps of:

1. preparing a first self-sustaining green tape of substantially uniform thickness consisting essentially of ceramic composition in thermoplastic polymeric binder at a volume percentage of from about 70 to 90 percent.

2. preparing a second self-sustaining green tape of substantially uniform thickness consisting essentially of compositing material selected from the group of metallic and non-metallic refractory compositions in thermoplastic polymeric binder to about the same volume percentage of from about 70 to 90 percent and;

3. simultaneously punching predetermined areas from said second green sheet and said first green sheet so that each piece punched from said second sheet is lodged in said first sheet.

2. Process according to claim 1 in which first and second green tapes are of substantially the same caliper.

3. Process according to claim 1 in which the second green tape consists essentially of metal of the group of tungsten and molybdenum with up to about 30 percent by weight of the ceramic essentially constituting the first green tape.

4. Process according to claim 3 in which the first green tape contains alumina of at least 90 percent purity in polymeric binder.

5. Process according to claim 1 wherein the second green tape is an essentially non-metallic refractory of the group of ferrites, beryllia and titanates.