Kovar Glass-to-Metal Seals: The Definitive Engineering Guide

If you’ve ever designed a hermetic package for a pacemaker, a satellite connector, or a vacuum feedthrough, you already know the truth: the seal is everything. A single microscopic leak path can compromise an implant, silence a sensor, or take down an entire avionics module. And when the application demands absolute reliability, the material pairing that engineers return to again and again is Kovar glass-to-metal.

I’ve spent over a decade machining Kovar and Fe-Ni-Co alloys at KELTRYN, and I’ve seen what happens when engineers understand the materials science behind glass-to-metal sealing—and what happens when they don’t. This guide is the resource I wish I could hand every new customer before they send us their first drawing.

We’re going to cover the fundamentals of CTE matching, the chemistry of oxide bonding, the differences between matched and compression seals, glass selection, design rules, manufacturing processes, and qualification testing. By the end, you’ll have a practical framework for designing Kovar glass-to-metal seals that survive thermal cycling, mechanical stress, and decades of service.

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Kovar Glass-to-Metal Seals: At a Glance

| Parameter | Typical Value / Best Practice |

|---|---|

| Material | Kovar (ASTM F15 / 4J29): 29% Ni, 17% Co, balance Fe |

| CTE (30–300°C) | 5.2–5.5 ppm/°C |

| Matching Glass Families | Borosilicate (7052, 7070), aluminosilicate |

| Seal Types | Matched (CTE-aligned) or Compression (glass under compression) |

| Critical Process Step | Controlled pre-oxidation to form a 2–5 μm oxide layer |

| Typical Hermeticity Test | Helium mass spectrometry, < 1 × 10⁻⁹ atm·cc/s He |

| Key Standards | MIL-STD-883, ASTM F19 |

| Max Continuous Service Temp | ~400–450°C (dependent on glass and design) |

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What is a Kovar Glass-to-Metal Seal?

A Kovar glass-to-metal seal is a hermetic (vacuum-tight) joint formed by fusing molten glass to a specially prepared Kovar metal surface. The glass wets the oxidized Kovar surface, and upon cooling, the two materials form a chemical and mechanical bond that prevents the passage of gases, liquids, or ions.

The technology has been around far longer than most engineers realize. Early vacuum tubes in the 1930s relied on glass-to-metal seals to maintain the internal vacuum required for electron emission. Those early seals used platinum or tungsten, but the search for a cheaper, more workable metal with a compatible thermal expansion led to the development of Kovar by the Stupakoff Company in the 1930s. Today, the alloy is standardized as ASTM F15 and is used in everything from medical implants to semiconductor manufacturing equipment.

The applications are broad, but they share a common thread: failure is not an option. You’ll find Kovar glass-to-metal seals in:

• Military and aerospace connectors that must survive extreme vibration, thermal shock, and altitude changes
• Implantable medical devices like pacemakers and neurostimulators, where a leak could expose the patient to toxic materials or cause device failure
• Pressure sensors for automotive, industrial, and process control applications
• Laser diode packages and RF/microwave housings that require a controlled atmosphere or vacuum
• Vacuum feedthroughs for ion implanters, electron microscopes, and particle accelerators

So why Kovar specifically? It comes down to one number: the coefficient of thermal expansion (CTE).

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The Science Behind Kovar Sealing: CTE and Oxide Bonding

Understanding Coefficient of Thermal Expansion

Every material expands when heated and contracts when cooled. The coefficient of thermal expansion (CTE) quantifies this behavior—it’s the fractional change in dimension per degree of temperature change, typically expressed in parts per million per degree Celsius (ppm/°C).

When you seal glass to metal, the two materials are fused at high temperature (typically 900–1000°C for borosilicate glasses) and then cooled to room temperature. If the metal and glass have significantly different CTEs, the differential contraction creates stress at the interface. If that stress exceeds the glass’s fracture strength, the seal cracks.

