4J29 Alloy

4J29 (internationally known as Kovar®) is a high-performance iron-nickel-cobalt (Fe-Ni-Co) based hard glass-sealing alloy, specifically engineered for hermetic packaging applications requiring precise thermal expansion matching with hard glass (e.g., borosilicate glass) and reliable mechanical integrity. Unlike soft magnetic Permalloys (1J series, focused on magnetic permeability), 4J29 achieves its core function—hermetic sealing—through a precisely optimized composition (Ni≈29%, Co≈17%) that tailors its thermal expansion coefficient (CTE) to match hard glass in the critical temperature range (20-400℃). This alloy excels in vacuum-tight and moisture-proof packaging scenarios, making it the standard material for electronic components, optical devices, and aerospace sensors where hermeticity directly determines reliability.

Notably, 4J29 maintains excellent dimensional stability during glass-sealing processes (typically 500-650℃) and forms a strong, leak-proof bond with hard glass, ensuring long-term hermeticity (leak rate ≤1×10⁻⁹Pa·m³/s). Its good cold workability and weldability allow for manufacturing of complex packaging structures (e.g., transistor headers, optical fiber ferrules), while its moderate corrosion resistance suits indoor and controlled-environment applications. It is widely used in vacuum electronics, optoelectronics, aerospace instrumentation, and medical devices, where hermetic packaging protects sensitive components from moisture, dust, and gas contamination. The following is a comprehensive breakdown of its chemical composition, physical properties, mechanical properties, and application products.

1. Chemical Composition (Mass Fraction, %)

2. Physical Properties

2.1 Basic Physical Parameters

• Density: Approximately 8.30g/cm³ at room temperature (25℃), higher than soft magnetic 1J50 (8.20g/cm³) due to cobalt addition, but lower than high-alloyed superalloys (e.g., GH4738: 8.45g/cm³). This density is suitable for compact electronic packaging (e.g., transistor cases) where weight is not a primary constraint.
• Melting Temperature Range: 1430-1480℃ (liquidus: ~1480℃; solidus: ~1430℃). The narrow melting range ensures uniform composition during casting, avoiding segregation that would cause CTE variations (critical for consistent glass-sealing performance across batches).
• Thermal Expansion Coefficient (CTE) – Core Performance Indicator:

◦ 20-100℃: 4.6-5.0×10⁻⁶/℃

◦ 20-300℃: 4.8-5.2×10⁻⁶/℃

◦ 20-400℃: 5.0-5.5×10⁻⁶/℃ (matches borosilicate hard glass: 5.0-5.5×10⁻⁶/℃)

◦ 20-600℃: 5.8-6.3×10⁻⁶/℃

The CTE in the 20-400℃ range (glass-sealing and service temperature) is precisely matched to hard glass, minimizing thermal stress during cooling after sealing and preventing glass cracking or bond failure.

• Thermal Conductivity (λ):

◦ 100℃: ~16.5W/(m·K)

◦ 300℃: ~18.5W/(m·K)

◦ 400℃: ~20.0W/(m·K)

Moderate thermal conductivity facilitates heat dissipation from packaged components (e.g., high-power transistors) while avoiding excessive heat transfer that could disrupt glass-seal integrity.

• Electrical Resistivity (ρ):

◦ Room temperature (25℃): 49-53×10⁻⁸Ω·m

◦ 400℃: 60-65×10⁻⁸Ω·m

Higher resistivity than pure iron (9.7×10⁻⁸Ω·m) reduces eddy current losses in high-frequency applications (e.g., microwave tubes), protecting sensitive electronic components.

2.2 Magnetic Properties

Unlike 1J series soft magnetic alloys, 4J29 is not optimized for magnetic performance, but its magnetic properties are relevant for magnetically sensitive applications:

• Magnetic Permeability (μ): ~100-200μ₀ (at H=800A/m, room temperature) — much lower than 1J50 (≥25,000μ₀), making it suitable for non-magnetic packaging (e.g., magnetic sensor enclosures) where magnetic interference must be minimized;
• Coercivity (Hc): ~160-240A/m (room temperature) — significantly higher than 1J50 (≤5.0A/m), ensuring the alloy does not act as a magnetic core and avoids interfering with internal components;
• Magnetic Saturation (Bs): ~0.75-0.85T (room temperature) — lower than 1J50 (≥1.45T), further reducing magnetic interference risks.

3. Mechanical Properties (After Standard Heat Treatment: 850-900℃ annealing for 1h, air cooling)

Key Notes:

• Excellent Cold Workability: High elongation (δ₅ ≥30%) and reduction of area (ψ ≥60%) allow the alloy to be rolled into ultra-thin sheets (minimum thickness ≥0.05mm) for miniaturized packages (e.g., microelectronic chip carriers) and drawn into fine wires (0.1-1.0mm) for lead frames;
• Adequate Strength for Packaging: Tensile strength (σᵦ ≥450MPa) is sufficient to withstand mechanical stress during assembly (e.g., soldering, mounting) and service (e.g., vibration in aerospace applications);
• Stable Strength at Service Temperature: At 400℃ (maximum service temperature for most glass-sealed components), retained yield strength (≥200MPa) ensures the package maintains structural integrity without deformation.

4. Glass-Sealing Performance (Core Application Performance)

Key Notes:

• Ultra-high Hermeticity: Leak rate ≤5×10⁻¹⁰Pa·m³/s ensures packaged components (e.g., vacuum tubes, MEMS sensors) are protected from moisture (≤100ppm) and gas contamination for 10+ years;
• Strong Glass-Alloy Bond: Shear strength ≥25MPa exceeds the mechanical strength of most hard glasses, ensuring the bond does not fail before the glass itself;
• Excellent Thermal Shock Resistance: Withstands extreme temperature cycles without bond degradation, critical for aerospace and automotive electronics exposed to harsh environments.

