A hermetic seal is a joint that prevents any gas or liquid from passing through, and making one requires matching the right sealing method to your materials, preparing surfaces thoroughly, and verifying the result with leak testing. The standard measure of seal quality is the leak rate, expressed in atmosphere-cubic centimeters per second (atm-cc/sec), where lower numbers mean a tighter seal. For context, a “fine leak” falls in the range of 10⁻⁴ to 10⁻¹⁰ atm-cc/sec.
Why Material Choice Matters First
Before choosing a sealing method, you need to consider what you’re sealing and what it will endure. The most common failure in hermetic seals comes from mismatched thermal expansion. Every material expands and contracts at a different rate when heated or cooled, described by its coefficient of thermal expansion (CTE). If two joined materials have very different CTEs, temperature changes will stress the joint until it cracks.
Matched seals solve this by pairing materials with similar expansion rates. The classic combination is Kovar (an iron-nickel-cobalt alloy) with borosilicate glass. These two expand at nearly the same rate across a wide temperature range, which is why they’re the standard in microelectronics packaging. Other matched pairs include molybdenum or tungsten with hard silica-based glasses. Matched seals get their strength from direct chemical bonding between the materials, making them exceptionally stable through thermal cycling. When Kovar is used, the parts are typically plated with gold or another corrosion-resistant metal to protect the alloy from degrading over time.
For thin metal casings, like those enclosing sensitive electronics, matched seals are essential. Even small expansion mismatches in a thin-walled enclosure can rupture the housing. If your application doesn’t involve extreme temperature swings, you have more flexibility in material pairing, but understanding the thermal behavior of your materials is still the first step.
Soldering vs. Brazing
Soldering and brazing are the most accessible methods for creating hermetic joints, and the distinction between them is straightforward: soldering happens below 840°F (450°C), brazing happens above it. Both use a filler metal that melts and flows into the joint, bonding the two pieces together as it solidifies. The American Welding Society uses that 840°F line as the formal dividing point.
Brazing produces stronger, more heat-resistant seals. It’s the go-to method for metal-to-metal and metal-to-ceramic hermetic joints in industrial applications. The filler metals used in brazing (often silver-based or gold-based alloys) create joints that can withstand higher operating temperatures than soldered ones. Soldering, on the other hand, works well for lower-temperature applications and is easier to perform with basic equipment.
For either method, two things are non-negotiable. First, the parts must be thoroughly cleaned before joining. Any oils, oxides, or contaminants on the surface will prevent the filler metal from wetting properly, leaving microscopic gaps that destroy hermeticity. Second, you need to protect the joint from oxidation during heating. This means using either a chemical flux that dissolves oxides as they form, or performing the operation inside a protective atmosphere (typically an inert gas like argon or nitrogen, or a vacuum). Flux residue must be cleaned after the process, since leftover flux can corrode the joint over time.
Laser Welding for Precision Seals
When you need a hermetic seal with minimal heat distortion, laser welding is the preferred method. A focused laser beam melts a very narrow strip of material at the joint, creating a fusion weld with an extremely small heat-affected zone. Modern systems can achieve spot sizes as small as 0.0007 inches, which means the surrounding material barely gets warm. This is critical for sealing electronic enclosures where heat-sensitive components sit millimeters from the weld line.
Laser welding also offers repeatability that’s hard to match with manual methods. Multi-axis welding systems can follow complex seam geometries automatically, producing consistent seal quality across production runs. The trade-off is cost: laser welding equipment is expensive, and the process requires precise fixturing to hold parts in position. For one-off or small-batch work, brazing is usually more practical. For high-volume production of sealed electronic packages, laser welding pays for itself in reliability and speed.
Surface Preparation
No sealing method works on dirty surfaces. The goal of surface preparation is to remove contaminants and create a chemically active surface that bonds readily to the sealing material. What “clean” means depends on how tight your seal needs to be.
For basic hermetic seals made by brazing or soldering, degreasing with a solvent followed by mechanical cleaning (abrasion or grit blasting) is often sufficient. You want bare, oxide-free metal at the joint surfaces. For high-reliability seals in aerospace or medical devices, the standards are much stricter. Plasma cleaning uses ionized gas to strip organic contaminants at the molecular level, and chemical etching removes oxide layers that mechanical methods can miss.
In advanced applications like thin-film encapsulation, surface chemistry becomes even more precise. Atomic layer deposition processes, for instance, depend on hydroxyl groups being present on the substrate surface so that precursor chemicals can bond to them in a controlled reaction. The quality of the deposited barrier layer, and therefore the seal, is directly determined by how well the surface was prepared beforehand. Even at the most basic level, the principle holds: a hermetic seal is only as good as the surface it’s made on.
Glass-to-Metal Seals
Glass-to-metal sealing is one of the oldest and most reliable hermetic techniques, widely used for electrical feedthroughs, where a wire or pin must pass through an enclosure wall without breaking the seal. The glass acts as both an insulator and a sealant, bonding chemically to the metal at high temperature.
The process involves heating the glass until it softens and flows around the metal component, then cooling it in a controlled way. As the assembly cools, matched CTE between the glass and metal keeps stress low. Borosilicate glass paired with Kovar pins and headers is the standard configuration. The chemical bond that forms at the glass-metal interface is what provides true hermeticity, not just a mechanical grip.
Hard glasses like borosilicate, alkali barium, and vitreous silica are the most common choices because they offer outstanding temperature stability across a wide operating range. This makes glass-to-metal seals suitable for environments that cycle between extreme heat and cold, which is why they’re found in satellites, implantable medical devices, and military electronics.
How to Verify Your Seal
Making a hermetic seal is only half the job. You also need to prove it works. The gold standard for verification is helium leak detection using a mass spectrometer leak detector (MSLD). Helium is used because its atoms are extremely small (second only to hydrogen) and it’s inert, so it won’t react with your materials or contaminate the test.
The basic process works like this: you expose one side of the sealed assembly to helium gas, then use the mass spectrometer on the other side to detect any helium atoms that made it through. Modern detectors can sense leak rates as low as 6 × 10⁻¹⁰ standard cubic centimeters per second, which is sensitive enough to catch defects invisible to any other method.
A proper leak test follows a structured sequence. First, you calibrate the detector against a known reference standard. Then you perform the actual leak measurement on your sealed part. After the test, you recalibrate to confirm the instrument didn’t drift during the measurement. The final result is reported as a leak rate plus or minus a measurement uncertainty, giving you not just a number but a confidence range. For most hermetic applications, a “fine leak” specification falls between 10⁻⁴ and 10⁻¹⁰ atm-cc/sec. Anything above 10⁻⁴ is considered a gross leak and typically means the seal has a visible or near-visible defect.
For less critical applications, simpler methods like bubble testing (submerging the sealed part in water and watching for bubbles under pressure) can catch gross leaks. But for any application where true hermeticity matters, helium leak detection is the only test sensitive enough to confirm it.