Glass To Metal Seal Hermetic Feedthrough
An electrical conductor that carries signals and electric power through an enclosure or environmental barrier is named a feedthrough. For some applications like aerospace and defence, the feedthrough must be protected against extreme conditions and be engineered for very high reliability. They require robust seals that maintain electrical integrity and long life environmental sealing. This is often the kind of application where glass-to-metal-seal technology is best applied. Feedthroughs require hermetic or airtight seals that can withstand severe conditions such as high temperature, high pressure, vibration, moisture, and difficult chemical environments.
Glass-to-metal-seal (GTMS) feedthroughs have high reliability and high-performance capabilities. Glass-to-metal sealing is a technique to hermetically insulate electrical conductors that pass from one side of a barrier to the other side using special controlled expansion alloy wires called glass-to-metal sealing alloy wires. The glass melts to both the package and the pin and acts as an airtight barrier while providing the insulation needed between the housing and the pins. Glass is an excellent electrical insulator and also has low thermal conductivity. This combination enables efficient electrical signal and power transmission in severe situations like high pressure and exposure to highly corrosive chemicals.
There are hermetically sealed feedthroughs that are used to conduct power or signals through the metal walls of vessels of liquified gas tanks or through the walls of power plants. The seals for these applications must operate reliably for the long term.
An airtight hermetic seal between glass insulator and a metal conductor or package is called a glass-to-metal seal (GTMS). These seals are commonly used when reliability and long life are important, but the hash use environment makes conventional electronic packaging inadequate.
Why Use Hermetic Seals
Unlike plastic, hermetic products protect against various environmental conditions, including changes in atmospheric pressure, humidity/moisture, soil/grime, and other natural hazards that would otherwise disrupt electrical connections or damage delicate electronics within a hermetic product. Hermetics are constructed from materials that offer continuity of use over a long period of time so you can be rest assured that your electronics are secure and safe.
Product applications that are susceptible to harsh environmental conditions and whose disrupted functionality could cause catastrophic outcome require the use of hermetics. For example:
- The Petrochemical industry uses hermetics to protect sensors and electronics in downhole drilling exploration equipment. Exposed to dirt, oil, grime and extreme pressure, hermetic connectors, terminals and headers, as well as microelectronic packages are able to meet these stringent requirements
- The Aerospace industry uses hermetic connectors, hermetically sealed terminals and hermetically sealed headers in many systems within an aeroplane. Hermetic connectors are used to ensure that the integrity of delicate electronics within a black box are maintained in case the contents of the box are ever needed. Additionally, hermetic connectors are used in sensor systems within fuel systems – fuel tanks – to help notify and prevent the seepage of fuel
- The Optical Networking industry uses hermetics to protect delicate circuitry in the infrastructure currently being installed underground
- The Medical industry uses hermetics within the human body to protect ensure electronic functionality in pacemakers and hearing devices
- The Automotive industry uses hermetics to ensure sensor functionality in rollover devices and airbag equipment
Hermetic Products Process
In the basic manufacturing process, Hermetic Seals are formed by assembling sets of component parts on graphite fixtures that hold components in place:
- Metal contacts or pins
- Glass beads or high temperature co-fired ceramic (HTCC) feedthroughs
- Housing, shell or composite material
- Special electronic low-temperature braze preforms
- Glass-metal sealing alloy wires with coefficient of thermal expansion similar to glass
- Assemblies are put through controlled atmospheric furnaces at temperatures that vary from brazing temperatures to approximately fusing temperatures to high temperature brazing.
- In the case of glass-to-metal seal, as an assembly cools, a physical and/or chemical bond is formed between the glass and the shell, as well as the glass and the sealing alloy wire
Hermetic products are designed to meet strict specifications and offer protection of sensitive, high precision devices. High reliability of hermetics in providing protection in harsh environmental conditions is unmatched. Corrosion proof hermetic packages, hermetic connectors, and hermetically sealed terminals and headers are designed to withstand rugged surroundings and are perfectly closed or sealed to offer airtight and watertight protection to meet today’s hermetic requirements.
Glass-to-metal-seal feedthroughs can be broadly classified as power or signal feedthroughs.
Power feedthroughs are mainly to transmit energy, often at high voltage or current. Its conductor is generally large to support 5 to 150 Amps applications. This kind of feedthrough consists of metal-glass joints wherein the glass serves as an electrical insulation between the connection flange and the sealing alloy conductor. The seal provides isolates the environments on either side of the feedthrough barrier.
