Choosing the Right Vacuum Feedthroughs, Flanges & Hermetic Components

Vacuum feedthroughs, flanges, and hermetic components are where most real-world vacuum systems either succeed or fail over time. These are the parts that must create true hermetic seals between the inside of a chamber and the outside environment while still letting power, signals, fluids, and light pass through the wall.

In high and ultra-high vacuum (HV/UHV) systems, the wrong feedthrough or flange choice can lead to a chronic leak, a contamination source, or a bakeout limitation. The right combination of hermetic sealing, flange type, and materials acts like an extension of the chamber wall itself.

This guide provides a practical framework for choosing between CF and KF, selecting feedthrough types, selecting viewport materials, and accounting for cryogenic and ITAR constraints – all through the lens of hermetic seals and long-term leak integrity.

How Feedthroughs and Flanges Create Hermetic Seals

A vacuum feedthrough is any component that lets something cross a vacuum boundary – power, signals, thermocouples, fluids, motion, or light – while maintaining a hermetic seal at the chamber wall. It relies on ceramic-to-metal or glass-to-metal hermetic sealing inside the body, not only on external gaskets.

A flange is the standardized interface (CF, KF/NW, etc.) that joins chambers, feedthroughs, viewports, pumping lines, and other vacuum hardware. In UHV systems, those flanges must work together with internal hermetic seals to keep leak rate within tight limits over long service lives.

If the internal hermetic seals are strong but the flange joint is weak, the assembly still leaks. If the flange seals well but the feedthrough’s ceramic-to-metal joint fails, leak rate creeps up over time.

To understand how these principles apply to system design, read on for a comparison of CF vs. KF flanges, or, for a deeper dive into the physics and materials behind hermetic sealing, check out our technical guide, The Science of Hermetic Sealing.

CF vs. KF: Flange Choices for Hermetic Sealing in UHV

CF Flanges: All-Metal Hermetic Seals for UHV

CF (ConFlat) flanges use a knife-edge metal sealing surface and a soft metal gasket – most commonly oxygen-free high conductivity (OFHC) copper, though aluminum gaskets are available for process-sensitive applications where copper contamination is a concern. When properly torqued in the correct star-pattern bolt sequence, this creates a metal-to-metal deformation seal with an extremely low leak rate, ideal for high- and ultra-high-vacuum applications.

CF flanges are usually the right foundation for hermetic sealing when:

• You need true UHV base pressure and very low leak rates.
• You plan high-temperature or repeated bakeout cycles.
• Long-term hermetic seal stability matters more than quick changeover.
• The boundary is mission-critical: main chamber, process chamber, beamline, or TVAC chamber.

In many UHV systems, CF-based joints (with metal gaskets) plus ceramic-to-metal or glass-to-metal internal seals give you a fully hermetic sealing stack from inside to outside.

KF Flanges: Elastomer Seals with Limits

KF (also QF or NW) flanges use an elastomer O-ring compressed by a clamp. These seals are vacuum-tight and convenient, but they are not hermetic seals in the strict sense used for long-life UHV or hermetic packaging. The limitation is not that gas leaks around the O-ring – it is that gas molecules, particularly helium and hydrogen, permeate directly through the elastomer bulk and outgas from its surface. That distinction matters in UHV and long-life applications.

KF flanges are useful when:

• You are working in high vacuum rather than genuine UHV.
• The connection is non-critical (roughing line, diagnostic branch, temporary tooling).
• Speed and reconfiguration matter more than maximum bakeout or minimum permeation.

Elastomer O-rings have higher gas permeation, lower bakeout limits, and more outgassing than metal gaskets. That makes them less suitable as the primary hermetic sealing interface in demanding UHV or long-life systems.

For most engineers, the CF vs. KF decision comes down to a few practical differences in sealing performance, temperature limits, and maintenance. The table below puts those tradeoffs side by side.

