This invention relates generally to glass compositions and more particularly to a high-thermal-expansion, high-durability glass with a sufficiently low dielectric constant to be suitable for use in hermetically-sealed, radio-frequency (RF) applications.
Typically, high reliability RF circuits are packaged in hermetically-sealed steel housings to protect the circuits from corrosive environments and humidity. These sealed housings use electrical interconnections that are also hermetic that penetrate the housing, and are needed to transfer electrical signals into and out of the housing. Hermetic RF-feedthrough interconnections are usually incorporated to transfer the high-frequency electrical signals. The RF interconnections generally comprise an electrically-conductive center conductor pin, an outer electrically conductive housing, and an electrically insulating material, such as an insulating glass, hermetically sealed to the center conductor and the outer housing. These interconnections are typically manufactured from weldable stainless steels and high-temperature, silicate-based, insulating glasses that require sealing temperatures above 900.degree. C. The RF interconnections are installed into the hermetically-sealed housings, which are also made of steel. These packages are used in aerospace applications, such as communications satellites, microwave communications equipment, and military communications and radar systems which require a hermetic seal to avoid contamination and corrosion of the RF devices inside. Being made of steel, these packages contribute to the heavy and undesirable weight of the final application assembly.
Aluminum alloy connectors and housings, being of lighter-weight, are preferred but have not been used for these hermetic assemblies because, heretofore, direct, weldable, hermetic, RF connections with suitable electrical and chemical durability properties joined with aluminum bodies could not be made. A hermetically-sealed, aluminum, RF interconnection must employ an insulating material with both suitable chemical properties and suitable electrical properties. In particular, the dielectric constant of the material must be sufficiently low to permit use in RF applications.
Previous RF connections were generally either made with steel bodies or made with aluminum bodies with a transition joint made of steel between the insulating glass and the aluminum body. The transition joint is a seal made between steel or iron-nickel alloy rings and pins where the seal is explosively bonded to an aluminum ring and then welded into the aluminum housing package. The use of transition joints requires additional processing steps compared to the traditional manufacture of hermetic steel RF connections.
Sharp et al., in U.S. Pat. Nos. 5,041,019 and 5,109,594, issued on Apr. 20, 1991 and May 5, 1992, respectively, describe a connector that utilizes a transition joint where a layer of steel is laser welded to a layer of aluminum, thus enabling a connector to be made that has substantially an aluminum alloy body. These inventions are hermetically sealed transition joints for use with a microwave package. The inventions are seals between steel or Kovar rings and pins. These seals are then explosively bonded to an aluminum ring, which is then welded into the aluminum package. Electronic signals are allowed to enter and exit the package via pins contained within the feed-throughs and power connectors. The feed-throughs contain a pin of desired metal surrounded by a bead of glass which is surrounded by a layer of cold rolled steel, stainless steel and/or iron-nickel alloy. This layer is laser-welded to a second layer of an aluminum alloy. The pin serves as an electrical connection to communicate with the electronic circuit inside the package. The glass provides electronic isolation between the pin and the package. Manufacture of these connectors with transition joints requires additional processing steps for making the steel-to-aluminum joint compared to connectors where the glass is directly attached to a steel or aluminum package. Moreover, the designed package is a microwave package and is not designed for RF applications, where the impedance must be closely matched along the entire connector length.
The reliability of the feed-through with a transition joint is typically very poor. Besides the difficulty of a good attachment during manufacture, these joints commonly fail upon thermal cycling. There are two recognized reasons. First, poor nickel and/or gold plating of the packages, feed-throughs and power connectors results in excessive leaching of the plated metals during soldering, thereby inhibiting soldering. The second reason is mismatched expansion between the aluminum or aluminum alloy of the package and the feed-throughs and power connectors. The coefficient of thermal expansion of aluminum/aluminum alloys is approximately 22.times.10.sup.-6 in/C/in vs that of cold rolled steel and stainless steel at approximately 12-18.times.10.sup.-6. This mismatch in expansion during thermal cycling creates stresses which causes loss of the hermeticity and expensive rework and repeat of testing. In frequent situations upon multiple recurrence, the package becomes useless and is discarded.
