US 3768048 A
An extremely lightweight microwave circuit comprising stripline printed circuitry which is highly efficient, inexpensive, and suitable for use in electronics systems for space and military applications. The microwave circuit is formed by printing on a very thin substrate such as Mylar or alumina approximately 0.005 inches thick. The circuit is assembled between two thin sheets of a foam material approximately 0.040 to 0.050 inches thick. This assembly forms a sandwich which has its outer surface copper-plated. The thin sheet of material containing the circuitry and the lightweight dielectric foam sheets on each side of the circuit are bonded together with a low loss adhesive. Holes are drilled into the assembly and these holes are also plated throughout for additional mechanical support. In addition, these holes, properly located, provide mode suppression and thus, reduce the amount of radiation of electromagnetic energy. Thick film or printed circuit techniques are used to provide the electrical paths on the thin substrate. The conductive metallic surface on the outside of the sandwich is appositioned by the use of electroless copper-plating techniques. The connectors for the electrical paths of the circuit are secured to the assembly by means of a conductive epoxy or adhesive.
Description (OCR text may contain errors)
Unite States Patent [1 1 Jones, Jr. et al.
[ 1 Oct. 23, 1973 SUPER LIGHTWEIGHT MICROWAVE CIRCUITS  Inventors: Howard S. Jones, In, Washington,
DC; Richard J. Norris, Falls Church, Va.
 Assignee: The United States of America as eme iil l l fieiet m the Army, Washington, D.C.
 Filed: Dec. 22, 1971  Appl. No.: 210,634
 US. Cl 333/84 M, 333/98 M 51 Int. Cl HOlp 1/l6,1-101p 3/08  Field of Search 333/84 R, 84 M  References Cited UNITED STATES PATENTS I 3,310,748 3/1967 Putnam.. 333/84 M X 3,654,573 4/1972 Graham 333/84 M X 2,994,050 7/1961 Ayer et al. 333/84 M 3,142,808 7/1964 Gonda 333/84 M X 2,872,391 2/1959 l-lauser et al. 333/84 M UX OTHER PUBLICATIONS Dinella, Method ,of Grounding a Printed Circuit Board, Western Electric Technical Digest No. 16, Oct. 1969, page 1 Patrick, Flexible Strip Transmission Line, IBM Technical Disclosure Bulletin, Vol. 2, No. 6, Apr. 1960, pp. 35,36
Primary ExaminerPaul L. Gensler Attorney-Harry M. Saragovitz  ABSTRACT An extremely lightweight microwave circuit comprising stripline printed circuitry which is highly efficient, inexpensive, and suitable for use in electronics systems for space and military applications. The microwave circuit is formed by printing on a very thin substrate such as Mylar or alumina approximately 0.005 inches thick. The circuit is assembled between two thin sheets of a foam material approximately 0.040 to 0.050 inches thick. This assembly forms a sandwich which has its outer surface copper-plated. The thin sheet of material containing the circuitry and the lightweight dielectric foam sheets on each side of the circuitare bondedtogether with a low loss adhesive. Holes are drilled into the assembly and these holes are also plated throughout for additional mechanical support. In addition, these holes, properly located, provide mode suppression and thus, reduce the amount of 1 radiation of electromagnetic energy. Thick film or "printed circuit techniques are used to provide'the electrical paths on the thin substrate. The conductive metallic surface on the outside of the sandwich is appositioned by the use of electroless copper-plating techniques. The connectors for the electrical paths of the circuit are secured to the assembly by means of a conductive epoxy or adhesive.
1 Claim, 3 Drawing Figures SUPER LIGHTWEIGHT MICROWAVE CIRCUITS 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 Conventional microwave circuit assemblies are made from copper cladded epoxy, Teflon, Kapton, and fiber glass boards; and have high energy losses and the dielectric constant varies from 2.5 to 4.0.
Since all printed circuits employ the conventional techniques of construction wherein heavy boards are housed in heavy metal housings, there has been no reasonable way of reducing their weight. However, in the last few years, there has been an increasing demand for lighter weight, higher efficiency, microwave andconventional circuit networks for various applications among which space and military uses are prime. Heretofore, there has been no technique described or invented which makes full use of a material with good physical and electrical characteristics applicable to the forescribed problem, and at the same time, adaptable to electroless plating.
It is therefore an object of this invention to provide a super lightweight microwave circuit.
It is also an object of this invention to provide a super lightweight microwave printed circuit using thick film techniques.
It is a further object of this invention to provide a super lightweight microwave circuit for use in electronics system forspace and military application.
It is a further additional object of this invention to provide a super lightweight microwave circuit which is highly efficient and comprised of low loss dielectric materials. 1 I
It is also a further object of this invention to provide a super lightweight microwave circuit using metalizationscomprising thin films.
It is moreover an object of this invention to provide a super lightweight microwave circuit which is inexpensive;
It is an additional object of this invention to provide a new and novel super lightweight microwave printed circuit manufactured on a Mylar substrate and having mechanical support, mode suppression and radiation control as a matter of structure.
It is a further additional object of this invention to provide a new and novel super lightweight microwave circuit sandwiched between a lightweight foam material and electromagnetically shielded with a copperplating appositioned to the assembly by electroless deposition.
These and other objects of this invention will become apparent from the foregoing specifications and appended claims.