A Practical Rule for Hermetic Seals

• For true hermetic seals at critical UHV boundaries, build around CF flanges and all-metal gasket joints.
• Use KF sparingly on non-critical branches where a vacuum-tight but not fully hermetic seal is acceptable and easy maintenance is a priority.

Leak Paths and Failure Modes in UHV Feedthroughs and Flanges

Even when you choose CF over KF, hermetic seals can still fail at several points:

• Knife-edge and gasket surfaces (scratches, contamination, over-torque or incorrect bolt sequence).
• Ceramic-to-metal brazes in electrical and thermocouple feedthroughs.
• Glass- or sapphire-to-metal hermetic seals in viewports.
• Welded or brazed joints in subassemblies and manifolds.
• Elastomer O-rings (if used) through permeation, aging, or damage.

In UHV systems, the acceptable leak rate is so low that even microscopic flaws in hermetic seals can extend pump-down time, raise base pressure, or destabilize sensitive processes. That is why component choice, material compatibility, and manufacturing quality are core parts of hermetic sealing, not just the end-of-line leak test.

Types of UHV Feedthroughs and Their Hermetic Seals

Electrical Feedthroughs

Electrical feedthroughs provide sealed paths for power and signals while maintaining a hermetic seal around each conductor. High-quality UHV electrical feedthroughs use ceramic-to-metal joints to hermetically seal pins in alumina insulators, then seal the ceramic into a stainless or Kovar body.

Selection factors:

• Voltage and current rating.
• Number of pins and required configuration – multi-pin consolidated feedthroughs minimize chamber penetrations but require more upfront design work; arrays of single-pin feedthroughs offer more flexibility but introduce more potential leak points.
• Insulation and creepage distances.
• Temperature and bakeout rating of the ceramic-to-metal seals.
• CF vs. KF mounting and flange size.

Thermocouple Feedthroughs

Thermocouple feedthroughs must carry temperature signals through a hermetically sealed barrier so bakeout profiles, process temperatures, or cryogenic conditions can be monitored.

Key selection points:

• Thermocouple type and pair count.
• Connector style (miniature vs. standard).
• Flange style (CF vs. KF) and bakeout rating.
• Hermetic seal materials and leak-rate performance.

Fluid and Gas Feedthroughs

When gases or fluids cross into the chamber, the interface becomes both a process boundary and a hermetic boundary. Feedthroughs designed for fluid or gas service must deliver reliable hermetic seals under pressure, thermal cycling, and exposure to specific media.

Selection should account for:

• Media compatibility with metals, brazes, and seals.
• Cleanability and contamination control.
• Impact of any leak on both the vacuum and the process.

Motion Feedthroughs

Rotary and linear motion feedthroughs allow mechanical manipulation inside a vacuum chamber – sample positioning, shutter actuation, source alignment, and similar functions – without breaking the hermetic boundary. They are a staple of research chambers, surface science instruments, and process tools.

Selection factors include rotary vs. linear motion, travel range, torque or force capacity, operating speed, and whether continuous rotation or limited-angle movement is required. Bakeout rating and leak-rate specification are just as important here as for electrical feedthroughs, since motion feedthroughs are permanent chamber penetrations.

Fiber Optic Feedthroughs

Fiber optic feedthroughs allow optical fiber to pass through a vacuum wall for laser delivery, illumination, or spectroscopic signal collection inside the chamber. They are increasingly common in research systems and semiconductor process tools where optical diagnostics or laser access through a fiber is preferable to a free-space viewport.

Key considerations include fiber type and core diameter, operating wavelength, connector style, and whether single or multi-fiber configurations are needed. The hermetic seal around the fiber itself is a precision ceramic-to-metal or glass-to-metal joint and should be specified with the same leak-rate and bakeout requirements as any other feedthrough.

RF and Specialized Signal Feedthroughs

RF and high-speed signal feedthroughs must maintain controlled impedance while still providing hermetic seals through the chamber wall. That often involves carefully engineered ceramic-to-metal transitions combined with CF-mounted interfaces.