Kovar’s CTE is approximately 5.2–5.5 ppm/°C from 30°C to 300°C. That’s remarkably close to the CTE of certain borosilicate glasses, which fall in the 4.6–5.5 ppm/°C range depending on the specific formulation. This close match minimizes residual stress in the finished seal, which is why Kovar has become the default choice for hermetic glass sealing.

The Critical Role of the Oxide Layer

Here’s where the materials science gets interesting. Glass does not bond to bare metal. For a true hermetic seal, the Kovar surface must be oxidized first.

When Kovar is heated in an oxidizing atmosphere, it forms a layered oxide structure. The outer layer is primarily iron oxide (Fe₂O₃), and beneath that sits a spinel layer of nickel and cobalt ferrites (NiFe₂O₄ and CoFe₂O₄). It’s this inner oxide layer that actually wets the molten glass and forms the chemical bond.

During sealing, the molten glass dissolves the outer iron oxide layer and reacts with the inner spinel layer. This creates a glass-to-oxide chemical bond that is genuinely hermetic—not just a mechanical interlock. The oxide becomes part of the seal structure.

The thickness and composition of that oxide layer are critical. If the oxide is too thin (under-oxidation), there isn’t enough material to form a robust chemical bond, and the seal may delaminate or leak. If the oxide is too thick (over-oxidation), the oxide layer becomes brittle and can fracture under thermal stress, creating leak paths.

In practice, we aim for a total oxide thickness of roughly 2 to 5 micrometers. This is controlled by the pre-oxidation temperature, time, and atmospheric conditions. It’s a process that sounds simple but requires tight process control to execute repeatably.

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Matched Seals vs. Compression Seals: Which to Choose?

When you’re designing a Kovar glass-to-metal seal, the first major decision is whether to use a matched seal or a compression seal. The choice affects your glass selection, geometry, stress profile, and manufacturing process.

Matched Seals

In a matched seal, the CTE of the glass is closely aligned with the CTE of the Kovar metal. Because the two materials expand and contract at nearly the same rate, residual stress in the finished seal is minimized.

Best for:

• Precision electrical feedthroughs where pin alignment and dimensional stability are critical
• High-reliability applications subject to repeated thermal cycling
• Designs with thin glass sections that can’t tolerate high compressive stress

Trade-offs:

• Limited glass selection—you’re constrained to glasses formulated to match Kovar’s CTE
• Higher precision required in both the metal part and the glass preform
• Generally more expensive due to tighter tolerances and specialized glass

Compression Seals

In a compression seal, the glass has a lower CTE than the surrounding metal shell. After cooling from the sealing temperature, the metal contracts more than the glass, placing the glass in a state of compressive stress. Glass is strong in compression but weak in tension, so this stress state can actually strengthen the seal.

Best for:

• High-pressure environments where the seal must withstand significant mechanical loads
• Larger terminals and feedthroughs where matched seals would be impractical
• Applications where a lower-cost glass can be used

Trade-offs:

• Residual stress must be carefully managed—too much compression can crack the glass
• Limited to certain geometries (typically annular or cylindrical)
• The metal shell must be thick enough to generate the required compressive stress

Comparison Table

| Factor | Matched Seal | Compression Seal |

|---|---|---|

| CTE Relationship | Glass CTE ≈ Metal CTE | Glass CTE < Metal CTE |

| Stress Profile | Minimal residual stress | Glass under compressive stress |

| Typical Applications | Precision feedthroughs, RF packages, medical devices | High-pressure sensors, power terminals, large connectors |

| Glass Selection | Limited to matched borosilicates | Broader range of glasses |

| Geometry Flexibility | High | Limited to cylindrical/symmetric shapes |

| Relative Cost | Higher | Lower to moderate |

| Thermal Cycling Resistance | Excellent | Good, but stress-sensitive |

Decision Flowchart

1. Is the application subject to extreme thermal cycling? → Yes: matched seal is safer
2. Does the design require a large-diameter terminal? → Yes: compression seal may be more practical
3. Is the operating pressure above 5,000 psi? → Yes: compression seal provides added strength
4. Are there tight dimensional tolerances on pin spacing? → Yes: matched seal offers better stability
5. Is cost a primary driver? → Yes: compression seal may reduce glass and processing costs

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Kovar Glass Formulations: Choosing the Right Glass

The glass you choose is just as important as the Kovar itself. The glass must have a CTE compatible with your seal type, a softening point that fits your manufacturing process, and electrical properties that meet your application requirements.

