KOVAR

1. Thermal expansion coefficient matching hard glass and ceramics (core advantage): Within the temperature range of -80°C to +450°C, the average linear expansion coefficient is 4.5 - 6.5 × 10⁻⁶ /°C, which is almost perfectly matched with borosilicate glass and alumina ceramics, ensuring the hermeticity and long-term reliability of the sealed assembly.
2. Excellent hermetic sealing performance: After pre-oxidation treatment, KOVAR forms a chemical bond with molten glass. The leakage rate after sealing can reach < 1 × 10⁻⁹ Pa·m³/s, meeting the requirements for high-reliability electronic packaging.
3. Good plasticity and workability: With an annealed elongation of 20-30%, it can undergo various forming operations such as cold rolling, cold drawing, stamping, and machining.
4. Superior welding and sealing performance: It can be connected using methods such as TIG welding, resistance welding, electron beam welding, laser welding, and brazing, with high sealing strength when bonded to glass and ceramics.
5. Good dimensional stability: After stabilization treatment, dimensional changes during long-term service are minimal, making it suitable for precision electronic components.
6. Complies with multiple international standards: ASTM F15, UNS K94610, GB/T 15018 (4J29), MIL-I-23011, etc.

A:

The expansion-matching properties of KOVAR stem from its precisely controlled nickel-cobalt content and ferromagnetic characteristics:

1. Extended Applications of the Invar Effect:
KOVAR's expansion properties are based on the Invar effect but extend and shift the originally narrow low-expansion temperature range (-100°C to +200°C) to a higher temperature range (room temperature to +450°C) by adding approximately 17% cobalt and adjusting the nickel content to about 29%. This aligns perfectly with the softening temperature range of borosilicate glass.
2. Synergistic Action of Nickel and Cobalt:
The approximately 29% nickel and 17% cobalt in the alloy work together to make the alloy's expansion curve almost completely coincide with that of hard glass within the room temperature to glass annealing temperature range. This precise match ensures that during the cooling process of sealing, the metal's contraction matches the glass exactly, preventing excessive residual stress at the seal interface.
3. Relationship Between Curie Point and Operating Temperature:
KOVAR has a Curie point of approximately 415°C, and its upper operating temperature limit (around 450°C) falls precisely within the annealing temperature range of borosilicate glass. This is no coincidence but a core aspect of the alloy design-when the alloy transitions from ferromagnetic to paramagnetic, its change in expansion coefficient synchronizes with the change in glass expansion characteristics, achieving matching across the entire temperature range.

The heat treatment of KOVAR has a decisive impact on the final microstructural stability, dimensional accuracy, and hermetic sealing performance:
1. Annealing (Stress Relief, Homogenization of Structure):
Heat to 800-900°C, hold for 0.5 to 1 hour, then cool in the furnace or air-cool at a rate ≤ 300°C per hour to obtain stable austenitic structure and uniform expansion characteristics.
2. Stabilization Treatment (Recommended for High-Precision Parts):
Hold at 300-400°C for 1 to 4 hours, then slowly cool to room temperature to eliminate residual stresses, stabilize the austenitic structure, and minimize dimensional drift during subsequent use.
3. Pre-Oxidation Treatment (for Glass Sealing):
Hold at 900-1000°C for 15 to 30 minutes in a wet hydrogen atmosphere or nitrogen atmosphere with an appropriate amount of oxygen to form a dense oxide film with a thickness of 2-5 μm, ensuring good sealing with glass. The main component of the oxide film is (Co,Ni)Fe₂O₄ spinel structure.
4. Stress Relief Treatment (After Machining):
Hold at 200-300°C for 1 to 2 hours, then air-cool to prevent deformation caused by cutting stresses.
Note: KOVAR is prone to grain growth at high temperatures, so annealing temperature and time must be strictly controlled. Pre-oxidation treatment is a critical process in glass hermetic sealing, as the thickness and uniformity of the oxide film directly affect the bonding strength and airtightness of the seal.