Viewports as Hermetic Seals for Light

In contrast to feedthroughs that manage the flow of power, signals, or materials, viewports provide the hermetic seal necessary for optical access, treating light itself as the substance crossing the boundary.

Viewports are not just windows – they are hermetic sealing components for light. A viewport must allow optical access for cameras, lasers, and diagnostics while maintaining UHV conditions and withstanding thermal and mechanical stresses.

High-quality UHV viewports rely on:

• Glass-to-metal or sapphire-to-metal hermetic seals.
• Compatible CTE between window and metal ring.
• All-metal CF joints to the chamber wall.

If either the internal hermetic seal or the flange seal fails, the viewport becomes a leak source rather than a reliable optical boundary.

Fused Silica Hermetic Viewports

Fused silica viewports are widely used where UV or visible transmission matters, moderate to high bakeout temperatures are needed, and clean, stable hermetic seals are required for laser or diagnostic access. Fused silica transmits from approximately 150 nm in the deep UV through about 2.5 μm in the near-infrared, making it suitable for most common laser wavelengths.

Sapphire Hermetic Viewports

Sapphire viewports offer high mechanical strength, scratch resistance, and robust hermetic sealing when joined correctly to compatible metals. Sapphire’s transmission range, approximately 0.15 to 5.5 μm, extends well into the mid-infrared, making it the preferred choice for IR laser access as well as for applications where mechanical abuse risk, high pressure differential, or aggressive thermal environments make other glasses less reliable.

Borosilicate and Other Glasses

Borosilicate glass can be adequate in less demanding roles, but has a lower annealing point and more limited UV transmission than fused silica, and is less tolerant of the bakeout temperatures required in true UHV systems. It is more commonly found in high vacuum rather than UHV viewports. In all cases, the hermetic seal between glass and metal and the CF flange interface are what keep the vacuum boundary secure.

Materials and Joining Methods for Hermetic Sealing in UHV

Hermetic sealing in UHV components is ultimately a materials and joining problem:

• Metals (e.g., stainless steel, Kovar, titanium) must match ceramics and glasses well enough in CTE to avoid cracking hermetic seals during bakeout or cool-down.
• Ceramics like alumina provide electrical insulation and are central to ceramic-to-metal hermetic seals in feedthroughs.
• Optical materials (fused silica, sapphire, etc.) must be joined to metals without leaving leak paths.

Joining methods such as ceramic-to-metal brazing (most commonly the moly-manganese, or Mo-Mn, process), active-metal brazing, glass-to-metal sealing, and precision welding are what transform these materials into reliable hermetic seals that can survive UHV, bakeout, and thermal cycling.

Cryogenic UHV: Hermetic Seals at Low Temperatures

Cryogenic systems push hermetic seals into extreme CTE mismatch territory. Metals, ceramics, and glasses shrink at different rates when cooled – from liquid nitrogen temperatures (77 K) down to liquid helium (4 K) or the sub-kelvin temperatures of dilution refrigerators – and poorly matched joints can develop microcracks or lose hermeticity after repeated cycles.

Thermal conductivity is an additional consideration in cryogenic feedthrough design. Engineers often need feedthroughs that are electrically conductive but thermally resistive to minimize heat load on the cold stage. Wiring choices inside cryostats – such as Kapton-insulated wire and other low-thermal-conductivity wiring options – are influenced by the same concern.

For cryogenic UHV applications:

• Favor all-metal CF seals over elastomers. Where indium gaskets are specified in place of copper, they offer better ductility and compliance at low temperatures.
• Use feedthroughs and viewports with cryogenically validated hermetic seals and documented low-temperature leak testing.
• Pay close attention to material combinations – for example, sapphire with specific metal alloys – and their tested behavior through the relevant temperature range.

Installation, Cleaning, and Handling

In UHV work, how a component is installed and handled is nearly as important as what it is made of. Contaminated feedthrough or viewport surfaces are a significant source of outgassing and can prevent a system from reaching its target base pressure regardless of the components’ rated performance.

