Although the environmental test and the reliability test are closely related, they have distinct differences in the purpose of the test, the amount of environmental stress used, the selection criteria of the environmental force value, the type of test, the test time, and the test termination criteria.
Test purposes:
The environmental test examines the adaptability of the product to the environment, determines whether the environmental adaptability design of the product meets the contract requirements, and provides decision-making basis for receiving and rejecting. The reliability test is to quantitatively evaluate the reliability of the product, that is, the probability that the product will complete the specified function within the specified time under the specified environmental conditions.
Number of environmental stresses used:
For environmental testing, 19 test items are specified in GJB 150, 20 environmental test items are specified in MIL-STD-810 D, and 810F is added to 24 test items, including environmental stresses that are more important to the product, such as : temperature, wet bar, salt spray, vibration shock, pressure, solar radiation, sand dust, rain and so on. The test product should be selected according to its future environmental conditions and the degree of impact. Generally, more than 10 environmental stresses should be investigated. The reliability test, based on comprehensive simulation, only combines the combined environmental stress (temperature, humidity, vibration) with electrical stress. It can be seen that the number of environmental stresses selected for the reliability test is much less than the environmental test.
Guidelines for the selection of environmental stress values:
The environmental test basically adopts extreme conditions and uses harshness instead of mildness, that is, the most extreme environmental conditions that the product may encounter during the life cycle are used as test conditions. Many tests are somewhat destructive and generally do not require simulated product operation during the test. The reliability test adopts the effective test, which is to simulate the main environmental conditions and dynamic changes encountered during storage, transportation and use. In GJB-889 and the corresponding MIL-STD-781 D, a set of methods for determining the environmental profile according to the task profile and then simplifying the environmental profile to the test profile for long-term reliability assessment of the product is specified. The reliability test generally does not cause damage to the product. It needs to simulate the working state of the product. The test conditions used are mostly simulated in the milder stress environment often encountered in the work, and the value is much lower than the environmental test.
Test type:
Environmental tests usually use a single factor test and a multi-factor combination test, which are applied to the product in a certain order. The reliability test uses a comprehensive stress test to apply multiple environmental stresses in the same space and apply them to the sample at the same time, which more realistically simulates the influence of environmental conditions. In order to improve the accuracy and reliability of the test results, environmental tests have begun to focus on the development of comprehensive tests, and actively develop large-scale multi-functional environmental test equipment capable of simultaneously applying temperature, humidity, vibration, radiation, dust, wind, rain and other stresses.
Test time:
In environmental testing, the time of each test depends on the selected test and the specific test procedure. It is only because the time required for performance testing at each stage is different, and the test time is much shorter than the reliability test. The reliability test time depends on the reliability index value to be verified and the statistical test plan selected and the quality of the product itself. The time cannot be determined, and the total number of hours of the test product reaches the specified value or can be accepted, and the judgment is rejected.
Test termination criteria:
Environmental tests are not allowed to fail. If a product fails, it is considered to pass the test. The test is stopped and the failure analysis is carried out, and corrective measures are taken to improve the design. This is the TAAF process for environmental testing. The reliability test is a test that expresses the results with a certain statistical probability. The number of faults allowed is determined according to the reliability quantitative indicators and the selected statistical schemes required by the contract. The test must be carried out until the specified total number of hours is reached. A Fault Reporting, Analysis and Corrective Action System (FRACAS) should be established throughout the test.
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