Common Glass Families for Kovar

Borosilicate Glasses (7052, 7070)

These are the workhorse glasses for Kovar sealing. They offer a good CTE match, excellent electrical resistivity, and reasonable thermal shock resistance.

• 7052 has a CTE of approximately 4.6 ppm/°C and is commonly used for matched seals with Kovar. It has a working temperature around 700°C and offers good chemical durability.
• 7070 has a lower CTE (~3.2 ppm/°C) and is often used in compression seal designs or where lower dielectric loss is required.

Aluminosilicate Glasses

These glasses have higher softening points and better high-temperature performance than borosilicates. They’re used in applications that see continuous service above 400°C. However, they require higher sealing temperatures, which can be challenging for Kovar due to its relatively low melting point (~1450°C).

Key Glass Properties

When selecting a glass, pay attention to:

• CTE: Must match your seal type (matched vs. compression)
• Softening point: Determines the sealing temperature and process window. Kovar can be sealed to glasses with softening points up to ~1000°C, but higher temperatures increase the risk of oxide over-growth and Kovar grain growth.
• Electrical resistivity: Critical for feedthrough applications where insulation resistance matters. Borosilicates typically offer volume resistivity above 10¹⁴ Ω·cm at room temperature.
• Dielectric constant and loss tangent: Important for RF and microwave applications.
• Chemical durability: Relevant for medical or corrosive environments.

Frit vs. Pre-Formed Glass Beads

You have two main options for introducing the glass into your assembly:

Pre-formed glass beads are solid glass parts machined or pressed to your exact dimensions. They sit between the Kovar pin and the Kovar housing, and the entire assembly is heated until the glass flows and wets the oxide surfaces. Pre-forms offer excellent dimensional control and are ideal for high-volume production.

Glass frit is a powdered glass mixed with a binder to form a paste. The paste is dispensed or screen-printed onto the Kovar part, then fired to melt the glass. Frit sealing is more flexible for complex geometries and allows for sealing at lower temperatures using devitrifying frits. However, frit seals can be more prone to voids and require careful process control.

Matching Glass Transition Temperature to Your Process Window

The glass transition temperature (Tg) defines the temperature at which the glass begins to behave less like a solid and more like a supercooled liquid. For sealing, you need to heat the glass well above Tg—typically to its softening point or higher—so it can flow and wet the oxide layer.

The processing window is bounded on the low end by the glass’s softening point and on the high end by Kovar’s metallurgical limits. Excessive temperatures can cause Kovar grain growth, which weakens the metal, or excessive oxide growth, which creates a brittle interface.

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Kovar Processing and Surface Preparation

The success of a Kovar glass-to-metal seal is determined long before the glass ever melts. Surface preparation is the foundation.

Cleaning Protocols

Kovar parts arrive at the sealing operation with machining oils, handling residues, and native oxides. These must be completely removed before pre-oxidation.

Our standard protocol at KELTRYN is:

1. Degreasing: Ultrasonic cleaning in an alkaline detergent or solvent to remove oils and organic residues.
2. Acid etching: A brief immersion in a dilute acid solution (typically hydrochloric or sulfuric acid) to remove the native oxide and any surface contamination.
3. Rinsing: Multiple deionized water rinses to remove all acid residues.
4. Drying: Clean, dry air or nitrogen drying to prevent water spotting.

Surface roughness also matters. A slightly roughened surface (approximately 0.4–0.8 μm Ra) provides mechanical keying that enhances the oxide-to-glass bond. However, excessive roughness can create stress concentrations that weaken the seal.

Pre-Oxidation Process

The pre-oxidation step forms the critical oxide layer. The goal is to produce a uniform, adherent oxide of controlled thickness.