5. Application Products & Industry Scenarios

5.1 Vacuum Electronics Field

As the standard hermetic packaging material for vacuum electronic components, 4J29 is used for:

• Vacuum Tube Headers: Pin headers and bases for electron tubes (e.g., klystrons, magnetrons), forming a hermetic seal with hard glass to maintain vacuum (≤1×10⁻⁵Pa) inside the tube; the CTE match prevents glass cracking during tube operation (up to 400℃);
• X-ray Tube Enclosures: Ceramic-glass-sealed enclosures for medical X-ray tubes, withstanding high voltage (up to 150kV) and maintaining hermeticity to prevent dielectric breakdown;
• Microwave Component Packaging: Hermetic packages for microwave transistors and diodes, protecting components from humidity (which degrades microwave performance) and ensuring stable operation at frequencies up to 20GHz.

5.2 Optoelectronics & Optical Communication Field

In optoelectronics requiring hermetic protection for optical components, 4J29 is applied to:

• Optical Fiber Ferrules & Connectors: Glass-sealed ferrules for single-mode optical fibers, ensuring precise fiber alignment (≤0.1μm) and hermeticity to prevent dust and moisture from degrading signal loss (≤0.1dB/km);
• Laser Diode Packages: Hermetic cans for high-power laser diodes (e.g., 1064nm infrared lasers), maintaining a dry environment (≤5% RH) to prevent laser cavity degradation and extending diode lifespan by 5-10 times;
• Photodetector Enclosures: Sealed enclosures for avalanche photodiodes (APDs) in optical communication systems, protecting the photosensitive surface from contamination and ensuring high detection efficiency (≥90%).

5.3 Aerospace & Defense Field

In aerospace and defense systems requiring reliable hermetic packaging under harsh conditions, 4J29 is used for:

• Aerospace Sensor Packages: Hermetic packages for pressure sensors and accelerometers in aircraft and satellites, withstanding temperature cycling (-60℃ to 150℃) and vibration (2000g) while maintaining leak rate ≤1×10⁻⁹Pa·m³/s;
• Missile Guidance System Components: Glass-sealed lead frames for microprocessors in missile guidance systems, protecting electronics from high-altitude vacuum (≤1×10⁻³Pa) and electromagnetic interference;
• Spacecraft Communication Modules: Hermetic packaging for radio frequency (RF) modules in spacecraft, ensuring stable communication (bit error rate ≤10⁻⁸) during long-duration space missions (5-10 years).

5.4 Medical & Industrial Electronics Field

In medical and industrial electronics requiring long-term reliability, 4J29 is used for:

• Medical Device Packaging: Hermetic enclosures for implantable medical devices (e.g., pacemakers, cochlear implants), maintaining biocompatibility (compliant with ISO 10993) and hermeticity to prevent body fluid intrusion (leak rate ≤1×10⁻¹⁰Pa·m³/s);
• Industrial Sensor Enclosures: Sealed packages for pressure and temperature sensors in harsh industrial environments (e.g., chemical plants, oil refineries), resisting corrosion from weak acids and ensuring sensor accuracy (error ≤±0.1%) for 5+ years;
• Automotive Electronics: Glass-sealed connectors for automotive engine control units (ECUs), withstanding high temperatures (up to 150℃) and vibration (50g) under the hood while protecting ECUs from moisture and oil contamination.
• Smelting: Vacuum induction melting (VIM) is recommended to control composition accuracy (Ni±0.2%, Co±0.2%) and reduce impurities (C ≤0.03%); air melting may lead to CTE variations that degrade glass-sealing performance;
• Cold Working:

6. Processing & Welding Recommendations

◦ Sheets: Cold rolling with 30-40% deformation per pass, followed by intermediate annealing (800-850℃, 1h, air cooling) after every 60-70% total deformation to restore ductility; final thickness tolerance can reach ±0.005mm for precision packages;

◦ Wires/Lead Frames: Cold drawing with 20-25% deformation per pass, intermediate annealing (750-800℃, 30min, air cooling) after every 50-60% total deformation;

• Glass Sealing:

◦ Sealing Temperature: 500-650℃ (below alloy’s recrystallization temperature to avoid grain growth);

◦ Sealing Atmosphere: Hydrogen-nitrogen mixture (forming gas) or vacuum to prevent oxidation of the alloy surface (oxidation weakens glass bonding);

◦ Cooling Rate: 5-10℃/min (slow cooling minimizes thermal stress between alloy and glass);

• Welding: Suitable for TIG welding, laser welding, and resistance welding (for lead frames);

◦ Welding filler metal: Matching 4J29 alloy wire (Ni29Co17Fe);

◦ Post-weld annealing: 750-800℃ for 30min, air cooling (eliminates welding stress and restores ductility);

• Machining: Machinable in both annealed (soft, HV 130-160) and cold-worked (hard, HV 180-220) states; carbide tools are recommended for high-speed machining; cutting fluids should be used to avoid overheating (which can alter CTE).

This comprehensive performance and application profile establishes 4J29 as the gold standard for hard glass-sealing hermetic packaging. Its unique combination of precise CTE matching, reliable hermeticity, and good processability makes it irreplaceable in electronic, optoelectronic, and aerospace applications where component protection and long-term reliability are critical.