A propellant tank has a port. Connect to it, pressurise it, measure what escapes. A hermetically sealed avionics box offers nothing of the kind, and the same holds for a hybrid microcircuit, an optoelectronic package or an RF module. The cavity under test is closed by design, and that single fact reshapes the whole approach to verification.
Our overview of helium leak detection in the aerospace sector covers the ground rules, and our guide to spacecraft and satellite testing touches on this method briefly. We go deeper here, because the equipment implications catch procurement teams out far more often than the physics does.
Getting helium into a closed cavity
Two routes exist. One pressurises the finished component in a chamber filled with helium, so the gas forces its way through any defect and collects inside. Move the part to an evacuated vessel afterwards, and we can measure what comes back out. For that pressure, ESCC Basic Specification No. 21100 (the official European Space Components Coordination standard) sets a floor of 207 kPa absolute when the flexible method applies.
The other route seals the component in a helium atmosphere in the first place, so the pressurisation step disappears entirely. Cleaner and quicker, and available only when you control the sealing operation yourself. ESCC practice then requires the fine leak measurement within one hour of removal from the sealing atmosphere, before any other test.
Either way, the measurement carries a clock. Helium that entered the cavity starts leaving the moment ambient pressure returns. Four of the inputs to the Howl-Mann equation, used to derive the equivalent standard leak rate, come from how you run the test: the pressure applied, the dwell time under pressure, the elapsed time before measurement and the internal free volume of the cavity. Change the transfer time and the result changes with it.
Equipment selection follows directly from that. Your handling sequence has to be quick and consistently repeatable, and the system has to record the timings against the serial number of the part. If you ask any prospective supplier how their machine captures and logs elapsed time between stations, the answers vary considerably.
The failure mode that reads as a pass
A product with a large defect fills quickly under pressure, then empties just as quickly. By the time it reaches the detector, the helium has gone, the instrument reports a very low leak rate, and the worst product in the batch looks like the best one.
Standards handle this by insisting on a second, physically different test. Fine leak measurement comes first, gross leak testing follows, and MIL-STD-883 Test Method 1014, the military seal test procedure for microelectronic devices, requires the gross leak test within one hour of removal from pressurisation.
Verifying hermeticity takes two capabilities, and a quotation covering only the helium station leaves the qualification incomplete.
Small cavities reach the limit of the method
Internal free volume drives everything. A large cavity accumulates a useful quantity of helium and holds it long enough to measure. Shrink that volume, and both effects weaken, until the fine leak test can no longer resolve the acceptance threshold you need.
Typical aerospace practice sets fine leak limits around 1×10⁻⁷ atm cc/s helium, with flight hardware often specified at 1×10⁻⁸. For very small products, ESCC documentation routes you to residual gas analysis, where a measured helium content above 1500 ppmv indicates a leak rate beyond the acceptable value. Establish the internal volume of your product before anyone writes a specification around it.
What to require from the equipment
Start with calibration. The mass spectrometer needs a certified standard leak and calibration at least once per working shift. A system that keeps running outside its calibration window is a liability; it should stop instead.
Traceability matters just as much. Pressure applied, both time intervals, the measured leak rate and the serial number all belong in a record that survives to the flight readiness review. Auditors ask for the calibration certificate behind a single measurement taken months earlier, so plan the data architecture for that question.
Then there is the test condition itself. Different part families fall under different documents: Method 1014 covers microcircuits, while MIL-STD-750 Method 1071 applies to discrete semiconductors. Name the applicable specification and condition in the purchase requirement, and leave nothing to interpretation.
Fixturing and chamber design deserve the last word. Small components need tooling that aligns them repeatably without damage, and a chamber volume matched tightly to the part protects sensitivity by keeping background low.
Equipment engineered around your products
MVS Technologies designs and manufactures this kind of station to order. More than thirty years of custom vacuum and helium leak detection engineering sit behind that capability, spanning single R&D set-ups through to fully automated production lines, and every system starts from a specific component and a specific acceptance criterion.
A hermeticity station brings together a pressurisation vessel, an evacuated measurement chamber, a controlled transfer sequence and the timing records that Method 1014 depends on. Around those we apply the practices that run through all our systems: tooling designed to the component, chamber volume matched to the part, automatic fixture recognition, and logging traceable to serial numbers and calibration certificates.
If you are working out what your hermeticity verification requires, or what a system built around your products would involve, get in touch.