UHV-grade components should arrive clean and individually packaged. During installation, clean-room handling practices apply: handle components with clean gloves, avoid touching sealing surfaces, clean with appropriate solvents if required, and minimize exposure time before closing the system. For CF flanges, following the correct star-pattern bolt torque sequence – in accordance with the gasket manufacturer’s specification for the flange size and gasket material – is essential. Uneven torquing is one of the most common causes of CF leaks in practice.

ITAR, Export Controls, and Hermetic Components

Many hermetic UHV components end up in defense, space, or dual-use systems. When a project falls under ITAR or other export controls, the hermetic seals themselves may not change, but how they are specified, documented, and shipped certainly does.

As a US-based manufacturer, MPF Products maintains ITAR compliance capabilities including secure documentation, design confidentiality, and controlled communication channels – which simplifies the regulatory path compared to sourcing from international suppliers who may require additional export approvals or present technology-transfer risks.

Engineering and purchasing teams should:

• Flag export-control or ITAR needs early in the supplier engagement.
• Confirm supplier capabilities for secure documentation, traceability, and controlled communication.
• Understand that certain custom hermetic assemblies may require additional approvals or review steps.

Standards, Testing, and Proving Hermetic Sealing

Helium Leak Testing for UHV Assemblies

Helium leak testing is the primary method for qualifying UHV feedthroughs, viewports, flanges, and hermetic subassemblies. In high- and ultra-high-vacuum systems, acceptable leak rates are orders of magnitude tighter than in rough vacuum or industrial pressure applications, because even tiny leak paths can extend pump-down time, raise base pressure, and destabilize sensitive processes.

For UHV components, buyers should ask about several key points:

• What helium leak test method is used (e.g., integral/vacuum mode vs. sniffer testing).
• What minimum detectable leak rate and pass/fail thresholds are specified.
• Whether every device is tested or only samples from each lot.

When assemblies such as manifolds, subassemblies, or connector blocks are welded or brazed together, it is often worth testing both the individual hermetic devices and the integrated assembly to ensure no new leak paths have been introduced.

For a deeper explanation of leak physics, leak-rate units, and how helium testing relates to long-term reliability, see the testing section in our Science of Hermetic Sealing guide.

Relevant Vacuum and Component Standards

In hermetic electronics and aerospace components, standards such as MIL-STD-883 Method 1014 define hermeticity test methods, fine-leak and gross-leak procedures, and how allowable leak rates scale with internal volume and mission requirements. Many vacuum customers adopt similar expectations for UHV feedthroughs and viewports, even when not strictly required, to ensure robust hermetic performance over long service lives.

Beyond formal standards, program-specific specifications and internal customer requirements often impose tighter limits on leak rate, temperature cycling, and documentation. When you compare suppliers, ask for written leak-test specifications, method descriptions, and representative data so you can verify that feedthroughs, viewports, and hermetic assemblies are tested to the level your UHV environment demands.

MPF in Practice: Proven UHV Leak Performance

Many of MPF Products’ UHV electrical feedthroughs are qualified to helium leak rates at or below 1×10⁻⁹ atm·cc/sec He after bakeout – firmly within the range required for demanding UHV work – with 100% of devices helium-leak tested before shipment. This level of verification helps your feedthroughs behave like an extension of the chamber wall rather than a hidden leak source.

Where Hermetic Seals and UHV Feedthroughs Are Critical

The combination of hermetic sealing and UHV hardware is central to several sectors:

• Semiconductor manufacturing – chambers and tools rely on hermetically sealed feedthroughs and viewports to isolate process gases, maintain base pressure, and protect yield.
• Aerospace and space simulation – thermal-vacuum chambers and space hardware need hermetic seals that survive long test campaigns and extreme conditions.
• Research and accelerators – beamlines and high-resolution instruments require stable, ultra-clean environments with reliable hermetic seals at every boundary.
• Quantum and cryogenic systems – cryostats and quantum devices depend on hermetic feedthroughs and viewports that stay leak-tight at very low temperatures.

