Kovar 4J29 Plate: Sealing Alloy for Electronics and Vacuum Devices

Introduction

Kovar 4J29 Plate is an iron-nickel-cobalt controlled-expansion alloy plate used to manufacture hermetic sealing components for electronics, vacuum devices, sensors, microwave equipment and scientific instruments. Its thermal expansion behavior is engineered to closely follow selected borosilicate glasses and ceramic materials over a defined temperature range. This compatibility reduces thermal stress at a glass-to-metal or ceramic-to-metal joint during sealing, cooling and service.

Key Takeaways: Kovar 4J29 is normally selected for matched-expansion sealing rather than for general structural strength or atmospheric corrosion resistance. ASTM F15 and UNS K94610 are widely used procurement references, while 4J29 and material number 1.3981 are common regional designations. Plate condition, rolling direction, heat treatment, surface preparation, oxidation control and the actual sealing cycle can all affect final joint reliability.

A complete purchase specification should identify the alloy designation, governing standard, plate thickness, width, length, condition, flatness, surface finish, machining allowance, coefficient-of-expansion requirement and inspection documentation. Ordering only “Kovar plate” does not adequately control the material for a critical vacuum or electronic package.

What Is Kovar 4J29 Plate?

Kovar 4J29 plate is a flat product made from an alloy containing nominally about 29% nickel, 17% cobalt and iron as the balance. The nickel-cobalt relationship is tightly controlled because relatively small chemistry variations can alter the coefficient of thermal expansion.

The alloy was developed for metal components that must be sealed to hard glass or compatible ceramic without creating excessive stress during temperature changes. Ordinary carbon steel, copper or stainless steel expands at a substantially different rate from common sealing glasses. That mismatch can crack the glass, separate the interface or reduce vacuum integrity.

Kovar plate is commonly cut, stamped, deep drawn, machined, brazed or welded into housings, lids, frames, feedthrough bases and vacuum-device components. Depending on the order, it may be supplied hot rolled, cold rolled, annealed, pickled, ground or polished.

Typical Product Data

Specification Item	Typical Supply Options	Buyer Check
Alloy	Kovar / 4J29 / FeNi29Co17	Verify chemistry and expansion requirements rather than relying on a trade name.
Common Designations	ASTM F15, UNS K94610, W.Nr. 1.3981	Use the designation required by the drawing or end-user specification.
Product Form	Plate, sheet, strip, flat bar and machined blank	Confirm whether the order requires plate or thinner cold-rolled sheet.
Condition	Annealed, cold rolled, stress relieved or customer specified	Match the condition to stamping, machining and sealing operations.
Surface	Pickled, bright, ground, polished or prepared for oxidation	State acceptable scale, scratches, oil and oxide conditions.
Processing	Cutting, stamping, deep drawing, machining, brazing and welding	Provide the final component drawing and sealing process where possible.

How Kovar Supports Hermetic Sealing

A hermetic seal restricts the passage of gases or moisture through an enclosure or feedthrough. In electronic and vacuum devices, the seal must remain intact while the assembly experiences heating during manufacture, cooling to room temperature and repeated thermal cycles during operation.

Kovar works because its controlled thermal expansion is close to that of selected hard borosilicate glasses and certain ceramics. When the metal and glass expand and contract at similar rates, the interface experiences lower differential stress than it would with ordinary engineering metals.

The oxide layer formed on Kovar also influences sealing. A controlled, adherent oxide can support bonding with glass. Excessive oxidation, loose scale, contamination or an unsuitable oxide chemistry can weaken the interface. The sealing supplier must therefore control cleaning, atmosphere, oxidation time, temperature and glass composition.

The alloy alone does not guarantee a leak-tight seal. Component geometry, glass type, surface preparation, furnace cycle, cooling rate and residual stress must be qualified as one manufacturing system.

Chemical Composition

The composition below represents typical ASTM F15-type chemistry for Kovar sealing alloy. Nickel, cobalt and iron are adjusted within controlled limits to meet the required thermal-expansion behavior. Final acceptance must follow the ordered standard edition and the actual heat analysis.

Element	Typical Content or Limit, %	Technical Significance
Nickel, Ni	28.5-29.5	Controls thermal expansion and metallurgical stability.
Cobalt, Co	16.8-17.8	Works with nickel to establish the required expansion curve.
Iron, Fe	Balance	Forms the primary alloy matrix.
Manganese, Mn	Typically 0.50 maximum	Controlled to maintain sealing and expansion consistency.
Silicon, Si	Typically 0.20-0.30 maximum, depending on specification	Affects oxidation, melting control and interface behavior.
Carbon, C	Typically 0.02-0.04 maximum	Restricted to support ductility and dimensional consistency.
Cr, Cu and Mo	Controlled residual limits	Excess residual elements may affect expansion and sealing behavior.

