Kovar alloy (ASTM F15) is an iron–nickel–cobalt sealing alloy whose coefficient of thermal expansion must be tightly controlled to ensure reliable glass-to-metal and ceramic-to-metal seals in electronic packages. Accredited third-party laboratories provide comprehensive Kovar physical property and expansion testing—including CTE measurement by push-rod dilatometry, thermomechanical analysis, and laser-flash thermal conductivity—in accordance with ASTM, ISO, and GB standards. This article explains the sample types, testing items, methods, and quality criteria involved in Kovar expansion testing.
Technical Overview
Kovar alloy is a controlled-expansion nickel–cobalt–iron alloy containing approximately 29% nickel, 17% cobalt, and a balance of iron, as specified in ASTM F15. Its thermal expansion behavior closely matches that of borosilicate glass and certain alumina ceramics between room temperature and approximately 400 °C, which makes it the material of choice for hermetic sealing applications in microelectronics, vacuum devices, and precision instrumentation. Because seal integrity depends directly on the matching of expansion curves between the metal and the glass or ceramic, accurate measurement of the coefficient of thermal expansion (CTE) is the single most critical physical property test for Kovar.
- Composition range (ASTM F15): Ni 28.5–29.5%, Co 16.8–17.8%, Fe balance, with controlled Mn, Si, and C levels
- Typical mean CTE: approximately 4.6–5.2 × 10⁻⁶/°C between room temperature and 300 °C
- Curie temperature: approximately 415–450 °C, above which the expansion coefficient rises sharply to roughly 9–10 × 10⁻⁶/°C
- Density: approximately 8.36 g/cm³; thermal conductivity: approximately 17 W/(m·K); electrical resistivity: approximately 0.49 μΩ·m
Physical property and expansion testing verifies that incoming material, semi-finished products, and finished components conform to specification, that heat treatment has produced the intended stable metallurgical condition, and that batch-to-batch consistency supports long-term seal reliability. Testing is typically performed by accredited third-party laboratories using calibrated dilatometers and traceable reference materials.
Testing Samples
Raw and Semi-Finished Materials
Testing laboratories accept Kovar in all common mill forms, including hot-rolled and cold-drawn bars, rods, wires, strips and foils, tubes, plates, and forged blanks. For strip products, specimens are normally taken in the longitudinal (rolling) direction because cold rolling can introduce slight anisotropy in the expansion behavior.
Finished and Near-Net-Shape Components
Finished parts such as transistor headers and lids, integrated circuit packages, lead frames, eyelets, feedthroughs, battery cover assemblies, and complete glass-to-metal seal subassemblies can be evaluated either directly (when geometry permits) or through representative coupons machined from the same production lot and processed through the identical annealing and sealing thermal cycle.
Powder Metallurgy and Additively Manufactured Parts
Sintered Kovar, metal-injection-molded (MIM) components, and additively manufactured Kovar parts are increasingly tested for expansion behavior, since porosity and microstructural differences can shift the CTE and density relative to wrought material. These parts are usually tested after the final sintering or HIP consolidation cycle.
Specimen Preparation Requirements
For push-rod dilatometry per ASTM E228, typical specimens are 25–125 mm long (50 mm preferred) with flat, parallel end faces (generally within 10–25 μm) and adequate cross-section to resist load-induced deformation. For thermomechanical analysis, much smaller specimens of a few millimeters are acceptable. All specimens must be free of residual machining stress; a stabilization or degassing anneal prior to testing is recommended when the service condition involves prior heat treatment, because residual stress produces irreversible length changes on first heating.