Typical pre-oxidation parameters:

• Temperature: 700–800°C
• Time: 5–15 minutes (adjusted to achieve target oxide thickness)
• Atmosphere: Air or a controlled oxygen-containing atmosphere

The oxide thickness is usually verified by visual inspection (the part should have a uniform dark gray to black appearance) or by measuring weight gain. For critical applications, we recommend establishing a correlation between process parameters and oxide thickness using cross-section microscopy.

Hydrogen Firing vs. Air Firing

Hydrogen firing (also called bright firing or wet hydrogen firing) is used to reduce surface oxides and produce a clean, active surface. This is typically done at 900–1000°C in a wet hydrogen atmosphere. Hydrogen firing is often used before nickel plating, as it produces a surface that accepts plating more readily.

Air firing is the standard method for pre-oxidation before glass sealing. The part is heated in air to form the controlled oxide layer.

Some processes use a combination: hydrogen firing to clean, followed by a controlled air oxidation step.

Plating Options

After sealing, Kovar parts are often plated to improve solderability, corrosion resistance, or wire bonding characteristics.

• Nickel plating: The most common choice. It provides a solderable surface and protects against corrosion. Typical thickness is 2.5–5 μm.
• Gold plating: Often applied over nickel for applications requiring excellent corrosion resistance or gold wire bonding. Typical gold thickness is 1–3 μm.
• Copper plating: Used occasionally for specific soldering requirements.

Handling and Storage

Oxidized Kovar parts are sensitive to contamination. Handle them with clean, lint-free gloves. Store them in sealed bags with desiccant to prevent moisture absorption or further oxidation. Ideally, seal the glass within 24–48 hours of pre-oxidation.

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Design Guidelines for Kovar Glass-to-Metal Seals

Designing a Kovar part for glass sealing is different from designing a standard machined component. You’re not just designing for function—you’re designing for stress management, glass flow, and hermeticity.

Wall Thickness and Geometry

The Kovar housing must be thick enough to resist deformation during sealing but not so thick that it creates excessive thermal mass. As a general rule, wall thicknesses of 0.5–2.5 mm work well for most applications.

Avoid sharp corners wherever possible. Sharp internal corners create stress concentrations that can crack the glass during thermal cycling. Use generous fillet radii—aim for a minimum radius of 0.25 mm, and larger if the design allows.

Glass-to-Metal Ratio

The ratio of glass thickness to metal thickness affects the stress distribution in the finished seal. For matched seals, the ideal ratio depends on the specific glass and Kovar CTEs. A common starting point is a glass thickness of 0.5–1.5 mm for a Kovar wall thickness of 0.5–1.0 mm.

For compression seals, the metal wall must be thick enough to generate the required compressive stress on the glass. This typically requires a metal-to-glass thickness ratio of at least 2:1.

Lead and Wire Diameter

The diameter of the center lead or wire affects both the electrical performance and the mechanical stress in the seal. For a matched seal, the lead diameter should be matched to the glass preform’s inner diameter with a controlled interference fit.

Larger leads require thicker glass walls to manage stress. As a rule of thumb, the glass wall thickness around a lead should be at least equal to the lead diameter.

Dimensional Tolerances

Glass sealing requires tighter tolerances than typical machining. Plan for:

• Kovar part tolerances: ±0.05 mm for critical dimensions, ±0.13 mm for non-critical
• Surface finish: 0.4–0.8 μm Ra on sealing surfaces
• Flatness: Within 0.05 mm for sealing surfaces that contact the glass

Designing for Thermal Cycling

If your application will see thermal cycling, design for it from the start. Avoid abrupt changes in cross-section that create stress concentrations. Use symmetrical designs where possible to keep stress evenly distributed. And consider adding stress-relief features like stepped transitions between thick and thin sections.

Common Design Mistakes

1. Sharp corners at the glass-to-metal interface — always use radii
2. Uneven wall thickness — creates differential thermal stress
3. Excessive lead length — can cause alignment issues during sealing
4. Insufficient glass thickness — leads to electrical breakdown or mechanical failure
5. Contaminated surfaces — any residue will prevent proper oxide bonding

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Manufacturing Processes: Sealing Techniques Explained

Once your Kovar parts are machined and prepared, the actual sealing process begins. There are several methods, each with its own advantages.