Standard vs. Custom Hermetic UHV Components

When Standard Hermetic Components Are Enough

Standard UHV feedthroughs, viewports, and flanges provide:

• Proven hermetic seals and published ratings.
• Faster sourcing and often lower engineering overhead.
• Compatibility with common CF and KF sizes.

When Custom Hermetic Assemblies Are Required

Custom or modified hermetic UHV components make sense when:

• Geometry or space constraints are unusual.
• Multiple functions (power, signals, optics, motion) must be combined in a single hermetic assembly.
• The environment involves unusual gases, extreme temperature cycles, radiation, or heavy mechanical loads.
• Additional documentation or ITAR/export control considerations apply.

A key benefit of working with a supplier like MPF Products is the ability to design, build, and test custom hermetic seals – not just sell catalog parts – so the component matches the real vacuum and environmental conditions, not just the drawing. Learn more about MPF Products’ capabilities at mpfpi.com/company/mpf-capabilities/.

Key Takeaways: UHV Feedthroughs, Flanges & Hermetic Seals

• Treat every feedthrough, flange, and viewport as part of a single hermetic sealing stack, not as isolated components, so you avoid hidden leak paths at joints and interfaces.
• Use CF flanges with all-metal gaskets for critical UHV boundaries and long-life applications, and reserve KF O-ring joints for high-vacuum, non-critical lines where fast changeover matters more than ultimate hermeticity.
• Match materials and joining methods – metals, ceramics, glasses, and brazes – to your vacuum level, temperature profile, and any cryogenic cycling to prevent CTE-driven cracking or gradual loss of hermeticity.
• For demanding environments, choose UHV-rated electrical, thermocouple, fluid, motion, and RF feedthroughs with specified leak-rate limits, proven ceramic-to-metal or glass-to-metal seals, and bakeout ratings that align with your process.
• Consider standard catalog hermetic components first for speed and cost, then move to custom assemblies when geometry, combined functions, extreme conditions, or ITAR/export requirements push beyond standard offerings.
• Always confirm helium leak testing methods, leak-rate thresholds, and any relevant standards up front, so your selected components behave like an extension of the chamber wall across the full operating life of the system.
• Handle and install UHV components with care: clean-room practices, correct CF bolt torque sequence, and proper cleaning before installation are as important as the component’s rated performance.

Decision Framework and Selection Workflow for UHV Components

Use the checklist below to structure your decisions about feedthrough types, CF vs. KF flanges, viewport materials, cryogenic requirements, ITAR/export controls, and standard vs. custom hermetic components before you lock in a design.

1. Define the environment. Vacuum level, gases, bakeout and operating temperatures, any cryogenic or radiation exposure.
2. Define what crosses the boundary. Power, signals, thermocouples, fluids, motion, fiber optics – all require different hermetic sealing strategies.
3. Choose the flange standard. Use CF for critical hermetic seals at UHV boundaries; use KF selectively on less demanding, non-critical lines.
4. Match materials and joining methods. Confirm metals, ceramics, and optical materials are compatible and joined using appropriate hermetic processes.
5. Decide standard vs. custom. Use catalog hermetic components when fit; escalate to custom hermetic assemblies when requirements are unique.
6. Verify testing and documentation. Ensure the hermetic seals are backed by helium leak testing, temperature ratings, and any required standards or export controls.
7. Plan for installation and handling. Confirm components are supplied clean, and follow correct torque sequence and handling procedures during integration.

Hermetic sealing is not a “nice to have” in UHV systems; it is the difference between components that behave like solid chamber walls and components that quietly become the weakest link. By choosing CF over KF where it really matters, matching materials and joining methods to your environment, and working with proven hermetic feedthroughs and viewports, you can design systems that pump down faster, hold pressure longer, and need less troubleshooting over their lifetime.

If you are defining a new chamber, upgrading a problematic boundary, or deciding between standard and custom hermetic components, reach out to MPF Products’ engineering team at mpfpi.com/contact/ to review your vacuum level, gases, temperature profile, and regulatory needs – and get application-specific recommendations for hermetic seals, feedthroughs, and viewports that will keep your UHV system performing as designed.

