MIL-STD-202G
METHOD 112E
SEAL
1. PURPOSE. The purpose of this test method is to determine the effectiveness of the seal of a component part which has an internal cavity which is either evacuated or contains air or gas. A defect in any portion of the surface area of a sealed part can permit entrance of damaging contaminants which will reduce its effective life. This test will detect leaks due to the use of inferior sealing materials, or to the manufacturing processes used to form the seal. The degree of completeness of the seal can be verified by testing in the "as received" condition or after submission to other environmental or physical-characteristics tests, such as thermal shock, physical shock, or vibration. This test method does not include the many existing versions of lesser-sensitivity seal tests now appearing in some specifications, which verify such requirements as "water tightness", "moisture proofness", etc. In those lesser-sensitivity seal tests, parts are submerged to various depth in water, heated water, water or alcohol with a vacuum, water with a wetting agent, etc; all these tests depend on observation of bubbles as the criterion for failure.
1.1Definitions.
a.Standard leak rate. Standard leak rate is defined as the quantity of dry air at 25°C in atmospheric cubic centimeters flowing through a leak or multiple leak paths per second when the high-pressure side is at 1 atmosphere (760 mm Hg absolute) and the low-pressure side is at a pressure of not greater than 1 mm Hg
absolute. Standard leak rate shall be expressed in units of atmospheric cubic centimeters per second (atm cm3/s).
b.Measured leak rate. Measured leak rate (R1) is defined as the leak rate of a given package as measured
under specified conditions and employing a specified test medium. Measured leak rate shall be expressed in units of atmospheric cubic centimeters per second (atm cm3/s). For the purpose of comparison with rates determined by other methods of testing, the measured leak rates must be converted to equivalent standard leak rate.
c.Equivalent standard leak rate. The equivalent standard leak (L) rate of a given package, with a measured leak rate (R1), is defined as the leak rate of the same package with the same leak geometry, which would exist under the standard conditions of 1.1a. The formula (does not apply to procedure IIIb) in 5.4.3.2.3 represents the L/R1 ratio and gives the equivalent standard leak rate (L) of the package with a measured leak rate (R1) where the package volume and leak test conditioning parameters influence the measured
value of (R1). The equivalent standard leak rate shall be expressed in units of atmospheric cubic centimeters per second (atm cm3/s).
d.Sensitivity. (Minimum detectable leak rate). Sensitivity is the smallest leak rate that an instrument, method, or system is capable of measuring or detecting under specified conditions. To compare sensitivities of two or more different test conditions, the sensitivity should be converted to sensitivity under standard conditions by using the appropriate conversion factors.
e.Ambient background. For a scintillation-crystal counting station, ambient background is the oscillatory reading obtained on the meter readout due to electrical noise plus the reading in counts per minute due to cosmic radiation that penetrates the lead shielding of the closed crystal system. This value must be determined just before device counting.
METHOD 112E
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MIL-STD-202G
2. TEST CONDITIONS.
2.1 Selection. There are six test conditions (A, B, C, D, E, and F) in this method. Test conditions A and B utilize oil as a means to detect gross leaks by the observation of bubbles. Both of these tests will detect leaks of a nominal value of 10-5 atm cm3/s. Test condition B is used to test parts which will not withstand the temperature required in test condition A. Test condition C detects fine leaks by using a tracer gas and apparatus to measure a leakage rate (R1) to a nominal of 10-8 atm cm3/s. The apparatus can be calibrated for any leakage rate within its range. Test condition D utilizes a fluorocarbon liquid at 125°C ±5°C (257°F ±9°F) at ambient pressure and detects gross leaks by the observation of bubbles. This latter test condition provides an alternate gross leak test to test condition A. Test condition E utilizes two fluorocarbon liquids; one under pressure followed by the other at 125°C ±5°C (257°F ±9°F) at room ambient pressure and detects gross leaks by the observation of bubbles. This test condition provides a measure of leakage rate of a nominal value of 10-5 atm cm3/s. Test condition F utilizes a fluorocarbon liquid and a fluorocarbon vapor detector to detect gross leaks. Test condition F provides a measure of leakage rate of a nominal value of 10-5 atm cm3/s. Both test conditions C and F require more expensive commercial equipment and trained personnel, but the equipment provides automatic and quantitative readings. The selection of a test condition to perform the seal test depends on the state of the art of component part manufacturing, the leakage rate which can be tolerated for the desired reliability, the frequency of testing, range of test conditions, cavity size, and whether the testing is to be on a 100 percent production or on a periodic basis. The individual specification shall specify the test condition letter required. When test condition C is specified, the procedure number (see 5.4) and the degree of leakage rate sensitivity required shall be included. The following is included as a sensitivity and applicability guide:
Test condition A (bubble test - mineral oil or peanut oil at 125°C ±3°C (257°F ±6°F)). Use when a nominal sensitivity of 10-5 atm cm3/s is sufficient.
Test condition B (bubble test - silicone oil at room ambient temperature 25°C ±2°C (77°F ±3.6°F), with a vacuum resulting in an absolute pressure of not greater than 1.5 inches of mercury (not greater than 38.1 torr). Use to test parts which cannot withstand the temperature of test condition A, and when a nominal sensitivity of 10-5 atm cm3/s is sufficient.
