Helium leak testing uses helium as a tracer gas and a mass-spectrometer-based detector to evaluate gas flow through a package or seal. It is valuable for laser-welded hermetic enclosures because it can detect leakage that is too small to locate visually. A useful result, however, requires more than placing a part in a machine and reading the display.
The test method, package free volume, helium exposure, pressure differential, transfer time, background, fixture, calibration and acceptance calculation must match the product. Fine-leak testing should also be paired with an appropriate gross-leak method when a large opening could let the tracer gas escape before measurement.
What the helium detector measures
A mass spectrometer leak detector identifies helium entering its vacuum system and reports a flow-related leak-rate value. Depending on the setup, the helium may:
- be sealed inside the package before closure;
- be forced into a sealed package through an existing leak path during pressurization, often called bombing;
- be supplied around a pressurized assembly while a probe searches for escaping helium;
- be introduced on one side of a seal while the opposite side is connected to the detector.
These configurations do not produce interchangeable results automatically. The report should identify the test gas, mode, pressure, timing, temperature, package volume and any conversion to an equivalent standard leak rate.
ASTM F2391 describes vacuum-mode and sniffer-mode procedures for package and seal integrity using helium. It emphasizes that method sensitivity must be appropriate to the application and that acceptance depends on the individual package requirement. For microcircuits, MIL-STD-883 Method 1014 provides several seal-test conditions. Use only the method and revision required by the governing drawing or contract.
Why helium is used
Helium is useful as a tracer because sensitive instruments can distinguish its partial pressure against a controlled background. Calibrated helium leaks are also available for instrument verification and traceability.
Helium does not make the test immune to error. Background helium can raise the baseline, permeation standards can be temperature-sensitive, and the response time depends on system volume and pumping conditions. NIST notes that calibrated leak artifacts are used to calibrate helium leak detectors and that their flow can vary with temperature. Calibration and thermal stabilization therefore belong in the written test process.
Common test arrangements
| Arrangement | How it works | Useful for | Main cautions |
|---|---|---|---|
| Prefilled or sealed-in helium | The package is closed with helium in its internal atmosphere and then placed under detector vacuum | Non-destructive screening when helium can be part of the closure process | Internal concentration, time since sealing and allowable package atmosphere must be controlled |
| Bombing and fine-leak test | A sealed package is exposed to pressurized helium, then transferred to the detector | Packages that can safely withstand the exposure and retain enough tracer for measurement | Bomb pressure, exposure time, free volume, leak size and transfer delay affect the reading |
| Vacuum mode with product-specific fixture | Helium escaping from a package is collected in an evacuated fixture | Quantitative or comparative package testing with suitable fixtures | Fixture leakage, dead volume, background and response time require validation |
| Sniffer mode | The package contains or is supplied with helium while a probe scans the outside | Locating leaks and testing assemblies that cannot be placed in a vacuum chamber | Operator speed, probe distance, ambient flow and background increase variability |
| Local seal test | A fixture isolates a particular seam or feedthrough | Separating closure-weld performance from other package boundaries | Fixture sealing can mask or create apparent leakage |
The best method is the one capable of detecting the specified failure mode without damaging the product or creating an unmanageable false result.
Why gross- and fine-leak tests are complementary
A fine-leak test can miss a very large leak in a bombed package. If the opening is large enough, helium admitted during pressurization may escape rapidly during transfer to the detector. By the time the part is measured, little tracer remains and the reading can appear deceptively low.
A gross-leak test addresses larger leakage paths, while the fine-leak method addresses smaller flows. NASA's hermeticity guidance describes gross- and fine-leak testing as complementary tools for determining the effectiveness of microelectronic package seals.
The sequence, liquids or tracer gases used for gross-leak testing, and compatibility with the product must follow the applicable procedure. Some packages cannot tolerate immersion, pressure or particular test media. Do not substitute a convenient gross-leak screen without evaluating product safety and method sensitivity.
Information required before choosing the test
Package free volume
Internal volume affects how much helium can enter, how rapidly it can escape and how a measured rate is converted or interpreted. Small cavities can behave differently from large housings. Use the actual free volume, not only external dimensions.
Maximum safe pressure differential
The lid, walls, feedthroughs, windows and internal structures must withstand the specified bombing, evacuation or pressurization condition. Thin lids may deflect; brittle seals may be damaged. Define a safe pressure and rate of pressure change.
Target leak-rate requirement
The acceptance limit must be stated with units, reference conditions and method. A detector's lowest displayed value is not automatically a valid product limit. The method must have usable sensitivity, repeatability and uncertainty around the acceptance threshold.
Internal atmosphere
If helium is sealed into the product, confirm that its concentration is compatible with product function and long-term requirements. If helium is introduced only by bombing, establish exposure and transfer timing.
