trends in engineering applications for hermeticity

Trends in engineering applications for

hermeticity by Dan Harnett & David Marsden

December 2016

http://martec.solutions/[email protected]

Who we areAt Martec, our connectors and penetrators are used for power and data, at the forefront of technology, offering the highest performance, quality and reliability where they are needed most.

HistorySince 1987, we have been designing and man-ufacturing high-quality precision components for harsh and demanding environments. Our products are developed using an application-focussed approach, building on our extensive systems and materials expertise, to create world-class, performance engineered solutions at the forefront of technology. This is why many companies continue to trust our penetrators and interconnectors for a broad range of ap-plications in sectors such as oil and gas, aero-space, marine, nuclear and defence.

Complete serviceYet it is not just the quality of our product that sets us apart, but also our commitment to ser-vice and exceeding customers’ expectations. At Martec, as well as supplying a range of the standard connectors, we can provide a com-plete interconnection solution from concept to supply.

We are able to design and develop unique and unusual connectors in partnership with engineering teams, in large or small quantities and we are particularly experienced in one-off projects.

Having an enviable reputation for being an extraordinarily dependable partner, you can rest assured our products will be on time and to specification.


Hermeticity

Trends in Hermetic DevicesRecent developments in materials and process-ing technology have broadened the potential range of applications for hermetically sealed devices over a wide variety of performance engineered systems. Whether the primary de-sign goal is corrosion resistance, pressure capa-bility in underwater systems or lightweight and moisture ingress prevention in avionic systems, Martec 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 lev-els of moisture, and that specific performance and reliability requirements are achieved and maintained.Also, aerospace and defence systems incor-porate high performance optical assemblies sealed to avoid contamination of optical path-ways. In addition these hermetic products en-able avionic devices to maintain hermeticity and avoid ingress of moisture, allowing the sys-tems to achieve and maintain specified perfor-mance.These hermetic devices can be found across a wide spectrum of avionic applications such as aircraft, missiles and space programs to pro-vide system integrity and ensure no damage is caused by the ingress of moisture or contami-

nants. Hermetic devices are also incorporated within avionic systems such as landing gear, fuel management, engine condition monitor-ing including, speed and temperature probes pressure sensors, turbine temperatures monitor-ing, blade tip clearance measurement, torque measurement, filter performance, actuators and many others.

In Oil & Gas applications increasingly harsh en-vironments 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 bar-rier 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 as-semblies 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 semi-conductor component manufacturing pro-cesses.New material developments have provided new glasses that enable hermetic sealed de-vices to be produced in a variety of housing materials, including aluminium, titanium and super duplex alloys ensuring that specific per-formance requirements and end user environ-ments are met. These include galvanic, cor-rosive, bio-compatibility and reduced system mass for weight critical applications.

High strength super duplex alloys address spe-cific 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 re-quirements.The development of low temperature glasses allows Martec to design hermetic devices to a wider range of conductor materials for im-proved conductivity and non magnetic as-semblies (e.g. paliney, beryllium copper & mo-lybdenum). These may be found in medical, industrial & chemical applications addressing the specific industry standards and challenging operating environments.

Through Martec’s research and development, hermetic devices can now incorporate im-pedance matched materials to reduce inser-tion 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 require-ments. Martec’s hermetic designs provide ion-ization sources for advanced security devices such as analytical equipment and chemical trace equipment.

Where space is at a premium, Martec’s her-metic devices can provide a compact solution in place of an environmental style device.

Hermeticity


HermeticityIntroduction At Martec, the devices we design and manu-facture incorporate glass, glass ceramic or ce-ramic 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 un-der high differential pressure and or extremes of temperature whilst providing means of pass-ing electrical signals and or power through the barrier.

The most common example of this is the exclu-sion of moisture from sensitive electronic sys-tems.

A hermetic device can also be remarkably compact and is often specified where avail-able space is limited. Both internal and external protrusions of a hermetic device can be signifi-cantly less than those of an equivalent environ-mental style connector.

Hermetic connectors are specified for a vari-ety 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 & medical sensing, naval to nuclear submarines.

Within the industry glass to metal seals are also

referred to or known as penetrators or feed-throughs. 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 re-peated mate and unmate characteristics are required.