SUMMARY OF THE INVENTION This invention relates to an assembly of a microwave circuit, and more particularlyQto a lightweight assembly for a microwave circuit appositioned to a substrate for use in electronics systems for space and military applications. A plurality of conductive paths for microwaves are appositioned onto a thin substrate comprising either Mylar or another suitable material approximately 0.005 inches thick. This circuit is assembled between two thin sheets of foam material forming a sandwich structure. These sheets are held together by means of a low loss fluid adhesive. The entire assembly or sandwich is plated with 'a conductive metal coating. Holes are drilled into the sandwich and plated throughout to provide mechanical support, mode suppression and reduction of the amount of electromagnetic radiation therefrom. Conductive and resistive materials define the electrical paths appositioned to the substrate. These materials are applied using thick-film, thin-film, or conventional printed circuit techniques. Plating of the outer surface of the sandwich is provided by electroless copper-plating techniques. Connectors are secured to the sandwich and connected to the electrical power input paths by a conductive epoxy.
I BRIEF DESCRIPTION OF THE DRAWINGS 1 The specific nature of this invention as well as other objects, aspects, uses, and advantages thereofwill clearly appear from the following description and from the accompanying drawings, in which:
FIG. 1 is a perspective drawing of one embodiment of the circuit assembly.
FIG. 2 is a cross-section of the assembly shown in FIG. 1 along the line 22.
FIG. 3 is a view of the interface between the connector and the electrical paths on the substrate along the line 3-3.
DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is cut away view of a particular embodiment of a super lightweight microwave circuit using thick film metalizing techniques. The circuit assembly in this figure comprises a fiat thin sheet 'of Mylar l3 approximately 0.005 inches thick sandwiched between two thin sheets polystyrene foam 12 which are approximately 0.040 to 0.050 inches thICk.'ThIS sandwich assembly is held together by means of a dielectric, low energy loss, 'epoxy bonding material 11. This bonding material is applied to the inside surfaces of the polystyrene sheets 12 (See FIG. 2). The Mylar substrate with the electrical paths of metalization 15 thereon is inserted between said sheets 12 and said sheets are pressed together and thebonding epoxy allowed to solidify. Of course, before the sandwich is assembled, the metalizations defining the conductive electrical paths 15 are appositioned to the Mylar substrate 13 by silk screening (See FIG. 3). The tab which is the center termination of the connector 14 is epoxy bonded to the electrical-path 15 prior to insertion of the substrate 13 between the polystyrene sheets 12.
Now referring to FIG. 2, once the assembly is complete with respect to the substrate 13 sandwiched and bonded between the polystyrene sheets 12 and the connector 14 attached to the substrate 13 and to the polystyrene sheets 12, the polystyrene 12 is plated. Plating of the polystyrene foam material is accomplished by electroless deposition techniques already dis-closed in the referable art. The connector 14 is suitably covered before plating to prevent undue buildup of metalization on the closely controlled dimensions thereof. The. plating material 10 applied or appositioned to the surface of the sandwich is copper. It is applied by electroless copper plating techniques.
Once the initial layer of copper is deposited onto the outer surface of the sandwich, additional layers of other materials such as silver gold, ead, etc., are deposited by electroplating onto the copper layer.
Substrates that are used in developing circuits of this type include those made of Mylar, Kapton, glass, beryllia, and alumina. Various foams are available as dielectric mediums into which the circuit may be inserted. Among these materials are polyurethane foam, certain acrylics, polystyrene foam, and rubber foam. Polystyrene foam is particularly suited for this application because of its low loss and relatively stable dielectric constant of 1.05 and because of its closed cell structure is particularly applicable to polishing and electroless plating.
In addition to using silk screening and thick film printing techniques to deposit the metalization 15 which provides the electrical paths on the substrate, other techniques such as vacuum deposition of thin films and sputtering of thin films onto a substrate selected from the group above is useful in the development of certain types of circuits. Furthermore, the electrical paths of metallization 15 may be also developed and defined by substractive etching techniques.
When the metalization 15 on the substrate 13 is able to withstand soldering temperatures the connector 14 amy be soldered to the substrate 13. This situation is particularly true in the case of the use of alumina or beryllia as a substrate. It is noteworthy to point out at this point that the epoxy adhesive adjoining the thin sheets of polystyrene foam is only applied to those surfaces not touching the substrate 13. Furthermore, it may be noted that a recess in one of the polystyrene members 2 conforms to the shape and thickness of the substrate 13 (See FIG. 2).
As pointed out earlier and referring again FIGS. 1 and 2, once the assembly is completed the outer surface of the sandwich is plated with copper using electroless chemical deposition. This provides electromagnetic shielding of the internal microwave circuitry without the use of heavy and expensive machined steel or aluminum boxes. Holes 20 are drilled into the sandwich structure and plated throughout to provide mechanical support for the assembly. These holes are drilled before plating of the assembly and at locations along the periphery of the circuit substrate 13. Other holes 21 when properly located, in addition, provide for mode suppression and also tend to reduce the radiation of electromagnetic energy from the circuit on substrate 13. This styrofoam material in addition to providing a lightweight means for electromagnetically shielding the circuit provides a relatively stable dielectric environment for the microcircuit.
It is to be understood any adhesive suitable for adjoining styrofoam may be used toadjoin the styrofoam sheets herein described. The inventor wishes it to be understood furthermore, that he does not desire to be limited to the exact detail of construction shown and described, herein, for obvious modifications will occur to a person skilled in this art.
What is claimed is:
1. A super lightweight microwave circuit comprising a dielectric substrate, at least one electrical path appositioned to said substrate, a conductive material defining said electrical path, two rigid sheets of foam, said substrate being rigidly held between and surrounded by said sheets, said substrate and said sheets forming a sandwich having an outer surface, a conductive metal plating appositioned to said outer surface, means for providing mode suppression and control of radiation from said electrical path, said means comprising at least one hole having a conductive metal plating on the surface thereof, said hole further including a closure at the bottom thereof, said closure being rigidly located above said electrical path and within said foam, and further comprising a plurality of holes extending completely through said foam, last said holes having conductive metal plating throughout for providing mechanical support for said sandwich.