Trends in engineering applications for hermeticity

Whether the primary design goal is corrosion resistance, pressure capability in underwater systems or lightweight and moisture ingress prevention in avionic systems, Martec - part of Amphenol Military & Aerospace Operations - is able to provide an effective custom-design solution.

The current trends involving high-performance sensors and signal processing electronics are already incorporating hermetic technology to ensure stable atmospheres with controlled levels of moisture, and that specific performance and reliability requirements are achieved and maintained. Also, aerospace and defence systems incorporate high-performance optical assemblies sealed to avoid contamination of optical pathways. In addition these hermetic products enable avionic devices to maintain hermeticity and avoid ingress of moisture, allowing the systems to achieve and maintain specified performance. These hermetic devices can be found across a wide spectrum of avionic applications such as aircraft, missiles and space programmes to provide system integrity and ensure no damage is caused by the ingress of moisture or contaminants. Hermetic devices are also incorporated within avionic systems such as landing gear, fuel management, engine condition monitoring including, speed and temperature probes pressure sensors, turbine temperatures monitoring, blade tip clearance measurement, torque measurement, filter performance, actuators and many others.

In oil and gas applications increasingly harsh environments demand higher levels of pressure sealing and temperature tolerance. The use of hermetic technology can provide protection to either electronics or act as a pressure barrier between sensors and electronic packages. Martec’s hermetic devices have recently been qualified to 90,000 psi and our feed-throughs have demonstrated capability to withstand a fire test at +950°C.

In other applications, vacuum systems and assemblies benefit from improved sealing made possible by hermetic devices to achieve faster pump down, lower vacuum hold power and improved base vacuum levels. These devices can be found in scientific, electronic and semiconductor component manufacturing processes.

New material developments have provided glasses that enable hermetic sealed devices to be produced in a variety of housing materials, including aluminium, titanium and super duplex alloys ensuring that specific performance requirements and end user environments are met. These include galvanic, corrosive, bio-compatibility and reduced system mass for weight critical applications.
High-strength super duplex alloys address specific industry needs for strong materials in the oil and gas and subsea applications. For instance, subsea and oceanographic hermetic devices require careful material selection to address the systems material compatibility for corrosion as well as galvanic differential requirements. These applications can also benefit from the high strength super duplex alloys to meet the systems pressure and fluid containment requirements. The development of low temperature glasses allows Martec to design hermetic devices to a wider range of conductor materials for improved conductivity and non-magnetic assemblies (e.g. paliney, beryllium copper and molybdenum). These may be found in medical, industrial and chemical applications addressing the specific industry standards and challenging operating environments.

Through Martec’s research and development, hermetic devices can  now incorporate impedance matched materials to reduce insertion loss through Martec’s patented design particularly in the case of high-speed signals. Martec’s design capabilities enable complex designs and meet customer-specific requirements. Martec’s hermetic designs provide ionization sources for advanced security devices such as analytical equipment and chemical trace equipment.
Where space is at a premium, Martec’s hermetic devices can provide a compact solution in place of an environmental style device.

Introduction

At Martec, the devices we design and manufacture incorporate glass, glass ceramic or ceramic materials to achieve high levels of fluid sealing, or hermeticity within connectors and glass to metal seals.
Hermetic devices are typically designed for use in harsh and demanding environments. They are capable of maintaining fluid separation under high differential pressure and or extremes of temperature whilst providing means of passing electrical signals and or power through the barrier.
The most common example of this is the exclusion of moisture from sensitive electronic systems.
A hermetic device can also be remarkably compact and is often specified where available space is limited. Both internal and external protrusions of a hermetic device can be significantly less than those of an equivalent environmental style connector.

Hermetic connectors are specified for a variety of environments and can be found across a wide and diverse range of applications such as satellites, aircraft, missiles, oil and gas down hole tools, industrial and medical sensing, naval to nuclear submarines.
Within the industry glass-to-metal seals are also referred to or known as penetrators or feedthroughs. A feed-throughs is a single-piece component providing direct termination on both faces whereas a connector requires a secondary mating half and is used when repeated mate and unmate characteristics are required.

