Converting Vacuum Bombing Helium Signal Data into Air Leak Equivalents
Converting helium bombing signal data to standard air leak rates requires applying the Howl-Mann equation with explicit cavity volume and dwell parameters.
Bombing Pressure
Meaning
Hermetic seal testing subjects sealed electronic components or battery cell cans to elevated external helium gas pressure inside a sealed vessel. Applying this external stress creates the bombing pressure, which forces tracer gas through microscopic defects into internal cavity voids prior to mass spectrometer leak detection. Standardized testing dictates specific pressure thresholds and duration limits based on package internal volume and shell wall thickness.
The protocol stops applying once the component exits the pressurization vessel and undergoes gross leak testing or fine leak evacuation.
Tracer Gas
High pressure helium permeates tiny structural fissures in laser welds or glass-to-metal feedthroughs. Pressurization forces gas molecules into the internal space, establishing a concentration gradient necessary for subsequent vacuum extraction detection. Small internal free volumes require higher gas pressures to achieve detectable accumulation levels before back-out occurs.
Exposure Duration
Dwell time under pressure determines the total helium mass forced into internal cavities. Prolonged exposure causes tracer gas saturation in outer seal materials, which can create false positive leak readings during the subsequent analysis phase. Rapid depressurization following the pressurization cycle prevents premature back-out gas loss before measurement begins.
Leak Rate
Standard formulas convert measured helium flow rates into equivalent standard air leakage rates based on pressure differentials. Specifying incorrect bombing pressure yields invalid leak rate calculations, potentially passing compromised cell packages into production assemblies.