Furnace Sealing

Furnace sealing is the most common method for production volumes. The assembly—Kovar housing, glass preform, and lead—is fixtured and passed through a controlled atmosphere belt furnace.

Typical temperature profile:

1. Preheat zone: 400–600°C to burn off binders and slowly heat the assembly
2. Sealing zone: 900–1000°C where the glass melts and wets the oxide
3. Annealing zone: Controlled cooling to prevent thermal shock

The atmosphere is typically nitrogen with a small amount of hydrogen or oxygen, depending on the glass and oxide requirements. Total cycle time ranges from 30 minutes to several hours, depending on the part size and thermal mass.

Direct Sealing vs. Frit Sealing

Direct sealing uses solid glass preforms that melt and flow to fill the gap between the Kovar parts. This is the preferred method for high-reliability applications because it produces a dense, void-free seal.

Frit sealing uses powdered glass paste. It’s more forgiving of dimensional variations and can be used for complex geometries, but the resulting seal may have more porosity and requires careful process control.

Laser Sealing

For prototyping or low-volume production, laser sealing offers speed and precision. A focused laser beam heats the glass locally, melting it without heating the entire assembly. This is particularly useful for sealing heat-sensitive components or for rework.

The trade-off is that laser sealing requires precise beam control and is generally slower for high-volume production.

Quality Control During Production

In-process inspection is essential. Key checkpoints include:

• Visual inspection of the oxide layer before sealing
• Dimensional verification of the assembled stack-up before sealing
• Process parameter monitoring (temperature, atmosphere, belt speed)
• Visual inspection of finished seals for cracks, bubbles, or dewetting

Automation and Scalability

For high-volume production, automation can improve consistency and reduce labor costs. Automated dispensing of glass frit, robotic assembly, and vision-based inspection systems are all available. However, the initial investment is significant, so automation makes the most sense for mature products with stable designs.

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Testing and Qualification of Kovar Seals

You can’t ship a hermetically sealed component without proving it’s actually hermetic. Testing and qualification are critical steps.

Hermeticity Testing

The gold standard is helium mass spectrometry leak testing, performed per MIL-STD-883. The process is straightforward:

1. Place the sealed part in a pressurized helium chamber for a specified time
2. Remove the part and place it in a vacuum chamber connected to a mass spectrometer
3. Measure the helium that escapes from the part

Acceptance criteria vary by application, but for high-reliability applications, leak rates below 1 × 10⁻⁹ atm·cc/s He are typical.

Thermal Cycle Testing

Thermal cycling tests expose the seal to repeated temperature extremes to verify its ability to withstand thermal stress. A typical test might be:

• Temperature range: -65°C to +150°C (or wider for aerospace)
• Number of cycles: 100 to 1,000
• Dwell time: 15 minutes at each extreme
• Transfer time: Less than 1 minute between extremes

After thermal cycling, parts are re-tested for hermeticity and inspected for cracks.

Mechanical Strength Testing

Depending on the application, you may need to verify:

• Pull tests on leads (typical acceptance: 5–20 pounds depending on lead diameter)
• Torque tests on threaded terminals
• Burst pressure tests for high-pressure applications
• Vibration testing per MIL-STD-202

Electrical Testing

For feedthroughs, you’ll need to verify:

• Insulation resistance: Typically > 10¹⁰ Ω at 100 VDC
• Dielectric withstanding voltage: No breakdown at specified voltage (often 500–1,500 VAC)
• Capacitance: If relevant for RF applications

Visual Inspection Criteria

Per industry standards, reject parts with:

• Cracks in the glass (any size)
• Bubbles larger than 0.25 mm or clusters of smaller bubbles
• Dewetting (glass pulling away from the metal)
• Oxide discoloration indicating process deviation
• Glass flow that doesn’t meet the specified wetting criteria

Industry Standards

The most relevant standards for Kovar glass-to-metal seals are:

• MIL-STD-883: Test methods for microelectronics, including hermeticity testing
• ASTM F19: Standard test method for tension and vacuum testing of metallized ceramic seals
• MIL-STD-202: Test methods for electronic components
• Customer-specific requirements: Many aerospace, medical, and automotive customers have their own qualification standards

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Applications and Case Studies

Let’s look at how Kovar glass-to-metal seals are used in practice across different industries.

