COMPRESSIBLE COAXIAL INTERCONNECTION WITH INTEGRATED ENVIRONMENTAL SEAL

Abstract: A coaxial RF interconnect structure that is compressible in the z-axis and provides its own environmental seal against moisture and coolant. The structure (50) includes three components, a center conductor (60), dielectric spacer structure (70), and outer conductor shield (80), all fabricated of compressible materials. The center conductor is a compressible metal interconnection element formed by die compressing 1 mil diameter fine wire to a desired shape and density. The coaxial dielectric structure supports the compressible center conductor, and is fabricated from a fluorinated elastomer (FPM) known as fluorosilicone. The compressible coaxial outer conductor shield functions as an RF gasket in the form of a round flat washer surrounding the dielectric structure and center conductor.

Description: COMPRESSIBLE COAXIAL INTERCONNECTION WITH INTEGRATED ENVIRONMENTAL SEAL TECHNICAL FIELD OF THE INVENTION This invention relates to microwave interconnection devices, and more particularly to a compressible coaxial interconnection device with an integrated environmental seal . BACKGROUND OF THE INVENTION Electrically interconnecting circuit boards has conventionally been accomplished with cables or ribbons. The disadvantage to these methods are size, weight, and cost. Other transmission interconnections require a more permanent attachment, such as solders and epoxies, and have relatively narrow operating bandwidths . Removable RF interconnects typically require considerable depth and additional elements, and add weight. Moreover, separate materials and processes are conventionally required to environmentally protect these conventional interconnect devices before or after they are installed in a microwave assembly . SUMMARY OF THE INVENTION A coaxial RF interconnect structure is described that is compressible along its longitudinal axis, and provides an environmental seal. The structure includes a compressible, electrically conductive center conductor member, and a compressible dielectric member surrounding the center conductor member. A compressible coaxial outer RF conduc- tor shield surrounds the dielectric member and center conductor member. Thus, all elements of the structure are compressible along the longitudinal axis. This invention offers a new, compact approach to microwave packaging. Separate, individual hybrids can now be packaged vertically, saving valuable real estate. Other vertical bends require several process steps and a more permanent attachment such as epoxies and solders . The interconnect is completely shielded for electromagnetic interference (EMI) and coolant with little or no leakage. BRIEF DESCRIPTION OF THE DRAWING These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: FIG. 1 is an end view of a compressible interconnect structure in accordance with the invention. FIG. 2 is a side view of the structure 50. FIG. 3 is a graph illustrating insertion loss data as a function of frequency for an exemplary interconnect structure in accordance with the invention. FIG. 4 is a graph of the return loss as a function of frequency for the interconnect structure as in FIG. 3. FIG. 5 is an exploded view showing elements of an array antenna system embodying the interconnect structure of this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is a coaxial RF interconnect structure that is compressible in the z-axis and provides its own environmental seal against moisture and coolant. An RF interconnection structure 50 in accordance with the invention is illustrated in FIGS. 1 and 2, and includes three components. The center conductor 60, dielectric spacer structure 70, and outer conductor shield 80 are fabricated of compressible materials. The first component is the center conductor 60 which is a compressible metal interconnection element formed by die compressing 1 mil diameter fine wire to a desired shape and density. The resulting interconnection element pro- vides a coaxial center conductor contact that has low resistance, good redundancy of contact and mechanical compliance . The second component of the interconnection structure 50 is the coaxial dielectric structure 70 supporting the compressible center conductor 60. This compressible dielectric structure in this exemplary embodiment is fabricated from a fluorinated elastomer (FPM) known as fluorosilicone . This material is typically used as compressible environmental seals and O-ring gaskets to seal joints within various heat exchanger assemblies for automotive, aerospace and industrial applications. The material resists degradation from exposure to a wide range of fluids, including hot oils, gasoline, jet fuels and coolants. Fluorosilicone remains effective over a broad range of temperatures up to 600 deg . F. As an electrical insulator, fluorosilicone has good low frequency characteristic and is comparable to silicone in terms of thermal stability and aging. The third component 80 of the interconnection struc- ture is the compressible coax outer conductor shield which functions as an RF gasket in the form of an round flat washer surrounding the fluorosilicone dielectric structure 70 and center conductor 60. Typical RF gasket material uses either an silver or copper filled elastomer, typically silicone rubber or fluorosilicone. FIG. 2 is a side view of the structure 50. In exemplary implementations, the interconnection structure 50 can have a thickness dimension T, measured along the Z axis, in the range of 0.030 inch to 0.060 inch. The center conduc- tor 60 can have a diameter of 0.018 inch, and the dielectric structure 70 a diameter of 0.140 inches. FIG. 3 is a graph illustrating insertion loss data as a function of frequency for an exemplary interconnect structure in accordance with the invention, with a thick- ness of 0.030 inch and utilized with SMA connectors. FIG. 4 is a graph of the return loss as a function of frequency for the same device. The interconnection structure has good RF performance . The combination of the three components 60, 70 and 80 forms a coaxial interconnection structure 50 with good RF performance, and will allow up to 10 mil tolerance in the z-axis under compression. RF losses as low as 0.2 dB is achievable up to 12 Ghz with an interconnection structure in accordance with the invention. Since the connection is made under compression, this interconnect will provide its own environmental seal while maintaining the same good RF performance . An exemplary application for this invention is to provide a vertical RF interconnect between the T/R modules and planar RF feed assembly for an active array antenna as shown in FIG. 5. To reduce the risk of the compressible center conductor smearing across the face of the coaxial interconnect and potentially short circuiting the transmission line during installation, the invention is configured so that the end of the compressible center conductor 60 is recessed below the face of ther fluorosilicone dielectric 70. A solder ball or pin 116 protruding from the exposed dielectric 116 of the T/R module I/O ports 118 and the pin 102 attached to the RF feed I/O port 108 are then respec- tively inserted into the dielectric 70 to DC contact the compressible center conductor 60 while holding it in place by compression. A dielectric spacer 124 fits below the compressible interconnect. The outer shield 80 of the coaxial interconnect is in DC contact with the correspond- ing outer shields 112, 122 of the T/R module 110 and the RF feed 108 located on the surface of their housing packages. In this example, the RF feed is a strip line transmission line carried within a cold plate shown as housing 122. Finally the exposed dielectric 116 separating the pinned I/O ports and outer shield 112 of the T/R module and the dielectric substrate 108A of the RF feed will contact the fluorosilicone dielectric 70 at opposite ends of the interconnect structure. The interconnection structure 50 can operate from DC to greater than 18 GHz with reasonable loss and good match. The interconnection structure can be employed to interconnect stacked multi- layer microwave hybrid assemblies by solderless vertical interconnects with self-sealing capability against moisture and coolant. Because of the solderless nature of the interconnection provided by the invention, stacked microwave hybrid printed wiring assemblies can be realized which are easy to assemble and disassemble for rework. Exemplary applications include vertical interconnects between stacked microwave sub- strates, which can be found in radar receiver/exciter assemblies, communication subsystems, and other microwave circuitry, found in radar systems, satellites, microwave automotive electronics, missile systems and other systems where size is important, such as cellular telephones. It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.