Material permeability and surface condition
Polymers, elastomers, porous materials and some package constructions can produce permeation or outgassing behavior that complicates interpretation. The method may be intended for rigid, nonporous barriers rather than every enclosure type.
Production purpose
Qualification, process development, 100-percent production screening, sampling and failure analysis may require different fixtures, cycle times and records.
A controlled helium-test workflow
- Confirm the governing specification, current revision and acceptance limit.
- Identify package free volume, materials and all possible seal boundaries.
- Select a test condition that is safe and sensitive enough for the product.
- Verify the detector with a calibrated leak in the relevant range.
- Establish a stable baseline and acceptable helium background.
- Control package conditioning, bombing or internal-helium concentration as applicable.
- Control the time between exposure, removal, cleaning and measurement.
- Record the raw reading and all conversion inputs.
- Perform the specified gross-leak screen as part of the complete sequence.
- Review failures with weld inspection and localization rather than changing welding parameters from the leak number alone.
The supporting equipment overview places leak detection within the wider welding workflow. The detector verifies package performance; it does not replace control of cleaning, fit-up, atmosphere or weld parameters.
Calibration, background and system checks
A quantitative test needs a detector response that is traceable and stable near the decision range. Include:
- calibrated standard leak identification and due date;
- standard leak value and reference temperature;
- instrument warm-up and zero procedure;
- background before and after the measurement group;
- response check at the beginning and end of a run or shift as required;
- fixture blank test;
- maintenance and calibration status;
- operator and program revision.
NIST calibration work shows why temperature matters: the flow of a helium permeation leak artifact can change measurably with temperature. Let reference leaks stabilize and follow their calibration certificate rather than treating the nominal label as constant under all conditions.
Common causes of misleading results
| Result pattern | Possible test-related cause | Possible package-related cause |
|---|---|---|
| Unexpectedly high reading on every part | Elevated helium background, dirty fixture, fixture seal leak or insufficient pump-down | Repeated process defect or shared component leak |
| Very low reading after bombing | Tracer escaped through a gross leak during transfer, insufficient exposure or wrong volume calculation | Truly tight package |
| Reading falls rapidly during repeated tests | Tracer depletion, changing transfer time or insufficient re-bombing | Leak path with time-dependent behavior |
| Reading rises slowly | Large fixture volume, trapped helium, outgassing or slow detector response | Small leakage path or permeation |
| Only some orientations fail | Fixture sealing or probe access | Location-dependent crack, feedthrough or seam defect |
| Passes leak test but has poor cross-section | Leak path is not through-connected under test conditions | Non-through porosity, incomplete fusion or local metallurgical defect |
Treat the test as a measurement system. A result near the limit should trigger review of uncertainty, repeatability and method conditions, not only a binary pass/fail decision.
Leak testing does not identify the root cause
Helium testing can show that gas crossed a boundary under the test conditions, but it may not reveal whether the path is:
- a start-stop discontinuity in the laser weld;
- a pore network connected to the surface;
- a crack;
- an incomplete-fusion region;
- a glass-to-metal or ceramic-to-metal feedthrough;
- a braze, solder joint, fill port or fixture seal.
Combine leak results with visual inspection, microscopy, dimensional checks, localized probing, radiography or CT where appropriate, and destructive cross-sections during investigation. The laser weld quality inspection guide explains how these methods answer different questions.
Reporting a defensible result
A useful leak-test record should include:
- part and lot identification;
- package free volume;
- detector and fixture identification;
- calibrated leak and calibration status;
- test mode and applicable procedure;
- helium concentration and pressure;
- exposure, dwell and transfer time where applicable;
- test temperature and background;
- measured rate, converted rate and units;
- acceptance limit and result;
- gross-leak result;
- any deviation, retest or anomaly.
Avoid reporting only “helium leak test passed.” The missing method conditions make the result difficult to reproduce or compare.
Connecting leak testing to laser welding
During process development, test representative welds from the nominal setting and from relevant window limits. Correlate leak results with seam images, parameter records and cross-sections. If a leak is found, locate the boundary before changing the weld recipe. A feedthrough or fixture problem will not be corrected by increasing laser power.
JMLASER configures laser hermetic sealing systems around the required welding atmosphere, package handling and verification route. The leak detector, fixtures and data flow should be planned with the package design and production sequence so that hermeticity is verified consistently rather than treated as an isolated final check.
Reviewing gross- and fine-leak results together
Use the gross-leak versus fine-leak comparison when selecting complementary coverage or investigating conflicting results. It distinguishes a low helium reading from a complete package acceptance decision and provides a result-review table.
When comparing detector reports with a drawing, use the leak-rate units and acceptance guide to normalize units and identify missing gas, reference-condition or decision-rule information. Unit conversion alone does not convert a raw helium reading into an equivalent standard leak rate.