HermeticityTechnologyA hermetic seal is defined as being air or gas tight, impermeable to fluids, excluding the pas-sage 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 imper-meable 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 ox-ide bond formed between the two at elevat-ed 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 par-ticular intermetallic compounds across the in-terface 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 illustra-tive 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 permit-ting undesired flow of gas, moisture or other contaminates. However, in some cases water vapour migra-tion through materials can be sufficient to constitute a significant leak path, particularly in the case of plastics or elastomers.[2] Irrespec-tive 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 spe-cies 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 mo-lecular size is arguably more invasive due to its electron charge configuration allowing it to be readily adsorbed by many materials, includ-ing 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 ex-cluded.

Leak tests are defined within Mil-Std-883.[3]

The size of the leak is determined by the vol-ume of gas that passes through the leak per

10-6 cc/sec 1cc of air flow in 11.6 days 10-7 cc/sec 1 cc of air flow in 16.5 weeks 10-8 cc/sec 1 cc of air flow in 3.2 years 10-9 cc/sec 1 cc of air flow in 31.8 years 10-10 cc/sec 1 cc of air flow in 318 years 10-11 cc/sec 1 cc of air flow in over 3000 years


Hermeticitysecond under a 1 atmosphere differential pres-sure. Leaks greater than 1x 10-5 atm cc/secare generally considered gross leaks whereas those smaller than 1x10-5 atm cc/sec are con-sidered fine leaks. In practice gross leaks are determined, isolated and quantified using liq-uid bath and bubble detection. As an indica-tor for gross leaks a dye penetrant test can also be used. For fine leaks mass spectrometers are used with Helium gas. Modern mass spec-trometers are now capable of detecting some gross leaks.

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

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

Types of Sealing

There are two categories of glass to metal seals: compression seals and matched seals. The for-mer, as the name suggests, have materials of differing coefficients of thermal expansion nor-mally 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, produc-ing a robust seal capable of tolerating high levels of shock and vibration. Due to glass be-ing 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 ex-pansion, the seal derives its strength from the chemical bond between the glass and the ox-ide 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 man-ufacture of matched seals cannot resist large differential pressures.


Alternative materials for hermetic sealing

Ceramic glasses (or glass ceramics) are a rel-atively new class of material discovered by Corning in the 1950s. A correct thermal pro-cessing of these materials results in developing ceramic crystals in a surrounding glassy matrix. Some versions may be used to provide insulat-ing glasses with coefficient of thermal expan-sion 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, ceram-ic 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 her-metic 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 outgas-sing and permeability rate than glass or met-al and a more limited operating temperature range typically +70°C to +125°C (200°C for spe-cial epoxy).

A typical, sealed avionics LRU will have nu-merous interconnects, and a lid or removable panel to facilitate assembly. All will be poten-tial 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 elastomer-ic 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 installa-tions can allow firings to take place at sched-uled 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 compres-sion seal design due to the need to avoid axial stress variations with temperature.

Hermeticity


BenefitsUsing 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 sub-system enclosure or housing. As such, it forms part of the overall environmental control mea-sures included in the design. The hermetic de-vice 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 wa-ter vapour or moisture ingress into electronics enclosures. Such water vapour will degrade dielectric materials thereby affecting mod-ule performance and or degrading reliability. Even if a sealed module is backfilled with dry gas (e.g.; Nitrogen) the hermeticity of the en-closure 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 her-meticity and dryness of the interior.[2] Hermetic connectors are used to ensure overall perme-ation rates are maintained as low as possible.A non hermetic device mounted in an unpres-surised 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, va-pours, oils, 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 minimize 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 elevat-ed 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 perfor-mance depends on both thermal excursion and pressure differential; requirements should be discussed with Martec’s design team to en-sure compliance.

Compact Size - Where space is at a premium, a hermetic device can also be remarkably compact and is often specified where avail-able space is limited. Both internal and external protrusions of a hermetic device can be signifi-cantly less than those of an equivalent environ-mental style connector.