Test condition C (tracer gas test). Use when a nominal sensitivity of 10-8atm cm3/s is sufficient.
Procedure I |
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(For parts having an evacuation tube which is unsealed and is to be connected to the leak |
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detector for test purposes.) |
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Leak from outside of part to inside. Check the seal of evacuation tube by backfilling with |
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air or gas. Then subject to procedure IIIa, IIIb, IIIc or IV. |
Procedure II |
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(For parts having an evacuation tube which is unsealed to permit pressurizing with a |
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tracer gas.) |
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Leak from inside of part to outside. Check the seal of evacuation tube by backfilling with |
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air or gas. Then subject to procedure IIIa, IIIb, IIIc, or IV. |
Procedure III |
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(For parts which are to be tested without breaking their seals.) |
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Fixed method (IIIa) - Part not filled with tracer gas. Force gas into leaks, then detect its |
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escape. Then subject to test condition A, B or D (see 5.4.3.2.1 permitting use of water), |
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as applicable, to check for gross leaks. |
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IIIb - Part not filled with tracer gas. Force radioactive gas into leaks, then use a |
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radioactivity counter to determine gas that has entered part. Then subject to test condition |
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A, B, or D (see 5.4.3.2.2.4 permitting use of water), as applicable, to check for gross |
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leaks. |
METHOD 112E |
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11 October 1988 |
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MIL-STD-202G
Flexible method (IIIc). Part not filled with tracer gas. Force gas into leaks then detect its escape. Then subject to test condition A, B or D (see 5.4.3.2.3 permitting use of water), as applicable, to check for gross leaks.
Procedure IV - (For parts which are to be tested without breaking their seals.) Parts backfilled with tracer gas as normally supplied.
Leak from inside of part to outside, then subject to test condition A, B or D (see 5.4.4 permitting use of water), as applicable, to check for gross leaks.
Test condition D (gross leak bubble test - Fluorocarbon liquid at 125°C ±5°C (257°F ±9°F). Use as an alternate when test condition A is specified as a gross leak test.
Test condition E (bubble test - two fluorocarbon liquids - one at pressure followed by immersion in a second liquid at 125°C ±5°C (257° ±9°F). Use when a nominal sensitivity of 10-5 atm cm3/s is sufficient.
Test condition F (fluorocarbon vapor detection test). Use when a nominal sensitivity of 10-5 atm cm3/s is sufficient.
2.2 Substitution. The individual specification should specify the lowest sensitivity test condition which is practical; a higher sensitivity test which can be calibrated shall be permitted, i.e., if a test is specified such as test condition A requiring a sensitivity of 10-5 atm cm3/s, test condition C may be used since it can be calibrated to read this leakage rate. Substitution of test condition B or C should be made to test condition A when parts are rated at a temperature lower than 130°C (266°F). Test condition D may be substituted for test condition A.
3. TEST CONDITION A.
3.1 Materials.
3.1.1 Mineral oil or peanut oil. The oil used for the bath shall be clear mineral oil or peanut oil having a universal Saybolt viscosity of 175 to 190 seconds when tested at 38°C (100°F).
3.2 Apparatus.
3.2.1 Heated oil container. The container for the oil bath shall be of sufficient depth to immerse the uppermost portion of the enclosure or seal to be tested to a depth of 1 inch (25.4 mm) below the surface of the bath. The container shall be capable of maintaining the oil at the required temperature.
3.3Precautions. This condition shall not be used for parts rated at a temperature lower than 130°C (266°F). It shall not be used for parts which contain seals made of materials which will outgas due to the temperature of the bath.
3.4Preparation of specimens. Immediately before immersion it shall be determined that the specimen is at room ambient temperature and free of any foreign (including labels) matter. If applicable, the critical side of the specimen shall be determined. The critical side is defined as the side having the greatest number of seals or length of seal.
METHOD 112E
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MIL-STD-202G
3.5 Procedure. This test consists of one immersion of the specimen or groups of specimens into a bath of clear mineral oil or peanut oil (see 3.1.1) maintained at a temperature of 125°C ±3°C (257°F ±6°F). The specimen shall be placed in the oil bath with the critical side (or side of special interest) in a horizontal position facing up. If the specimen has no critical side, it shall be placed in the bath with its major axis in a horizontal position. The specimen shall be completely submerged in the bath, with the uppermost portion of the enclosure or seal at a depth of 1 inch (25.4 mm), and left in that position for a minimum duration of 1 minute. The specimen shall be carefully observed during the entire duration of the immersion for indication of a poor seal as evidenced by a continuous stream of bubbles emanating from the specimen. After the test is completed, the specimen shall be cleaned in a suitable degreaser and permitted to dry thoroughly before any additional tests are performed.
4. TEST CONDITION B.
4.1 Materials.
4.1.1 Silicone oil. The oil used for the bath shall be clear silicone oil having a viscosity of 20 centistokes at 25°C (77°F).
