Exploring the limits of helium spray tests

Helium-spray testing poses a simple technique to investigate the hermeticity of joints and unsealed packages in general by sealing the device under test to a mass spectrometer by way of an O-ring seal, drawing a mild vacuum within the sample and exposing the outside to helium while measuring the leak rate through the joints, surfaces and seals. Military Standard 883 describes this test and establishes a pass-fail criterion, but fails to give distinct instructions on how to setup the test and to interpret the results. The aim of this study is therefore to explore the limits of spray testing unsealed packages by evaluating the standard deviation of the method in combination with the maximum achievable sensitivity as a function of the permeability of the O-ring material to helium gas. It could be shown (1) that nitrile O-rings exhibited the lowest permeability to He gas under steady state conditions, (2) the O-ring itself usually limits the detection rate and (3) that the achievable sensitivity lies in the range of 10^-8 atm*cc*s^-1 and, thus, may merely be used for gross leak detection.