Mechanical and Physical Properties

Kovar properties depend on plate thickness, rolling reduction and heat-treatment condition. The values below are typical engineering references and should not replace the mechanical and thermal requirements in the purchase specification.

Property	Typical Character	Engineering Relevance
Tensile Strength	Condition-dependent; increased by cold working	Controls forming, handling and component strength.
Yield Strength	Lower in annealed plate and higher in cold-worked material	Influences permanent distortion during stamping and assembly.
Elongation	Higher after suitable annealing	Important for deep drawing, bending and formed sealing parts.
Density	Approximately 8.3-8.4 g/cm³	Used for component weight and shipping calculations.
Thermal Conductivity	Relatively low compared with common structural metals	Affects sealing-cycle temperature distribution and machining heat.
Coefficient of Expansion	Controlled to match compatible hard glasses over a specified range	Reduces thermal stress in hermetic seals.
Magnetic Condition	Ferromagnetic below its Curie region	Must be evaluated near magnetically sensitive electronics.

Standards and International Designations

Reference System	Common Designation	Procurement Note
ASTM	ASTM F15	Covers iron-nickel-cobalt sealing alloy in specified wrought forms.
UNS	K94610	Widely used material designation for ASTM F15-type Kovar alloy.
Chinese Designation	4J29	Confirm applicable national standard, chemistry and expansion limits.
European Material Number	1.3981	Commonly associated with Fe-Ni-Co controlled-expansion sealing alloy.
Commercial Names	Kovar, Alloy 29-17, FeNi29Co17 and Nilo K	Trade-name materials require technical equivalence review.
Inspection Documentation	EN 10204 3.1 or agreed mill certificate	State the document type and thermal-test requirements before production.

Kovar Compared with Other Sealing Alloys

Material	Primary Benefit	Main Limitation	Best-Use Direction
Kovar 4J29	Expansion match with selected hard glasses and ceramics	Limited corrosion resistance and process-sensitive oxidation	Hermetic glass-to-metal and ceramic-to-metal sealing.
Alloy 42	Low and controlled expansion without cobalt	Different expansion curve from Kovar	Electronic lead frames, glass seals and thermostat parts after compatibility review.
Invar 36	Very low expansion near ambient temperatures	Not optimized for the same hard-glass sealing curve as Kovar	Precision instruments, optical structures and dimensional standards.
Dumet-Type Wire	Specialized lead-in wire for selected glass systems	Generally supplied as composite wire rather than structural plate	Lamp and electronic feedthrough applications.
304 Stainless Steel	Better general corrosion resistance and broad availability	Expansion is generally too high for direct matched hard-glass seals	General housings without a critical matched-expansion interface.

Industrial Applications

Application Sector	Typical Kovar Plate Component	Critical Requirement
Electronic Packaging	Headers, package bases, frames and sealing lids	Thermal-expansion compatibility, flatness and controlled surface oxidation.
Vacuum Tubes and Devices	Tube bases, windows, feedthrough supports and envelope components	Hermeticity, low outgassing and seal reliability.
Microwave and RF Equipment	Waveguide windows, package frames and connector bases	Dimensional stability, brazability and leak-tight sealing.
Sensors and Transducers	Sensor housings, diaphragms, headers and feedthrough plates	Stable seal geometry and moisture exclusion.
Scientific Instruments	Vacuum-chamber windows, detector packages and optical-device frames	Vacuum integrity, cleanliness and dimensional control.
Aerospace Electronics	Hermetic relay, sensor and avionics package components	Thermal cycling, traceability and qualified leak testing.

Fabrication and Sealing Guidance

Machining and Forming

Kovar can be machined using practices developed for nickel-iron alloys. It tends to work harden, so tools should remain sharp and should cut rather than rub. Rigid setups, positive tool geometry, steady feed and suitable coolant help control heat and surface finish.

Annealed plate is suitable for stamping and forming, but bend radius, grain direction and thickness must be considered. Severe forming may require intermediate annealing. Excessive cold work can change mechanical behavior and introduce residual stress before sealing.

Cleaning and Oxide Preparation

Oil, fingerprints, machining residue and loose scale can interfere with sealing. Components are normally degreased and cleaned before a controlled oxidation treatment. The required oxide thickness and appearance depend on the glass formulation and sealing process.

