6 - Understanding Helium Fine Leak Testing in Accordance with Method 1014, MIL-STD-883

Abstract

This chapter explains the relationship between the fixed and flexible methods of testing. The purpose of the test is to measure the hermeticity of sealed packages. Method 1014 includes leak rate limits for different ranges of internal package volumes. If a package has a leak and it is placed in a helium pressurized vessel (bomb), some of the pressurized gas will enter the package through the leak channel. The absolute amount of helium entering the package depends upon the size of the leak channel, the time under pressure, and the pressure difference between the helium in the bomb and the helium pressure in the package. After removal from the bomb, the package is connected to a helium leak detector. Helium now escapes from the package and is detected. The absolute amount of helium escaping depends upon the size of the leak channel and the helium pressure within the package. The fixed method requires specific bombing pressures and times for specific package size ranges. Several calculations are necessary to understand the exact numerical relationship between the limits of the fixed method with that of the flexible method. The primary reason for the gross leak test is to detect large leak rates. Packages with large leaks can pass the fine leak test because most of the helium has leaked out of the package between bombing and test. In most cases, the gross leak test will fail the part.