Measuring hermeticity can be performed a number of ways but the concept is the same; a pressure difference between the internal package volume and the external atmosphere causes gas or liquid to diffuse through the seal material, or through a path created between the seal-to-surface interface, or a crack or pore. The mechanism for the seal to be breached and the leakage level (gross vs. fine leak) will determine the type of technique for measuring hermeticity. The standard for fine and gross leak testing of cavity style packages has been MIL-STD-883G, Method 1014.11.

Test A: Fine Leak Testing using Helium
A1: Fixed method - The sealed package is pressurized or "bombed" in helium at a specific time and pressure. The test package is removed and connected to a mass spectrometer which determines the amount of helium escaping. Rejection criteria will vary with test conditions and cavity size.

A2: Flexible method - This test utilizes the same procedure as the fixed method except that the bomb conditions are chosen based upon the minimum detection limit of the mass spectrometer. These chosen conditions, internal volume of the package, and the maximum equivalent standard leak rate limit are utilized to calculate the measured leak rate. As with the fixed method, the failure criterion is internal cavity size dependent. The fixed method has been superseded by the flexible method in the most recent revision of the TM 1014 unless specified differently in procurement documents.

A4: Open Can Leak - Does not involve pressurizing the package, but exposing the package to a flow of helium and observing any localized leakage greater than 1 x 10-8 atm cc/s.

Test B: Radioisotope Fine Leak Test - This test is similar to the helium leak test except the tracer gas is a mixture of radioactive krypton-85 and dry nitrogen. Leaks are determined through measuring the response of a scintillation detector which counts the krypton-85 particles. As with the helium leak test, cavity size will determine failure leak rates.

Test C: Fine and Gross Leak Test Techniques
C1: Gross Leak "Bubble" Test - Leaks are determined by exposing the package to a lower boiling fluid (type I fluid, boiling point < 100° C) followed by a higher boiling fluid (type II fluid, boiling point > 125° C) and elevating the temperature of the higher boiling fluid while the package is immersed in it. Failures are indicated by the presence of bubbles of type I fluid. This test is combined with a fine leak test as fine leaks are not often able to be detected with this method.

C3: Gross Leak, Perfluorocarbon Vapor Detection - This test is similar to C1 except that immersion in the type II fluid is not done and leak detection is done in a 125° C heated chamber. Failures are considered when more than 0.167 microliters of type I fluid is observed evolving from the package.

C4/C5: OLT Optical Leak Detection (Gross and Fine) - This test measures lid deflection in response to a change in ambient pressure. If the device has a good hermetic seal, then the lid will deflect in response to the pressure difference between the cavity and the outside atmosphere. A gross leak will be indicated by a lack of deflection as the pressure inside the package and chamber come to equilibrium quickly. One requirement of the test is that some observable lid movement is necessary to perform this test.

Test D: Gross Leak using a Dye Penetrant - This destructive test utilizes a fluorescent dye in which the device is immersed under pressure. After a set time and pressure the package is opened and a UV light source is used to detect the presence of dye within the package.

Test E: Gross Leak by Weight Gain Measurement - Through weighing the package before and after pressurizing in a bath of a low VOC type III detector fluid, any weight gain of more than 1.0 milligram for a package of less than 0.01 cm3 and more than 2.0 milligrams for a package of greater than 0.01 cm3 are considered failures.

MIL-STD-883 Method 1014, MIL-STD-750D Method 1071.7 and JEDEC Standard No. 22-A109-A are variants of each other. Recently, issues with these tests have been brought forth. All the tests are limited by the fact that a cavity is necessary for the test or test conditions to be applicable. Even then, the large-volume cavities are a problem for the RGA (Residual Gas Analysis) tests like the helium leak tests as the cavity volumes mask the test by diluting the helium. Packages with high cavity vacuums have the same problem as the large-volume cavities. With extremely small cavity volumes (nano-liter cavities), a different problem arises, that of being able to detect such small volumes of residual gas upon its removal from the cavity. "One-way leakers" are a problem because once the pressure differential is removed, the leak reseals itself and the RGA test becomes invalid. The Radioisotope Test Method is the exception as lower detection capabilities are possible.

With these tests and today's changing reliability requirements, establishing what level of hermeticity is acceptable and how that level is established is the challenge. The need to perform environmental stress screening (ESS), as performed routinely at the EMPF, is the key to measuring reliability. The RGA limits are based upon an inert fluid or gas which measures the degree of the leak, but not the effect of the leak in a given environment. As a result, a number of screening tools from recognized standards (JEDEC, IPC, ASTM, etc.) are available which address reliability concerns. These include, but are not limited to: temperature and humidity exposure, thermal cycling, highly accelerated stress test (HAST), high accelerated life test (HALT), vibration salt fog exposure, high temperature operation life (HTOL), humid sulfur environment, and mixed flowing gas.