Ceramic-to-metal seals are manufactured through a multi-step process: the ceramic is formed, fired, and then metalized on the bonding surfaces using the molybdenum-manganese (Mo-Mn) process or active metal brazing (AMB). The metalized ceramic is then brazed to the metal component in a controlled-atmosphere furnace using a filler metal – most commonly silver-copper eutectic or a gold-based alloy. The result is a fully metallurgical joint with no organic materials present.
Step 1: Ceramic Forming and Firing
The ceramic body begins as a powder compact of the ceramic material – most commonly alumina (aluminum oxide – Al₂O₃) in purities ranging from 94% to 99.9%, depending on the application requirements. The powder is mixed with organic binders and formed into the desired shape by dry pressing, isostatic pressing, injection molding, or tape casting, depending on the geometry and production volume.
The green (unfired) ceramic body is then sintered in a kiln at temperatures typically between 1400°C and 1650°C, depending on the alumina purity and desired final density. During sintering, the organic binders burn off, the ceramic particles bond, and the part shrinks predictably. Achieving the correct final dimensions requires careful control of powder particle size, binder content, pressing uniformity, and sintering temperature profile. Tight dimensional tolerances on the bonding surfaces are essential because the braze joint gap must be controlled to ensure proper filler metal flow and joint quality.
Step 2: Surface Preparation
After sintering, the ceramic surfaces that will be metalized are ground or lapped to the required flatness and surface finish. Surface roughness affects metallization adhesion and braze flow. Surfaces are then cleaned ultrasonically in solvents and deionized water to remove grinding debris, oils, and particulates before metallization.
Metal components are also prepared – machined to final dimensions with the mating surfaces finished to control joint gap geometry, cleaned, and in some cases plated (nickel or other metals) to promote braze wetting and protect against oxidation during the brazing cycle.
Step 3: Metallization (Mo-Mn Process)
The most widely used metallization method is the molybdenum-manganese (Mo-Mn) process. A paste containing finely divided molybdenum and manganese powders, along with glass-forming oxides and an organic vehicle, is applied to the ceramic bonding surfaces by screen printing, brushing, or spraying.
The coated part is then fired in a wet hydrogen atmosphere furnace at 1200–1500°C. During this firing, the manganese oxidizes and reacts with the alumina surface and the glass phase, creating a chemically bonded interface between the ceramic and the Mo-Mn layer. The molybdenum provides the metallic matrix for subsequent plating and brazing. The result is a thin, adherent metallic layer on the ceramic surface that is compatible with conventional brazing processes.
After Mo-Mn firing, the metalized ceramic is typically nickel-plated by electroless or electrolytic deposition to improve braze wetting and protect the Mo layer from oxidation during handling and brazing.
Step 4: Active Metal Brazing (Alternative Process)
For applications where Mo-Mn metallization is impractical – complex geometries, non-oxide ceramics such as silicon nitride or aluminum nitride, or small production quantities – active metal brazing (AMB) is used as an alternative. AMB uses a braze alloy containing an active element, most commonly titanium (Ti) or zirconium (Zr), which reacts directly with the ceramic surface during the brazing cycle without requiring a pre-applied metallization layer.
AMB is performed in high vacuum (typically 10⁻⁵ torr or better) or in an argon atmosphere to prevent oxidation of the active element before it reacts with the ceramic. The active element wets and bonds the ceramic in a single brazing step, simplifying the process flow but requiring more precise control of brazing atmosphere and temperature.
Step 5: Brazing
Whether using Mo-Mn or AMB metallization, the brazing step joins the prepared ceramic to the metal component using a filler metal. For Mo-Mn metalized parts, the assembly is fixtured to maintain the correct joint gap (typically 0.025–0.075 mm), filler metal in the form of preformed foil, wire, or paste is placed at the joint, and the assembly is loaded into a furnace.
Brazing is performed in a controlled atmosphere – dry hydrogen, forming gas (hydrogen/nitrogen mixture), or high vacuum – to prevent oxidation of the braze alloy and base materials during the thermal cycle. The furnace ramps to the brazing temperature (above the filler metal liquidus), holds at that temperature for a controlled time to allow filler metal flow and wetting, and then cools at a controlled rate to solidify the joint and minimize residual thermal stress.
Silver-copper eutectic (Ag-Cu, 72/28, liquidus 779°C) is the most common filler for alumina-to-Kovar assemblies. Gold-copper, gold-nickel, and copper-based fillers are used in specific applications. Braze selection affects final joint strength, ductility, corrosion resistance, and re-melting temperature.
Step 6: Post-Braze Processing and Inspection
After brazing, assemblies are cleaned to remove flux residues (if flux-brazed) or furnace contamination. Final machining or plating may be applied. All hermetically sealed assemblies are leak-tested using helium mass spectrometer leak detection per ASTM E498 or E499, with acceptance criteria typically in the range of 1×10⁻9 to 1×10⁻¹¹ atm·cc/sec depending on application requirements.
Additional inspection steps may include dimensional inspection, pull or shear strength testing of brazed joints, electrical testing of feedthrough pins, and visual inspection for braze voids, cracks, or incomplete fill. Documentation packages including material certifications, process records, and leak test data are provided for applications in aerospace, defense, and semiconductor markets.
Quality Control Considerations
- Ceramic lot traceability: raw material purity and batch number tracked through firing and metallization.
- Metallization thickness and adhesion: peel tests or cross-sectional metallography verify Mo-Mn layer integrity.
- Braze joint inspection: X-ray radiography or destructive cross-section reveals voids or incomplete fill.
- Hermeticity testing: 100% helium fine-leak test on finished assemblies, with gross-leak screening for large batches.
- Thermal cycling qualification: sample assemblies cycled across the application temperature range to verify joint fatigue life.
MPF Products Manufacturing Capability
MPF Products manufactures ceramic-to-metal hermetically sealed feedthroughs and assemblies using both Mo-Mn and active metal brazing processes. In-house metallization, brazing, and helium leak-testing capabilities enable full process control and traceability of documentation for semiconductor, space, defense, and scientific instrumentation customers.