Technology

A hermetic seal is defined as being air or gas tight, impermeable to fluids, excluding the passage of air, oxygen or other gases. The degree of hermeticity is usually specified as a measure of leakage, into or out of the enclosed system where the specified level is usually stated in conjunction with a specific test method and condition of use.

One of the primary requirements of hermetic packaging is to maintain the leak tightness of an electronics unit by maintaining an impermeable barrier between different fluids (usually water vapour) or vacuum. The leak tightness of any interconnect device is therefore a key contributor to the overall leak tightness of the sub-system enclosure or module.
The hermeticity of a glass or glass ceramic seal is achieved by both the impermeability of the materials (i.e. glass & metal) and by the oxide bond formed between the two at elevated sealing temperatures. One of the benefits of glass in this application is its ability to form a strong, well adhered  bond to appropriate metal oxides. Thus, a key enabler for a high-performance glass to metal seal is the ability to form a well adhered metal oxide on the parent metal substrate prior to sealing.
Image to the right shows the migration of particular intermetallic compounds across the interface of a typical glass to metal seal.
As with ingress protection defined by the IP code[1] there are different levels of hermeticity or leakage.

Hermeticity is typically defined as the amount of a substance moving per unit of time through a given cross section, otherwise known as a leak, either as a fine or a gross leak. For illustrative purposes a leak value can be compared to an indicative flow of 1cc of air over a period of time.
As a quick indicative way of converting to a helium leak rate, divide the above time periods  by three. i.e.; 10-11 cc/sec equates to 1 cc of helium flow in over 1000 years.
A leak may often be assumed as some kind of hole, or porous or gas permeable area  permitting undesired flow of gas, moisture or other contaminates. However, in some cases water vapour migration through materials can  be sufficient to constitute a significant leak path, particularly in the case of plastics or elastomers.[2] Irrespective of the source or unit of measure chosen, a ‘leak’ can cover a range of 10-1 to 10-11; which represents 11 orders of magnitude and covers several different modes of flow. Both gas species and mode of flow influence the amount of gas flowing through a given leak path. Helium, for example is a smaller molecule than air and will permeate a given leak path more readily. However, water vapour, in spite of a large molecular size is arguably more invasive due to its electron charge configuration allowing it to be readily adsorbed by many materials, including both metals and plastics, and to migrate through solid material barriers. It is for this reason that a hermetic connector should be used as part of an overall enclosure sealing strategy if moisture is to be reliably excluded.
Leak tests are defined within Mil-Std-883.[3]
The size of the leak is determined by the volume of gas that passes through the leak per second under a 1 atmosphere differential pressure. Leaks greater than 1x 10-5 atm cc/sec are generally considered gross leaks whereas those smaller than 1x10-5 atm cc/sec are considered fine leaks. In practice gross leaks are determined, isolated and quantified using liquid bath and bubble detection. As an indicator for gross leaks a dye penetrant test can also be used. For fine leaks mass spectrometers are used with helium gas. Modern mass spectrometers are now capable of detecting some gross leaks.

It is recognised that connectors made from glass, metals and ceramics are considered hermetic. However, with all types of connectors over a length of time they will allow the passage of moisture and or other gases through the hermetic barrier as all materials are permeable to gases to some degree. To ensure that your system meets all specified needs discuss your exact hermetic requirements with Martec.

In addition to maintaining hermeticity, Martec’s connectors and glass-to-metals seals can be specifically designed to withstand high pressures both at ambient and elevated temperatures.

Types of sealing

There are two categories of glass-to-metal seals: compression seals and matched seals. The former, as the name suggests, have materials of differing coefficients of thermal expansion normally arranged such that the outer shell has the higher value. Cooling from the high glass forming temperatures necessary to achieve wetting of the shell surface therefore results in residual compressive stress in the glass, producing a robust seal capable of tolerating high levels of shock and vibration. Due to glass being extremely strong in compression, such seals can withstand very high pressures.