Viret et al., in U.S. Pat. No. 5,367,125, issued on Nov. 22, 1994, describe an hermetic connector consisting of an aluminum shell, a phosphate-based glass seal and a Cu/Be connecting pin. The connectors described utilize a vitreous or glass material with a required modifying agent for increasing the working temperature range of the glass material. Various embodiments of the invention require pre-oxidation of the aluminum body in a toxic chromic acid bath and Ni-plating and pre-oxidation of the conducting Cu/Be pin. The glass material composition comprises approximately 20 to 50 mole percent of Na.sub.2 O, approximately 5 to 30 mole percent of BaO, approximately 0.5 to 3 mole percent of Al.sub.2 O.sub.3 and approximately 40 to 60 mole percent of P.sub.2 O.sub.5. The embodiments are designed for electrical applications, rather than RF applications.
Compositions formed from glass and ceramics for sealing to molybdenum are disclosed in U.S. Pat. No. 3,957,496, issued to R. J. Eagan on May 18, 1976, and for sealing to stainless steel are disclosed in U.S. Pat. No. 4,135,936, issued to C. P. Ballard, Jr., on Jan. 23, 1979. These compositions each require temperatures in excess of 900.degree. C. to form the seal. The high seal-forming temperatures of these compositions preclude their use in practice with aluminum and aluminum-alloys, since the seal-forming temperatures are greater than the melting points of these metals.
Wilder, Jr., in U.S. Pat. No. 4,202,700, issued on May 13, 1980, discloses a glassy composition adaptable for hermetically sealing to aluminum-based alloys. The composition may either be employed as a glass or glass-ceramic and includes from about 10 to about 60 mole percent Li.sub.2 O, Na.sub.2 O, or K.sub.2 O, from about 5 to about 40 mole percent BaO or CaO, from 0.1 to 10 mole percent Al.sub.2 O.sub.3 and from 40 to 70 mole percent P.sub.2 O.sub.5. Although this composition has a thermal expansion coefficient which closely matches the thermal expansion coefficient of stainless steel, its aqueous durability or dissolution rate is relatively poor. This characteristic precludes its use in applications which require long operating lifetimes in humid environments.
Some progress has been made in developing a glass that can be hermetically sealed to materials such as aluminum alloys. Brow et al., in U.S. Pat. No. 5,262,364, issued on Nov. 16, 1993, describe a glass composition for hermetically sealing to high thermal expansion materials such as aluminum alloys, stainless steels, copper, and copper/beryllium alloys. The composition includes between about 10 and about 25 mole percent Na.sub.2 O, between about 10 and about 25 mole percent K.sub.2 O, between about 5 and about 15 mole percent Al.sub.2 O.sub.3, between about 35 and about 50 mole percent P.sub.2 O.sub.5, and between about 5 and about 15 mole percent of one of PbO, BaO, and mixtures thereof. The composition may also include between 0 and about 5 mole percent Fe.sub.2 O.sub.3 and between 0 and about 10 mole percent B.sub.2 O.sub.3.
The dielectric constant of the insulator material used to make any coaxial RF interconnection is important since the dielectric constant, .kappa., technically derived from the electrical relative permittivity of the material, controls the electrical characteristic impedance of the interconnection for a given physical geometry.
The electrical Characteristic Impedance (Z.sub.0) of a coaxial geometry RF interconnection is inversely proportional to the dielectric constant and directly proportional to diameter of the interconnection, as given by the relationship,
Z.sub.0 .varies..sqroot..kappa. log.sub.10 (D.sub.l/.sub.o) PA1 where:
______________________________________ Z.sub.0 is the Characteristic Impedance of the geometry; .kappa. is the dielectric constant of the insulator material; D.sub.i is the inner diameter of the outer conductor; d.sub.o is the outer diameter of the inner conductor. ______________________________________
Therefore, given a constant center conductor diameter and desired Characteristics Impedance (e.g., 50 ohms), a higher dielectric constant means that the outer conductor must have a larger diameter and the overall weight of the RF interconnection will increase. Advantageous are therefore glasses that can be sealed to aluminum alloys and that have low dielectric constants.
To make a direct, weldable, hermetic RF interconnection with aluminum bodies, insulating glass compositions are required that can be sealed to the aluminum at temperatures below the melting point of aluminum alloys, that have thermal expansion coefficients that can be matched to the conducting pin material, that can be impedanced-matched to make a suitable RF interconnections and that have high chemical durability, mechanical strength and low gas permeability. The melting point of typical aluminum alloys is about 550.degree. C. compared to that of a conventional silicate glass which has a sealing temperature generally higher than about 1000.degree. C. Glass transition temperatures of less than approximately 450.degree. C. are desired. Furthermore, the thermal expansion coefficient of copper and copper-beryllium alloys preferred for high electrical conductivity pins is generally higher than that of conventional silicate glasses. Thermal expansion coefficients between about 160 and about 210.times.10.sup.-7 /.degree. C. are desired.