4.2 Apparatus.
4.2.1Reduced pressure vessel. The vessel for the oil bath shall be of sufficient depth to immerse the uppermost portion of the enclosure or seal to be tested to a depth of 1 inch (25.4 mm) below the surface of the bath, over which can be drawn a vacuum resulting in an absolute pressure of not greater than 1.5 inches of mercury (not greater than
38.1torr).
4.2.2Vacuum pump. The vacuum pump shall be capable of evacuating and holding a vacuum resulting in an absolute pressure of not greater than 1.5 inches of mercury (not greater than 38.1 torr) for a minimum duration of 1 minute in the reduced pressure vessel (see 4.2.1).
4.2.3Degassing of silicone oil. The silicone oil shall be placed in the test fixture and a pressure of 1 inch (25.4 mm) of mercury or less attained over the fluid for as long as is necessary to degas the fluid. Such degassing is complete when no further bubbling or frothing is present in the fluid. Throughout the test, components shall be lowered gently in the fluid to prevent aeration of the fluid. The fluid shall not be poured from one container to another without first being degassed again before testing.
4.3Preparation of specimens. As specified in 3.4.
4.4Procedure. This test consists of one immersion of the specimen or group of specimens into a bath of clear silicone oil (see 4.1.1) maintained at room ambient temperature. The specimen shall be placed in the oil bath with the critical side (or side of special interest) in a horizontal position facing up. If the specimen has no critical side, it shall be placed in the bath with its major axis in a horizontal position. The specimen shall be completely submerged in the bath with the uppermost portion of the enclosure or seal at a depth of 1 inch (25.4 mm). A vacuum resulting in an absolute pressure of not greater than 1.5 inches of mercury (not greater than 38.1 torr) shall be drawn and held over the bath for a minimum duration of 1 minute. The specimen shall be carefully observed during the entire duration of the immersion for indication of a poor seal as evidenced by a continuous stream of bubbles emanating from the specimen. After the test is completed, the specimen shall be cleaned in a suitable degreaser and permitted to dry thoroughly before any additional tests are performed.
4.5Precaution. This test condition should not be used either as a separate test or a gross leak test for component parts which are to be subsequently attached to printed circuit board assemblies. Since complete removal of silicone oil residues is difficult, the oil will be transferred unknowingly to other parts during assembly processes. Traces of silicone can result in poor solder wettability of component part leads, poor adhesion to sealants, and mealing of the conformal coating on the printed circuit board.
METHOD 112E
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MIL-STD-202G
5. TEST CONDITION C.
5.1 Materials.
5.1.1 Tracer gases. When performing tests in accordance with procedures I, II, and IV of this condition, tracer gases, i.e., helium, argon, or other rare gas, or a mixture of a gas with nitrogen (such as 90 percent nitrogen and 10 percent helium) shall be used. The tracer gas used in procedures IIIa and IIIc shall be helium. The tracer gas used in procedure IIIb shall be the radioactive gas, krypton 85.
5.2 Apparatus. For all the procedures of this test condition, the test apparatus, exclusive of pressurization equipment, shall be calibrated using a diffusion type calibrated standard leak at least once each working shift.
5.2.1 Mass-spectrometer-type leak detector. For procedures I, II, IIIa, IIIc, and IV of this test condition, a commercially available mass-spectrometer-type leak detector, preset to read a tracer-gas content, shall be used to measure the leakage rate of gas through a faulty seal. Another instrument may be used if it can be demonstrated to the Government that the instrument, properly calibrated to read tracer-gas content, has the required leakage-detection sensitivity (see 2.1).
5.2.1.1Chambers. Depending on which procedure is used (see 5.4), suitable pressure or vacuum chambers are
required.
5.2.1.2Pumps. Depending on which procedure is used (see 5.4), suitable pressure or vacuum pumps are
required.
5.2.2 Radioactive-gas detection apparatus. Apparatus for procedure IIIb shall consist of:
a.Radioactive tracer gas activation console.
b.Counting equipment consisting of a scintillation crystal, photomultiplier tube, preamplifier, ratemeter, and krypton 85 reference standards. The counting station shall be of sufficient sensitivity to determine through the device wall the radiation level of any krypton 85 tracer gas present within the device. The counting
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station shall have a minimum sensitivity, in c/m per Ci, corresponding to a leak rate of 10 atm cm / s of krypton 85 and shall be calibrated at least once every working shift using krypton 85 reference standards and following the equipment manufacturer's instruction.
c.A tracer gas consisting of a mixture of krypton 85 and dry nitrogen. The concentration of krypton 85 in dry nitrogen shall be no less than 100 microcuries per atmospheric cubic centimeter. This value shall be determined at least once each 30 days and recorded in accordance with the calibration requirements of this standard.
5.3Supplementary tests. When parts to be tested are normally evacuated through a tube and are sealed in some manner prior to delivery, procedures I and II (see 5.4.1 and 5.4.2) will require a separate verification of the seal of the evacuation tube in conjunction with this test method, using the mass-spectrometer-type leak detector (see 5.2.1).
The verification may be accomplished by backfilling the specimen with air or gas at a specified pressure and then submitting the specimen to either procedure IIIa, IIIb, IIIc, or IV.
METHOD 112E
11 October 1988
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