Testing Items
Depending on the specification and application, Kovar physical property and expansion testing may include the following measurable items:
- Mean (average) coefficient of thermal expansion over defined temperature intervals, for example RT–100 °C, RT–200 °C, RT–300 °C, RT–400 °C, RT–450 °C, RT–500 °C, and RT–600 °C
- Instantaneous (tangent) coefficient of thermal expansion as a function of temperature
- Percent linear thermal expansion (ΔL/L₀) and the complete expansion curve from room temperature to the maximum service or sealing temperature
- Curie temperature, identified as the inflection point of the expansion curve
- Thermal hysteresis and irreversible length change during heating–cooling thermal cycling
- Density by hydrostatic (Archimedes) weighing
- Thermal diffusivity and thermal conductivity by the laser-flash method
- Specific heat capacity by differential scanning calorimetry
- Electrical resistivity by the four-terminal DC method
- Young's modulus and Poisson's ratio by dynamic (resonance) methods
The most commonly specified acceptance criterion is the mean CTE between room temperature and 300 °C, which must fall within the narrow window that matches borosilicate glass so that seal stresses remain compressive and within safe limits on cooling from the sealing temperature.
Testing Methods
Push-Rod Dilatometry (ASTM E228)
Push-rod dilatometry is the primary reference method for metallic materials. The specimen is placed in a fused-silica or alumina dilatometer, a small constant force is applied through a push rod, and length change is measured by a displacement transducer while the furnace heats at a controlled rate of typically 3–5 °C/min under argon, nitrogen, or vacuum. The mean CTE is calculated as α = (L₂ − L₁) / [L₀ × (T₂ − T₁)]. The instrument is calibrated and verified with certified reference materials traceable to national metrology institutes, such as fused silica, sapphire, copper, and tungsten standards, achieving repeatability on the order of 0.05 × 10⁻⁶/°C.
Thermomechanical Analysis (ASTM E831)
Thermomechanical analysis (TMA) applies a very low static force to a small specimen resting on a quartz platform and records dimensional change up to approximately 1000 °C. TMA is well suited to finished components, coated parts, and material availability limited to small coupons, offering rapid turnaround with modest precision compared with push-rod dilatometry.
Laser Interferometry (ASTM E289)
Laser-Flash Thermal Diffusivity (ASTM E1461)
A short laser pulse heats the front face of a disk specimen, and an infrared detector records the rear-face temperature rise to determine thermal diffusivity. Combined with measured density and specific heat, thermal conductivity is derived as k = α × ρ × cp. This characterizes heat dissipation in packaged devices and verifies material consistency.
Dynamic Elastic Modulus (ASTM E1875)
The impulse excitation technique measures resonant frequencies of a specimen to compute Young's modulus, shear modulus, and Poisson's ratio non-destructively. These values support stress calculations for seal design and finite-element modeling of packages.
Specific Heat and Transition Analysis (ASTM E1269)
Differential scanning calorimetry determines specific heat capacity and can independently confirm the Curie transition temperature, complementing the inflection observed on the dilatometric expansion curve.
Testing Standards
- ASTM F15 — Standard Specification for Iron-Nickel-Cobalt Sealing Alloy (composition and expansion limits)
- ASTM E228 — Linear Thermal Expansion of Solid Materials with a Push-Rod Dilatometer
- ASTM E831 — Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis
- ASTM E289 — Linear Thermal Expansion of Rigid Solids by Interferometry
- ASTM E1461 — Thermal Diffusivity by the Flash Method
- ASTM E1269 — Determination of Specific Heat Capacity by Differential Scanning Calorimetry
- ASTM E1875 — Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Sonic Resonance
- ASTM B63 — Resistivity of Metallic Conductor Materials
- GB/T 4339 — Determination of thermal expansion characteristics of metallic materials
Testing Process
A typical expansion test campaign at an accredited third-party laboratory follows a controlled workflow to ensure traceable, repeatable results:
- Sample reception, lot identification, and review of material certification and heat-treatment history
- Specimen machining, cleaning, and dimensional verification; optional stabilization anneal to relieve residual stress
- Instrument calibration and verification against certified reference materials
- Programmed heating under inert atmosphere or vacuum at 3–5 °C/min, with simultaneous recording of length change and temperature
- Optional cooling segment to evaluate hysteresis and irreversible length change
- Data reduction: mean and instantaneous CTE by temperature interval, expansion curve, and Curie point determination
- Reporting with measurement uncertainty, specimen orientation, heating rate, atmosphere, calibration traceability, and a pass/fail judgment against ASTM F15 or client-specified limits
Applications
Microelectronic and Semiconductor Packaging
Kovar headers, lids, and lead frames are used for hermetic transistor-outline (TO) cans, dual-in-line packages, hybrid circuits, surface-mount headers, and optoelectronic packages. Verified CTE ensures that glass seals and ceramic bodies remain crack-free through solder reflow, temperature cycling, and long-term service.