Hermeticity


Hermeticity


Design & Performance ConsiderationSeveral 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 posi-tion and orientation of welded seams. Depending on applications, other require-ments, such as creepage and clearance dis-tances, dielectric constant etc. may also be relevant. In some cases it may be advanta-geous to match the body material of the her-metic connector to that of its housing. Alu-minium, titanium, stainless steel or high strength duplex alloys may be specified for this reason, although sealing to aluminium or titanium al-loys require specialised glasses and sealing pro-cesses.

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 re-quired, copper cored nickel iron alloy contacts can be used, or seals can be made with berylli-um copper, paliney or molybdenum although, in these cases, specialized glasses and sealing processes are required.

At Martec we can design a complete solu-tion to ensure system integrity by incorporating electronic filters within the connector housing to protect the devices from lighting strike, volt-age 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 incorpo-rated together with cable attachment features to ensure minimal untwisting (of twisted pairs) and cross talk.

ApplicationsGlass to metal seals and hermetic connectors are used across a wide range of electrical and electronics industries within numerous appli-cations 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 dam-aged or compromised by the ingress of mois-ture or contaminants.

Hermetic devices can also be incorporated within systems such as:- landing gear, - fuel management, - engine condition monitoring including, speed & 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 en-sure system integrity by incorporating electron-ic filters within the connector housing to protect

the devices from lighting strike, voltage spikes, EMP and EMC compliance.

Oil & Gas applications range from wire line tools to secondary containment seals with typi-cal 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 mi-gration of fluids and gases passing through.

Martec specialist connectors and penetrators are specifically designed to meet extreme pres-sure 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 & Subsea interconnects Working in one of the world’s most treacherous operating environments, Martec’s specialist connectors and penetrators have been de-signed to meet specific environmental require-ments 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 in-conel, hastelloy and titanium for the housing material.

Hermeticity


In an innovative and advancing field, Martec has been designing superior engineered solu-tions for the Automotive industry in regards to safety, performance, reliability and compli-ance.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 penetra-tors have been used on naval vessels, subma-rines, hull penetrators and marine power plants. Designed to meet the specific environmental conditions and functionality; through the de-sign, materials and coating selection.

Interconnects for Medical applications are used on equipment such as MRI scanners, sens-ing, monitoring and analytical equipment. They are also used as implantable connectors used within the body. Through understanding the re-quirements and careful material selection Mar-tec is able to supply to specific interconnection needs.

IndustrialWorking in one of the world’s most far-reaching and diverse industries, you will know that supe-rior 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 resis-tance, shock and vibration. Our products can be found in a variety of industrial applications across a wide variety of industries from food to pharmaceutical including condition monitor-ing equipment (pressure, vibration, level and flow) Vacuum applications and process con-trols.

Scientific & ResearchWorking 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, instru-mentation and detectors with specifications and requirements to operate at cryogenic temperature and or at ultra high vacuums with fine rates of hermeticity.

SecurityAt the cutting edge Martec has designed her-metic interconnect solutions (including ioniza-tion sources) for advanced security devices, such as analytical equipment and trace equip-ment used within the airport and border con-trol for explosives and drug detection.

Hermeticity


ManufactureTo illustrate the manufacturing process we will consider the production of a standard con-nector:

• 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 & 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 tem-peratures of up to 1000°C and a reducing at-mosphere for a predetermined time whilst the glass undergoes the nucleation and crystalliza-tion stages.

• Post firing, the connector will then be disas-sembled from the jig and then plated before undergoing various tests and inspection stages.

Hermeticity


References

1. IEC Standard 60529

2. Foundation of Vacuum Science and Tech-nology, Ch 9. Ed. J.M.Lafferty

3. Mil-Std 883 – Test method for micro electron-ics environmental test methods (1001 – 1034)

4. Foundation of Vacuum Science and Tech-nology, Ch 5. Ed. J.M.Lafferty

5. http://martec.solutions

Contact usMartec LimitedSt Augustines Business ParkSwalecliffe, Whitstable, Kent CT5 2QJ, United Kingdom

Tel: +44 (0)1227 793 733Fax: +44 (0)1227 793 735

Email: [email protected]: http://martec.solutions/

www.amphenol.co.ukwww.amphenolmao.com

trends in engineering applications for hermeticity

Engineering