Composite Epoxy Glass-Microsphere-Dielectrics for Electronic Coaxial Structures

Coaxial structures such as cables and hermetic R.F. connectors include inner and outer cylindrical conductors separated by a dielectric medium, typically of glass. It has been difficult to achieve optimum electrical performance of these devices because of lack of uniformity in the meniscus of the glass-to-metal seals which terminate the connectors, and also lack of parallelism of the glass end surfaces. Since glass has a relatively high dielectric constant (.epsilon..sub.r =5), small physical variations can lead to large variations in electrical performance.

In the prior art it is known to utilize polymeric materials such as teflon or polyethylene as the dielectric material. However, large differences in the coefficient of thermal expansion between these polymers and the surrounding metal make it impossible to obtain a hermetic seal.

It would therefore be desirable to have a low dielectric constant material for use in coaxial structures, particularly in sub-miniature type-A (S.M.A) R.F. connectors so that design tolerances could be relaxed and R.F. performance and ease of manufacturability be increased. These improvements should be accomplished without sacrificing hermiticity or mechanical strength.

In accordance with the illustrated preferred embodiments, the present invention provides a dielectric material particularly useful as the dielectric in coaxial structures such as R.F. connectors. The material utilizes an epoxy base which can be easily molded into the connector to form a mechanically rigid hermetic seal between dielectric and inner and outer conductors comparable to glass-to-metal seals. The electrical and physical properties of the material are precisely varied and controlled by introducing a predetermined concentration of hollow glass microspheres into the epoxy. In preferred embodiments of the invention, silane coupling agents are also introduced to improve performance.

FIG. 1 shows an uncured epoxy dielectric composition injected into a hollow outer conductor.

FIG. 2 shows a pair of caps with guiding central slots for the center conductor.

FIG. 3 shows an inner conductor positioned centrally by the caps and forced through the uncured epoxy dielectric.

FIG. 4 shows an R.F. connector configuration.

Initially an epoxy base is prepared by mixing an appropriate epoxy resin with a suitable curing agent. Table I shows several suitable resins, identified by their tradenames, R-400 (from Abelstik Laboratories, Gardena, California) and Epon-825 (from Shell Chemical Co., New York, New York). The chemical formulations are also shown in Table I.

Suitable curing agents are listed in Table II, again by their tradenames and chemical formulations. EMI-24 is available from Okura Co., New York, New York, Shell D and Shell Z are both available from Shell Chemical Co., and NMA is manufactured by Union Carbide, New York, New York, while POPDA can be obtained from Jefferson Chemical Co., Houston, Texas.

The several resins listed in Table I may be combined with any of the curing agents of Table II in the weight ratios shown in Table III.

A silane coupling agent such as those listed in Table IV (all available from Dow Corning Chemical Products Division, Midland, Michigan) is incorporated into the mixture in the range of 0.50% to 1.00% by weight.

At this point there is incorporated into the epoxy-silane matrix a desired density of glass microspheres. Glass microspheres are thin-walled (1-2 .mu.m) hollow air-filled spheres, typically with a particle size between 10 and 300 .mu.m. They are available, for example, from 3M Company, Saint Paul, Minnesota or Emerson & Cuming Inc., Canton, Massachusetts, and are typically fabricated of materials such as sodium borosilicate, silica, or alumina silicate. For applications in R.F. connectors, low alkaline sodium borosilicate microspheres are preferred. The size of the microspheres may be selected to produce any desired amount of electrical phase shift at the connector interface. To produce less then 2.degree. phase shift at about 25 GHz it has been found that glass microspheres in the size range 10 .mu.m-63 .mu.m are preferred. These are introduced into the epoxy-silane matrix in a ratio of about 38% by weight, with a range of between 33 wt% and 40 wt% producing acceptable results.

When the above-described composition has been thoroughly mixed, excess air is removed and the dielectric material inserted into a hollow metallic conductor. For example, in FIG. 1 a dielectric material 11 is inserted into a hollow metallic conductor 13. In FIG. 2, a pair of caps 15 and 17 including hollow central portions 19 and 21 are snapped onto the outside of conductor 13 to position a central conductor. FIG. 3 shows a solid center conductor 23 having been inserted through slots 19 and 21 in caps 15 and 17 and pushed through the uncured dielectric medium 11.

At this point the connector is placed in an oven to cure the epoxy under a pressure of 60-80 psig. Curing times and temperatures appropriate for each of the illustrative resin curing-agent combinations are shown in Table III. After curing, caps 15 and 17 are removed leaving a basic connector configuration shown in FIG. 4.

Of the various combinations of materials fabricated and tested, the preferred embodiment consists of an R-400/EMI-24/silane/microsphere composite. The weight ratio of R-400 to EMI-24 is fixed by stoichiometry at 96.15/3.85. The ratio of silane to the R-400, EMI-24 mixture should be in the range 0.9/99.1 to 1.1/98.9, with a preferred ratio of 1.0/99.0. Finally, the weight ratio of glass-microspheres to the R-400, EMI-24, silane mixture should be in the range 33/67 to 40/60, with a preferred ratio of 38/62.

In addition to a desirable low dielectric constant, the preferred composite was found to exhibit a coefficient of thermal expansion very close to that of metal conductors such as aluminium or beryllium-copper typically used in R.F. connectors. This property makes it possible to obtain a simple hermetic seal at the conductor-dielectric interfaces. Some electrical and physical properties of this preferred composite are tabulated in Table V.