When the glass and the metal devices have the same or similar coefficient of thermal expansion, the seal derives its strength from the chemical bond between the glass and the oxide formed on the metal parts. This “matched seal” can be the weaker of the two glass-to-metal hermetic seals which is dependent on material combinations.
Some materials combinations used in the manufacture of matched seals cannot resist large differential pressures.

Alternative materials for hermetic sealing

Ceramic glasses (or glass ceramics) are a relatively new class of material discovered by the company Corning in the 1950s. A correct thermal processing of these materials results in developing ceramic crystals in a surrounding glassy matrix. Some versions may be used to provide insulating glasses with coefficient of thermal expansion matched to a wide variety of materials. Some ceramic glasses have higher chemical durability and can resist higher temperatures than conventional glasses.
Ceramic seals are often used in high voltage applications as large stand-off or clearance distances can be achieved by protruding or self-supporting insulators. In general, ceramic materials have a high dielectric constant and can perform well as electrical insulators, however some cannot be machined and are formed by either moulding or pressing powder. The majority of engineering insulating ceramics have low thermal expansion coefficients and do not form ‘wetted’ bonds to most metals. Metallisation or active brazing is necessary to form hermetic seals to these materials. The braze or metallised joint is therefore subject to cyclic stress if thermal cycling is experienced by the assembly.
Epoxy - Certain epoxy resins can create a hermetic bond to copper, brass, or epoxy itself with similar coefficients of thermal expansion and are used in the manufacture of hermetic electrical and fibre optic seals. Epoxy hermetic seal designs can be used in applications for low or high vacuum or differential pressures. With careful design leak rates approaching those of glass or ceramic seals can be achieved. Epoxy seals also offer the design flexibility of sealing either copper alloy wires or pins, instead of the much less electrically conductive nickel-iron pin materials required in glass hermetic seals. However, epoxy seals have a higher outgassing and permeability rate than glass or metal and a more limited operating temperature range typically +70°C to +125°C (200°C for special epoxy).

A typical, sealed avionics LRU will have numerous interconnects, and a lid or removable panel to facilitate assembly. All will be potential leak paths and the overall hermeticity of the package will be the sum total. A number of sealing methods are available, including, in approximate order of effectiveness
• fused metal seals (soldered / weld). Note, that fused metal seals can be designed to be re-usable, for a limited number of times. A system can be designed to have elastomeric seals functioning during test / qualification which are augmented by an EB weld or laser welded seal at the final stage of production.
• metal rings (solid, c profile) with or without soft metal plating.
• soft metal seals (e.g. solder / indium)
• elastomeric seals (gaskets / ‘o’ rings / ‘x’ rings)
• Getters ( adsorption material for capture of gas molecular species).[4] Often used in high vacuum systems, rare earth getters can be one-time fired electrically to absorb contaminants such as water vapour. Multiple getter installations can allow firings to take place at scheduled intervals throughout the service life of the system.

The choice of the seal type and the materials used is normally determined by the specified functionality of the part. Any requirement for long glass will normally preclude a compression seal design due to the need to avoid axial stress variations with temperature.

Benefits

Using hermetic technology can add a number of benefits to the product. Hermeticity – A hermetic device, particularly a connector usually forms part of a system or subsystem enclosure or housing. As such, it forms part of the overall environmental control measures included in the design. The hermetic device itself is usually one of a number of potential leak paths into or out of an enclosure, and its method of attachment may also contribute to the total leakage, particularly in the case of an o-ring sealed bulkhead mounted connector. Water vapour permeability through the o-ring will constitute an additional leak path. The sum total of all such leak paths will give the overall sealing level of the system.

Fluid separation – It is important to avoid water vapour or moisture ingress into electronics enclosures. Such water vapour will degrade dielectric materials thereby affecting module performance and or degrading reliability. Even if a sealed module is backfilled with dry gas (e.g.; nitrogen) the hermeticity of the enclosure is important as the permeation of water vapour is driven by partial pressure differences i.e.; the overall “leak rate” of water vapour into the system is a function of the assembly’s hermeticity and dryness of the interior.[2] Hermetic connectors are used to ensure overall permeation rates are maintained as low as possible. A non-hermetic device mounted in an unpressurised part of an aircraft will contain air at at mospheric pressure when the plane is on the ground. At altitude the decrease in external pressure will cause a partial evacuation of the device; on returning to the ground the device will be refilled with the surrounding air, which may contain amounts of contaminants, vapours, oils and moisture, which over a period of time may damage the electronics within the device.