Aerospace and Defense

Hermetic connectors for avionics and missile guidance systems must survive extreme thermal cycling, vibration, and altitude changes. Kovar’s CTE match with borosilicate glass ensures that the seal remains intact from -65°C to +200°C.

Case example: A missile guidance connector with 37 pins, each requiring a matched glass-to-metal seal. The challenge was maintaining pin-to-pin spacing within ±0.05 mm after sealing. By designing a Kovar housing with precision-machined pin holes and using pre-formed glass beads with tight tolerances, we achieved the required dimensional stability.

Medical Devices

Implantable pacemaker feedthroughs are among the most demanding applications. The seal must remain hermetic for decades inside the human body, resisting corrosion from bodily fluids.

Case example: A pacemaker feedthrough with a platinum-iridium lead sealed through a Kovar ferrule. The Kovar ferrule was machined to a wall thickness of 0.38 mm, and the glass was sealed under vacuum to eliminate voids. The finished assembly passed 1,000 thermal cycles from 0°C to 100°C with no measurable leak rate.

Semiconductor and Vacuum Equipment

Power feedthroughs for ion implanters carry high voltages and currents into vacuum chambers. These require robust insulation and reliable hermeticity.

Case example: A high-voltage feedthrough for an ion implanter required a 20 kV rating. We machined a Kovar flange with a ceramic insulator and a Kovar center conductor. The design used a compression seal to manage the stress from the larger diameter, and the finished assembly passed dielectric testing at 30 kV.

Automotive and Industrial

Pressure sensors for automotive applications see harsh environments: temperature extremes, vibration, and exposure to chemicals.

Case example: A manifold absolute pressure (MAP) sensor housing required a Kovar-to-glass seal that could withstand 1,000 thermal cycles from -40°C to +150°C. The key was selecting a glass with a CTE slightly lower than Kovar to create a mild compression seal, providing additional mechanical strength.

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Kovar vs. Alternatives: When Not to Use Kovar

Kovar is the right choice for many applications, but it’s not always the right choice. Here’s how it compares to alternatives.

Alloy 52

Alloy 52 (52% Ni, balance Fe) has a higher CTE (~10 ppm/°C) than Kovar. It’s used with certain sealing glasses that have a higher CTE, but it’s not as versatile as Kovar for glass sealing.

Other Ni-Fe Alloys

Nickel-iron alloys with lower nickel content have lower CTEs and can be used for specialized applications. However, they typically don’t form the same quality of oxide layer as Kovar, making them more difficult to seal.

Stainless Steel

Stainless steel is less expensive than Kovar and offers better corrosion resistance. However, its higher CTE (~10–17 ppm/°C depending on grade) makes it difficult to match with glass. Stainless steel is sometimes used for compression seals, but the design constraints are more severe.

Ceramic-to-Metal Seals

For extreme environments—very high temperatures, aggressive chemicals, or high voltages—ceramic-to-metal seals are often superior to glass-to-metal seals. Alumina ceramics offer higher temperature capability, better electrical insulation, and superior mechanical strength.

However, ceramic-to-metal sealing is more expensive and requires specialized processes like molybdenum-manganese metallization. For many applications, Kovar glass-to-metal seals offer the best balance of performance and cost.

Decision Guide

| Factor | Choose Kovar Glass Seal | Choose Ceramic Seal |

|---|---|---|

| Temperature | Up to ~450°C | Up to ~1,000°C+ |

| Electrical requirements | Moderate voltage/insulation | High voltage, low loss |

| Budget | Lower cost | Higher cost |

| Mechanical strength | Adequate for most | Superior |

| Chemical resistance | Good | Excellent |

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Common Defects and Troubleshooting

Even with careful process control, defects can occur. Here’s a guide to the most common issues and their root causes.

