Implication of Multiple Leak Tests and Impact of Rest Time on Avionic Hybrids

Abstract

Since 2007 Northrop Grumman's Baltimore facility has leak tested Hybrid packages primarily through the Optical Leak method. The Optical Leak systems allow for individual part readings performed in parallel and they eliminate the need for separate fine and gross leak tests. As with most leak test operations, there are occasionally parts that need to be retested. One of the benefits of Optical Leak Test is that there is no exposure to liquids, greases, or other contaminates that may plug fine leaks. However, there is concern about the aggregate impact of re-leak testing, and interest in any time effects or “charging” that may occur from repeated test exposure. To investigate the impact of repeated test exposure and determine the effect on test results of retesting parts, NGES Baltimore performed testing on a dozen parts (both leakers and good seals) to measure the amount of distortion repeated testing generates. This paper describes the parts tested and the results from a series of tests spaced approximately 24 hours apart as well as tests performed immediately in conjunction – exposing the parts to repeated pressure cycling without a rest period for recovery. The results show that rapid, repeated testing will yield a slight worsening of the leak measurements (i.e. the test may falsely fail a good part), although the overall magnitude of the shift was not great. Allowing a rest time of 4x the test time worked as a rule of thumb for eliminating the “pressure-charging” effect of prior tests, and that testing performed on different days yielded very consistent results. The current method 1014 leak test MIL-STD-883 has specific guidance against retesting parts. At Northrop Grumman we believe there are instances in which retesting parts is useful, and have shown that with appropriate handling, retest will yield consistent results.