Instrumentation feedthroughs, on the other hand, are for electrical signals including thermocouples that are low-current (milliamp), low- or high-voltage signals from low- or high-impedance sources.
RF feedthroughs are a special type and are used for high frequency electrical signals. The design is a single pin with a metal shroud calculated on the basis of the glass dielectric constant to achieve a 50 or 75 ohm impedance at frequencies approaching 100 GHz. This kind of feedthrough uses GTMS technology and is often used in commercial and military applications with environments that need reliable and rugged hermetic sealing.
Glass-to-metal sealing has been in existence for a long time, with the vacuum tubes being one of the earlier applications. The technology is applied to a wide range of uses from glass diodes to hermetic electrical feedthroughs used in vacuum packages. The result is a seal that can withstand high operational pressure or extreme temperature.
Creating a robust seal requires a strong chemical bond between the glass and the metal as well as a matching of the coefficient of thermal expansion (CTE) between the different materials. CTE is a measure of how the component’s size changes as its temperature varies. Certain speciality glass-metal sealing alloys, and controlled expansion alloys and dumet wires. GTMS feedthroughs are formed at high temperatures in excess of 900°C. Management of any thermal mismatch that could happen during the sealing process is important to avoid any residual tension or stress on the joint that could lead to voiding or separation between the glass and the metal.
Glass to Metal Fusing Processes
Since most applications required devices to operate in a wide range of temperatures, it is vital for the components to expand and contract at very similar rates. This would ensure that no stresses or misalignments occur during production or operation. These are called matched glass-to-metal seals.Feedthroughs must work over the full operating temperature ranges, and generally must work flawlessly over many thousands of thermal cycles. A common material system used in the industry is Alloy K per ASTM F-15 (Fe-Ni-Co alloy) with a Borosilicate glass. The coefficient of thermal expansion of Alloy K is a very good match to glass over a wide range of temperatures.
Compression Glass-To-Metal Sealing
If the feedthrough needed focuses on pressure capability, corrosion resistance or conductor strength then a compression type of GTMS may be a better option. Glass has the interesting property of getting stronger when it is compressed. The materials are specifically engineered to result in positive pressure between the glass to metal joints which can enhance overall system strength in difficult mechanical environments.
Applications of GTMS Feedthrough
Engineers in some advanced industrial applications utilize glass-to-metal seal feedthroughs in the most critical applications. GTMS seals protect sensors from highly corrosive chemicals and excessive high temperature. These feedthroughs have the hermeticity to prevent electronic failure due to extreme environmental conditions. Example applications include humidity, pressure, and flow sensors.
Various sensors use GTMS feedthroughs in the automotive industry. Acceleration, gyro, flow and pressure sensors rely on the high-pressure and resistance capability, i.e., vibrations, temperature fluctuations.
The glass-to-metal seal feedthrough has proven to be effective for implantable medical devices. The feedthroughs are corrosion resistant and can be biocompatible. Biocompatibility means that the material is compatible with a living tissue and does not produce any toxic or immunological results when exposed to body tissues and fluids. The use of glass-to-metal seals feedthroughs in medical devices prevent the diffusion of gases which results in degradation of the electronic component or malfunction or corrosion of other parts. In autoclaves, the feedthroughs can withstand shock and high temperature as well as isolate humidity from the autoclave environment.
GTMS feedthroughs are also commonly utilized in endoscopes and surgical instruments. Endoscopes are used to examine the interior of a hollow organ or cavity of the body.
Aerospace and Defense Applications
The aerospace and defense sectors find use for glass-to-metal seal feedthroughs in a wide range of components where system reliability is important: sealed relays, vacuum feedthroughs, pin feed-through terminals, thermal sensors, beam-line feedthroughs and injector tubes, LED's, thermocouple feedthroughs, AC/DC power distribution assemblies, packages for electronic components and time delay devices.
Space-grade connectors are spin-offs of military-grade connectors. As the space program becomes more and more extensive and sophisticated, hermetic connector design engineers are now modifying mounting and shell formats based on specific requirements. Adjustments to existing formats are made in consideration of high shock, long flight duration and vibrations.Based on "Sleeves to replace grease in ground glass joints" Article in Journal of Scientific Instruments.