Pressure barrier - Both glass and glass ceramic form exceptionally well adhered  bonds with metal oxides. This ability means that a well formed glass to metal seal can withstand high pressure differentials with a very compact seal length. Devices have been successfully tested up to 90,000 psi differential, although careful design of the seal body is required to minimise deflection under such high loads.

Fire barrier – Once successful glass wetting to the metal oxide has been achieved, the bond formed is remarkably persistent. Even elevated temperatures, above the glass softening point do not normally weaken the bond, with the result that glass-to-metal seals can provide an effective barrier at temperatures up to the working point of the glass, typically 900°C to 1,000°C. The high temperature barrier performance depends on both thermal excursion and pressure differential; requirements should be discussed with Martec’s design team to ensure compliance.

Compact size - Where space is at a premium, a hermetic device can also be remarkably compact and is often specified where available space is limited. Both internal and external protrusions of a hermetic device can be significantly less than those of an equivalent environmental style connector.

Design and performance consideration

Several factors contribute to the design of hermetic devices. The functionality required, in terms of pressure capability and electrical performance primarily Insulation Resistance (IR) and dielectric withstand voltage (DWV) or breakdown voltage (BV) together influence the insulator sizing. The conductor performance requirements will influence pin size and other environmental functionality may influence the materials used and the method of attachment to the housing. Other parameters, such as the need for low outgassing and minimal trapped volumes in vacuum applications may determine the position and orientation of welded seams. Depending on applications, other requirements, such as creepage and clearance distances, dielectric constant etc. may also be relevant. In some cases it may be advantageous to match the body material of the hermetic connector to that of its housing.  Aluminium, titanium, stainless steel or high strength duplex alloys may be specified for this reason, although sealing to aluminium or titanium alloys requires specialised glasses and sealing processes.
The most commonly used alloys for glass sealed contacts are nickel / iron alloys such as Alloy 48, Alloy 52 or Kovar. These materials are matched to commonly used sealing glasses but their conductivity is relatively low at about 30% that of copper.

In applications where high conductivity is required, copper cored nickel iron alloy contacts can be used, or seals can be made with beryllium copper, paliney or molybdenum although, in these cases, specialized glasses and sealing processes are required.

Martec can design a complete solution to ensure system integrity by incorporating electronic filters within the connector housing to protect the devices from lighting strike, voltage spikes, EMP and EMC compliance.

High integrity interconnects offer low insertion loss and thereby avoids loss of data or signal. Several types of interconnect can be optimised for high integrity data and can be compatible with various network cables e.g. Cat 5 or Cat 6 or for use on 100BaseTX / 1000BaseT / USB 3. Martec has a patented design to achieve these.
Standard “data” connector interfaces e.g.; RJ45 and coaxial (N Type etc.) can be incorporated together with cable attachment features to ensure minimal untwisting (of twisted pairs) and cross talk.

Applications

Glass-to-metal seals and hermetic connectors are used across a wide range of electrical and electronics industries within numerous applications ranging from sensors, transducers and electronic systems. They are primarily used to pass an electrical signal or power through the hermetic barrier.

Aerospace environments require hermeticity within avionic style products for applications such as aircraft, missiles and space. Hermetic connectors are used to ensure that the avionic systems integrity is maintained and is not damaged or compromised by the ingress of moisture or contaminants.
Hermetic devices can also be incorporated within systems such as: -landing gear, -fuel management, -engine condition monitoring including speed and temperature probes, -pressure sensors, -turbine temperatures monitoring, -blade tip clearance measurement, -torque measurement, -filter performance, -actuators -and many other avionic systems.
In addition to providing the hermetic solution Martec can design a complete solution to ensure system integrity by incorporating electronic filters within the connector housing to protect
the devices from lighting strike, voltage spikes, EMP and EMC compliance.