FAQ: UHV Feedthroughs, Flanges & Hermetic Seals

What is a vacuum feedthrough?

A vacuum feedthrough is a component that lets power, signals, fluids, motion, or optics cross a vacuum wall while maintaining a hermetic seal at the chamber boundary. It typically uses ceramic-to-metal or glass-to-metal hermetic sealing internally, combined with a CF or KF flange on the outside.

What is the difference between CF and KF flanges?

CF flanges use metal gaskets and knife edges to create all-metal hermetic seals suitable for UHV and high-temperature bakeout. KF flanges use elastomer O-rings for quicker, tool-light connections but have higher permeation – gas passes through the elastomer bulk itself – and lower temperature limits, making them better for high vacuum and non-critical connections.

When should I choose CF flanges instead of KF flanges?

Choose CF flanges when you need UHV, high bakeout, long-term leak stability, or resistance to radiation and aggressive gases. KF can be used on non-critical branches and roughing lines where a vacuum-tight but not fully hermetic seal is acceptable and fast maintenance matters.

Which feedthroughs are best for UHV systems?

Ceramic-to-metal electrical feedthroughs, CF-mounted designs, and UHV-rated thermocouple and power feedthroughs are usually best for UHV. Look for components with specified leak-rate limits, robust internal hermetic seals, and bakeout ratings that match your environment.

How do I choose viewport materials (fused silica vs. sapphire vs. borosilicate)?

Fused silica transmits from deep UV through the near-infrared (~150 nm to 2.5 μm) and suits most UHV laser and diagnostic applications. Sapphire extends further into the mid-infrared (~0.15 to 5.5 μm) and offers higher mechanical strength for harsh environments. Borosilicate can be adequate in less extreme, high-vacuum roles but has lower bakeout tolerance and more limited UV transmission. Match the viewport material to wavelength, temperature, mechanical load, and cleaning requirements.

What special considerations apply to cryogenic UHV feedthroughs?

Cryogenic applications amplify CTE mismatch between metals, ceramics, and glasses, so poorly matched joints can crack and lose hermeticity. Use cryo-rated hermetic feedthroughs, favor CF/all-metal seals (with indium gaskets where appropriate), and look for suppliers who can demonstrate low-temperature leak testing and experience in cryogenic UHV. Also consider thermal conductivity – feedthroughs that minimize heat load on the cold stage are often required in dilution refrigerator and liquid helium systems.

Can I use KF components in ultra-high vacuum?

You can sometimes use KF on non-critical branches, roughing lines, or diagnostic ports in systems that include UHV regions, but KF O-rings are not ideal as the primary hermetic seals for demanding UHV boundaries. For long-term UHV stability, CF/all-metal joints plus internal hermetic seals are strongly preferred.

What is the difference between standard and custom UHV feedthroughs?

Standard UHV feedthroughs follow catalog geometries, pin-outs, and ratings, giving you off-the-shelf hermetic seals for common needs. Custom hermetic feedthroughs are designed around specific geometries, extreme environments, unusual pin configurations, motion or fiber optic requirements, or ITAR and documentation requirements, trading more upfront engineering for a better long-term fit.

How do ITAR and export controls affect hermetic and UHV components?

Some hermetic and UHV components used in defense and space programs may fall under ITAR or other export controls. Working with a US-based, ITAR-compliant manufacturer like MPF Products simplifies documentation, traceability, and controlled communication compared to sourcing internationally. Engage your supplier early to confirm they can handle relevant regulatory requirements.

How do I get help selecting the right feedthroughs and flanges for my system?

The best approach is to share your environment (vacuum level, gases, temperature, cryogenic or bakeout needs), electrical, optical, and mechanical requirements, and any standards or ITAR/export requirements with a qualified hermetic supplier. MPF Products’ engineering team can use that information to recommend appropriate hermetic seals, standard catalog components, or custom hermetic assemblies for your specific UHV system.