Hermeticity Testing — Is the Package Truly “Hermetic”?

1. How Do You Know a Package Is Truly Hermetic? — Dual Testing for Gross and Fine Leaks

Once a hermetic package has been assembled, how do you prove that it is truly “sealed”? Not by eye — molecular-level leaks are completely invisible. The industry adopts a two-stage testing strategy: gross leak testing and fine leak testing; neither can be omitted.

Gross leak testing targets larger leaks (>1×10⁻⁴ Pa·m³/s). Common methods include fluorescent dye penetration and bubble testing. The product is immersed in heated fluorescent dye liquid or glycerin; if a leak path exists, the dye penetrates or bubbles emerge — directly visible.

Fine leak testing targets minute leaks. The classic method is helium mass spectrometry leak detection: the package is first “bombarded” with helium at high pressure, allowing helium to enter the package cavity; it is then taken out and placed in a leak detector. If helium escapes from the cavity, the instrument measures it precisely, with sensitivity down to the 1×10⁻⁹ Pa·m³/s level.

2. Testing Standards and Acceptance Criteria

The industry benchmark is MIL-STD-883 Method 1014, “Seal”. The standard specifies that the fine leak acceptance threshold depends on the package cavity volume — the smaller the cavity, the stricter the allowable leak rate. Typical requirements are a fine leak rate ≤1×10⁻⁸ Pa·m³/s and zero leakage in gross leak testing. The TO56 headers, optical communication headers and other products that Xuri (Rizhao Xuri Electronics) ships in volume are tested by helium mass spectrometry leak detection before leaving the factory.

The core logic of the acceptance criteria: at this leak rate, over a service life of more than 20 years, the amount of external moisture permeating into the package is too small to harm the internal chip.

Hermeticity testing is the “acceptance gate” of hermetic packaging — only after passing this gate can a product move on to the next stage of reliability verification.

[Next Article Preview] Next article: how sealed products survive even more demanding environmental and mechanical reliability testing.

[Glossary]

1. Gross Leak Rate: the leak rate magnitude of larger leaks, typically >1×10⁻⁴ Pa·m³/s, detected by fluorescent dye penetration or bubble testing

2. Fine Leak Rate: the leak rate magnitude of minute leaks, down to 1×10⁻⁹ Pa·m³/s, detected by helium mass spectrometry

3. Helium Mass Spectrometry: a high-sensitivity leak detection method that uses helium as the tracer gas; the standard method for hermeticity testing

4. MIL-STD-883 (Military Standard 883): the U.S. military standard of test methods for microelectronic devices, widely used as the industry benchmark for high-reliability packaging