TABLE OF CONTENTS Some developers have tried to define hermeticity on just the basis of the fine leak test method, but this is a misapplication of the test and an incomplete definition. The helium fine leak test, true to its name, is designed to test for leaks. It is not intended to measure barrier properties of packaging materials. The test is typically applied to metal-hermetic packages to ensure that the lid and pass-through points for conductors do not have leaks as the result of faulty processing. Since solid metal is a near-perfect barrier to gases, any helium that passes through a metal package is due to one or more leaks at an interface point. Plastic cavity packages made from high-performance plastics, such as LCP type resin, can generally pass the fine leak test but this only indicates that the lid seal or metal conductor interface is secure. Passing the fine leak test should not be interpreted to mean that the package is hermetic, nor that the seal is hermetic. The detection limits of this test method may seem extraordinary but the test is not sensitive enough to measure very low transmission rates of helium passing through the package walls or lid. There is no doubt that oxygen and water vapor pass through all plastics that are not coated with some type of barrier material such as metal. Helium balloons once had a lifetime of only a day or two as the gas leaked...
