The ASTM E493/E493M-11 Standard Practice for Leakage Detectors Using Mass Spectrometer Leak Detectors in Inside-Out Test Mode, developed by the American Society for Testing and Materials (ASTM), describes procedures for testing semiconductors, hermetically sealed relays, pyrotechnic devices, and similar devices for leakage through enclosure walls prior to testing. These can be used at varying sensitivity levels depending on the internal volume, enclosure strength, time available for test preparation, and the sorption properties of the enclosure material for helium. In general practice, sensitivity limits for helium range from 10−10 to 10−6 Pa m³ / s, although these limits may be exceeded by several decades in either direction in some cases.
This standard defines two test methods:
For the purposes of this standard, the terms and definitions in the standard “ASTM E1316-16 Standard terminology for non-destructive testing” apply.
The test methods discussed in this standard require the test piece to contain helium at a calculable pressure during the test. If the apparatus cannot be sealed with a known helium pressure inside, it is necessary to bombard the part in a helium chamber to introduce helium into the test piece if there is a leak.
After the test piece has been subjected to helium pressurization, it is placed in a housing which is then evacuated and connected to a mass spectrometer leak detector. In the event of a leak, an output signal is obtained from the leak detector. If the actual leak rate of the test piece is to be known, this must then be calculated from the output reading and the test parameters.
Methods A or B are useful for testing hermetically sealed devices with internal volumes. Maximum acceptable leakage rates for microelectronic devices have been established to ensure that performance characteristics will not be affected by ingress of air, water vapor, or other contaminants during the expected estimated life. Care should be taken to control the bombardment pressure, bombardment time, and dwell time after bombardment, otherwise results may vary significantly.
Surface cracks, paint, dirt, and polymer seals can absorb helium during the bombing procedure and contribute to the output signal from the leak detector. Unless this process is uniform and consistent, i.e., test pieces that do not leak do not always give the same helium signal, the test sensitivity is limited by the background signal from the adsorbed helium. One or both procedures of nitrogen flushing or baking the pieces for 30 minutes between bombing and testing will sometimes help to reduce this background signal.
In the case of large leaks or very small internal volumes in test pieces, helium that was probably sealed or bombarded into the test piece may have escaped before the mass spectrometer leak detector can test it. For large leaks, testing should be done a few seconds after removing the piece from the bomb or an alternative test method for large leaks should be used: bubble testing, liquid bombardment, and weight change. If the method for detecting large leaks uses a liquid, the helium should be done after the leak test to avoid sealing small leaks.
During these tests, a mass spectrometer leak detector is used. In addition, a rough pump is used to remove air from the test enclosure in which the test piece is placed. This apparatus has pressure measuring instruments and appropriate valves to connect the test enclosure to the leak detector after evacuating it. On the other hand, a pressure vessel is used in the tests. This vessel can safely withstand the required helium bombardment pressure. Pressure vessels must meet the applicable safety codes for pressure vessels.
The tests also require the following materials:
In test method A (bombardment), a quantity of sealed parts to be tested is placed in a pressure vessel. The pressure vessel is then flushed with helium or evacuated (the latter is generally preferred). The vessel is sealed with helium and pressurized to the specified pressure. It is held at the specified pressure for the time required to provide the desired test sensitivity and pressure. Helium is released a considerable distance from the leak detector. (It may be piped out of the building or released into a holding tank so as not to increase the ambient helium pressure in the test area.) The test parts are removed from the pressure vessel and flushed with dry air or nitrogen without helium to remove the helium adsorbed from the surface. The flushing time can be determined experimentally.
The objects to be tested are placed singly or in multiples in the mass spectrometer leak detector test area and tested. The indicated leak rate of each part and the time in seconds that the part exits the pressure vessel are recorded. If more than one part is tested simultaneously and the mass spectrometer leak detector indicates a leak, the parts are tested individually and the leak rate of each part and the time in seconds that it exits the pressure vessel are noted. To find the actual leak rate of a particular part, the simplified equation given in the standard assuming molecular flow is used.
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