Over-oxidation can create a thick, weak or flaking layer. Under-oxidation may provide insufficient interaction with the glass. Production trials should establish the suitable furnace atmosphere, temperature and exposure time.

Welding and Brazing

Kovar can be welded or brazed using qualified procedures. Heat input, filler selection and joint geometry must be controlled because local overheating can create distortion or change the condition of the sealing surface. Brazing materials should be compatible with the required vacuum, service temperature and adjoining materials.

Limitations and Design Risks

Limited corrosion resistance: Kovar is not stainless steel. Unprotected plate can oxidize or rust during storage and service in humid environments.

Process-sensitive oxide: The sealing oxide must be controlled. A visually dark surface is not automatically suitable for glass bonding.

Material-name ambiguity: Commercially described Fe-Ni-Co alloys may not meet ASTM F15 chemistry or expansion requirements.

Thermal-range dependence: Expansion matching must be evaluated over the actual sealing and service temperature range, not at a single temperature.

Magnetic behavior: Kovar is ferromagnetic at normal temperatures and may influence magnetically sensitive components.

Residual stress: Heavy cold work, uneven machining or welding can cause distortion during the sealing heat cycle.

Quality Control, Certificates and Traceability

Heat-number control should link the finished plate or cut blank to the original melt and production lot. Markings, package labels, cut records, inspection reports and the MTC should show consistent identification. This control is particularly important where small Kovar blanks resemble ordinary nickel-iron or carbon-steel components.

An EN 10204 3.1 MTC may document the heat number, specification, chemistry, mechanical properties, dimensions and delivery condition. Critical sealing applications should also request a coefficient-of-expansion report covering the specified temperature interval.

PMI can support nickel and cobalt identification but does not prove that the plate meets the complete ASTM F15 thermal-expansion requirements. Dimensional inspection should include thickness, width, length, flatness and surface condition. UT may be relevant for thick plate or critical stock when internal soundness is specified, but it is not automatically required for every thin sealing component.

Kovar 4J29 Plate RFQ Checklist

✅ State Kovar, 4J29, UNS K94610 or W.Nr. 1.3981.

✅ Identify ASTM F15 or the governing project specification.

✅ Provide thickness, width, length, flatness and dimensional tolerances.

✅ Specify annealed, cold-rolled, stress-relieved or another required condition.

✅ Define surface finish and whether controlled oxidation will be performed by the supplier or customer.

✅ State the glass or ceramic type and sealing-temperature range where available.

✅ Request MTC, EN 10204 3.1, expansion test, PMI or UT as required.

✅ Define anti-rust protection, export packaging, delivery schedule and destination port.

FAQ

What is Kovar 4J29 plate used for?

Kovar 4J29 plate is used to manufacture hermetic sealing frames, electronic package bases, vacuum-device components, sensor headers, microwave housings and glass-to-metal or ceramic-to-metal feedthrough parts.

Is 4J29 equivalent to ASTM F15 Kovar?

4J29 and ASTM F15 Kovar are commonly treated as comparable iron-nickel-cobalt sealing alloys. They should not be substituted without comparing chemistry, coefficient of expansion, product condition and testing requirements.

Does Kovar need oxidation before glass sealing?

Many glass-sealing processes use a controlled oxide layer on the Kovar surface. The suitable oxidation cycle depends on the glass, component geometry and furnace process. Excessive or loose oxide can weaken the seal.

What certificates should be requested for Kovar plate?

Typical documents include an MTC or EN 10204 3.1 certificate, heat-number traceability, chemical analysis, mechanical-property results, dimensional inspection and a coefficient-of-expansion report when thermal matching is contractually required.

Related Sealing and Controlled-Expansion Alloys

Product	Typical Procurement Use
Kovar Precision 4J29 Plate	Controlled-expansion plate for glass-to-metal seals, electronic packages and vacuum-device components.
Precision Alloy Kovar 4J29	Fe-Ni-Co sealing alloy in multiple product forms for electronics, ceramics and hard-glass assemblies.
ASTM F15 4J29 Round Bar	Round stock for machined feedthroughs, pins, supports and sealing components.

Conclusion

Kovar 4J29 plate provides the controlled expansion required for reliable hard-glass and ceramic sealing in electronic, vacuum, microwave and scientific equipment. Its value comes from the relationship between tightly controlled nickel-cobalt chemistry, thermal expansion, surface oxidation and the qualified sealing cycle. Plate dimensions, condition and cleanliness must be specified with the same care as the alloy grade.