Effective Working Temperature (with stable expansion matching performance):
It is recommended to be -80°C to +450°C. Within this temperature range, it can maintain a thermal expansion coefficient matching hard glass and excellent gas-tight sealing performance. The optimal sealing temperature range is 300°C to 450°C.
Short-term exposure temperature:
Up to +500°C, but the coefficient of thermal expansion will significantly increase, which may affect the reliability of the seal.
Low-Temperature Performance:
KOVAR maintains good strength and toughness below -80°C, but its coefficient of thermal expansion may deviate slightly from that of glass. For ultra-low-temperature applications, the reliability of hermetic sealing should be evaluated based on specific operating conditions.
High-temperature limitations:
When the temperature exceeds 415°C (Curie point), the alloy transitions from ferromagnetic to paramagnetic, resulting in significant changes to its expansion characteristics and a reduced match with glass. Therefore, KOVAR is not suitable for high-temperature environments above 450°C.

KOVAR exhibits excellent hermetic sealing performance and is an irreplaceable key material in the electronic packaging industry:
1. Hermetic Sealing with Glass:
After pre-oxidation treatment, KOVAR forms a dense oxide layer (thickness 2-5 μm) on its surface. During the glass sealing process (at approximately 900-1000°C), molten glass chemically bonds with the oxide layer to create a high-strength, hermetically sealed interface. The leakage rate after sealing can reach < 1 × 10⁻⁹ Pa·m³/s (helium mass spectrometry leak detection).
2. Sealing with ceramics:
KOVAR can be sealed to ceramics such as alumina and beryllium oxide through active metal brazing (e.g., TiCuAg filler metals) or direct bonding, making it suitable for high-reliability ceramic packaging.
3. Key Control Points: The three key control elements for Kovar and glass hermetic sealing are:
Oxide film thickness: 2-5 μm. Too thin will result in insufficient sealing strength, and too thick will cause the oxide film to peel off.
Sealing temperature: 900-1000°C, matching the glass softening point
Cooling Rate: Slow cooling is required (≤ 300°C per hour) to prevent thermal shock from causing glass cracking.
4. Reliability:
KOVAR hermetic components can withstand over 1000 temperature cycles between -80°C and +450°C without exceeding leak rates or experiencing mechanical failure.

Partial substitution is possible, but it must be evaluated based on specific temperature ranges and sealing material requirements:
Advantages of KOVAR:

1. Matching the thermal expansion coefficient of borosilicate glass in the range of -80°C to +450°C, it is the most widely used and technically mature sealing alloy in the field of electronic packaging.
2. Good workability and weldability, allowing it to be formed into various shapes.
3. High seal strength and good airtightness, with a leakage rate as low as < 1 × 10⁻⁹ Pa·m³/s.
4. It has good thermal expansion matching properties similar to those of alumina ceramics.

Limitations of KOVAR:
1. Differences in compatibility with different types of glass:
It is incompatible with soda-lime glass (coefficient of expansion approximately 9 × 10⁻⁶ /°C) and other sealing alloys (such as 4J28) should be selected instead.
2. Differences from welding with ordinary stainless steel:
When welding with austenitic stainless steel, attention should be paid to the stress caused by differences in the coefficient of thermal expansion.
3. Inadequate high-temperature (>500°C) performance:
The coefficient of thermal expansion increases significantly beyond the Curie point, making it unsuitable for ultra-high-temperature environments.

4. Mismatch with low-expansion glass:
For quartz glass (with an expansion coefficient of approximately 0.5 × 10⁻⁶ /°C), low-expansion sealing alloys (such as 4J36) should be selected.

Material Selection Recommendations:
For specific material selection, please provide detailed operating conditions (sealant material type, temperature range, environmental medium, service life). We can assist with a technical and economic comparison analysis to help you make the optimal choice between KOVAR and other seal alloy grades.
For quotations on specific specifications, material test reports, or technical parameter sheets, please contact our sales engineers. We support customizing dimensions, heat treatment conditions, and surface treatments as required, and provide a full set of internationally certified documents.