Cracks in the Glass

Symptoms: Visible cracks radiating from the glass-metal interface or through the glass body.

Root causes:

• Excessive thermal stress during cooling (CTE mismatch)
• Thermal shock from too-rapid cooling
• Sharp corners on the Kovar part creating stress concentrations
• Glass thickness too thin for the application

Remedies:

• Slow down the cooling rate
• Add fillet radii to Kovar parts
• Increase glass thickness
• Switch to a glass with a closer CTE match

Leaks (Hermeticity Failures)

Symptoms: Helium leak test failure.

Root causes:

• Inadequate oxide layer (under-oxidation)
• Contamination on the Kovar surface before sealing
• Incomplete glass wetting
• Voids or bubbles in the glass
• Micro-cracks not visible under normal inspection

Remedies:

• Verify and adjust pre-oxidation parameters
• Improve cleaning protocols
• Increase sealing temperature or time
• Use vacuum sealing to eliminate voids
• Perform dye penetrant or fluorescent inspection to find micro-cracks

Glass Devitrification

Symptoms: Cloudy or crystalline appearance in the glass.

Root causes:

• Sealing temperature too high or time too long, causing crystallization
• Incompatible glass composition
• Contamination that nucleates crystal growth

Remedies:

• Reduce sealing temperature or time
• Use a more stable glass formulation
• Improve cleanliness

Oxide Discoloration

Symptoms: Non-uniform color on the Kovar surface (patchy gray, brown, or green).

Root causes:

• Non-uniform heating during pre-oxidation
• Atmosphere control issues
• Residual contamination before oxidation

Remedies:

• Verify furnace temperature uniformity
• Check atmosphere composition and flow
• Improve cleaning before oxidation

When to Redesign vs. Adjust Processing

If you’re seeing the same defect repeatedly, it’s time to look at the design, not just the process. Redesign if:

• The defect is related to geometry (sharp corners, uneven walls)
• You’re consistently seeing CTE mismatch failures
• The application requirements exceed what the current design can deliver

Adjust processing if:

• The defect is related to process parameters (temperature, time, atmosphere)
• You’re seeing batch-to-batch variation
• The defect appeared after a process change

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Frequently Asked Questions (FAQ)

What is the maximum operating temperature of a Kovar glass-to-metal seal?

For continuous service, most Kovar glass-to-metal seals are rated to 400–450°C. The limiting factor is usually the glass, not the Kovar. Borosilicate glasses begin to soften around 700°C, but long-term exposure above 450°C can cause gradual devitrification or stress relaxation. For higher temperatures, consider ceramic-to-metal seals.

How do I choose between matched and compression seals for my application?

Matched seals are the default choice for most applications because they minimize residual stress. Choose a compression seal when you need additional mechanical strength (high pressure), when you’re working with larger diameters, or when you want to use a lower-cost glass. If you’re unsure, start with a matched seal—it’s the safer choice for high-reliability applications.

What glass types are compatible with Kovar?

The most common glasses are borosilicates like Corning 7052 and 7070, which have CTEs closely matched to Kovar. Aluminosilicate glasses can be used for higher-temperature applications but require higher sealing temperatures. Your glass supplier or sealing partner can help you select the right formulation for your specific requirements.

How is hermeticity tested on Kovar seals?

The standard method is helium mass spectrometry leak testing per MIL-STD-883. The part is pressurized with helium, then placed in a vacuum chamber connected to a mass spectrometer. Helium escaping from the part is measured, and the leak rate is calculated. Acceptance criteria are typically below 1 × 10⁻⁹ atm·cc/s He for high-reliability applications.

Can Kovar seals be welded or soldered after sealing?

Yes. Kovar is designed to be weldable and solderable. After glass sealing, Kovar parts can be:

• Welded using TIG, laser, or resistance welding
• Soldered using tin-lead or lead-free solders (after nickel or gold plating)
• Brazed using silver-copper or gold-based braze alloys

Just be careful to avoid overheating the glass during welding—keep the weld zone away from the glass and use heat sinks if necessary.