Glass-to-Metal and Ceramic-to-Metal Seals
Matched expansion between Kovar and borosilicate glass is essential for connector inserts, feedthroughs, hermetic terminals, battery covers, igniters, and relay envelopes. Expansion testing of both the alloy and the seal assembly confirms that residual seal stresses are compressive and stable.
Aerospace, Defense, and Vacuum Devices
Traveling-wave tubes, photomultiplier tubes, infrared detector Dewars, x-ray tube components, and microwave assemblies rely on Kovar to maintain vacuum integrity across extreme thermal excursions. Expansion data over extended temperature ranges, including behavior above the Curie point, feed directly into reliability models.
Medical, Automotive, and Industrial Sensors
Implantable device housings, pacemaker feedthroughs, pressure and oxygen sensors, and automotive control-module packages depend on consistent Kovar expansion behavior for long-term hermeticity under sterilization, autoclave, and under-hood thermal environments.
FAQ
What CTE acceptance limits apply to Kovar?
For glass-sealing grades, the mean CTE between room temperature and 300 °C is typically required to fall between approximately 4.6 and 5.2 × 10⁻⁶/°C. Purchase specifications based on ASTM F15, or customer-defined limits, state the exact intervals and tolerance bands that the laboratory applies for pass/fail judgment.
Why does the expansion coefficient of Kovar increase above approximately 430 °C?
Kovar owes its low expansion to magnetically driven anomalous contraction (magnetostriction) below the Curie temperature. Above the Curie point—roughly 415–450 °C—this effect disappears and the alloy expands like a normal metal, with CTE rising to around 9–10 × 10⁻⁶/°C. Sealing processes must therefore account for the divergence that occurs above this transition.
What is the difference between mean and instantaneous CTE?
The mean CTE is the averaged slope between two specified temperatures and is the value cited in specifications. The instantaneous (tangent) CTE is the slope at a single temperature and is used to analyze seal stress distributions and identify transition points on the expansion curve.
What sample size and quantity are needed?
For ASTM E228 push-rod dilatometry, specimens of 25–125 mm length (50 mm typical) with parallel ends are preferred; TMA accepts specimens of only a few millimeters. Testing duplicate or triplicate specimens from the same lot is recommended to demonstrate repeatability, and finished parts can often be tested as-is or via coupons from the same heat-treatment batch.
Does heat treatment affect the expansion test result?
Yes. Cold work and residual machining stress cause an irreversible length change during the first heating cycle, which appears as hysteresis. Hydrogen annealing and degassing stabilize the microstructure; testing is therefore performed either in the as-delivered condition (to verify incoming material) or after the production anneal (to verify service behavior), depending on the specification.
Which method should be chosen: dilatometry or TMA?
Push-rod dilatometry (ASTM E228) offers the best accuracy and is preferred for material qualification and certification against ASTM F15. TMA (ASTM E831) is faster and suits small components, coated parts, and engineering development screening. Interferometry (ASTM E289) is reserved for the highest-precision applications.
Can other physical properties be measured together with expansion testing?
Yes. A combined test report commonly includes density, thermal diffusivity and conductivity, specific heat capacity, electrical resistivity, and elastic modulus, giving designers a complete physical property dataset for seal stress analysis and thermal simulation from a single sample submission.