Oil and gas applications range from wire line tools to secondary containment seals with typical requirements being around 30,000 psi and 150°C. The use of hermetic technology within these applications provide protection to either electronics or act as pressure barriers between sensors and the electronic packages. Within this environment the hermetic barrier not only offers a pressure barrier but also stops the migration of fluids and gases passing through.
Martec specialist connectors and penetrators are specifically designed to meet extreme pressure and temperature through a wide choice of materials. There have been instances where the Martec feed-throughs have been tested to 90,000 psi at elevated temperatures.

Oceanology and subsea interconnects

Working in one of the world’s most treacherous operating environments, Martec’s specialist connectors and penetrators have been designed to meet specific environmental requirements such as 60,000 psi at 150C, incorporating materials to accommodate the challenging operating conditions and to meet industry standard material expectations, i.e: corrosion resistance, hardness and yield strength.

Examples of material selection may include inconel, hastelloy and titanium for the housing material.

In an innovative and advancing field, Martec has been designing superior engineered solutions for the automotive industry in regards to safety, performance, reliability and compliance. Connectors and feed-throughs are used for a diverse range of applications such as air bags, pressure sensors, tyre pressure monitors, fluid condition monitoring and fuel flow rate sensors.

Marine

Martec’s specialist connectors and penetrators have been used on naval vessels, submarines, hull penetrators and marine power plants. Designed to meet the specific environmental conditions and functionality; through the design, materials and coating selection.
Interconnects for Medical applications are used on equipment such as MRI scanners, sensing, monitoring and analytical equipment. They are also used as implantable connectors within the body. Through understanding the requirements and careful material selection Martec is able to supply to specific interconnection needs.
 
Industrial

Working in one of the world’s most far-reaching and diverse industries, you will know that superior engineering solutions are vital in regards to safety and productivity. Martec’s specialist connectors, penetrators and interconnection solutions address requirements for hemeticity, effective gas and fluid barriers, chemical resistance, shock and vibration. These products can be found in a variety of industrial applications across a wide variety of industries from food to pharmaceutical including condition monitoring equipment (pressure, vibration, level and flow) Vacuum applications and process controls.

Scientific & research

Working at the forefront of technology, in both a challenging and innovative field Martec’s hermetic interconnects can be found in wide range of critical scientific equipment, instrumentation and detectors with specifications and requirements to operate at cryogenic temperature and or at ultra high vacuums with fine rates of hermeticity.

Security

Martec has designed hermetic interconnect solutions (including ionization sources) for advanced security devices, such as analytical equipment and trace equipment used within the airport and border control for explosives and drug detection.

Manufacture

To illustrate the manufacturing process we will consider the production of a standard connector:
• The piece parts of the connector are - metal shell (housing), glass preform and pin contacts.
• The glass preform is placed on a carbon jig and the pin contacts are inserted through the preform holes and into pre-drilled holes on the carbon jig. The carbon jig holds the pins in a specific orientation for a particular connector plan form. At this stage the pins and glass are a loose fit.
• The shell housing is then carefully placed over the jig containing the glass and pin assembly.
• The assembly is then placed on the belt of the furnace and is subjected to a predefined process cycle. The parts will see elevated temperatures of up to 1000°C and a reducing atmosphere for a predetermined time whilst the glass undergoes the nucleation and crystallization stages.
• Post firing, the connector will then be disassembled from the jig and then plated before undergoing various tests and inspection stages.

References

1. IEC Standard 60529
2. Foundation of Vacuum Science and Technology, Ch 9. Ed. J.M.Lafferty
3. Mil-Std 883 – Test method for micro electronics environmental test methods (1001 – 1034)
4. Foundation of Vacuum Science and Technology, Ch 5. Ed. J.M.Lafferty
5. http://martec.solutions

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Tel: +44 (0)1227 793 733

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Email: sales@martec.ltd.uk 

http://martec.solutions/
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