What is the typical lead time and minimum order quantity for custom Kovar seals?

At KELTRYN, we specialize in prototypes and low-volume production. Typical lead times for machined Kovar components are 2–4 weeks for prototypes and 4–6 weeks for production quantities. There’s no minimum order quantity for prototypes, and we work with customers on production volumes from tens to thousands of parts. The glass sealing itself is typically done by our customers or their sealing partners, though we can recommend qualified partners.

How does Kovar compare to ceramic-to-metal sealing for high-reliability applications?

Kovar glass-to-metal seals are generally less expensive and offer more design flexibility than ceramic-to-metal seals. However, ceramic-to-metal seals offer higher temperature capability, better electrical insulation, and superior mechanical strength. For applications above 450°C, high voltage, or extreme mechanical stress, ceramic-to-metal is the better choice. For most other applications, Kovar glass-to-metal seals are the more cost-effective solution.

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Conclusion

Kovar glass-to-metal sealing is a mature technology, but it rewards engineers who understand the underlying materials science. The key takeaways from this guide:

1. CTE matching is everything. Kovar’s CTE of 5.2–5.5 ppm/°C makes it uniquely suited to borosilicate glasses. Mismatches create stress that leads to cracks and leaks.

1. The oxide layer is the secret sauce. A controlled oxide layer of 2–5 μm on the Kovar surface is essential for creating a chemical bond with the glass. Too thin, and the bond is weak. Too thick, and the interface is brittle.

1. Choose your seal type deliberately. Matched seals minimize stress; compression seals add mechanical strength. Each has its place, and the choice affects everything downstream.

1. Design for manufacturability. Avoid sharp corners, control tolerances, and think about how the glass will flow and wet the surface. Good design prevents most defects.

1. Qualification is non-negotiable. Hermeticity testing, thermal cycling, and mechanical testing are essential for high-reliability applications. Don’t skip them.

1. Partner with experts. Kovar machining and glass sealing are specialized disciplines. Working with a partner who understands both can save you months of trial and error.

At KELTRYN, we’ve built our entire operation around the challenges of machining Kovar and Fe-Ni-Co alloys. We know that a glass-to-metal seal is only as good as the metal components that go into it. That’s why we control every aspect of our machining process—from material traceability to burr control to dimensional verification—to ensure that your Kovar parts are ready for sealing.

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Final Checklist for Engineers Designing a Kovar Glass-to-Metal Seal

Before you send your design out for quotes, run through this checklist:

• [ ] Material specified as Kovar (ASTM F15 / 4J29) with material certification
• [ ] CTE of glass confirmed to match your seal type
• [ ] Seal type selected (matched vs. compression) with rationale documented
• [ ] No sharp corners at glass-metal interfaces (minimum 0.25 mm radius)
• [ ] Wall thicknesses designed for stress management
• [ ] Glass thickness specified relative to metal thickness
• [ ] Lead diameter and glass wall thickness ratio verified
• [ ] Tolerances specified for critical dimensions (±0.05 mm or better)
• [ ] Surface finish specified (0.4–0.8 μm Ra on sealing surfaces)
• [ ] Cleaning and pre-oxidation protocols defined
• [ ] Testing and qualification requirements documented
• [ ] Thermal cycling and operating temperature ranges defined
• [ ] Plating requirements specified (nickel, gold, or none)
• [ ] Partner selected with proven Kovar machining experience

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Ready to turn your Kovar design into precision-machined components?

At KELTRYN, we specialize in custom Kovar housings, lids, caps, rings, flanges, and frames for hermetic and high-reliability applications. From early-stage prototypes to repeat production, we provide the engineering support, material traceability, and dimensional control your glass-to-metal seal demands.

Send us your 2D drawing, 3D CAD model, or even just your application requirements. We’ll provide manufacturability feedback, a detailed quote, and a delivery timeline that works for your project.

[Contact KELTRYN today] to discuss your Kovar component needs.