Claims: CLAIMS What s claimed is: 1. A coaxial RF interconnect structure that is compressible along its longitudinal axis, and provides an environmental seal, characterized by: a compressible, electrically conductive center conductor member (60) ; a compressible dielectric member (70) surrounding the center conductor member; and a compressible coaxial outer RF conductor shield (80) surrounding the dielectric member and center conductor member. 2. An interconnect structure according to Claim 1, further characterized m that said compressible center conductor member (60) comprises a mass of compressed fine metal wire. 3. An interconnect structure according to Claim 1 or Claim 2, further characterized in that said compressible dielectric member (70) is fabricated of a dielectric elastomer material . 4. An interconnect structure according to Claim 3 , further characterized in that said dielectric elastomer material is fluorosilicone. 5. An interconnect structure according to any preceding claim, further characterized m that said outer shield (80) comprises a metal filled elastomer. 6. An interconnect structure according to Claim 5, further characterized in that said metal filled elastomer (80) comprises silicone rubber filled with silver or copper . 7. An interconnect structure according to Claim 5, further characterized in that said metal filled elastomer (80) comprises fluorosilicone. 8. An interconnect structure according to any preceding claim, further characterized in that the interconnect structure is employed in an active array antenna to provide an RF connection between an input/output (I/O) port of a transmit /receive (T/R) module (110) and an RF feed port (108) of an RF feed assembly, said RF conductor shield making electrical contact with an outer shield (112) of said T/R module and with an outer shield (122) of said RF feed assembly.

Inventors: HASHIMOTO MARK Y, JORGENSON ROSIE M, QUAN CLIFTON, HASHIMOTO, MARK, Y., JORGENSON, ROSIE, M., QUAN, CLIFTON
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1998
Main classification: H01R13
IPC codes: H01P1, H01P5, H01R13